United States

Department of

Agriculture

Animal and

Plant Health

Inspection

Service

Veterinary

Services

Centers for

Epidemiology and

Animal Health

July 2008

Equine Herpesvirus

Myeloencephalopathy:

Mitigation Experiences,

Lessons Learned, and

Future Needs
Mention of companies or commercial products does

not imply recommendation or endorsement by the

USDA over others not mentioned. USDA neither

guarantees nor warrants the standard of any product

mentioned. Product names are mentioned solely to

report factually on available data and to provide

specific information.

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#N522.0708

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Cover photo courtesy of

Charles Kerlee
USDA APHIS VS / i

Acknowledgments

Contributors

USDA, Animal and Plant Health Inspection Service (APHIS), Veterinary Services

(VS), Centers for Epidemiology and Animal Health (CEAH).

Dr. Josie Traub-Dargatz, 970-494-7000, Josie.Traub-Dargatz@ColoState.edu

Dr. Barbara Bischoff, 970-494-7000, Barbara.A.Bischoff@aphis.usda.gov

We would like to thank those who provided their ideas and experiences related to

equine herpesvirus myeloencephalopathy (EHM) as well as those who reviewed

this report for content and readability:

Drs. Lutz Goehring, Paul Lunn, Barry Meade, and Peter Timoney provided their

opinions on the status of EHM and needs for further information related to this

disease.

Those who were interviewed to gather information related to mitigation of recent

EHM disease events included Dr. Rick Arthur, Dr. Tim Boone, Dr. David

Fitzpatrick, Rusty Ford, Dr. Kent Fowler, Dr. Nancy Halpern, Dr. Steve Halstead,

Dr. Guy Hohenhaus, Dr. Don Hoenig, Dr. Don Hopson, Dr. Mary Lis, Dr. John

Madigan, Dr. Paul Morley, Dr. Steve Reed, Dr. Alfredo Sanchez, Dr. Mike Short,

Dr. Robert Stout, and Dr. Nat White.

Dr. Stanley Bruntz for providing an update on status of reporting of EHM through

the National Animal Health Reporting System (NAHRS).

Drs. Alyson Wiedenheft, Lindsey Garber, Tom Kasari, and Barry Meade for

assistance in development of the example study design and survey instrument

for collection of epidemiologic data in future EHM outbreaks, and Dr. Ann

Seitzinger for input on components of a cost estimate.

Patti Rosenfelder for editing and revisions of this document.

Reviewers of the report included Drs. Timothy Cordes, Sunny Geiser, Albert

Kane, Barbara Porter-Spaulding, Keith Roehr, and Alyson Wiedenheft.

Larry M. Granger

Director

Centers for Epidemiology and Animal Health
ii / EHM 2008

Suggested bibliographic citation for this report:

USDA. 2008. Equine Herpesvirus Myeloencephalopathy: Mitigation Experiences,

Lessons Learned, and Future Needs

USDA-APHIS-VS, CEAH. Fort Collins, CO

#N522.0708

Contacts for further information:

Questions or comments: Dr. Josie Traub-Dargatz or Dr. Barbara Bischoff

(970) 494-7000

Information on reprints or other reports: Ms. Abby Fienhold (970) 494-7000

E-mail: NAHMS@aphis.usda.gov
USDA APHIS VS / iii

Table of Contents

Executive Summary 1

Part I: Interviews with Selected Scientists and Researchers Concerning

Equine Herpesvirus 7

General Observations Related to EHM 7

Research and Investigation Needs 9

Miscellaneous Comments and Suggestions 14

Needs Other Than Those Specifically Related to Research 14

Summary from Interviews with Selected Scientists and Researchers 15

Part II: Summary of Interviews with Mitigators of Recent EHM Disease

Events 17

Case Recognition and Person Responsible for Mitigation 19

Quarantines 20

Case Definition/Categorization 22

Testing or Monitoring Performed 22

Scope of the Outbreak or Investigation 25

General Biosecurity Procedures and Other Control Strategies 28

Identified Needs 30

Planning 30

Disease control/biosecurity measures 31

Diagnostics 32

Communication materials and plan 32

Educational material developed or needed 32

Communication Efforts 34

Cost of Outbreak and Management Issues 36

Lessons Learned 38

Planning for disease detection and response 38

Biosecurity and disease containment 39

Information about EHV-1 and EHM 41

Education and communication 41

Research Needs 42

EHV-1 epidemiology 42

Diagnostics 44

Vaccine development and treatment options 44

 Summary and Conclusions from Interviews with Mitigators of EHM

 Disease Events 45
iv / EHM 2008

Part III: Recent Publications That Address Needs Suggested in

Interviews 46

Part IV: EHM Reporting in the National Animal Health Reporting System 56

Method of Reporting EHV and EHM in NAHRS 57

Appendix A: Interview Form for Mitigators of Recent Outbreaks

of EHM 58

Appendix B: USDA:APHIS:VS Center for Emerging Issues Emerging Animal

Disease Notice 63

Appendix C: Example of a Study Design for Collection of Epidemiologic and

Cost Estimate Data from Future EHM Outbreaks 67

Appendix D: List of Selected Sources of Information on EHM 74
Executive Summary

USDA APHIS VS / 1

In January 2007, the Center for Emerging Issues (CEI) at USDA-APHIS-VSCenters for Epidemiology and Animal Health (CEAH) issued an Emerging

Disease info sheet which suggested that the neurologic manifestation of equine

herpesvirus-1 (EHV-1) met the criteria for an emerging infectious disease based

on: (1) the occurrence of a more virulent strain of EHV-1 than previously seen in

the United States, and (2) increased recognition of outbreaks of disease at

equine events with associated high case fatality rate. The neurologic

manifestation of EHV-1 is also referred to as equine herpesvirus

myeloencephalopathy (EHM). Equine herpesvirus (EHV) was a focus of

presentations at multiple equine industry and veterinary meetings in 2007,

illustrating concern within the equine industry related to EHV.

Much remains to be discovered about EHV-1 and the neurologic manifestation of

the disease that could help prevent or mitigate outbreaks. The objective of this

report is to provide information—from interviews conducted with selected

scientists and researchers concerning EHV, including EHM, and those involved

in mitigation of recent outbreaks or trace-outs related to EHM—that may be

useful in mitigating future outbreaks and developing research needs, response

plans, and communication materials related to EHM.

Response to a contagious disease situation begins with recognition of clinical

signs and identification of the disease agent, thereby allowing the private

veterinary practitioner to determine the optimal care for the sick horse or horses.

Response efforts then progress to mitigation efforts at the population level, to

prevent spread of disease to other horses at the facility or to horses at other

facilities. Along with information related to EHM in the individual horse, this report

contains two broad aspects of EHM outbreak prevention and control: (1) disease

specific aspects, primarily at the population level, and (2) the regulatory

framework needed to respond efficiently to EHM outbreaks. Not only is there still

much to learn about the disease, but also much discussion needs to take place

regarding what is known about EHM, what is not known, and how that affects

recommendations for disease prevention and control. Interviewees emphasized

the need for preplanning regarding information resources, identification of and

contact information for disease experts, defining and clarifying roles and

authorities of various entities potentially involved in a response, and determining

when action will be taken and by whom.

Executive Summary
Executive Summary

2 / EHM 2008

The interviewees expressed the need for:

• Critical evaluation of existing literature;

• Vaccine development and study of the efficacy of currently available vaccines

for the prevention of EHM;

• A rapid horse-side test that detects EHV-1 and distinguishes the

neuropathogenic strain of the virus from the wild-type (non-neuropathogenic

strain);

• Individual animal risk factors, transmission characteristics, latency studies,

and prevention options;

• Clinical trials of treatment efficacy to determine appropriate treatment

options; and

• Preplanning to facilitate the effectiveness of outbreak response efforts.

Dr. Peter Timoney, Professor, University of Kentucky, Gluck Equine Research

Center and Chair of the Infectious Diseases of Horses Committee of the United

States Animal Health Association (USAHA), pointed out that “...within the past

few years, a mutant of the wild-type of EHV-1 has been identified which evidence

would indicate is very frequently associated with outbreaks of EHM. Also, this

mutant has been identified among isolates of EHV-1 made prior to 2000. As the

distribution of this virus mutant becomes more widespread in the equine

population, the frequency and severity of outbreaks of EHM is likely to increase

further unless measures to control its spread and occurrence of the disease can

be developed.” Although this mutant strain of EHV-1 has been frequently

associated with EHM, the wild-type EHV-1 also has been known to be associated

with EHM cases. One of the interviewees stressed that “...we are missing an

opportunity to advance the knowledge of EHM by not gathering observational

data during outbreaks.” To address that issue, CEAH has developed an Example

Study Design and Survey Instrument for Collection of Epidemiologic and Cost

Estimate Data from Future EHM Outbreaks (Appendix C) that is available for use

by those responding to future EHV-1 outbreaks.

Telephone interviews, using open-ended questions (Appendix A), were also

conducted with those who had been involved in mitigation of recent EHM disease

events, to gather information that could describe the identification of EHM cases

and methods used to mitigate the outbreaks, and to gather comments related to

lessons learned, and need for additional educational materials and further

research related to EHM. Interviews were conducted from July through October

2007 relating to 16 EHM disease events—13 outbreaks and 3 trace-outs.
Executive Summary

USDA APHIS VS / 3

Results of these interviews demonstrate the variation of settings, persons

responsible for initial recognition, authorities responsible for taking action, level

of response, and approaches in mitigating EHM disease events. In some States,

EHV-1 was reportable; in others it was not. In some it was reportable, but not

actionable—that is, the State veterinary regulatory officials did not consider it a

disease for which they would take regulatory action in response to the disease

report, but they did want to be aware of the situation, and in some situations

offered advice. Action taken ranged from State mandatory quarantines to a

voluntary “stop-horse-movement” action recommended by a university clinician

to a private riding stable owner and the attending private veterinarian. The criteria

for release of quarantine and approaches to testing in-contact horses varied

among outbreaks. Nine laboratories were used for polymerase chain reaction

(PCR) testing during the outbreaks. A few mitigators said it would be beneficial to

understand how results from the laboratories compared and that validation of

testing by a reference laboratory or protocol would be beneficial. Several

mitigators indicated it would be beneficial to develop a consistent approach

across States to address future EHV-1 neurological disease events.

Although biosecurity and disease containment measures were implemented in

the face of the disease events, it wasn’t until someone with disease control

experience evaluated the situation on-site that some breaches in containment

practices were discovered. These observations demonstrated the need for

someone with a critical eye and attention to detail to verify adherence to

recommended biosecurity and disease containment measures.

Few respondents tracked all the costs associated with mitigating the EHM

disease events, but indications are that outbreak mitigation can be quite

expensive. Mitigators reported that associated categories of cost included human

resources, loss of income due to cancellation or reduction in scope of events/

racing, disrupted training schedules, diagnostics, treatment of horses, and

biosecurity procedures (supplies and, in some instances, personnel required to

assure compliance with protocols).

This report contains a comprehensive description of research, education, and

other needs expressed by mitigators related to EHM.

Next steps include:

• Discussion among veterinary regulatory officials (State veterinarians, racing

commission veterinarians, etc.) to determine areas of consensus regarding

case definitions, outbreak definition, and best management practices related to

EHM disease events. The USAHA Infectious Diseases of Horses Committee

meeting would be a logical venue for such a discussion.
Executive Summary

4 / EHM 2008

• Establishment of validation criteria for PCR testing for EHV-1 and distinction

between the neuropathogenic strain and the non-neuropathogenic strain. The

American Association of Veterinary Laboratory Diagnosticians (AAVLD) would

be a starting point for dialogue regarding guidelines or oversight of such

validation.

• To identify potential risk factors for EHM, consistent data will need to be

gathered across outbreaks. Compilation of data from multiple outbreaks will be

necessary due to the small number of EHM cases associated with individual

outbreaks. Data from EHM outbreaks collected by those in the field will need to

be submitted by outbreak responders to one entity to be stored. When

sufficient data are compiled, analyses can be performed.

• Evaluation of the efficacy of currently available commercial vaccines through

challenge models and detailed epidemiological studies.

• Development of new vaccines that are efficacious in preventing EHM by

research institutions and commercial interests.

Background and profile of disease emergence

Equine herpesvirus type 1 (EHV-1) is primarily a respiratory pathogen

associated with a variety of clinical manifestations in horses. In addition to

being a significant cause of respiratory illness and abortion in horses, EHV1 is responsible for neurological disease, called equine herpesvirus

myeloencephalopathy (EHM). EHV-1 is enzootic throughout the world and

almost all horses older than 2 years of age have been exposed. Following

initial exposure, EHV-1 has the ability to develop into an inapparent, latent

infection. It is this ability to reside as a silent and persistent infection in

horses which provides a reservoir of virus for continual transmission. The

incubation period of EHV-1 is 1–10 days; typically signs are seen within 1–3

days. Viral shedding usually occurs for 7–10 days, but can occur up to 28

days from the onset of signs. The neurologic signs include ataxia, urinary

bladder atony and reduced tail tone. In severe cases, horses will be unable

to stand; these cases have a very poor prognosis. Foals are rarely affected

with the neurologic form of EHV-1, and no sex predilection is seen.

Treatment is supportive and tailored to the specific case. Antivirals such as

acyclovir, valcyclovir, famiciclovir and penciclovir have been used, but

efficacy of these drugs has yet to be determined in equids.

Excerpt from: USDA-APHIS-VS-CEAH-CEI info sheet: “Equine Herpes

Myeloencephalopathy: A Potentially Emerging Disease.”
Executive Summary

USDA APHIS VS / 5

In January 2007, CEI released an Emerging Disease info sheet which suggested

that the neurologic manifestation of EHV-1 met the criteria for an emerging

infectious disease, based on: (1) the occurrence of more virulent strains of EHV1 than seen previously in the United States, and (2) increased recognition of

outbreaks of disease at equine events with associated high case fatality rate. In

this same summary, CEI indicated that many data gaps existed, and more

investigations were needed to better understand the disease and to identify

factors playing a role in this possible emergence. In addition, CEI suggested that

identifying such factors would assist with control of future outbreaks. The CEI

info sheet indicated that the general ecology of this disease is not fully

understood; additional research is needed. A standard case definition and

standardized reporting requirements for EHM would be good starting points for

further study, with subsequent studies comparing cases to controls to determine

risk factors for individual horses to develop EHM. The info sheet is included as

Appendix B in this report and is also available at: http://www.aphis.usda.gov/vs/

ceah/cei/taf/emergingdiseasenotice_files/ehv.pdf

Several recent reports (Allen et al., Goodman et al., Nugent et al.) describe a

point mutation in the equine herpesvirus-1 polymerase. Terms that have been

used in the scientific literature to describe the strain of the virus that contains this

point mutation are “neuropathogenic or mutant strain.” Terms used to describe

the strain of the virus that does not contain this mutation are “the nonneuropathogenic or wild-type.” Both are field strains of EHV-1. The mutant, or

neuropathogenic, strain of the virus was found to exist as early as 1970, based

on testing of archival samples. The frequency of paralytic outbreaks appears to

be increasing, especially in the period 2000–05. Outbreaks of EHM investigated

during this period were predominately associated with the mutant

neuropathogenic strain of the virus; however, the wild-type non-neuropathogenic

strain has been implicated as a cause of two EHM outbreaks (Appendix B).

Further clarification of the terminology used in recent scientific literature related

to the EHV-1 virus includes that the mutation has been reported to be a single

nucleotide polymorphism (SNP) within the EHV-1 gene encoding for the viral

DNA polymerase (open reading frame 30, ORF30). This point mutation has been

reportedly associated with neuropathogenicity and higher levels of viremia in

horses. A PCR test was developed that allows for differentiation of the nonneuropathogenic (wild-type) from the neuropathogenic strains (mutant) of EHV-1

by detecting the SNP genetic marker (ORF30 A2254 wild-type to G2254 mutant). An

SNP in the EHV-1 polymerase gene results in a single amino acid sequence
Executive Summary

6 / EHM 2008

difference in the DNA polymerase protein. The N752 Pol variant (Asparagine=N

or Asn) has been referred to as the non-neuropathogenic or N752 sequence

variant of EHV-1 DNA Pol. The D752 Pol variant (Aspartic acid=D or Asp) has

been referred to as the neuropathogenic variant or the D752 variant.

There is speculation about the evolutionary origin of the D/N752 sequence

variation of EHV-1. Goodman et al. suggested that the N752 variant arose from

the D752 progenitor and that the prevalence of the N752 variant is higher. These

authors suggested that the N752 variant may have selective advantage over the

D752 variant.

EHV was a focus at several meetings in 2007, including the American Horse

Council, the Infectious Diseases of Horses Committee of USAHA, the American

College of Veterinary Internal Medicine (ACVIM), the National Institute for Animal

Agriculture (NIAA) and the American Association of Equine Practitioners (AAEP),

illustrating the heightened interest in EHV.

The purpose of this report is to serve as a source of information and as a basis

for further discussion by veterinary officials and those in the equine industry

regarding the prevention and mitigation of EHM outbreaks. In addition, the report

highlights some of the research needed to provide information that would provide

scientific evidence on which to base best management practices for EHV-1, and

specifically EHM. The compilation of information obtained from interviewing

mitigators of recent EHM disease events also was undertaken in the hope of

providing lessons learned to benefit those responding to future EHM outbreaks.

References related to EHV-1 terminology:

1) Allen GP: Brief communications: Development of a real-time polymerase chain reaction assay

for rapid diagnosis of neuropathogenic strains of equine herpesvirus-1. J Vet Diagn Invest 19, 69–

72, 2007.

2) Goodman LB, Loregian A, Perkins GA, Nugent J, Buckles EL, Mercorelli B, Kydd JH, Palu G,

Smith KC, Osterrieder N, Davis-Poynter N: A point mutation in a herpesvirus polymerase

determines neuropathogenicity. PLoS Pathogens 3, 1583–1592, 2007.

3) Nugent J, Birch-Machin I, Smith KC, Mumford JA, Swann Z, Newton JR, Bowden RJ, Allen GP,

Davis-Poynter, N: Analysis of Equid Herpesvirus 1 Strain Variation Reveals a Point Mutation of the

DNA Polymerase Strongly Associated with Neuropathogenic versus Nonneuropathogenic Disease

Outbreaks, J Vir 80, 4047–4060, 2006.
USDA APHIS VS / 7

Part I: Interviews with Individuals Having Expertise Related to Equine Herpesvirus

Based on the emergence of EHM as a more virulent disease than previously

considered and interest from the equine industry, interviews were conducted with

selected scientists and researchers concerning equine herpesvirus infections

and, specifically, EHM. Interview topics included the current status of knowledge

related to EHM, need for EHM research, and actions to control the disease. The

individuals interviewed were: Dr. Lutz Goehring, Assistant Professor in Clinical

Sciences, Colorado State University; Dr. Paul Lunn, Professor and Department

Head of Clinical Sciences, Colorado State University; Dr. Barry Meade,

USDA:APHIS:VS Veterinary Medical Officer and Ph.D. candidate in equine

infectious diseases at the University of Kentucky; and Dr. Peter Timoney,

Professor, University of Kentucky Gluck Equine Research Center and Chair of

the Infectious Diseases of Horses Committee of the USAHA. Each person

interviewed was given a draft of his comments to review and edit prior to

finalization of the interview. Comments from those interviewed are organized

under several key headings addressed in the interview. The interviews with these

four people were conducted and finalized from July to September 2007.

General Observations and Comments Related to EHM

Dr. Peter Timoney:

1. EHM is not a new disease. For example, in 1972, the Irish Department of

Agriculture created an Equine Diseases Section at the Central Veterinary

Research Laboratory to work on equine diseases following a series of major

outbreaks of EHM that affected the Thoroughbred breeding industry that year.

2. This led to the establishment of an Irish Liaison Committee on Equine

Diseases with representation from various sectors of the horse industry,

government laboratory service, and university and equine veterinary

professions; so, historically, it can be said that outbreaks of EHM have been

the stimulus behind advancements in efforts to control not only EHM but also

other equine diseases, especially those that are contagious.

3. Since 2000, there has been an increase in the frequency of reported EHM

outbreaks in the United Kingdom, Western Europe, and also in the United

States, in a variety of settings including racetracks, show grounds, breeding

farms, and riding schools. This disease does not respect any venue.

4. Within the past few years, a mutant of the wild-type of EHV-1 has been

identified which evidence would indicate is frequently associated with

outbreaks of EHM. Also, this mutant has been identified among isolates of

EHV-1 from prior to 2000. As the distribution of this virus mutant becomes

Part I: Interviews with Selected Scientists and Researchers Concerning

Equine Herpesvirus
Part I: Interviews with Individuals Having Expertise Related to Equine Herpesvirus

8 / EHM 2008

more widespread in the equine population, the frequency and severity of

outbreaks of EHM are likely to increase further unless measures to control its

spread and occurrence of the disease can be developed.

Dr. Paul Lunn:

1. ACVIM formed a task force to develop a consensus statement related to

equine herpesvirus. In March 2007, Dr. Brad Smith, president of the Large

Animal Specialty for ACVIM, selected the members of the task force, which

includes Drs. Nick Davis-Poynter, Julia Flaminio, David Horohov, Klaus

Osterrieder, Nicola Pusterla, Hugh Townsend, and myself. In the consensus

statement, we plan to address nine different areas related to EHV and fully

reference each section. The statement, expected to be released in fall 2008,

will address all aspects of EHV, not just the neurologic form of the disease,

and will include the following topics:

• Outbreak response—What are the key things I need to know as I plan for,

and respond to, an outbreak of clinical EHV-1 infection?

• Diagnostic testing—What kinds of viral detection tests should I select for

diagnosis, prognosis, and screening of horses for EHV-1 and its strains?

• Identification of the D752 SNP—What are the clinical implications of the

DNApol SNP for veterinarians and horse owners?*

• Vaccination—How and when should I use current commercially available

vaccines to control EHV-1 infection and disease?

• Risk factors for disease—What are the risk factors for respiratory,

abortigenic, or neurologic disease caused by EHV-1?

• Pathogenesis—How and why does EHV-1 infection target the pregnant uterus

and CNS? Why do some horses but not others get neurological disease?

*The difference between the non-neuropathogenic (wild-type) and neuropathogenic (mutant)

EHV-1 field strains has been reported to be based on identification of the single nucleotide

polymorphism (SNP) within the EHV-1 gene encoding for the viral DNA polymerase (open reading

frame, ORF30) that is highly associated with neuropathogenicity for horses. A PCR test has been

reported to distinguish between the non-neuropathogenic (wild-type) and the neuropathogenic

strain (mutant) of EHV-1 by detecting the SNP genetic marker (ORF30 A2254 wild-type to G2254

mutant). This SNP in the EHV-1 gene results in a single amino acid sequence change in the DNA

polymerase protein from an N (Asparagine=Asn) to D (Aspartic acid=Asp).

References:

1) Nugent J, Birch-Machin I, Smith KC, Mumford JA, Swann Z, Newton JR, Bowden RJ, Allen GP,

Davis-Poynter, N. Analysis of Equid Herpesvirus 1 Strain Variation Reveals a Point Mutation of the

DNA Polymerase Strongly Associated with Neuropathogenic versus Nonneuropathogenic Disease

Outbreaks, J Vir 80, 4047–4060, 2006.

2) Allen GP. Brief communications: Development of a real-time polymerase chain reaction assay

for rapid diagnosis of neuropathogenic strains of equine herpesvirus-1. J Vet Diagn Invest 19, 69–

72, 2007.
USDA APHIS VS / 9

Part I: Interviews with Individuals Having Expertise Related to Equine Herpesvirus

• Disease control—Beyond vaccination, what are the key factors to consider in

controlling disease caused by EHV-1?

• Epidemiology—What, if anything, has changed about EHV-1 epidemiology?

• Treatment—What treatments are useful for EHM, beyond supportive care?

2. It is likely that the ACVIM task force also will provide a description of

limitations of the current knowledge of equine herpesvirus in the concluding

remarks of the consensus statement. These would be presented in general

terms and would not be considered as a list of recommended research

priorities.

Research and Investigation Needs

1. Critical evaluation of existing literature:

Dr. Peter Timoney: It would be worthwhile to compare EHM with herpesvirus

infections in other species with reference to risk factors, control methods, and

treatment (if any).

2. Vaccine development and study of impact of use of currently available

vaccines:

Dr. Barry Meade: While there are some data that would indicate the abortion

rates have gone down with the introduction of EHV vaccines, the current

available vaccines do not appear to stop viral shedding in animals with

respiratory disease. There is only limited evidence that vaccination is effective

against the neurologic form of EHV-1 and no commercial EHV vaccines are

currently labeled for prevention of this clinical condition. To my knowledge, they

cannot prevent the establishment of latency. Development of a vaccine that

could prevent latency and the neurological form of EHV should be a top priority

for research related to this disease.

Dr. Peter Timoney: A vaccine is needed to prevent or reduce the frequency of

latency as well as occurrence of EHM. Such a vaccine should stimulate the

appropriate cellular and humoral immune responses needed to control the cellassociated viremia.
Part I: Interviews with Individuals Having Expertise Related to Equine Herpesvirus

10 / EHM 2008

Dr. Lutz Goehring: There is a need to evaluate the impact of previous

vaccination against EHV-1 and occurrence of EHM. This would require

collection of detailed vaccination history on both EHM cases and non-cases

(controls) in naturally occurring outbreaks of disease.

3. Diagnostics:

Dr. Barry Meade: A rapid horse-side test similar to heartworm testing or the

Directigen test for equine flu—maybe something based on the nasal swab

PCR done at the University of Kentucky Veterinary Diagnostic Center or at

least a commercial version that can be done at other laboratories would help

manage outbreaks.

4. Epidemiology:

Dr. Barry Meade: We need to have a better handle on the EHV-1 neurological

events that occur yearly. They are probably handled fine from a regulatory

sense—such that I do not believe there has been documented spread from

one racetrack to another—but we are missing some good observational data.

There is more to an outbreak than when it starts and when it is over. In

general, there are benefits to be gained epidemiologically in developing best

practices for investigation and management of EHV-1 outbreaks. Specifically, I

think there are opportunities to determine risk factors that contribute to disease

spread, to gain a better understanding of dynamics of virus transmission in

natural populations, to evaluate the efficacy of response activities of regulators,

and to increase our response capacity to animal disease outbreaks.

Comments related to epidemiological research needs from the interviewed

EHV scientists and researchers are grouped into the following categories: risk

factor determination, transmission characteristics, response by regulatory

veterinary medicine to contain or limit the outbreak, latency studies, and

prevention and/or treatment options.

• Risk factor determination

Dr. Barry Meade: Gathering of information during disease outbreaks would

allow investigators to evaluate factors involved in EHM, such as recent

training, sales, EHV-1 vaccine use (date or type), whether acyclovir
USDA APHIS VS / 11

Part I: Interviews with Individuals Having Expertise Related to Equine Herpesvirus

decreases viral shedding, whether continued training increases the

isolation period, and the role of direct and indirect contact on spread of a

disease—grooms, tack, etc.

Dr. Peter Timoney: Determine risk factors for disease occurrence in

individual horses in order to generate ideas for future research on

prevention and treatment. If aged horses were to be at higher risk for EHM

during outbreaks, then investigation into the reasons for this predisposition

would be warranted.

Dr. Lutz Goehring: We need basic risk factor information related to EHM. It

seems that previous studies lacked large scope or were limited by

geographic occurrence of the outbreaks. Since there is currently no reliable

challenge model for EHM, study of naturally occurring EHM is our best

hope of gathering information regarding risk factors for the disease. My

EHM studies were conducted in Holland and were based on a small

number of cases of EHM, thus limiting their generalizability. However, these

studies do suggest potential risk factors to evaluate including age, breed,

and seasonality. The only other published field-based study looking at risk

factors for EHM was based on a single outbreak in Ohio—again, limiting

the application of the findings to other outbreaks. The study of multiple

outbreaks with collection of information on both cases of EHM and noncases (controls) could allow for determination of associated risk factors for

occurrence of EHM. If multiple outbreaks could be studied then some

comparisons across outbreaks could be made. There is a need to evaluate

the impact of previous vaccination against EHV-1 and occurrence of EHM.

This would require collection of detailed vaccination history on both EHM

cases and non-cases (controls). There is also a need to collect information

regarding transport or movement as this has been proposed to be

associated with EHM occurrence.

Dr. Paul Lunn: There is a need to understand why some horses get EHM

and others do not during an outbreak of naturally occurring disease. It

would require many different fields of expertise to investigate this. The

pathogenesis of EHM is still not well understood. Does EHM occur due to

an immune response? Why does the immune response not prevent this

disease or does it? It is difficult to study the pathogenesis of this disease as

there is not a good experimental challenge model for EHM. It is hard to

study EHM in a controlled challenge study setting when there is not a way

to reproduce EHM that is of equal severity as that observed in naturally

occurring outbreaks. Potential identification of risk factors for this disease
Part I: Interviews with Individuals Having Expertise Related to Equine Herpesvirus

12 / EHM 2008

as determined through epidemiologic studies of natural outbreaks may

allow us to control the disease without knowing the exact mechanism or

pathogenesis of the disease. In addition, there is a need to understand the

epidemiology of the virus better than we do now. Are there strain

differences that occur during an outbreak? Collection of samples during

outbreaks that would allow for strain typing of multiple isolates would be of

value. This would require sampling multiple horses during an outbreak.

Strain typing would require funding and someone with knowledge of the

technique for strain typing.

• Transmission characteristics

Dr. Barry Meade: How do we estimate transmission parameters? What is

the likelihood of contact between infected and susceptible animals? How

do we estimate the proportion of infection that is transmitted prior to

symptom onset and what role does this play in disease persistence or

spread?

• Response by regulatory veterinary medicine to contain or limit outbreak

Dr. Barry Meade: For those involved in regulatory veterinary medicine,

there needs to be some means of evaluating risk for a specific set of

circumstances so they can decide on an appropriate level of response. As

an example, what is the minimum length of time that horses will need to be

prohibited from moving and how do you evaluate your success in stopping

viral spread prior to resumption of movement? Should these criteria be

applied under all circumstances or is there a threshold for the number of

in-contact horses, the type of event, time of year, type of premises, etc.?

• Latency studies

Dr. Barry Meade: Does a productive immune response prevent latency and

can we easily identify latent carriers?
USDA APHIS VS / 13

Part I: Interviews with Individuals Having Expertise Related to Equine Herpesvirus

Dr. Lutz Goehring: Evaluation of the role of latency in EHM is needed.

Dr. Peter Timoney: It would be interesting to know if the two types of EHV-1

(wild-type and mutant)* can coexist in the same horse and if they can, what

the impact of one on the other may be, with regards to reactivation and

recrudescence.

• Options for prevention and treatment

Dr. Lutz Goehring: There is a need for evaluation of treatment and/or

prevention of EHM. Although not all horses that develop viremia go on to

develop EHM, viremia is necessary for EHM to occur. A method of

treatment, or a preventative that could limit viremia, may be protective

against the occurrence of EHM. Studies designed to test drugs in

prevention or treatment would be of value. In addition, drugs that may alter

coagulation could also be of benefit in prevention of EHM and might be

worth evaluation in treatment/placebo format in naturally occurring

outbreaks.

Dr. Peter Timoney: More needs to be known about the efficacy of various

treatment options (antiviral, aspirin, other) on the outcome of cases of the

disease in the field. We need to explore the options of treatment and

prophylaxis that hopefully could reduce shedding of virus and thus limit

spread of the virus once the disease has been identified. A vaccine

capable of preventing or reducing the cell-associated viremia is a critical

need. We must not close our minds off to any approach that might reduce

the impact EHM is likely to have in the future.

*The difference between the non-neuropathogenic (wild-type) and neuropathogenic (mutant)

EHV-1 field strains has been reported to be based on identification of the SNP with the EHV-1

gene encoding for the viral DNA polymerase (open reading frame, ORF30) that is highly

associated with neuropathogenicity for horses. A PCR test has been reported to distinguish

between the non-neuropathogenic (wild-type) and the neuropathogenic strains (mutant) of EHV-1

by detecting the SNP genetic marker (ORF30 A2254 wild-type to G2254 mutant). This SNP in the EHV1 gene results in a single amino acid sequence change in the DNA polymerase protein from an N

(Asparagine=Asn) to D (Aspartic acid=Asp).

References:

1) Nugent J, Birch-Machin I, Smith KC, Mumford JA, Swann Z, Newton JR, Bowden RJ, Allen GP,

Davis-Poynter N. Analysis of Equid Herpesvirus 1 Strain Variation Reveals a Point Mutation of the

DNA Polymerase Strongly Associated with Neuropathogenic versus Nonneuropathogenic Disease

Outbreaks, J Vir 80, 4047–4060, 2006.

2) Allen GP. Brief communications: Development of a real-time polymerase chain reaction assay

for rapid diagnosis of neuropathogenic strains of equine herpesvirus-1. J Vet Diagn Invest 19, 69–

72, 2007.
Part I: Interviews with Individuals Having Expertise Related to Equine Herpesvirus

14 / EHM 2008

Miscellaneous Comments and Suggestions

Dr. Lutz Goehring: In addition to collecting questionnaire data on both EHM

cases and non-cases (controls), there may be some benefit in collecting and

banking sera for future studies [such as]:

• Determination of EHV-1 titers at the onset of an outbreak and subsequent

occurrence of EHM;

• Determinations of some deficiency or factors in those horses that go on to

develop EHM. The lesion in the central nervous system of EHM cases is

compatible with vascular injury and coagulation. In humans, factors that are

risk factors for coagulation include age, ethnicity, and geographic location.

Dr. Barry Meade: There should be some kind of mechanism for investigation of

EHV-1 events nationwide. In the past, APHIS has provided assistance to State

veterinary regulators to investigate a variety of disease situations. As an

example, I was asked to investigate the first occurrence of West Nile virus

among horses in New York in 1999. At that time, it would have been helpful to

have access to content experts on equine neurological disease or questionnaire

design. We probably could have learned more had these types of resources

been available at that time. There are several things that must be taken into

consideration before assistance is offered: (1) What would be the structure and

framework of support? (2) Would these investigations be done by Federal

personnel? (3) Can the Centers for Epidemiology and Animal Health provide

personnel to be used in an investigation? (4) How could such a service be

marketed?

Needs Other Than Those Specifically Related to Research

Dr. Peter Timoney: There is a need for communication about what we know of

EHM and our current knowledge on control of the disease. This communication

needs to occur with and between all sectors of the horse industry, inclusive of

horse owners, trainers, breeders, racetrack personnel, and equine event

management staff. We must also admit to limitations in our current knowledge of

this disease. For example, we have limited scientific knowledge of the impact

that vaccination, with the variety of available EHV-1 vaccines, would have in a

field setting, in terms of restricting occurrence of this disease. To reduce losses

from occurrences of EHM, we need to work across multiple areas and look at a

variety of different approaches to achieve greater control of the disease.
USDA APHIS VS / 15

Part I: Interviews with Individuals Having Expertise Related to Equine Herpesvirus

All those interviewed indicated the importance of determining risk factors

associated with EHM. Some indicated there would be a benefit in collecting

epidemiologic data related to the risk for EHM outbreaks and the risk of

development of EHM at the individual horse level during outbreaks. The

information could be gathered through a survey related to factors in both cases

and controls during an outbreak. To evaluate some of the suggested factors,

sample collection with subsequent laboratory testing would be required.

Other points made included:

• We are missing an opportunity to advance the knowledge of EHM by not

gathering observational data during outbreaks.

• A horse-side test would be advantageous in mitigating an outbreak.

• Can the two types of EHV-1 (wild-type and mutant) coexist in the same horse

and if they can, what is the impact of one on the other with regard to

reactivation (recrudescence)?

It is apparent that those who were interviewed agree there is still much to learn

about EHM. These EHV scientists and researchers advocate gaining more

information about strategies for prevention of EHM and evaluation of treatment

options. They also suggest it would be beneficial to share, across various levels,

what we do know and what we do not know about the prevention and treatment

of EHM.

The need to evaluate strategies for prevention of EHM is one area that all

interviewees appear to agree upon, particularly as some of the interviewed

scientists and researchers indicated that, in their opinion, there was an

emergence of a more severe form of EHM during recent outbreaks. Two areas

suggested for further examination in EHM prevention are the impact of the use of

available vaccines and determining risk factors associated with EHM.

In addition to evaluating the efficacy of available vaccines in reducing the risk for

EHM, scientists and researchers suggested investigating the development of

new vaccines that may reduce the occurrence of latency, viral shedding, and

onset of EHM. Several interviewees mentioned the challenge of developing such

a vaccine and the fact that there is currently no challenge model that parallels the

severity of naturally occurring disease. Testing of such a vaccine will be difficult,

except through field trials, unless appropriate challenge models are developed.

Summary from Interviews with Selected Scientists and Researchers
Part I: Interviews with Individuals Having Expertise Related to Equine Herpesvirus

16 / EHM 2008

• Evaluation of treatment to prevent neurologic signs in EHV-exposed horses,

as well as the efficacy of treatment of horses with EHM, is a priority for

further investigation.

• There is need for discussion of the options for involvement of APHIS in

gathering needed epidemiologic data related to EHM.

• ACVIM is developing a consensus statement on EHV-1, addressing multiple

key areas and supported by extensive scientific references, that is expected

to be posted on the ACVIM Web site by fall 2008.

Photo by Chad Mendell
USDA APHIS VS / 17

Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

Part II: Summary of Interviews with Mitigators of Recent EHM Disease

Events

Interviews were conducted with those who had been involved in mitigation of

recent outbreaks of EHM to gather information that could describe the

identification of EHM cases and methods used to mitigate the outbreaks, and to

gather comments related to lessons learned, and need for additional educational

materials and further research related to EHM. A set of open-ended questions

(Appendix A) was used for the telephone interviews. Notes from this interview

were then prepared and provided for each person to review for accuracy.

Interviews were conducted from July through October 2007. Sixteen interviews

were conducted relating to 16 disease events—13 outbreaks and 3 trace-outs.

1. Dr. Rick Arthur, the California Horse Racing Board Equine Medical Director,

was interviewed related to his experience with two EHM situations: the Los

Alamitos Quarter Horse and Golden Gate Fields Thoroughbred racetracks

EHM outbreaks (started late December 2006). [n=2 outbreaks, n=1

interview]

2. Drs. Kent Fowler and Tim Boone, California Department of Food and

Agriculture, provided comments related to their experience of receiving a

horse that had been recently imported to the United States and developed

EHM after arriving in California (December 2006). [n=1 trace-out, n=1

interview]

3. Dr. John Madigan, Professor, University of California at Davis, provided

comments on his experience with an EHM outbreak at a riding stable in

Pacifica, California (started in fall 2006). [n=1 outbreak, n=1 interview]

4. Dr. Paul Morley, Director of Hospital Biosecurity for the James L. Voss

Veterinary Teaching Hospital at Colorado State University, provided

comments related to an EHM outbreak that occurred in the veterinary

teaching hospital (started end of October 2006). [n=1 outbreak, n=1

interview]

5. Drs. Alfredo Sanchez (Tufts University Equine Ambulatory Veterinarian) and

Mary Lis (State Veterinarian in Connecticut) provided comments related to

an EHM outbreak among polo ponies at the University of Connecticut

(started early January 2007). Dr. Lis also provided general comments

related to mitigation, for comparison purposes, about an EHV-1 disease

event at a private veterinary practice in Connecticut that occurred within

several weeks of the University of Connecticut outbreak. [n=1 outbreak, n=2

interviews]
Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

18 / EHM 2008

6. Dr. Mike Short (Veterinarian with the State Veterinarian’s Office in Florida)

provided comments related to an EHM outbreak at multiple venues including

show facilities and veterinary clinics in the West Palm Beach area of Florida

(started mid-December 2006). [n=1 outbreak, n=1 interview]

7. Dr. David Fitzpatrick, Chief Veterinarian for the Illinois Racing Board,

provided comments related to his experience with the EHM outbreak at

Balmoral Park racetrack among Standardbred horses in Illinois (started in

early April 2007). [n=1 outbreak, n=1 interview]

8. Dr. Robert Stout, Kentucky State Veterinarian and Rusty Ford, Equine

Program Manager with the Kentucky Department of Agriculture, provided

comments related to an EHM outbreak that occurred at Turfway Park

Thoroughbred racetrack (started December 21, 2005). [n=1 outbreak, n=1

interview]

9. Dr. Don Hoenig, State Veterinarian in Maine, provided comments related to

an EHM outbreak at a boarding facility in Maine. (started mid-March 2007).

[n=1 outbreak, n=1 interview]

10. Dr. Guy Hohenhaus, State Veterinarian for Maryland, provided comments

about his involvement in tracing out horses that had left the Marion DuPont

Scott Equine Medical Center and returned to premises in Maryland. [n=1

trace-out, n=1 interview]

11. Dr. Steve Halstead, State Veterinarian in Michigan, provided comments

related to an EHM outbreak that occurred at Northville Downs Standardbred

racetrack (started mid-December 2004). [n=1 outbreak, n=1 interview]

12. Dr. Nancy Halpern, State Veterinarian for New Jersey, provided comments

related to an EHV outbreak with an epidemiologic link to EHM cases that

occurred at Monmouth Park Thoroughbred racetrack (started October 20,

2006). [n=1 outbreak, n=1 interview]

13. Dr. Steve Reed, a faculty member at The Ohio State University at the time

of the outbreak, provided comments related to an outbreak of EHM that

occurred at the veterinary hospital at The Ohio State University (started

January 2003) that had an epidemiologic link to the EHM outbreak at

University of Findlay Equine Center. [n=1 outbreak, n=1 interview]
USDA APHIS VS / 19

Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

14. Dr. Nat White, director of the Marion DuPont Scott Equine Medical Center

(EMC), provided comments related to the EHM outbreak that occurred at

the EMC in Virginia (started February 20, 2007). [n=1 outbreak, n=1

interview]

15. Dr. Don Hopson, a veterinarian with the Virginia Department of Agriculture

and Consumer Services, provided comments related to tracing out of

horses that had left the Marion DuPont Scott Equine Medical Center and

returned to premises in Virginia. [n=1 trace-out, n=1 interview]

Case Recognition and Person Responsible for Mitigation

Who notified you of the first case of EHM?

Those involved in the recognition or reporting of the first or index case varied by

outbreak. In addition, the position of the person who eventually managed the

outbreak varied. In some situations a State veterinary official was notified and

took action to investigate and/or manage the outbreak; in other situations the

racing commission or racing board veterinarian was notified and managed the

situation; and in yet other situations, the director of hospital biosecurity, an

equine infectious disease specialist, or chief of equine services was notified of

the index case and managed the outbreak.

Those who identified the index case and then notified the State Veterinarian,

racing commission or racing board veterinarian, biosecurity director, or equine

infectious disease specialist were primarily private veterinary practitioners or

university veterinary clinicians. In one situation, the stable owner, along with the

attending veterinarian, contacted the State Veterinarian’s office. In some

situations, the State Veterinarian was notified of the situation but was not the one

to take action in managing the outbreak. For example, in three different

outbreaks on racetrack sites (two in California and one in Illinois), it was the

veterinarian working for the racing commission or racing board who managed the

outbreak. In two of the university clinic outbreaks, although the State Veterinarian

was notified, the management of the situation was left to the director of hospital

biosecurity or equine medicine faculty and hospital directors.
Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

20 / EHM 2008

Quarantines

Was quarantine imposed as part of mitigation and, if so, who imposed it,

what criteria were used to lift the quarantine, and what were the dates for

placement of quarantine and lifting of quarantine?

A “stop-horse-movement” action was implemented in all outbreaks discussed by

interviewees, but the persons or authorities responsible for stopping movement

varied greatly. Official quarantines were imposed by the State Veterinarian’s

offices of Connecticut, Florida, Kentucky, Maine, Maryland, Michigan, New

Jersey, and Virginia. The Connecticut State Veterinarian described an alternative

approach used to respond to an EHV-1 situation at a private veterinary clinic.

The veterinary clinic personnel notified Dr. Lis of the diagnosis and the actions

they were taking (voluntary quarantine). Based on these actions, Dr. Lis

communicated with the Vermont State Veterinarian (the State from which the

index case originated) and the Connecticut Veterinary Medical Association

regarding the management of the situation. Dr. Lis provided this comparison in

order to illustrate her flexibility in applying management strategies depending on

the situation. Educational and outreach meetings included discussion of both the

private veterinary practice EHV-1 situation and the EHM situation at the

University of Connecticut. Veterinary teaching hospitals at Colorado State

University and The Ohio State University quarantined their equine facilities. The

California Horse Racing Board and the Illinois Racing Board quarantined the

affected racetracks in their States. In some instances, initial responders, such as

private veterinarians or university faculty, who suspected an EHV case

recommended a stop-horse-movement order prior to official action being taken.

Although most States considered equine herpesvirus reportable, some, such as

California, did not consider the disease as being actionable. The California Horse

Racing Board placed “voluntary” quarantines at the racetrack, and the owner of

the California riding stable voluntarily stopped horse movement. At the time of

the outbreaks, interviewees indicated EHV-1 was not reportable in Illinois or

Ohio; however, mitigators of both of these outbreaks informed the State

Veterinarian’s office.

The impact of the quarantines varied due to the length of time they were in place,

the number of facilities and horses affected by the quarantines, and whether or

not events/admissions were cancelled. The length of quarantines (stop-horsemovements) varied, from 14 days to 3 months. Mitigators of outbreaks at

racetracks tried to restrict the quarantine to the affected barn/s as long as the

disease was contained; in a few outbreaks, as the disease spread, the entire

track was quarantined. This quarantine strategy was implemented in order to
USDA APHIS VS / 21

Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

maintain racing and economic stability while managing the risk of spread of the

disease. At Golden Gate Fields, the transport of clinically normal horses was

restricted, but allowed to occur, between three tracks (Golden Gate Fields, Bay

Meadows, and Pleasanton) in order to keep an adequate number of horses in

the races. Some outbreaks and quarantines were limited to 1 or 2 facilities with

as few as 19 horses, whereas others, such as the Florida outbreak, involved

quarantine (or partial quarantine) of 2 show facilities with approximately 1,000

horses. Other quarantines consisted of portions of 3 private equine veterinary

clinics, and 5 other premises with fewer than 40 horses on each premises. The

Colorado State University Veterinary Teaching Hospital, The Ohio State

University Veterinary Teaching Hospital, and the Marion DuPont Scott Equine

Medical Center closed their teaching hospitals to equine admissions and

quarantined horses in the hospital.

The criteria for release of quarantines were not consistent among outbreaks. A

majority of mitigators (Lis, Short, Stout/Ford, Hoenig, Halstead, Halpern, Reed,

and Hopson) reported that they waited 21 days after all quarantined horses

appeared clinically normal and then required a negative test on all exposed

horses to release the quarantine. One mitigator (Madigan) indicated the

voluntary stop-horse-movement on the premises was in place for 21 days after

no fevers were detected, and then all previously EHV-positive horses were

retested by PCR to confirm negative results. At Golden Gate Fields, the

California Horse Racing Board required horses to be afebrile for at least 14 days

followed by two negative PCR tests (on both whole blood and nasal swabs taken

2 days apart). At the Los Alamitos racetrack, all horses in exposed barns had to

be afebrile for 7 days, then two tests (on both blood and nasal swabs) were

taken 48 hours apart, with negative results required in order to release

quarantine; many of the test-positive horses had been moved off-site as part of

the mitigation. The Colorado State University Veterinary Teaching Hospital and

the Marion DuPont Scott Equine Medical Center waited 14 days after any horse

had clinical signs (after all were afebrile for 14 days and no new neurologic cases

were identified), then required negative PCR tests on nasal swabs from all

horses in the hospital on samples collected on 4 consecutive days at the

Colorado State University Veterinary Teaching Hospital, and 3 consecutive days

at the Marion DuPont Scott Equine Medical Center. The State of Maryland tested

all horses with an epidemiologic link to the Equine Medical Center; if no horses

were febrile within 7 days, they tested all horses that had been at the Equine

Medical Center and a subset of in-contact horses twice, by nasal swab and blood

PCR, in order to release hold orders. The State of Virginia’s criteria to release

quarantine for trace-outs to premises with horses with contact with Marion

DuPont Scott Equine Medical Center were: (1) 21-day period after the date of
Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

22 / EHM 2008

detection of the last febrile horse (with no history of nonsteroidal drug treatment);

and (2) all horses on the premises were then tested for EHV on blood and nasal

swabs with all test results negative on PCR.

Case Definition/Categorization

Did you have a case definition at the outset or eventually develop one and

if so what was it?

Across outbreaks different criteria were used to define a “case.” More

specifically, several of those interviewed made a distinction between a case of

EHM and a herpesvirus infection with and without clinical signs, such as fever.

Several of those interviewed indicated that their case definition for EHM evolved

over the course of the outbreak. The case definition initially used or that evolved

for EHM for most of those interviewed was “clinical signs of neurologic disease

along with laboratory confirmation of EHV infection or a horse with neurologic

signs that was part of an outbreak where at least one other horse had laboratory

confirmation of EHV infection.” Several of those interviewed indicated the need

for further case criteria beyond EHM during an outbreak, such as for

categorization of: (1) horses with fever and laboratory confirmation of EHV

infection; (2) horses that had laboratory evidence of infection without clinical

signs of disease; (3) horses with clinical signs of disease that were not tested,

but had an epidemiologic link to confirmed cases; and (4) horses with fever and

laboratory confirmation of EHV-1 infection that had an epidemiological link to

EHM cases on another premises.

Testing or Monitoring Performed

Once you recognized the first EHM case(s) how did you identify

subsequent cases?

Divergent approaches were taken at the onset of the outbreak regarding

identification of infection beyond the index EHM case. In all situations, enhanced

surveillance for clinical disease was implemented. Another approach was to test

all potentially exposed horses for EHV as soon as the index case was identified

in order to determine the scope of exposure/infection. A second approach was to

enhance surveillance through monitoring of body temperature and observation

for neurologic signs, but to test only those horses on the premises that

developed signs consistent with EHV, including fever and/or neurologic signs. In

some of the outbreaks, testing of exposed horses for exposure to EHV was
USDA APHIS VS / 23

Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

conducted after all signs of disease had resolved in order to lift a quarantine or a

stop-horse-movement order. The specifics of this approach are discussed under

the quarantine section of this report.

Once the index case was diagnosed in the majority of the outbreaks/disease

events or for trace-outs, monitoring of body temperature was conducted as part

of surveillance for fever. When asked what defined a fever, eight of those

interviewed indicated that it was a temperature above or equal to 101.5°F; five

indicated a temperature of greater than or equal to 102°F; one indicated greater

than 101°F in the morning; one indicated greater than or equal to 101°F; and one

indicated 1°F above the horse’s normal temperature. All those interviewed

indicated action was taken if a fever was detected in exposed or at-risk horses

which included those horses being tested for EHV infection.

For some outbreaks a single laboratory was used for PCR testing, whereas other

outbreaks used multiple laboratories to perform PCR testing. Interviewees

reported nine different laboratories involved in PCR testing during the outbreaks:

California Veterinary Diagnostic Laboratory, the Lucy Whittier Molecular and

Diagnostic Core Facility, Colorado State Veterinary Diagnostic Laboratory,

Cornell Animal Health Diagnostic Center, University of Florida Emerging Disease

and Arbovirus Research and Test Program Laboratory, University of Kentucky

Livestock Disease Diagnostic Center, Michigan State University Diagnostic

Center for Population and Animal Health, Gluck Equine Research Center, and

the Ohio Animal Disease Diagnostic Laboratory.

For three of the EHM outbreaks or trace-outs in California, samples were tested

either at the California Veterinary Diagnostic Laboratory or the Lucy Whittier

Molecular and Diagnostic Core Facility. In the situation with the index case that

was necropsied at the San Bernardino Veterinary Diagnostic Laboratory, tissues

were examined histologically and with immunohistochemistry, and further testing

on the spinal cord was performed. The California Animal Health and Food Safety

Laboratory tested some collected samples as part of the outbreak at a boarding

facility in Maine (used multiple laboratories). Testing was done at the Kentucky

Livestock Disease Diagnostic Center, and/or the Gluck Equine Research Center

at the University of Kentucky for the outbreaks that occurred at the racetrack in

Kentucky, the outbreak in Florida (used multiple laboratories including the

Kentucky laboratory), the outbreak at the Standardbred track in Illinois, the traceout of horses that left the Marion DuPont Scott Equine Medical Center and

returned to facilities in Virginia and Maryland, the outbreak on a boarding facility

in Maine (used multiple laboratories), the outbreak at Monmouth Park racetrack

in New Jersey, and the outbreak that occurred at The Ohio State University
Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

24 / EHM 2008

Veterinary Hospital (used multiple laboratories). The Michigan State University

Diagnostic Center for Population and Animal Health performed the testing done

as part of the management of the outbreak at the Standardbred track in Michigan

and for the outbreak in Florida (used multiple laboratories). The Animal Health

Diagnostic Center at Cornell was used for testing of samples from the Florida

outbreak (used multiple laboratories), the outbreak at the University of

Connecticut, and the boarding facility in Maine (used multiple laboratories). The

Colorado State University Veterinary Diagnostic Laboratory tested samples from

the outbreak at the Colorado State University Veterinary Teaching Hospital and

also the Marion DuPont Scott Equine Medical Center outbreak.

Across outbreaks, the method used to diagnose a case of EHV infection varied

and included one or more of the following: clinical findings consistent with EHV

infection (fever and/or neurologic signs); necropsy and testing of tissues with

presence of EHV detected; positive testing of nasal swabs for EHV by PCR;

positive testing of whole blood for EHV by PCR; virus isolation on tissues, swabs

and/or whole blood; and seroconversion or a fourfold rise in titer based on testing

of paired sera samples. As indicated earlier, the approach to management of

some outbreaks entailed sampling of all exposed horses at the onset of

managing the outbreak while others tested only horses that developed fever or

neurologic signs. Across outbreaks, the majority of testing of clinical or exposed

horses occurred once the index case was identified, and entailed the collection

and testing of nasal swabs and whole blood (in 12 of the outbreaks and/or traceout situations) by PCR at 1 or more of the laboratories listed in the preceding

paragraph. In two of the outbreaks (2004 in Michigan and 2006 in Colorado), only

nasal swabs were collected and tested by PCR as a means of managing the

outbreaks.

Testing for the neuropathogenic variant of EHV was reported to have been done

as part of the investigation of all four California outbreak/disease events or traceouts (testing done at California laboratories), the outbreak in Florida (testing

done at the Kentucky Livestock Disease Diagnostic Center and/or Gluck Equine

Research Center), the outbreak in New Jersey (testing done at Kentucky

Livestock Disease Diagnostic Center ), the outbreak in Illinois (testing done at

the Kentucky Livestock Disease Diagnostic Center and Gluck Equine Research

Center), the outbreak at the boarding facility in Maine (testing done at a

California laboratory, the Kentucky Livestock Disease Diagnostic Center and

Gluck Equine Research Center), and the trace-out of horses in Virginia (testing

done at the Kentucky Livestock Disease Diagnostic Center and/or Gluck Equine

Research Center). In all situations in which testing for the neuropathogenic strain

was reported to have been conducted, positive results were obtained on at least
USDA APHIS VS / 25

Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

some of the horses tested. Several of those interviewed indicated that the

outbreaks they were summarizing (Michigan racetrack, The Ohio State

University, and Kentucky Turfway racetrack) occurred prior to the availability of

the test for the neuropathogenic strain of EHV, while others indicated the result of

such testing was not critical to their management of the outbreak (Colorado State

University Veterinary Teaching Hospital).

Some mitigators indicated that during the testing they received results from the

laboratory indicating that the sample was suspect (neither negative nor positive).

Most responded to this result by assuming the horse was positive for EHV and

implementing whatever biosecurity practices were used for test-positive horses

to prevent disease transmission. The horse was then retested to determine its

test status. One mitigator indicated conflicting results from two different

laboratories on the same horses for EHV PCR positive or negative status;

another reported that two different laboratories reported different results related

to the neuropathogenic variant of the virus on the same horses. Yet another

mitigator indicated that one type of sample (blood or nasal swab collected at the

same time) was positive while the other was negative on PCR test for EHV.

Scope of the Outbreak or Investigation

When asked, why did you consider the situation an outbreak?

In the California situation, in which a horse that was recently imported to the

United States was the index case, those interviewed were not sure they would

call this an outbreak since there was only one case of EHM (for this report,

classified as a trace-out situation).

For Golden Gate Fields, Dr. Arthur was not sure he would call this an outbreak,

in the traditional clinical usage of the term, with only one clinical EHM case. Only

with extensive monitoring of temperatures and the use of PCR testing was there

any sense of an outbreak at Golden Gate Fields. However, he did consider the

event at Los Alamitos to be an outbreak, as there were multiple EHM cases,

multiple febrile EHV-infected horses, and a spread from the original or index

case barn.

At the boarding stable in California there were multiple horses with EHM, two

horses that tested positive for EHV, and the detection of additional febrile horses

after the identification of the index case that tested positive by PCR for EHV.
Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

26 / EHM 2008

At the University of Connecticut polo facility there were three acutely neurologic

horses on same premises within a short period of time. This was considered an

outbreak because of multiple cases of EHM in a public venue and the concern

that it needed to be contained.

At Balmoral Park there were several horses with fevers and two with EHM, all

within one barn at the racetrack over a short period of time; PCR tests were

positive for EHV in the initial neurologic case and in additional cases of EHV

infection.

For the boarding stable in Maine, Dr. Hoenig said, “...there were three neurologic

cases on this one private boarding stable premises, which was not normal in the

equine population in our State [Maine].”

At the Michigan racetrack, Dr. Halstead stated ”...[he] did not initially consider

this an outbreak but when they recognized additional EHM cases beyond the

index case, based on heightened surveillance that was implemented after

recognition of the index case, they considered it an outbreak. In other words

occurrence of multiple cases of EHM at the racetrack over a short period of time

made us consider it an outbreak.”

Monmouth Park was considered an outbreak because there were multiple febrile

horses with a positive PCR test for the neuropathogenic variant of EHV, following

a direct epidemiological link to a confirmed neuropathogenic EHV outbreak in

Canada.

During the Florida EHM situation, Dr. Short indicated “...we considered this an

outbreak because there were two cases of severe neurological disease, one with

confirmed contagious disease [that is, EHV-1] and there was a lot of risk for

extensive exposure based on the situation we were facing.”

Nosocomial spread of EHV occurred within the three veterinary hospitals

reporting outbreaks (Colorado State University Veterinary Teaching Hospital, the

Marion DuPont Scott Equine Medical Center, and The Ohio State University).

Dr. Hopson indicated concern because horses had returned to Virginia farms

after having potential exposure to the index case of EHM at the Marion DuPont

Scott Equine Medical Center.

Dr. Hohenhaus indicated he considered one case of EHM as an outbreak and

EHM is reportable in his State (Maryland).
USDA APHIS VS / 27

Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

How many EHM cases occurred and what was the outcome for these

cases?

All the outbreaks except one had one or more horses with neurologic signs and

with laboratory confirmation of EHV infection. The exception was an EHV

outbreak with an epidemiologic link to cases of EHM and confirmation of EHV

infection with the neuropathogenic variant of the virus in febrile horses with no

neurologic disease. For several of the outbreaks, trace-outs of potentially

exposed horses resulted in identification of additional horses with infection, some

of which showed signs of fever and/or neurologic signs. The number of EHM

cases per disease event/outbreak or trace-out ranged from one to seven for

those that had any EHM cases as part of the outbreak. The median number of

EHM cases was three per disease event. Of the initial outbreak sites and traceouts, all but two identified one or more horses with fever and laboratoryconfirmed EHV infection, but without neurologic signs. Across the outbreaks and

trace-out situations, the case fatality rate among the horses with neurologic signs

(EHM cases) ranged from 0 to 100 percent. In one outbreak with five laboratoryconfirmed EHM cases, no animals died; in another outbreak with seven EHM

cases, six animals died or were euthanized. In one outbreak there was concern

whether the cause of death or reason for euthanasia was actually due to EHM in

horses that tested positive for the virus. This made calculation of a case fatality

rate across all reported outbreaks/disease events and trace-outs difficult. Seven

of those interviewed who had been involved in outbreaks with more than one

EHM case were asked if the severity of disease was consistent across the entire

outbreak: two indicated that the severity had been consistent across the

outbreak and the remaining five indicated that the more severe cases occurred

early in the course of the outbreak.
Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

28 / EHM 2008

General Biosecurity Procedures and Other Control Strategies

What biosecurity or control procedures were used?

The table below summarizes responses to an open-ended question related to

methods used to control the outbreak including biosecurity procedures

implemented.

General management and biosecurity 

procedures reported in response to openended question related to procedures that 

were implemented as part of control 

Data relates to 16 disease events: 

n=13 outbreaks and n=3 trace-outs 

Quarantine of individual barn or entire facility or 

voluntary stop-horse-movement 

State Veterinarian imposed quarantine, n=9 

Mandatory quarantine other than State 

Veterinarian imposed, n=1 

Voluntary stop horse movement, n=6 

Trace-out or some form of contact or notification of 

those who may have had horses exposed through 

contact with the EHM case 

 n=16 

Guard to enforce quarantine or stop-horsemovement n=4 (all were at racetrack venues: two at 

Thoroughbred tracks and two at Standardbred 

tracks) 

Monitored body temperature to detect new cases n=16 

Number of times temperature was taken per day 

on horses in quarantine or restricted area 

Daily, n=1 

Twice per day, n=9 

Three times per day for horses in quarantined 

barn n=2 (horses outside of the quarantined area 

but on premises, temperatures taken twice per 

day) 

Required log of body temperatures n=11 

Temperature logs inspected by regulatory 

authority 

n=6 

Used biosecurity precautions in quarantine area n=16 

Footbaths at entry point of quarantined barn or 

facility 

n=12 

Restricted persons entry to quarantined/restricted 

facility to only those necessary to care for horses 

n=13 

Allowed normal but potentially exposed horses 

from restricted/quarantined area to exercise but 

with protocols to reduce risk of them spreading 

disease 

n=8 

Moved all of the clinical cases (neurologic as well 

as febrile horses) off-site 

n=2 

Moved EHM cases off-site or into isolation facility n=6 

Moved all or some of the EHV test positive horses 

off site 

n=4 

Moved test positive or clinical cases to a separate 

quarantine facility/barn on-site 

n=2 

Wore gloves when working with horses in 

quarantined or restricted site (for some or all of the 

personnel) 

n=7 
USDA APHIS VS / 29

Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

Wore dedicated clothing or disposable coverall in 

quarantined area 

n=10 

Stop sharing of equipment including tongue ties, 

tack, buckets, feeders 

n=12 

Vaccination against EHV as part of management 

of outbreak 

Advised against vaccination of exposed horse, 

n=2 

Not addressed one way or other by those 

interviewed although private practitioners may 

have implemented vaccination, n=10 

Recommended for horses that commingle, n=1 

Required for new arrivals to venue where cases 

had been identified, n=3 

Required for horses at venue but not in direct 

contact with cases of EHM/EHV, n=1 

Required for both new arrivals and those already 

at venue, n=1 

Tested potentially exposed horses for EHV as part 

of initial management of outbreak at onset 

n=6 (two of these were trace-out situations) 

Tested potentially exposed horses for EHV as part 

of research project but not for purposes of 

managing outbreak 

n=2 

Tested only clinical cases (neurologic or febrile 

horses) for EHV in order to manage the outbreak 

n=10 

Tested both clinical cases (neurologic and febrile) 

and exposed as part of quarantine or stop horse 

movement release process 

n=12 

Had feed deliver truck park outside quarantine 

barn to unload feed 

n=3 

Recommendation to work with horses with clinical 

signs of disease last for service providers or other 

personnel 

n=4 

Starters or horse identification personnel wore 

gloves and changed between each horse 

examined 

n=4 (This was at racetrack venues and applied to 

contact with horses outside of the quarantine 

area) 

Used disinfectant wipes to clean off hands of the 

rider of pony horse and surfaces of tack on pony 

horse between horses ponied 

n=2 (This was at racetrack venues and applied to 

pony horses used outside of the quarantined area) 

Implemented special protocols for handling of 

manure and soiled bedding for quarantined 

barns/horses 

n=3 

Spray vehicle tires with disinfectant when leaving 

the premises or quarantine area 

n=2 

Periodically cleaned and disinfected common 

areas, such as starting gate, warm-up area, test 

barn buckets, etc.) 

n=6 (This applied to areas outside the quarantine 

as a general biosecurity practices begun because 

of outbreak) 

General management and biosecurity 

procedures reported in response to openended question related to procedures that 

were implemented as part of control 

Data relates to 16 disease events: 

n=13 outbreaks and n=3 trace-outs 
Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

30 / EHM 2008

Planning

Several respondents said it would have been helpful to have consistency

between States and to have disease program standards. One respondent

indicated that it would have been good to have discussed outbreak response in

advance of the outbreak and to have a pre-existing plan. Some States had case

definitions prepared; in other States, the case definitions evolved during the

outbreak.

Dr. White said, “We needed clearer guidelines from the State regarding what

they were going to do about containment of the problem on farms where horses

that had left the hospital were now housed (trace-outs). Owners were really

concerned and confused what criteria would be used by the State to contain the

problem. It would be good for those in a role with the State to meet with experts

on the disease and come up with best management practices and how those will

be implemented and enforced.” Another respondent indicated that it would have

been beneficial to have clearer State lines of authority and explanation of how

outbreak response measures would be executed at the racetrack.

Three of the respondents said that having designated isolation barns or a

separate facility where infected horses could be moved would reduce the impact

of the quarantines and the potential spread of disease. One person indicated he

would like to have an isolation area with equipment available to sling neurological

horses, if necessary.

Knowing which supplies are needed and having them on hand to respond to an

EHV-1 outbreak would enhance the timeliness of response. Interviewees

commented that the following items should be readily available:

• Disinfectant and footbaths to get started on the containment of the disease to

a given barn;

Identified Needs

What would have been helpful for you to have that you did not have at the

time of the outbreak?

Responses regarding what would have been helpful in addressing the outbreak

or disease event fell into four primary categories: Planning, Disease Control/

Biosecurity Measures, Diagnostics, and Communications.
USDA APHIS VS / 31

Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

• Appropriate swabs for collection of nasal samples from suspect cases; and

• Supply of quarantine orders in the hands of State field veterinarians.

Communication needs expressed by mitigators that could be addressed in

advance planning include:

• Rapid, confidential communication system among States and between States

and Federal regulatory officials;

• Private practitioner awareness of EHV and sampling methods;

• Access to EHV researchers to answer questions about test methods and

interpretation of results; and

• Submission forms for the laboratory and all the contact information for the

laboratory.

Mitigators also emphasized the need for both human and financial resources.

One respondent indicated that instituting an Incident Command System (ICS),

specifically tasked with logistical aspects of the response, would have provided

additional resources. However, since no emergency was declared, resources

available through ICS were not an option. Availability of funding to assist with

testing was another concern.

Disease control/biosecurity measures

Much is yet to be understood about EHV-1, particularly the neuropathogenic

variant. One respondent said it would have been beneficial to have information

regarding the duration of virus shedding in a field setting—to know a mean and

maximum for number of days of virus shedding, as much of what we know now

is based on experimental challenge studies. A better understanding of the

distance that the virus might spread via aerosol transmission would help

mitigators identify likely exposure risks. Other issues to be addressed include:

(1) the meaning of an EHV-1-positive test in a clinically normal horse, and (2) the

implications of detecting the neuropathogenic variant of the virus via testing of

nasal swab from a clinically normal horse.

Scientific validation for recommended biosecurity and containment measures

was highlighted by one respondent who asked the following questions: “What

recommendation should we give to manage the basic things on the farms where

exposed horses then went? For instance, what precautions should be taken for

feed delivery vehicles and personnel at farms? Is it acceptable for the farrier to

come to the farm that was quarantined and then go to other farms? What

precautions should be taken? Should they wear dedicated clothing? Do truck

tires need to be sprayed with disinfectant? If we cannot give detailed guidance on

these topics then we should say so.”
Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

32 / EHM 2008

Diagnostics

Issues regarding diagnostics for EHV-1 were also raised by mitigators. “Is there a

reference laboratory for validating tests for EHV-1? For example, which PCR is

the best one to use and should we be doing the testing for the neuropathogenic

variant?” One respondent indicated that the availability of a rapid test, such as a

stall-side test, would allow for more prompt management decisions. Others felt

that having the ability to run the test in their home States and having additional

funding to assist with testing would have been beneficial.

Communication materials and plan

Although a few respondents indicated that they had developed some guidelines

prior to the outbreak, several others indicated that having information prepared in

advance for veterinarians, equine owners, and the general public would have

been advantageous. In addition, implementation of communications early in the

outbreak would have facilitated the outbreak response. When asked what would

have been beneficial to have that they did not have when addressing their

outbreak, respondents commented on the need for:

• “Clear concise communication points to try to alleviate panic,”

• “Earlier implementation of communication system to reduce rumors and

expedite communication. We were capable of implementing this; we just did

not do it as early as we should have. It is hard to know how things will evolve

and this situation escalated and evolved more quickly than we anticipated.”

• “Clearer guidelines from the State. It would be good for those in roles with the

State to meet with experts on this disease and come up with best

management practices and how those will be implemented and enforced.”

• “Private practitioners to be more aware of EHV and sampling methods.”

Educational material developed or needed

Several sources of information were cited by respondents when asked about the

need for and availability of educational material. Some materials had been

developed prior to outbreaks and others were developed during outbreaks.

Sources of existing information that proved beneficial were:

• California Center for Equine Health (CEH) (n=31);

• AAEP infection control guidelines (n=3): one person indicated that the “AAEP

Web site information was very helpful. It gave me a clear idea of how to go

about testing and initial actions to contain the disease. It was step by step

helpful guidelines that I could immediately use to help me manage this

outbreak.”

1

 Refers to number of respondents who mentioned this resource.
USDA APHIS VS / 33

Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

• “Dr. Long at University of Florida helped a lot with good educational info.”

• The Horse magazine (n=2); EHV backgrounder;

• University of Florida Web site;

• “An article in Equine Veterinary Education by Dr. George Allen.”

• Journal articles;

• Material Safety Data Sheets on disinfectants used;

• “USDA Biosecurity brochure for equine owners, but it does not specifically

address EHM containment...” and

• Marion DuPont Scott Equine Medical Center Web site.

Information developed in response to outbreaks included:

• A biosecurity handout for trainers and horseman (n=2);

• “An educational program through the University of Connecticut. The AAEP

infection control guidelines were very helpful; included EHV info from AAEP

on the U of Conn Web site.”

• “Information for track management about EHV; the University of California at

Davis equine owner information was a very good start on this type of

material.”

• Information for grooms in both English and Spanish regarding basic hygiene

protocols and infection control methods; and

• “Question and Answer sheets” on EHV (n=2); one with assistance from Dr.

Hal Schott at Michigan State University.

When asked what educational material they would have liked for EHV and EHM,

some respondents indicated that they had the information they needed; however,

others said it would be beneficial to have some information prepared in advance

of an outbreak and to have an organization, like AAEP or USDA, develop a

consensus on best management practices for EHV. In addition, a few

respondents said they would like to have equine owner-level best management

practices specific for EHV, as well as general biosecurity practices, such as

keeping horses isolated for a period of time after they return from events or

veterinary hospitals.

Another person said that having information on EHV in one fact sheet would be

beneficial as he had to consult several sources to compile appropriate

information for equine owners. Respondents identified a couple of points that

should be highlighted in fact sheets:

• The horse with the neurological form of EHV poses a contagious disease risk

to other horses, and

• Clarify that the virus type does not correlate 100 percent with clinical disease

manifestation, e.g., neurological variant can be associated with abortion and

vice versa.
Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

34 / EHM 2008

Communication Efforts

What communication efforts did you carry out?

Respondents provided several pointers related to communication efforts in the

face of an outbreak, including the use of press releases, Web sites, e-mails,

equine practitioner meetings, owner/trainer meetings, meetings with workers at

outbreak sites, racing commission meetings, and conversations with animal

health authorities in other States.

A key point is to have clear, open communication to minimize anxiety and

confusion. Two respondents highlighted the value of having a public relations

(PR) person involved from the beginning of an outbreak. One of these said that

PR personnel were involved about 1 week into the outbreak, but should have

been involved sooner. The other respondent indicated that a PR person was

involved in meetings, worked with media contacts, reviewed press releases, and

updated the Web site with communication points.

Several of the respondents reported that designated spokespersons, many with

prior media training, served as communication contacts during the outbreak.

Most of these spokespersons had predetermined talking points. One person

stressed that responding entities work together to ensure that everyone

communicated the same message.

Media relations efforts ranged from interviews with local newspapers to national

equine publications. Respondents issued press releases and posted outbreak

information on Web sites, some with daily updates.

Face-to-face communications with equine veterinarians, trainers, owners, and

facility managers proved to be important in several of the disease events. One

respondent said that the combination of media interviews and a public meeting

seemed to bring people together and help them understand the plan of action,

convince them that the disease could be controlled, and helped control rumors.

Personnel involved with one of the disease events held three meetings with

equine veterinarians, and three separate meetings for owners, trainers, and

anyone else who wanted to attend. Those addressing an outbreak at a racetrack

held weekly meetings with trainers to discuss progress and plan of action. One

neighborhood stable meeting drew 100 attendees.

In fact, one respondent said, “It would be good to have educational material on

all likely equine contagious diseases but it’s difficult to predict which we will need

next.”
USDA APHIS VS / 35

Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

One respondent used the outbreak as an opportunity to improve relations with

two major racetracks in the State. “We also went to the two major racetracks and

spoke to them about EHV and the need to have a control plan. We never had

any EHM cases there, but we were able to use the outbreak at a boarding stable

as a teachable moment, to come up with an emergency response plan for the

tracks not just for EHV but any contagious disease outbreak. It allowed us to

meet the track administration and come up with a workable plan and do a walkthrough and identify an isolation barn and response plan, including monitoring for

clinical signs of disease, so we can make a rapid response if need be. We

discussed if there is an outbreak we might need to suspend activities at the track

for a few days to get a handle on the situation, so if we ever need to take this

action we have laid the ground work to make this a more likely successful plan.”

Communication with veterinary peers and equine advisory groups was another

important aspect of outbreak management, both in gaining information on how to

best address the outbreak and to inform others about the plan of action to control

the outbreak. Most respondents communicated with State Veterinarians, horse

councils, equine advisory groups, event and track management personnel,

racing commissions, and State veterinary and equine practitioners associations.

In addition, some held veterinarian meetings to share information about outbreak

control, included lessons learned from the outbreak in subsequent continuing

education seminars, and conducted a seminar for veterinary students, faculty,

and graduate students regarding the outbreak. Some mitigators also provided

updates on the equine industry monthly conference calls facilitated by Dr. Tim

Cordes of USDA:APHIS Veterinary Services.

Web sites, e-mails, and a toll-free telephone hot line were used to provide

information to horse owners and the general public. One respondent indicated

that they did get information onto their Web site but were late doing so; it would

have been good to have it up sooner to reduce the rumors and misinformation

that occurred early in the outbreak. The Marion DuPont Equine Medical Center

used a toll-free telephone hot line that equine owners could call in and ask

questions. This was for individuals, not the media, and the resident veterinarians

answered these calls.

The key message conveyed by a majority of respondents was to explain the plan

of action and biosecurity measures so people understood the actions being

taken by authorities and measures they could take to control the disease.

Sharing this information helped to relieve anxiety and decrease rumors and

misinformation. Two other key messages were to: (1) detect cases by finding
Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

36 / EHM 2008

horses with fever or neurologic signs and moving these horses to an isolated site

for care, and (2) explaining that the disease did not pose a risk to humans.

Opinions about the use of vaccine in the face of an outbreak were mixed. Some

respondents recommended vaccination in the face of an outbreak, whereas

others did not.

Cost of Outbreak and Management Issues

Do you have any estimate of costs associated with the outbreak or traceout?

a. Testing, cost per test, number of tests, cost of shipping samples, cost of

collection of samples for testing;

b. Closure or lost business;

c. Number of days of stopped-horse-movement and per-day estimate of loss

number;

d. Type of personnel involved in mitigation of outbreak and number of days

each contributed to this outbreak situation;

e. Cost of treatment of cases and number of cases treated; if there is a range

of treatment cost, then supply: (1) the dollar amount of the highest cost of

treatment and number of cases, (2) the dollar amount of mid-range

treatment and number of cases, and (3) the dollar amount of mild-case

treatment and number of cases;

f. Cost of deceased horses due to this disease outbreak;

g. Cost of biosecurity procedures, PPE supplies, disinfectant, hand hygiene

products, barriers, and signage; and

h. Other costs: specify what this cost was and the amount.

Few of the respondents had tracked costs of mitigating their outbreaks or traceouts in detail. However, in some of the interviews various categories of costs

were delineated. These included human resources costs, loss of income due to

cancellation of events/racing, diagnostic costs, cost of communication and

education efforts, and supply costs. The source of funding for outbreak response

varied.

Dr. Arthur reported that the testing involved in mitigation for the Golden Gate

Fields and Los Alamitos EHM outbreaks cost $20,000 in laboratory fees alone,

and was paid for by the racing associations. This included a reduced cost per

test from the laboratory.
USDA APHIS VS / 37

Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

Connecticut has funds to investigate any sudden death or neurological cases,

and helped with the cost of testing horses. In Florida, Illinois, and New Jersey,

samples were collected by private veterinarians and the horses’ owners paid for

testing. One respondent indicated an individual trainer at a quarantined outbreak

site had a $12,000 bill for laboratory testing.

In Kentucky, where the cost was $14,700 to perform approximately 420 PCR

tests, the testing was performed at the Kentucky Livestock Diagnostic Laboratory

at no charge to the Kentucky Department of Agriculture or the horses’ owners.

This was provided by the laboratory as a service to the equine industry of the

State of Kentucky. In addition, due to the stopped movement, equine owners

incurred additional board for 50 horses from December 21 to January 10, and an

additional 50 horses from January 6 to February 1, with board costing $50-$100

per day per horse.

Dr. Hoenig: “IDEXX paid for some of the testing in Maine because they were

working on a new test and they wanted samples to work with and to use in

validation of their test.”

Dr. Halpern: “There were 24,962 rectal temperatures taken at Monmouth Park

during their outbreak. So the time for personnel to do this monitoring is only part

of the cost of this outbreak.”

Dr. Reed: “All testing was paid by The Ohio State University, which paid for

deceased client horses. Estimated board costs for horses kept in the hospital as

part of self-imposed quarantine and drugs used to treat some of EHM cases

were approximately $20,000.”

Dr. White: “Hospital budget person estimated cost of lost income due to hospital

being closed, treating horses, testing, etc., was $755,000. No horses died from

EHV at the Marion DuPont Scott Equine Medical Center.”

Dr. Hohenhaus: “If the State Veterinarian of Maryland ordered the test, the State

paid for it. Approximately $3,500 in test costs. There has been lots of disruption

to Maryland and Virginia equine sporting events associated with EHM situations.”

Dr. Short: “[In Florida]...we tracked the time our Division employees spent on the

outbreak and it was just over 4,000 hours.”

Several respondents indicated that there were several days of loss associated

with stopped or limited horse movement, but accurate estimates on those costs

were not provided by respondents. Additionally, in multiple outbreaks, some
Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

38 / EHM 2008

horses died or were euthanized, but the value of such horses or the cost of

treatment given was not provided by these respondents—in part because they

were not the veterinarians in charge of the medical treatment of these cases, but

instead were the people involved in mitigation of the outbreak or trace-out.

One respondent indicated it would have been good to collect detailed information

related to the various costs for the EHM outbreak.

Lessons Learned

What were lessons learned from your experience in mitigating the outbreak

or trace-out?

Responses are grouped in the following categories:

• Planning for Disease Detection and Response;

• Biosecurity and Disease Containment;

• Information about EHV-1 and EHM; and

• Education and Communications.

Planning for disease detection and response

Several respondents stressed the importance of being prepared for an outbreak

by having a response plan and guidelines worked out prior to an outbreak. One

person said, “Have a plan for how you will manage such an outbreak, from being

prepared to collect samples appropriately to how to contain the situation and

having the supplies on hand to do this.” Other key planning needs mentioned by

respondents were to:

• Have case definitions and quarantine criteria ready before an outbreak or

within the first 1–2 days of an outbreak and stick with the quarantine criteria

once they are established;

• Know what you are going to do with results before testing. Specifically, this

person said, “Do not test till you know what you will do with the result if it

comes back positive on a normal horse”; and

• Determine who will be paying for the testing.

The need for planning was summarized by one respondent: “It is important to

have a plan for how we will manage the next EHM case or outbreak, as there will

be another one.”
USDA APHIS VS / 39

Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

The lack of consistency in how different States addressed outbreaks was

problematic for several respondents and created confusion among horse

owners. One respondent suggested that regulatory officials work together using

sound science to come up with a consistent response to EHM management.

The various options for testing for EHV-1 and the neuropathogenic variant of the

virus led to some result discrepancies among tests. Two respondents suggested

that the National Veterinary Services Laboratories or some other official

laboratory criteria be established that would certify or validate the testing

methods.

Biosecurity and disease containment

The implementation of additional day-to-day biosecurity practices at a racetrack

was the result of one of the outbreaks. This was carried out in cooperation with

racing commission and track management.

The early recognition of signs consistent with EHM along with a proactive

approach in treatment and containment efforts are important strategies in

responding to an EHM outbreak. Dr. Paul Morley, Director of Biosecurity at the

Colorado State University Veterinary Hospital, who managed an EHM outbreak at

the Veterinary Hospital, said: “The Findlay outbreak information was a great

resource for me. The implementation of measuring rectal temperature in all

potentially exposed horses once a suspect case of EHM occurs was very

important in our management of this outbreak. We plan to implement an

electronic medical record system that will flag any animal with a fever in our

hospital and this will alert me, the director of biosecurity. We would then institute

barrier precautions and work up the patient to determine cause of fever. This is

not only important in control of EHV but of all contagious disease events.”

Several respondents indicated that it is important to be able to remove EHM

cases from the population to reduce viral challenge to other horses on the

premises. The removal of an EHM case from the population requires having the

ability to isolate the horse and provide necessary care for that horse while

complying with biosecurity procedures. Those who addressed outbreaks at

racetracks agreed that having an off-site facility or a quarantine barn on the track

would facilitate management of the outbreak.

Although some respondents also believed it was important to segregate EHV-1

test-positive horses from potentially exposed but test-negative horses, others

believe there is no reason to test clinically normal horses, even if they have been

exposed, since some of these may test positive for EHV and it may be difficult to
Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

40 / EHM 2008

then determine how to manage them. One respondent said, “We do not believe

horses that are normal—no clinical signs of disease—should be tested for EHV,

as it is challenging to know how then to interpret a positive test in a normal

horse. In other words, what action should be taken based on such testing?”

One of the respondents who dealt with an outbreak at a racetrack acknowledged

the need to be very aggressive in response to the first EHM case(s) and put

control measures in place, but said it is important to try to have racing continue,

even if on a limited basis.

Several lessons were learned by respondents in regard to biosecurity and

disease containment. After trying to use private practitioners to implement control

efforts at a racetrack, one respondent concluded: “It is best to have veterinarians

who are not employed by the trainers in charge of the control efforts, such as

those that work for the track management.” Respondents reported that some of

the common practices that have been in place for years at racetracks were reevaluated and changes made as follows:

• “We minimize commingling of horses at the detention barn as much as

possible. We have done away with the common-use urine collection stall at

the detention barn; this stall was reused to collect urine from horses. Now we

pick the stall and spray it down when done with each horse. We recommend

trainers bring their own water buckets to the detention barn.”

• “We have the ID person not touch the horses, but have the handler show

them the ID on the horse’s lip.”

• “The person checking tattoo on lip changes gloves between horses.”

Implementation of biosecurity and disease containment measures at racetracks

proved challenging as evidenced by the respondents’ comments below:

• “The workers live like the 1930s, in that the backside of the track is their life.

They live there and have very little in the way of change of clothes and

footwear, so we need to take this into consideration when trying to implement

biosecurity and restrict access to the barns.”

• “The trainer in [a quarantined barn] had the coffee pot on all the time so

people would come in for a cup and to chat—as that is what they always

did—yet we were trying to limit access to this barn, so it is hard to get the

message across in this setting.”

• “The racetrack management putting people in charge of being sure protocols

were followed really helped, but it took multiple cases of EHV infection to

convince them this was necessary. As we identified more infected horses

based on testing in-contact horses, we got more buy-in from trainers for the

control protocols we were putting into place.”
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Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

Information about EHV-1 and EHM

Another category of lessons learned from these EHM outbreaks centers around

the herpesvirus itself and the neurological presentation of the disease. Although

we have learned a lot from those who mitigated EHM outbreaks, there still is

much to learn.

Perhaps one of the most important lessons learned regarding EHM case

management was recommendated by one of the respondents: “If a horse or

horses have a history of neurological disease that was preceded by fever, EHM

is high on the differential list and these horses should be managed as though

they pose a risk to other horses.”

Another mitigator pointed out that this disease agent can spread rapidly with a

few less-than-optimal control efforts, such as the pony horse leaving the

quarantine barn at a racetrack and having contact with horses in other barns.

This respondent pointed out that the trainer did not understand that the pony

horse was part of the quarantine as the trainer simply did not think of the pony

horse as a “horse.”

Dr. Steve Reed, faculty clinician at The Ohio State University Veterinary Hospital

during an outbreak in 2003, reported that EHM, as observed during this

outbreak, was a much more severe disease than he had observed in the past.

Some cases experienced ascending paralysis with progression to encephalitis.

Since there is still much to learn about this disease, such as risk factors, vaccine

efficacy, and treatment, one respondent recommended the archiving of samples

for future study. [Note: The suggested research needs will be addressed in more

depth later in this report.] “The equine industry wants to know when they can

safely move their horses and we as regulatory officials need to have the answer

if we are involved,” one respondent emphasized. “The owners are very

cooperative the first time around, but the next time around they expect us to

have more answers.”

Education and communication

Much was learned about communication needs and efforts during these EHM

outbreaks. Several mitigators emphasized the importance of open and timely

communication through multiple venues, such as meetings, Web sites, and

media interviews, to minimize rumors and give people confidence that the

situation is being handled effectively. One respondent learned that they should
Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

42 / EHM 2008

have implemented the communication system earlier to share current facts and

the plan for managing the situation. He said, “We found that not giving timely,

accurate and complete information will cause rampant speculation, rumors, and

panic.” Another respondent confirmed that “you can’t please everybody,”

reporting that “...some people thought we overreacted in our control efforts, and

others thought we should have quarantined all test-positive horses even if they

did not have clinical signs of disease.”

Other respondents used their outbreak as a teachable moment to share

information about disease control and vaccination in general. “We saw this as an

opportunity to educate horse owners and event organizers about importance of

disease monitoring and having a plan in place to isolate ill horses,” commented

one respondent. “This disease event provided a planning and educational

opportunity.”

Another communication initiative that resulted from an EHM outbreak is the

development of e-mail lists and a phone tree. “We are currently working on an

e-mail list so we can do alerts and share info, and then in some situations there

is a phone tree that we would call or alert designated people, and then they

would contact others in their group or organization,” said one State Veterinarian.

“I have always had an open-door policy, so if people are wondering what the

situation is on any given disease they can call my office and they will get to

speak to someone or get a prompt call back.”

Research Needs

What would you say are needed areas of research related to EHM?

Several research needs regarding EHV-1 and EHM were identified by

respondents. These research needs have been grouped into three categories:

• EHV-1 epidemiology;

• Diagnostics; and

• Vaccine development and treatment options.

EHV-1 epidemiology

Several research needs were identified regarding epidemiology of EHV-1 and

EHM as illustrated by the respondents comments listed below:

• Conduct surveillance to determine if this disease really is getting worse or is it

just heightened awareness and concern about it. (Hohenhaus)
USDA APHIS VS / 43

Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

• Identify non-horse reservoir of EHV. “I think there may be some reservoir of

the EHV that can somehow modify it, either in number of viral particles or

pathogenicity—some kind of cytokine promoter gene phenomenon. In several

outbreaks I have dealt with, mules have visited or been moved onto the

premises in the proceeding 21 days. I do not think they are carriers of the

virus but somehow they might modify the virus or shed high numbers for a

short time.” (Madigan)

• Why recent outbreaks have been occurring. (Sanchez)

• “What is the percent of horses that, in a stressful situation, would shed EHV

and of those, which shed the neurological form of the virus?” (Short)

• Epidemiologic studies to determine the percentage of horses that shed

 EHV-1 at shows and other equine events, both the neurologic variant and the

wild-type as determined by PCR testing of nasal swabs. (Madigan)

• “We need to understand this virus better and understand the role of latency in

the outbreaks of EHM.” (Arthur)

•“Are the isolated single EHM cases a result of reactivation of a latent infection

or due to exposure to another horse that is shedding?” (Arthur)

• Test interpretation of “garden variety” EHV versus the mutant strain; risk

posed by horses that have one type versus the other. (Hohenhaus)

• “What is the significance of the neuropathogenic variant of the virus being

shed by a normal horse?” (Hoenig)

• More epidemiologic studies from outbreaks to identify risk factors for EHM

and impact of vaccination on outcome of cases and controls. (Morley)

• “We need more research on risk factors for EHM so collection of casecontrol study info during naturally occurring outbreaks would be valuable.”

(Morley)

•“It would be good to know if there are biosecurity practices that could reduce

the risk of this disease or not.” (Lis)

• “How far can virus be spread via aerosol mode of transmission?” (Fowler and

Boone, Morley)

• “What is the incubation period in naturally occurring cases and how virus

spreads?” (Halpern)

• Duration of shedding of EHV-1 from field-based situations. (Morley)

Identification of risk factors associated with the development of EHM is one of

the primary areas for epidemiologic research mentioned by a majority of

respondents:

• Which horses are at risk for EHM, versus those that only develop a fever or

exhibit no signs of disease despite being exposed?
Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

44 / EHM 2008

• Correlation of positive PCR test with virus isolation results and if there is a

way to predict the risk test-positive but clinically normal horses pose to other

horses;

• What are risk factors for reactivation of EHV?

• Is there a risk posed by certain types of horses as carriers? Determine if

there is a way to identify the horses that are going to pose a risk before there

is an index case or outbreak.

• What is the risk posed by a PCR-positive, EHV-shedding horse that is

clinically normal?

Diagnostics

Some of the suggestions regarding research needs for EHV diagnostics were to:

• Have the National Veterinary Services Laboratories or some other

standardizing system recommend or validate the tests that are being done to

identify EHV and the neuropathogenic variant;

• Develop a rapid test/stall-side test for the variant that causes the neurological

form of the disease;

• Establish a better description of the immunopathology of the central nervous

system lesions; and

• Determine the most appropriate diagnostic samples to test: nasal swabs

versus blood in naturally occurring outbreaks.

Vaccine development and treatment options

Vaccine development and treatment options were other areas in which additional

research was deemed necessary by respondents. Two respondents indicated

that a study of the efficacy of the available vaccines in prevention of EHM would

be beneficial. A majority of respondents stated that development of a vaccine to

prevent the neurological form of the disease should be a priority. Three

respondents stated that the efficacy of treatment options needs to be assessed,

particularly the efficacy of antiviral drugs.
USDA APHIS VS / 45

Part II: Summary of Interviews with Mitigators of Recent EHM Disease Events

 Summary and Conclusions from Interviews with Mitigators of EHM Disease Events

Mitigators of each of the EHM disease events stopped horse movement, either

through voluntary cooperation or mandatory restrictions; however, the approach

taken by mitigators, and the positions of authority of the mitigators, was quite

varied. Several of the respondents mentioned a need for more consistent

guidelines from State to State on how best to contain EHM once an index case is

identified. Perhaps a “Best Management Practices” document could be prepared

using the current state of knowledge about EHV-1 and more specifically EHM,

but acknowledging that much dialogue and research is needed to determine

where consensus exists and to fill existing information gaps.

If a consensus regarding best management practices were developed, it would

provide a starting point for mitigators of future outbreaks, as well as eliminate

some of the confusion among those involved in the equine industry about why

outbreaks were managed differently. Although guidelines would potentially

provide consistency and add credibility to mitigators’ actions, it is important to

note that criteria for release of quarantines or other stop-horse-movement

actions can be assured through different approaches. Shortening the quarantine

period (e.g., from the AAEP-recommended 28 days after there is no evidence of

disease), would require more testing than would basing the status on disease

detection only.

Nearly all respondents mentioned the need to develop a vaccine that is

efficacious in preventing EHM. In addition, they stressed the need to identify risk

factors for EHM and transmission capabilities of EHV-1 in order to apply

scientific knowledge to the development of best management practices. Several

respondents indicated that they believed it ideal to move EHM cases to a barn or

facility separate from other horses. Efficacy of biocontainment procedures also

needs to be evaluated to make recommendations for best management

practices. Few of the respondents had tracked all of the costs associated with

the EHM outbreaks. To better document the financial impact of EHM outbreaks,

and to prepare for the financial costs of mitigating EHM outbreaks, costs need to

be tracked in future outbreaks.
Part III: Recent Publications That Address Needs Suggested in Interviews

46 / EHM 2008

Part III: Recent Publications That Address Needs Suggested in Interviews

The purpose of Part III is to provide information from a few selected recent

publications that address some of the questions posed by those interviewed for

this report. This section does not provide an extensive and all-inclusive review of

the literature regarding EHV-1 or EHM, as multiple reviews are presently

available and the ACVIM will have fully referenced responses to nine key areas

related to the understanding of equine herpesvirus available in fall 2008 on the

ACVIM Web site.

1) Recent Advances in Our Understanding of Equine Herpesvirus-1 (EHV-1)

Myeloencephalopathy. Allen G, Timoney PJ. Report for the Infectious Diseases

of Horses Committee of the U.S. Animal Health Association, 2007.

This proceedings paper contains as-yet unpublished information from these

authors as well as cited published information and is available on the USAHA

Web site at http://www.usaha.org/committees/reports/2007/report-hd-2007.pdf

Disease emergence:

Evidence of recent increase in EHV-1 myeloencephalopathy (EHM) in

assemblages of horses in the United States and discovery that the majority of

these outbreaks were caused by a mutant hypervirulent strain of the herpesvirus

suggests that EHM is an emerging disease.

Molecular characterization of 450 archived EHV-1 isolates recovered from

equine fetuses in Kentucky that were aborted over a 50-year period (1956-2005)

revealed a statistically significant increase in herpesviral isolates with the genetic

marker (ORF30 A2254 to G2254 mutation) for the neuropathogenic strain of EHV-1.

Enhanced pathogenicity:

Mechanistic basis for such increased virulence of the ORF30 mutant strain of

EHV-1 lies in the enhanced replicative capacity of this mutant strain to result in

tenfold higher levels of leukocyte-associated viremia, relative to those generated

by the wild-type EHV-1 infections.

Recent experimental inoculations with the mutant strain of EHV-1 resulted in a

higher attack rate and neurologic mortality rate than did challenge with the wildtype, abortion storm isolate of EHV-1.
USDA APHIS VS / 47

Part III: Recent Publications That Address Needs Suggested in Interviews

Diagnostic testing:

The laboratory test that yields the most rapid and sensitive diagnostic results for

EHV-1 is the PCR and the equine samples of choice are nasal swabs and buffy

coat leukocytes collected from whole blood samples. Since the post-exposure,

temporal profiles of the presence of EHV-1 in nasal secretions and circulating

leukocytes of the horse do not completely overlap, the collection and testing of

both clinical specimens is necessary to achieve the maximal diagnostic

sensitivity. Facilitation of EHV-1 genotyping in diagnostic laboratories has

recently been achieved with a real-time PCR-based test procedure.

In the context of an ongoing epizootic of EHV-1 neurologic disease, positive PCR

test results on horses associated with the epizootic by physical proximity, a

history of exposure, and/or clinical signs consistent with EHV-1 infection can

reliably be interpreted as evidence of active infection by herpesvirus. The

interpretation of positive PCR results on nasal swabs or blood samples from

horses not linked to an outbreak of EHV-1 infection and tested only for purposes

of general screening, health certificate requirements, or pre-transport monitoring

for virus shedding is more problematic and carries less certitude. A part of the

interpretative uncertainty derives from lack of any information on positive and

negative predictive values of EHV-1 PCR tests. There is strong evidence to

suggest that latent virus does not circulate in blood leucocytes. These authors

thus suggest that horses whose blood leukocytes test positive for EHV-1 DNA by

PCR are most likely to be actively infected by the virus.

Immunologic requirements for vaccine-induced protection from EHV-1

myeloencephalopathy:

These authors reported having developed an equine disease challenge model for

evaluation of the efficacy of vaccines for protection of horses against EHV-1

myeloencephalopathy. These authors suggested that to protect against EHM, a

vaccine would need to induce an immune response toward production of

cytotoxic T-lymphocyte precursors. They conducted proof-of-concept studies

using a virulent, wild-type EHV-1 intranasal vaccine that produced full protection

against challenge inoculation (90 days after vaccination) with a highly

neuropathogenic, mutant strain of equine herpesvirus.
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2) Prevalence of latent, neuropathogenic equine herpesvirus-1 in the

Thoroughbred broodmare population of central Kentucky. Allen GP, Bolin

DC, Bryant U, Carter CN, Giles RC, Harrison LR, Hong CB, Jackson CB,

Poonacha K, Wharton R, Williams NM. Eq Vet J 40:2, 105–110 (6), 2008. The

abstract below was excerpted from the Web of Science.

 Abstract:

Reasons for performing study: An emerging problem of equine herpesvirus-1

(EHV-1) infection in horses in the USA is a high-mortality myeloencephalopathy

that commonly occurs where large numbers of horses are stabled. EHV-1

isolates recovered from recent neurological outbreaks represent a mutant virus

strain that possesses enhanced neuropathogenicity. A central question of EHV-1

myeloencephalopathy is the latency carriage rate for these mutants of EHV-1 in

USA horse populations.

Objective: To estimate the prevalence of neuropathogenic strains of EHV-1 as

latent infections in the Thoroughbred broodmare population of central Kentucky.

Methods: Submandibular lymph nodes (SMLN) were collected during post

mortem examination of 132 Thoroughbred broodmares. Total DNA purified from

SMLN tissue was tested for the presence of latent EHV-1 DNA by an

ultrasensitive magnetic bead-based, sequence-capture, nested PCR method.

Differentiation of active from latent infections by EHV-1 was achieved by

detection of transcripts of EHV-1 glycoprotein B by reverse transcription PCR.

Results: Latent EHV-1 DNA was detected in the SMLN tissues of 71 (54%) of the

132 mares submitted for necropsy. Thirteen (18%) of the 71 latently infected

horses harbored the neuropathogenic biovar of EHV-1. Of the 13 horses latently

infected with an ORF30 mutant strain of EHV-1, 11 also carried a latent, wild-type

strain of the virus in their SMLN tissues.

Conclusions: Neuropathogenic strains of EHV-1 have established a significant

presence in the Thoroughbred broodmare population of central Kentucky as

latently infected carrier horses. The data also indicate that a highly sensitive DNA

detection method is required to identify many instances of EHV-1 latency.

3) Equine Herpesvirus Type-1-Associated Myeloencephalopathy in The

Netherlands: A Four-Year Retrospective Study (1999–2003). Goehring LS,

van Winden C, van Mannen C, Sloet van Oldruitenborgh-Oosterbaan M. J Vet

Intern Med 20:601–607, 2006.

These authors described nine confirmed outbreaks of EHV-1-associated

myeloencephalopathy (EHM) in the Netherlands. The authors stated that the

typical presentation of EHM in The Netherlands is: (1) an index animal with
USDA APHIS VS / 49

Part III: Recent Publications That Address Needs Suggested in Interviews

neurologic signs; (2) fever occurs in horses other than those with signs of

neurologic dysfunction; (3) signs of mixed neurologic myelopathy; (4) seasonality

of outbreaks (late fall, winter, and early spring); and (5) specific risk factors

(breed, age, and sex) were associated with presence of neurologic signs in

horses. In this study, factors individually associated with severe neurologic

dysfunction included increasing age, horses having been febrile, and being

female. After adjusting for premises, breed, and fever, age, and sex were no

longer significant risk factors, suggesting that fever was the most important

factor related to the chance of developing severe neurologic dysfunction.

Neurologic dysfunction was never encountered in Haflingers, Fjords, Icelandic

horses, the archetypical pony breeds, and horses less than 3 years of age. Most

index cases in this study had a history of transportation or exercise under

stressful conditions. Most EHM outbreaks in The Netherlands occurred on

premises with unvaccinated horses and the authors conclude that today’s

vaccines are an unlikely risk factor for development of EHM.

4) Comparison of the efficacy of inactivated combination and modified-live

virus vaccines against challenge infection with neuropathogenic equine

herpesvirus type 1 (EHV-1). Goodman LB, Bettina W, Flaminio MJB, Sussman

KH, Metzger SM, Holland R, Osterrieder N. Vaccine 24, 3636–3645, 2006.

These authors described a challenge infection using aerosolized virus that had

been isolated from a fatal case of EHV-1 myeloencephalopathy in Findlay, Ohio

(strain OH03, kindly provided by Dr. G.P. Allen). Healthy female mixed breed

horses ages 3–10 years were randomly assigned to one of three groups for this

study: (1) the control (placebo) group received minimal essential medium, n=5;

(2) inactivated virus vaccine group (FluVac® Innovator 6 combination vaccine,

Fort Dodge), n=5; and (3) modified live virus (MLV) vaccine group (Rhinomune ®,

Pfizer), n=5. All vaccinated horses received two intramuscular injections of

vaccine per manufacturer recommendation at 30-day intervals and were

challenged with OH03 virus (5x106 plaque-forming units of cell-free virus

supernatant) 59 days after initial injection.

The authors designed this study to compare the efficacy of two commercially

available vaccines used to protect horses against the respiratory form of EHV-1

disease. The immunization regime was that recommended by the manufacturers’

instructions and the challenge was with a recent, neuroinvasive EHV-1 isolate.

Fever occurred in horses in all three groups but duration of fever was statistically

different among the groups. The mean duration of the fever in the modified live

vaccine group was 1.2 days; the mean duration in the controls was 3.4 days; and
Part III: Recent Publications That Address Needs Suggested in Interviews

50 / EHM 2008

the mean in the inactivated vaccine group was 2.25 days. Induction of EHV-1-

caused neurologic disease was observed in three of the five horses in the control

and inactivated vaccine groups while no neurologic signs occurred in the

modified-live vaccine group.

Nasal shedding peaked on days 3–5 in both control and inactivated vaccine

groups; in contrast, only marginal shedding was detected in one of the five

horses in the modified-live vaccine group. Comparable levels of viremia, up to

106 genome copies per million lymphocytes, were detected in all three groups of

horses. On days 6-8, fewer of the modified-live vaccine group horses had

detectable viremia when compared to the other two groups, although the

difference was not statistically significant. When the data on viremia were

compared to development of neurologic signs for the horses in all three groups

there was a suggestion that the duration of viremia was more important than the

number of infected lymphocytes.

These authors suggested that neutralizing antibodies are not correlated with

protection from abortion and extend those findings to clinical presentation as it

pertains to the neurological form of EHV-1. They indicated that an EHV-1 specific

serum IgG isotype assay may be predictive with respect to protection against

EHV-1 infection with the immune response in the modified live vaccine group

being characterized by IgGb antibodies after the second dose of vaccine and

horses vaccinated with the inactivated vaccine having a high proportion of IgG(T)

antibodies. These authors proposed that high IgG(T)/IgGa or IgG(T)/IgGb ratios

as induced by the inactivated vaccine seem to be unfavorable with respect to

protection against EHV-1 clinical disease while the EHV-1 specific responses

dominated by IgGb antibodies seemed to be correlated with protection.

In conclusion, these authors suggested that the quick recovery from fever and

the protection from neurological signs, despite the high challenge induced

viremia, indicate that the RacH-based MLV vaccine (Rhinomune®) is apparently

effective in protecting horses from EHV-1 disease, at least under conditions of a

relatively short interval (29 days) between the booster dose of vaccine and the

challenge infection. It is important to note that the results of their study related to

the “poor performance” of the inactivated vaccine evaluated in this study may not

be indicative of all inactivated vaccines and that other inactivated vaccines with

proven immunogenicity and efficacy in prevention of abortion and respiratory

disease in EHV-1 studies should be tested in future studies.
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5) Outbreak of Neurologic Disease Caused by Equine Herpesvirus-1 at a

University Equestrian Center. Henninger RW, Reed SM, Saville WJ, Allen GP,

Hass GF, Kohn CW, Sofaly C. J Vet Intern Med 21, 157–165, 2007.

These authors describe an outbreak of EHV-1 infection causing neurologic

disease among a group of horses at a university equestrian center beginning in

January 2003. Out of the 135 horses at this facility, 117 displayed signs of EHV-1

infection (117/134=87.3%). Forty-six horses developed neurologic disease (46/

135=34%) and 12 (12/135=8.9%) became recumbent and did not survive (12%).

There were two waves of fevers among horses on this facility with 42 horses

affected with neurologic signs in the first wave of EHV-1 infection. The authors

indicated that the time interval from the end of the febrile period to the onset of

neurologic signs ranged from 1-4 days and that all horses were afebrile at the

onset of neurologic signs. Factors associated with development of neurologic

disease at this facility included age of the horse (>5 years), location in south or

arena stall area, and highest rectal temperature on day 3 or later in the febrile

period. Resident horses at this facility were vaccinated every 3 months with an

inactivated EHV-1/EHV-4 vaccine product (FluVac® Innovator, Fort Dodge Animal

Health), with the last vaccine being administered 2 months prior to the outbreak.

The one pregnant mare had received two vaccinations with an inactivated

vaccine product containing EHV-1 abortigenic strains (EHV-1p and EHV-1b,

Pneumabort-K® +1b, Fort Dodge Animal Health). Newly arriving horses were

required to be vaccinated against influenza and herpesvirus 10-45 days prior to

arrival at this facility. The type of vaccine product to be administered to newly

arriving horses was not mandated. Only four of the horses at the facility at the

time of the outbreak had received the modified live EHV-1 vaccine (Rhinomune®,

Pfizer Animal Health). The authors indicated there was no apparent difference in

effect of vaccine type or manufacturer on the incidence of EHV-1 infection or

development of neurologic signs, but pointed out that only four horses had

received the modified-live vaccine prior to the outbreak. There did appear to be

an association between horses that were vaccinated three to four times per year

preceding the outbreak and the development of neurologic manifestations of

EHV-1. The authors suggested that this apparent association between more

frequent vaccination and development of neurologic disease needs to be

interpreted with caution as the majority of horses that were vaccinated three to

four times annually were those that were older resident horses to the facility,

whereas many young horses had been vaccinated for the first time before

entering the facility. They also pointed out that since age was also associated

with the development of neurologic disease, it is difficult to assess the impact of

vaccination alone.
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52 / EHM 2008

6) Transmission of EHV-1 by Horses with EHV-1 Myeloencephalopathy:

Implications for Biosecurity and Review. Kohn CW, Reed SM, Sofaly CD,

Henniger RW, Saville WJ, Allen GP, Premandan C. Clinical Techniques in Equine

Practice 5, 60–66, 2006.

This article described the occurrence of nosocomial EHV-1

myeloencephalopathy in hospitalized horses associated with admission of EHV-1

myeloencephalopathy cases into the hospital from a 2003 outbreak at the

University of Findlay. The authors pointed out that the transmission of EHV-1

from neurologic cases to other hospitalized horses not associated with the

original outbreak indicates that the previous assumption that horses with EHV-1

myeloencephalopathy are no longer shedding virus is erroneous. Although the

precise means by which EHV-1 was spread in this university veterinary hospital

is unknown, the authors suggested several potential means by which the virus

may have spread. These include: (1) the close proximity of the index cases and

subsequent EHV-1 myeloencephalopathy cases; (2) people and horse traffic

through the area with the index cases and their movement into other areas of the

hospital; and (3) movement of hospitalized cases around the hospital. The EHV-1

transmission occurred despite personnel wearing barrier precautions when

working with EHV-1 myeloencephalopathy cases.

These authors recommended that vigorous biosecurity precautions be instituted

when EHV-1 myeloencephalopathy cases are admitted for veterinary care

including strict isolation of cases, preferably in a dedicated facility well away from

the main hospital. Attendants should wear occlusive barrier clothing (coveralls,

rubber or plastic boots, and gloves) and access to EHV-1 myeloencephalopathy

cases should be restricted as much as possible. In addition, they suggested

strict quarantine of all horses having had recent contact with EHV-1

myeloencephalopathy case(s), with implementation of monitoring to detect signs

of EHV-1 infection for a minimum of 21 days following the last clinical evidence

of active EHV-1 disease. These authors reported that some outbreaks can be

characterized by new waves of disease, as reported at the University of Findlay,

and that this necessitates an even longer period of quarantine of the premises.
USDA APHIS VS / 53

Part III: Recent Publications That Address Needs Suggested in Interviews

7) Comparison of the Diagnostic Sensitivity of Nasopharyngeal and Nasal

Swabs and Use of Viral Loads for the Molecular Diagnosis of Equine

Herpesvirus-1 Infection. Pusterla N, Mapes S, Wilson DW. AAEP Proceedings

53, 220–224, 2007.

1. This study indicated there was a similar recovery and quality of viral DNA

between nasopharyngeal swab samples and nasal swab samples, when

collected from the same horse at the same sampling time in the course of

EHV-1 infection, during a naturally occurring outbreak of disease. There

were 19 study horses in which EHV-1 was detected on both nasopharyngeal

and nasal swabs. There were an additional 14 horses that were real-time

PCR test-positive on either the nasal swab or the nasopharyngeal swab. A

larger number of horses were test-positive for EHV on the nasal swabs than

on the nasopharyngeal swabs.

2. Viral load in nasal swab samples from neurologic EHV cases was high, and

the authors concluded this confirmed the importance of these horses as a

source of contagion to other horses.

3. Of 15 neurologic horses tested, viral loads were significantly higher in blood

than in nasal secretions, but only 9 of the 15 neurologic horses tested PCRpositive on blood samples, whereas EHV-1 was detected in all of the nasal

secretions. For the nine horses in which EHV-1 was detected in both blood

and nasal secretions, viral loads were higher in nasal secretions.

4. Of 41 subclinical horses tested, only 5 had positive PCR test on blood while

40 tested positive on PCR test of nasal secretions.

5. When viral loads in blood were compared between subclinical, febrile, and

neurologic horses, febrile horses had significantly higher viral loads than did

subclinical or neurologic horses. In contrast, viral loads in nasal secretions

of neurological horses were significantly higher than those of febrile or

subclinically infected horses.

6. The authors suggested that neurologic EHV-1 infected horses pose a

contagious risk to other horses based on high viral load in nasal secretions.

7. They also concluded that the random testing of healthy horses for EHV-1 by

PCR should be avoided, because practicing veterinarians and regulatory

veterinarians who submit such samples may be unaware of the

complexities involved in test interpretation. The situation is likely different

when healthy horses determined to be at high risk of exposure are tested

for surveillance purposes during active outbreaks of EHV-1 infection. Under

such circumstances, horses that test positive by PCR on nasal secretions

should be isolated and closely monitored for development of clinical

disease, because the viral load pattern of infected horses during the early

incubation period is similar to that of subclinical carriers. Followup
Part III: Recent Publications That Address Needs Suggested in Interviews

54 / EHM 2008

assessment of viral loads in blood and nasal swabs can be used to help

guide modification of infectious disease-control measures, including lifting

quarantine, for individual horses that test negative on subsequent sample.

8) Characterization of viral loads, strain and state of equine herpesvirus-1

using real-time PCR in horses following natural exposure at a racetrack in

California. Pusterla, N, Wilson WD, Mapes S, Inno C, Isbell D, Arthur R,

Farraro G. The Veterinary Journal, in press. Available online via Science Direct,

DOI number: doi:10.1016/j.tvjl.2007.09.018. Accessed online April 15, 2008.

Viral loads, strain type (neuropathogenic versus non-neuropathogenic) and state

(lytic, non-replicating, or latent) of EHV-1 were determined by real-time PCR on

whole blood and nasopharyngeal secretions of adult horses after natural

exposure. The index case of EHM and potentially exposed horses were sampled

as part of this study. The authors suggested that the PCR tests typically

performed at veterinary diagnostic laboratories do not differentiate the state of

the virus (lytic, non-replicating, or latent) or report viral load (quantification of

amount of virus detected).

Several key findings from this study included:

1. The index case that exhibited signs of EHM at the time it was initially tested

had high viral shedding, based on testing of nasal swab samples, with

concurrent low level viremia. This horse was afebrile at this time but had a

history of fever in the 3 previous days. The authors concluded that horses

with EHM pose a substantial risk to other horses and should be isolated

away from other horses in order to decrease the risk to other horses.

2. PCR assay targeting the gB gene of EHV-1 was associated with a higher

detection rate when compared to the assay that targets the ORF 30 gene

(used to distinguish the neuropathogenic from the non-neuropathogenic

variant). Typically those cases with low viral loads tested negative when the

ORF30 gene was targeted.

3. Only the index horse and the horse in the neighboring stall tested positive

solely for the neuropathogenic strain of the virus.

4. Other horses that were exposed to the index case were most commonly

found to be positive, based on PCR testing, for either the nonneuropathogenic strain or both the neuropathogenic and nonneuropathogenic strain. Some of these horses remained clinically normal

despite testing positive on the same sample for both the neuropathogenic

and non-neuropathogenic strain of EHV-1.

5. Horses with evidence of the lytic infection characterized by presence of the

transcriptional activity of the gB gene were associated with high viral loads at

the gDNA level, while non-replicating virus was routinely seen with low
USDA APHIS VS / 55

Part III: Recent Publications That Address Needs Suggested in Interviews

viral loads. To assist in defining the non-lytic state, samples were tested for

markers of functional latency, EHV-1 LAT; latency was detected in some of

the samples tested as part of this field investigation.

6. The authors concluded that there is a need for consensus on the use and

interpretation of molecular diagnostic techniques in the diagnostic evaluation

of field cases of suspected EHV-1 infection, since this information may

influence the risk of exposure to other horses and may help practitioners and

regulatory officials make appropriate decisions regarding the management of

horses that test positive on PCR.

9) Detection of Nasal Shedding of EHV-1 and 4 at Equine Show Events and

Sales by Multiplex Real-Time PCR. Yactor J, Lunn KF, Traub-Dargatz JL,

Morley PS, Barnett CD, Kohler AK, Kasper KS, Kivi AJ, Lunn DP. AAEP

Proceedings 52, 223–227, 2006.

1. This study was based on samples collected from horses attending four

equine events (shows and sales) and from horses at a farm with an

epidemiologic link to horses at one of the sales. Sampling was directed

toward juvenile horses attending these events. A total of 451 horses were

sampled on arrival at the event with 122 of these same horses having a

repeat sample several days later.

2. The testing performed on the nasal swab samples was a multiplex real-time

PCR test conducted in a research laboratory at Colorado State University.

3. Overall 3.8 percent of the samples were positive for EHV-1 on arrival and 1.8

percent of the repeat samples were positive for EHV-1.

4. These samples were not tested further to determine if the virus detected was

the neuropathogenic variant of EHV-1.

5. This study illustrated that horses at events can shed EHV-1 that is detectable

by real-time PCR, yet none of these horses showed signs of neurologic

disease while at the event. The authors indicated further analysis of data will

be conducted to determine if animal risk factors for shedding of virus can be

ascertained.
Part IV: EHM Reporting in the National Animal Health Reporting System

56 / EHM 2008

Part IV: EHM Reporting in the National Animal Health Reporting System

The National Animal Health Reporting System (NAHRS) is a joint effort of

USAHA, AAVLD, and APHIS. NAHRS was designed to provide data from State

animal health officials on the presence of confirmed OIE-reportable diseases in

specific commercial livestock, poultry, and aquaculture species in the United

States. It is intended to be one part of a comprehensive and integrated animalhealth surveillance system.

NAHRS is based on the reporting of recognized presence or absence of each of

the OIE-reportable diseases, but does not require the reporting of the prevalence

of these diseases. Confirmed disease is reported as present if the State animal

health official determines there was at least one case that met NAHRS reporting

criteria for that specific disease. If a case does not specifically meet all of the

NAHRS reporting criteria, the State animal health official may also use additional

information or epidemiological information to decide if the case should be

reported as a confirmed case. Within a State, data about animal disease

occurrences are gathered from as many verifiable sources as possible and

consolidated into a monthly report that is submitted to the APHIS-VS National

Surveillance Unit, where the information is verified, summarized, and compiled

into a national report.

Historically, the NAHRS category for reports of equine herpesvirus was “EHV-1

or EHV-4” (see B208 below). The States reported “Yes” or “No” to indicate the

detection (or lack of detection) of “EHV-1 or EHV-4,” with no further

differentiation. In August 2007 the NAHRS Steering Committee approved the

addition of reporting capabilities for neurological EHV-1 EHM for the NAHRS

monthly reports submitted by the States. In October 2007 this proposal was

approved by the USAHA/AAVLD Committee on Animal Health Information

Systems. This reporting was implemented in January 2008.
USDA APHIS VS / 57

Part IV: EHM Reporting in the National Animal Health Reporting System

Method of Reporting EHV and EHM in NAHRS

B208 Equine Rhinopneumonitis (Equine Herpes Virus, EHV 1 and 4):

Proposed Reporting Criteria: Presumptive diagnosis based on clinical signs,

histological lesions (inclusion bodies) and/or a positive fluorescent antibody.

Definitive diagnosis requires either virus isolation and/or PCR detection of

the virus, or a 4X rise in titre in paired serum samples on the serum

neutralization test.

If the response is “Yes” to B208, then States report “Yes” or “No” to

B208a.

B208a EHV-1 Myeloencephalopathy (EHM):

NAHRS reporting criteria for EHM:

A case of EHM is defined as a horse exhibiting signs of CNS dysfunction,

including most commonly posterior incoordination, weakness, and

bladder atony. Additionally, the horse tests positive for EHV-1 by virus

isolation and/or PCR assay on nasal swab or blood (buffy coat). In cases

of sudden death or where the horse dies as a result of neurological

complications, the postmortem lesions are consistent with those of

myeloencephalopathy and EHV-1 has been isolated, detected by PCR, or

demonstrated by immunohistochemical examination of the CNS.

Photo by Chad Mendell
Appendices

58 / EHM 2008

Appendix A: Interview Form for Mitigators of Recent Outbreaks of EHM

The goal is to gather information, through a telephone interview, about your

experience with managing or involvement in mitigation of recent outbreak(s) of

equine herpesvirus myeloencephalopathy (EHM). The purpose of the interview is

to gather background information about the outbreak, then ask some questions

that will allow me to understand how you investigated the outbreak, how the

outbreak was managed/controlled, and any lessons learned from dealing with the

outbreak. Lastly, I would like to get your opinion on research and education as

well as communication needs related to EHM.

First, I will ask you to describe the location, type of facility where outbreak took

place, and dates of the outbreak (from the date the initial case was identified until

the date the outbreak was declared resolved or the quarantine was lifted). Then I

will lead into the following outline of talking points listed below.

1. Describe how you recognized you had an EHM case(s)

a. Who recognized first occurrence of EHM?

i. Clinical signs and testing criteria for index case(s)

ii. Was EHV-1 actually isolated, using virus isolation technique,

from any of the samples?

iii. Was testing done to determine if the EHV-1 had mutation

associated with the neurologic form of EHV?

b. Did you have a specific case definition for EHM and if so what

was it?

c. What made you consider this an outbreak (disease occurrence

above the norm or expected)?

2. Once you recognized the first EHM case(s) what measures were taken

for as yet unaffected horses? List and explain all that apply.

a. If you instituted some form of active surveillance can you describe

what that entailed?

i. If they monitored body temperature, probe regarding what

criteria were used to describe a horse as febrile.

1. Temperature cut-off used to describe a fever

2. Did horses have to have a fever more than one

occasion to be called a “febrile horse?”

3. How often did they take temperatures and on which

horses?
USDA APHIS VS / 59

Appendices

4. Did they have a log or written record of actual temps

or just a fever? [yes/no]; outcome to be reported to

someone involved with control of the outbreak

b. If you instituted sampling and test protocols for as-yet nonclinical

horses, please describe how you decided which horses to sample, what

samples were collected, and what tests were performed.

i. Describe, during your outbreak of EHM, how you interpreted

various examinations and test results, including how the horses’

clinical signs were integrated into interpretation of laboratory test

results.

3. How did you classify or categorize horses relative to their EHV status?

a. For example, if I asked how many “cases” you had in your outbreak,

how would you report horses that only had a fever, or had no clinical

signs of disease but from which virus was isolated?

b. What action did you take based on the clinical and/or laboratory

findings?

c. Can you describe what you did to define “exposed” or “at-risk” horses

once you identified the first case of EHM?

d. Did you have any laboratory tests reported as “inconclusive” and, if so,

how did you interpret these test results?

4. What was done to mitigate the outbreak?

a. List all that apply and describe how it was implemented.

i. Potential responses might include:

1. Isolate confirmed cases of EHM away from other

horses.

a. If so, describe how you isolated these cases.

Did you move them to another location? Was

each case kept so it did not have physical

contact with any other equids? Describe

ventilation of isolation site, barrier precautions

taken, how you staffed the area used for

isolation.

2. Instituted biosecurity precautions for handling of

febrile horses.

a. If so, describe what these were and how you

did this.

3. Instituted biosecurity precautions for handling of all at

risk horses.

a. If so, describe what your procedures were.
Appendices

60 / EHM 2008

4. Did you trace horses that might have been exposed to

the EHM case(s)? [For example, those that might have

shared a housing area in the recent past, shared a trailer

or had the same exercise rider or farrier with the EHM

case?]

a. What criteria did you use to determine

exposure? [For example, how much contact was

considered exposure? (e.g., shared a pony

horse or shared a water bucket)]

b. If so, can you describe how you did this?

5. What were the challenges with doing this tracing?

6. Instituted surveillance to detect febrile horses.

a. If so, what temperature did you use to

describe a fever and how were you alerted to

occurrence of a febrile horse?

b. If a febrile horse was detected, what did you

do?

7. Performed surveillance testing to determine horses

shedding status of EHV-1.

a. Was this done on all horses, or just those with

fever, or just those with neurologic signs?

8. Instituted some form of formal identification system

during quarantine and testing and, if so, how was this

done?

9. Did you vaccinate at-risk horses to attempt to control

the outbreak?

5. How many cases of EHM did you have as part of the outbreak?

a. Were EHM cases recognized throughout outbreak, with equal severity

of signs, or only at beginning of the outbreak?

6. Did you have horses that had a fever and confirmed infection but that

did not develop neurologic signs? If so, how many?

7. How many horses were determined to be infected with EHV-1 during this

outbreak?

a. How many were tested?

b. How many were test-positive for EHV?

c. How many of these had any clinical signs of disease?

8. How many horses were at the premises when the outbreak occurred?

a. How many of these were at risk of exposure to EHV-1 as part of this

outbreak?
USDA APHIS VS / 61

Appendices

9. Did you collect any information about the cases such as signalment,

EHV-1 vaccination history, travel history, response to treatment or diet?

a.If so, did you collect this same information on horses that were

exposed during the outbreak but that did not become cases (e.g.,

controls)?

10. Was a quarantine imposed?

a. If so, what were the specific criteria for quarantine e.g. which horses

and for how long?

i. Describe if additional housing was required, or if horses were

quarantined on-site where the outbreak began and in original

housing

b. Who imposed the quarantine? How was the quarantine enforced and

by whom?

c. How long was the quarantine in place?

d. What criteria were used to remove the quarantine?

11. Who did you communicate with during the outbreak regarding the

status of the situation?

a. List all with whom you had to communicate--for example, owners of

horses that were cases, owners of horses that were exposed or

potentially exposed, private veterinarians, the general public, health

regulatory officials, equine-oriented media, general media.

b. If so, how did you conduct this communication?

i. Was there a designated spokesperson? If so, what was that

person’s position?

ii. Did you have selected talking points?

iii. Did you have written press releases?

iv. Did you post press releases on a Web site?

v. Did you conduct or participate in conference calls?

vi. Did you have meetings with equine owners/trainers to give

updates on the outbreak?

vii. Other?

12. Estimate of cost and please describe who paid for:

a. Testing, cost per test, number of tests, cost of shipping samples, cost

of collection of samples for testing;

b. Number of days of stopped horse movement and per day estimate of

loss number and type of personnel involved in mitigation of outbreak and

number of days each contributed to this outbreak situation;
Appendices

62 / EHM 2008

c. Cost of treatment of cases and number of cases treated—if there is a

range of treatment cost, then what was the number with upper cost of

treatment and the number of cases; the number with average cost of

treatment and number of cases; and lastly, the cost of treating a mild

case, the number with mild disease and number of cases;

d. Cost of deceased horses due to this disease outbreak; this would be

based on the number of horses that died and average or range of value

of these horses;

e. Cost of biosecurity procedures, PPE supplies, disinfectant, hand

hygiene products, barriers, and signage;

f. Other costs, specify what this cost was and the amount.

13. What would have been helpful to you in mitigation of this outbreak that

was unavailable at the time of this outbreak?

14. Are there needed educational materials about EHM? If so specify what

info is needed and format that would be helpful to have.

Examples: biosecurity guidelines specific to EHM, survey instruments/

forms for outbreaks, standard epi questionnaire

15. Was EHM a reportable disease prior to this outbreak? Did your

reporting criteria for EHM change based on this outbreak?

a. Do you feel reporting has changed or just reporting criteria have

changed? What are the criteria for reporting now?

16.What would you describe as lessons you learned from your involvement

in this outbreak?

a. Have you altered your methods of detection of first cases of EHM

based on the outbreak you were involved with? If so, describe.

b. What would you do differently based on the outcome of your outbreak

investigation and mitigation? This could include how you handled first

cases, surveillance, communication, treatment, etc.

c. What advice would you give others based on your experience?

17. Are there needed research areas for EHM? If so, what are they?
USDA APHIS VS / 63

Appendices

Veterinary Services 

Centers for Epidemiology and Animal Health January 2007 

_________________________________________________________________________________________________________________________ 

Equine Herpesvirus 

Myeloencephalopathy: A 

Potentially Emerging 

Disease 

Introduction 

Concern has been voiced within the U.S. horse industry 

that the neurologic (also known as 

myeloencephalopathic or paralytic) form of equine 

herpesvirus type 1 (EHV-1) may be increasing in 

prevalence and/or morbidity and mortality. This concern 

is based on an increased number of neurologic cases 

reported in recent years, as well as the occurrence of 

several high profile outbreaks of neurologic EHV-1 

affecting several sectors of the U.S. equine industry. 

These outbreaks are the first reported EHV-1 outbreaks 

at large facilities or events involving neurologic cases 

that resulted in euthanasia. At least part of the 

increased interest and concern are related to these 

highly publicized events. It is possible that reporting has 

increased, as opposed to an actual increase in number 

and severity of cases; more data is needed to determine 

the actual situation. 

Profile of Disease Emergence 

The question of whether neurologic EHV-1 is emerging 

can be evaluated using standard definitions of disease 

emergence. A disease is considered to be “emerging” 

when it meets at least one of three general criteria. The 

first is when a disease is identified for the first time. 

Recent examples of this would be SARS or Ebola. 

Another criterion is when a disease evolves and 

changes in virulence, host capable of being infected or 

other pathogen behavior. The monkeypox outbreak in 

prairie dogs in 2003 in the United States would be an 

example of this type of disease emergence, as a new 

species was infected. The third criterion is a change in 

disease geographic range or incidence within a range. 

The appearance of West Nile virus in the United States 

in 1999 is a good example of emergence in a new 

geographic location, as is the previously mentioned 

monkeypox example. The current EHV-1 outbreaks are 

of concern because they likely fit the criteria of a disease 

that is evolving and changing in virulence and behavior. 

It is possible that the disease has not changed in 

incidence or character, and that testing and/or reporting 

has been increasing awareness, or that the animals 

affected are higher profile, causing more interest. 

However, it is not possible at this time to make this 

distinction. 

All types of disease emergence are driven by a variety of 

emergence factors. The factors can be environmental, 

agent- or host-specific, related to use of the animal, 

industry standards or any other factor that can affect the 

health of an animal or place selective pressure on an 

infectious agent. Typically, multiple factors come 

together to create a situation where a disease can 

evolve, newly emerge or spread to new environments. 

In the case of neurologic EHV-1 in the United States, the 

disease appears to be increasing in incidence, mortality 

and morbidity. While there has not been much field 

research on neurologic EHV-1, mainly due to the 

sporadic nature of outbreaks, the available data support 

the theory of disease emergence and are cause for 

concern. Some research has been done that suggests 

that a mutation in the virus is playing a role in these 

changes in the disease behavior, which could indicate 

evolution of the viral agent. 

Source: USDA:APHIS:Veterinary Services: Centers for Epidemiology 

and Animal Health, Center for Emerging Issues 

Background of EHV-1 

Equine herpesvirus type 1 (EHV-1) is primarily a 

respiratory pathogen associated with a variety of clinical 

manifestations in horses. In addition to being a 

significant cause of respiratory illness and abortion in 

horses, EHV-1 is responsible for paralytic neurological 

disease. EHV-1 is enzootic throughout the world and 

almost all horses older than 2 years of age have been 

exposed. Following initial exposure, EHV-1 has the 

ability to develop into an inapparent, latent infection. It is 

this ability to reside as a silent and persistent infection in 

horses which provides a reservoir of virus for continual 

transmission. The incubation period of EHV-1 is 1–10 

days; typically signs are seen within 1–3 days. Viral 

shedding occurs for 7–10 days, but can occur up to 28 

days from the onset of signs. The neurologic signs 

include ataxia, urinary bladder atony and reduced tail 

tone. In severe cases, horses will be unable to stand; 

these cases have a very poor prognosis. Foals are 

rarely affected with the neurologic form of EHV-1, and no 

sex predilection is seen. Treatment is supportive and 

tailored to the specific case. Antivirals such as acyclovir, 

valcyclovir, famiciclovir and penciclovir have been used, 

but efficacy of these drugs has yet to be determined in 

equids. 

Sources:USDA-APHIS 2005 United States Animal Health Report, pp. 

78-79, 

http://www.aphis.usda.gov/publications/animal_health/2005animal_hea

lth.shtm; Equine Infectious Diseases, Sellon & Long; Chapter 13; Josh 

Slater; pp.150-152; Equine Herpesvirus Type 1 Virulence and Vaccine 

Efficacy, Osterrieder, Zweig Memorial Fund for Equine Research, 

Cornell; EHV-1 Outbreaks, Lenz, TR, 

www.xcodesign/aaep/displyArticles.cfm?ID=222;

United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture 

Appendix B: USDA:APHIS:VS Center for Emerging Issues Emerging Animal Disease Notice
Appendices

64 / EHM 2008

History of Recent Neurologic EHV-1 Outbreaks 

in the United States 

Prior to 2003, the U.S. reports of neurologic EHV-1 

outbreaks in the United States were sporadic, with 

typically none to few outbreaks identified annually. 

Table 1 shows outbreaks of equine herpesvirus; many of 

these cases may not have been typed for the 

neuropathogenic mutation, and there was not a standard 

method of defining a case across the time period. In 

2005, seven outbreaks of neurologic EHV-1 were 

reported in 5 different States. In 2006, the number of 

reported outbreaks grew to 11, and involved 8 States. 

The outbreaks have been primarily concentrated in the 

eastern United States, with a few midwestern and 

western States experiencing outbreaks. The last 

outbreak of 2006 involved a group of 15 horses shipped 

from Germany that were subsequently shipped to 8 

States. Five of the horses went to Florida; this resulted 

in 13 horses identified as infected, with neurologic signs 

of in seven cases and 6 associated deaths. Ten Florida 

premises were quarantined. One horse was shipped to 

California from the original group from Germany and 

died shortly after arrival due to the neurologic form of 

EHV-1. The exposed horses were identified, and the 

horses that were shipped to the remaining 6 States were 

quarantined and monitored for EHV-1 signs; none were 

found. 

Outbreaks of neurologic EHV-1 continue to be identified 

in early 2007. 

 

Table 1. Reported Neurologic EHV-1 Outbreaks in 

the United States Annually from 2001-2006 

State 2001 2002 2003 2004 2005 2006 

CO 1 

FL 1 

GA 1 

KY 1 3 

MD 1 1 4 

MI 1 1 

NH 1 

NJ 1 

NY 1 

OH 1 

OR 1 

PA 1 1 1 

VA 1 

WI 1 

WY 1 

Totals 1 1 4 2 6 11 

Sources: 2006 NIAA Proceedings, Emerging Diseases, “Neurological 

Equine Herpes Update”, Promed, The Horse, Pittsburgh Post Gazette, 

Kentucky Dept. Of Agriculture, 2005 U.S. Animal Health Report, WJZTV Baltimore, U.S. States News, Newark Star Ledger

The clustering of outbreaks in certain regions of the 

country could be related to where high-level 

performance horses are located or where they tend to 

travel, but other ecological factors could also play a role 

in outbreaks; this aspect of the disease is not well 

understood at this time. The number of reported 

outbreaks in the United States has increased, and the 

numbers infected in an outbreak appear to be much 

higher than what was seen previous to 2001. Data from 

6 U.S. neurologic EHV-1 outbreaks (2001–2005) 

involving 403 at-risk horses were combined and 

analyzed; a mean attack rate of 33%, and a mean case 

fatality rate of 40% were found (2006 NIAA 

Proceedings). 

Nationally, reports of neurologic EHV-1 have increased 

in recent years; this may be attributable to a strain of 

EHV-1 with a mutation that encodes for a particularly 

robust replicase enzyme. The result of this mutation is 

that the virus can reproduce rapidly with a predilection 

for nervous tissue, therefore the viremia occurs earlier, it 

reaches a higher peak, and it lasts longer. Beginning 

with the Ohio outbreak during January 2003, the 

progression of the disease in a population as well as in 

individual cases has been seen to be much more rapid 

than in the past. Response to currently available 

vaccines for EHV-1 does not appear to be strong 

enough to protect all immunized animals against the 

disease induced by the mutated strain of EHV-1. In 

some outbreaks, such as the Ohio outbreak of 2003, 

well-vaccinated populations of horses have experienced 

severe disease outbreaks and some animals have died. 

However, serious outbreaks have occurred in equine 

populations where no EHV-1 vaccines are used, such as 

in the Netherlands. It is still unknown what factors are 

involved in the emergence and/or maintenance of the 

viral mutants. It is also unclear at this time what role the 

poor immunogenic response to the mutated strain is 

playing in the outbreaks that have occurred in recent 

years. 

 

Variations in EHV-1 reporting requirements and case 

definitions make it difficult to accurately identify the true 

number of cases. Creating a standard case definition for 

both the first case on a premise, as well as for 

subsequent cases, would alleviate some of the 

discrepancies seen in reporting and facilitate better 

understanding of the epidemiology of the disease. 

Sources: Florida Department of Agriculture and Consumer Services, 

http://www.doacs.state.fl.us/ai/ehv1/daily_status_table_and_definitions

.shtml, Equine herpesvirus-associated myeloencephalopathy; Aspects 

of epidemiology and pathogenesis, Goehring, L.S., Univeritiet Utrecht, 

2005; Gluck Equine Research Center, University of Kentucky, 

http://www.ca.uky.edu/gluck/; 2006 NIAA Proceedings, Emerging 

Diseases, “Neurological Equine Herpes Update,” Harrison, LR; 

Promed; The Horse; Pittsburgh Post Gazette, Kentucky Dept. Of 

Agriculture; 2005 U.S. Animal Health Report, 

http://www.aphis.usda.gov/publications/animal_health/2005animal_hea

lth.shtml; WJZ-TV Baltimore; U.S. States News, Newark Star Ledger 

International EHV-1 Situation 

The United States is not the only region or country that 

has recognized the issue regarding neurologic EHV-1 

cases; other countries have also seen an increase in the 

number and virulence of reported neurologic EHV-1 

outbreaks. In 2005, significant outbreaks occurred in 

Canada, Ireland, South Africa and Switzerland as well as 

the United Kingdom and the United States. Studies 

United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture 
USDA APHIS VS / 65

Appendices

conducted by the Gluck Equine Research Center in 

Kentucky indicate that in both the United States and the 

United Kingdom, the number of paralytic outbreaks of 

EHV-1 has increased dramatically since 2001. Greater 

than 85% of the outbreaks identified over a 30-year 

period have been caused by the mutated strain of EHV-1 

as compared to the wild-type or “original” strain of 

EHV-1. 

Horse Industry in the United States 

Estimates on the value and numbers of equids of the 

U.S. horse industry vary widely. The Census of 

Agriculture estimates that there were 3.6 million horses 

and ponies in the United States in 2002 residing on 

operations fitting the definition of a “farm.” Although this 

source of equine population data only estimates a 

segment of the industry, one can compare across States 

for relative size of their equine population. Texas, 

California, Oklahoma, Kentucky, Missouri and Ohio were 

the 5 States with the largest equine populations based 

on the Census of Agriculture. The overall value of the 

horse industry in the United States is in the billions of 

dollars. The most valuable subgroups of the horse 

industry are racing, competition and recreation. 

In the United States, the recent outbreaks of the 

neurologic form of EHV-1 appear to be affecting race 

horses and other types of competition horses. The 

response to these outbreaks has been to identify 

potentially exposed animals and quarantine them. Since 

these sectors of the horse industry require horse 

movement in order to function, the EHV-1 outbreaks 

have had a major impact on these segments of the 

industry. 

Sources: American Horse Council Foundation, USDA-NASS Census 

of Agriculture 

Table 2. EHV-1 Paralytic Outbreaks: United 

States and United Kingdom caused by wild-type 

and mutant virus strains 

5-year 

interval 

No. 

outbreaks 

No. 

outbreaks 

caused by 

wild-type 

No. 

outbreaks 

caused by 

mutant 

1970–

1975 1 0 1 

1976–

1980 3 0 3 

1981–

1985 4 2 2 

1986–

1990 6 1 5 

1991–

1995 5 1 4 

1996–

2000 6 2 4 

2001–

2005 32 2 30 

Source: 2006 NIAA Proceedings, Emerging Diseases, “Neurological 

Equine Herpes Update,” Harrison, LR 

International Horse Trade in the United States 

The United States imported 25,000 to 30,000 live horses 

per year from 2003 through 2006 (statistics for 2006 are 

not yet complete). The vast majority (60–70%) of these 

live horse imports originated from Canada. Import 

requirements for horses from Canada are a health 

certificate and negative Coggins or ELISA test for equine 

infectious anemia (EIA) within 180 days preceding 

exportation to the United States.

The Netherlands, Mexico and Germany are the next top 

exporters to the United States. Horses from these 

countries are required to be detained at the United 

States port of entry while tests for dourine, glanders, 

equine piroplasmosis, and EIA are conducted. Test 

results from NVSL are generally available three days 

after the date of arrival of the horses. 

The index case for the recent outbreak affecting Florida 

originated from a shipment of 15 horses from Germany. 

Sources: World Trade Atlas, APHIS Veterinary Services National 

Center for Import-Export

Discussion 

Equine herpesvirus 1 neuropathogenic form is not a 

“new” disease, but the evidence currently supports the 

observation that paralytic EHV-1 is emerging as a more 

virulent strain that has a higher incidence, morbidity and 

mortality. In the past, outbreaks of neurologic EHV-1 

were sporadic and typically involved one or a few cases 

within a herd or on a premises. The recent outbreaks in 

the U.S. and Europe have involved large numbers of 

infected animals with many of these showing neurologic 

signs. In the EHV-1 outbreak in Ohio during 2003, more 

than 30% of the 138 horses on the premises developed 

neurologic disease (http://www.the horse.com, articles 

#4104 and #4272). This disease behavior is different 

than what was seen prior to 2001 in U.S. outbreaks. 

The factors involved in this emergence are not clearly 

identified, but several factors have been suggested. 

Animal movement appears to be associated with several 

of the recent outbreaks. Movement could result in 

introduction of the virus to a new population; it is also 

possible that transport stress plays a role in allowing the 

disease to express itself or re-emerge from the latent 

stage. The role of vaccination is controversial; based on 

available information from recent outbreaks, the disease 

has occurred in well-vaccinated animals. What the 

attack rate would have been had the population been 

unvaccinated is unknown. The disease has occurred in 

equine populations where no vaccination occurred. 

There is much yet to learn about the ecology of the 

disease. It is likely that other factors may be playing a 

role. Potential temporal associations also exist; most 

outbreaks have been seen in the fall and early spring, 

yet there is likely more horse movement nationally in the 

summer. 

The interest level in this disease remains high. The 

economic impact is substantial; not only are the horses 

United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture 
Appendices

66 / EHM 2008

that die from the disease lost, but there are many costs 

associated with treatment, quarantines, cancelled 

events, and the inability of horses to compete in events. 

Horse practitioners and horse owners should continue to 

quarantine suspect and diagnosed cases. Using strict 

biosecurity measures in day-to-day procedures, even 

when disease is not suspected, is a key strategy in 

preventing the introduction and spread of infectious 

diseases. APHIS Veterinary Services has developed a 

brochure titled Biosecurity – The Key to Keeping Your 

Horses Healthy, which can be found at 

http://www.aphis.usda.gov/lpa/pubs/HorseBioSecurity_fi

nal.pdf Another resource available to equine 

veterinarians who are members of the American 

Association of Equine Practitioners (AAEP) is Equine 

Infectious Disease Outbreak: AAEP Control Guidelines. 

The AAEP guidelines provide guidance for veterinarians 

who encounter contagious infectious disease in horses, 

and covers topics such as pre-outbreak considerations, 

considerations when equine infectious disease is 

suspected, and sign-based flowcharts and guidelines for 

response-plan recommendations. 

Conclusions 

It appears that neurologic EHV-1 is emerging as a more 

virulent strain of the disease than seen previously in the 

United States. Many data gaps exist, and more 

investigations need to be done to better understand the 

situation and to identify factors playing a role in possible 

emergence. Identifying such factors will also assist with 

control of future outbreaks. The general ecology of this 

disease is not fully understood; additional research is 

needed. A standard case definition and standardized 

reporting requirements for the neurologic form of EHV-1 

would be good starting points for further study, with 

subsequent studies comparing cases to controls to 

determine risk factors for the disease. ________________________________ 

For more information, contact: 

USDA:APHIS:VS:CEAH 

Center for Emerging Issues (CEI) 

NRRC Building B, M.S. 2E5 

2150 Centre Avenue 

Fort Collins, CO 80526-8117 

970.494.7000 

Or visit CEI on the Web at 

http://www.aphis.usda.gov/vs/ceah/cei/ 

___________________________________ 

The U.S. Department of Agriculture (USDA) prohibits 

discrimination in all its programs and activities on the basis of 

race, color, national origin, age, disability, and where 

applicable, sex, marital status, familial status, parental status, 

religion, sexual orientation, genetic information, political beliefs, 

reprisal, or because all or part of an individual’s income is 

derived from any public assistance program. (Not all 

prohibited bases apply to all programs.) Persons with 

disabilities who require alternative means for communication of 

program information (Braille, large print, audiotape, etc.) 

should contact USDA’s TARGET Center at (202) 720–2600 

(voice and TDD). To file a complaint of discrimination, write to 

USDA, Director, Office of Civil Rights, 1400 Independence 

Avenue, S.W., Washington, D.C. 20250–9410, or call (800) 

795–3272 (voice) or (202) 720–6382 (TDD). USDA is an equal 

opportunity provider and employer. 

United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture 
USDA APHIS VS / 67

Appendices

Appendix C: Example of a Study Design for Collection of Epidemiologic and Cost Estimate Data

from Future EHM Outbreaks

Data needed to make cost estimation for EHM disease event

To fully quantify the impact of EHM disease events, costs of various aspects of

management of the disease event would need to be collected and compiled. The

components which need to be collected in the field in order to estimate the

regional economic impact of an EHM disease event are listed below.

a. Testing costs:

1. number of tests

2. kit cost

3. sample shipment costs

4. sample collection costs

5. cost of running test

b. Lost use: average number of days horses unavailable for normal use and

 number of horses unavailable for use

c. Treatment costs, including number of cases treated. If there is a range of

 treatment costs, cases may be broken into categories (such as severe,

 moderate, and mild) and treatment costs calculated and applied according to

 severity.

d. Deaths:

1. number of horse deaths

2. value of each horse that died or was euthanized

e. Biosecurity costs (must be additional to normal biosecurity expenses):

1. PPE supplies

2. disinfectant

3. hand hygiene products

4. barriers and signage

5. additional labor

f. Facility (loss of income and expenses incurred):

1. betting revenue

2. ship-in lessons revenue

3. spectator revenue

4. hospital income lost due to closure

5. expense of keeping horses at the facility for a longer period due to

quarantine
Appendices

68 / EHM 2008

Example of study design and survey instruments related to EHM disease

events

Introduction

This example study design is included as an illustration of how critical data

related to description of the disease event and for eventual risk factor analysis

for EHM could be gathered. Utilization of a standardized method for data

collection on EHM disease events could meet two major goals:

1. To provide a better description of EHM disease events by describing

basic information on index EHM case as well as the likely exposed

 population, and

2. To gather data necessary to determine risk factors for EHM at the horse

 level.

To achieve these goals, two types of horse/equine-level questionnaires (available

upon request) were created--one captures basic descriptive information related

to the index case as well as all equids likely exposed to the EHM index case, and

the second focuses on collecting more detailed information about the index case,

the subsequent EHM cases (if they occur), any febrile horses, and selected

controls. To assure uniformity in data collection, a suggested definition to use for

likely EHM-exposed equines would include “equids that have shared airspace

and/or had direct physical contact with the index case.”

General information about the premises at which the outbreak occurred will also

need to be collected in order to describe the outbreak and help determine risk

factors. Form A: Premises Level Questionnaire provides background information

about the general practices of the premises, and a separate diagram (Premises

Diagram) describes spatial design of the premises.

The accurate classification of cases and controls is vital to assess the risk

factors for EHM. An EHM case is defined as an equid with neurological

manifestations and a positive biological test for EHV-1. Since fever may be an

indicator for a future neurological disease, daily temperature logs need to be kept

on all exposed equids, with neurological exams to be conducted on all febrile

(>101.5°F) equids. Controls would not be classified until the end of the outbreak

or disease event period (at least 28 days after the last identified EHV-1 clinical

case [fever or EHM].
USDA APHIS VS / 69

Appendices

Historically, in many outbreaks, only one to a few EHM cases are identified; thus,

the data should be collected across multiple outbreaks in order to attain the goal

of determining risk factors for EHM. Due to the time involved in the data

collection, it is recommended that someone other than those involved in

outbreak mitigation collect the data.

Explanation of documents for EHM data collection once index case is identified

1. Premises Diagrams: Aerial view and interior barn/housing schematic should

be created first in order to assign the “location on premises” numbers to the

horses. The interior barn/housing schematic diagram is used to determine if

the location of the horses on the premises plays a role in equine herpesvirus

transmission.

2. Form A=Premises Level Questionnaire. Detailed questionnaire about the

equine premises.

3. Form B=Initial Equine Index EHM Case and Likely Exposed to EHM

Index Case Chart. General chart to be filled out for each horse on the

premises.

4. Form C=Index Case Questionnaire. This form is to be completed for the

horse determined to be the index case on the premises.

5. Form D=Questionnaire for Horses with Fever, Neurologic Signs, and

Exposed to Index EHM Case at Initial Data Collection. To be filled out at

the initial visit by data collection personnel for horses with fever or neurologic

signs in the last 30 days. For each horse with a fever or neurologic signs,

three randomly selected controls need to be assigned and those controls

need to have Form F: Case/Control Questionnaire filled out for each of them.

6. Form E=Followup Chart for the Equine Index EHM Case, Subsequent

Cases, and Controls. General chart to be filled out for each horse for which

a Form D was completed.

7. Form F=Followup Questionnaire for the Equine Index EHM Case,

Subsequent Cases, and Controls. This questionnaire is to be completed

for the index case, subsequent EHM cases, and controls at the followup visit.

Followup, after outbreak is considered over:

1. Premises Diagram to be updated

2. Chart Code document to be updated

3. Case/Control Questionnaire to be updated for all equids that were subjected

to an initial Case/Control Questionnaire
Appendices

70 / EHM 2008

At time of initial data collection:

Step 1: Complete the premises diagrams (see example aerial view and interior

design diagrams.

Step 2: Complete Form A=Premises questionnaire

Step 3: Complete Form B=Chart for index EHM case and horses exposed to

index case

Step 4: Complete Form C=Index case questionnaire

Step 5: Complete Form D=Questionnaire for horses that have history of fever or

neurologic signs other than the index case.

At time of followup data collection (e.g., when outbreak has resolved):

Step 1: Update the premises diagram, as the horses’ locations may have

changed since the premises diagram was completed during the initial data

collection.

Step 2: Complete Form E (chart for index case and any exposed horses). Refer

to the chart that was completed during initial data collection in order to determine

which horses should be included in this followup data collection.

Step 3: Complete Form F for:

a) the index EHM case,

b) any horse that had a fever or neurologic signs during initial data

collection or based on data collected in Form E

c) three randomly selected controls for each horse that was an EHM

case or febrile horse based on data from Forms B or E.

Forms A through F are available upon request from:

Abby Fienhold 970.494.7000

USDA:APHIS:VS:CEAH

NRRC Building B, M.S. 2E7

2150 Centre Avenue

Fort Collins, CO 80526-8117

E-mail: NAHMS@aphis.usda.gov
USDA APHIS VS / 71

Appendices

Premises Diagram

The Premises Diagram provides a visual overview of the equine premises layout

and identifies the housing specifics for the index case and the locations of all

likely exposed equids at this premises. The Premises Diagram has two sections:

1. Aerial Overview

2. Interior Barn or Housing Schematic

In the Aerial Overview, the general layout of the premises can be obtained

through photography. Google EarthTM, www.earth.google.com, provides an aerial

picture of the premises after entering the address into the Web site. This image

should include neighboring farms and roads to better understand the location of

this premises in relation to other premises that may have equids. Once

produced, the image should be geographically oriented by labeling north,

property boundaries, and building names/identification. An example is provided

(page 72).

In the second section, an interior schematic of the buildings and/or other housing

types which contain equids, including the index case and likely exposed horses

and other equids,1 should be created. This schematic should include all interior

structures of each building, including stalls and common use areas, such as

wash racks or stocks, tack areas, feed storage, and indoor exercise arenas. This

schematic should also include the doors throughout the building with an

indication of the direction that the doors open. Each equid needs to be assigned

a location on the property, which can be accomplished by numbering stalls or

pens (for stalls, an example would be S1, S2, S3, etc.; for pens, an example

would be P1, P2, P3, etc.; for pastures, PA1, PA2, etc.). This number can be

included as a label on the schematic to identify the locations of all equids in the

barn, pens, and pastures. An Interior Building/Barn Schematic example is

provided to illustrate the optimal detail such a diagram would include (page 73).

This diagram can be drawn freehand or in a computer program.

The location designations that are assigned will be used as identifiers for the

equids in Form B. This, along with the two diagrams, will allow you to better

describe the outbreak or disease event and to track the location of horses as the

disease event is monitored. Specifically, you will have knowledge about the

location of the index case on the premises and the location of subsequent cases;

this documentation could be used in the determination of risk factors for

subsequent EHM cases in this disease event situation.

1. Exposed equids would include equines which have shared airspace and/or had direct physical

contact with the index case.
Appendices

72 / EHM 2008

Example of an Aerial View of an Equine Operation

with Google Earth

Neighbor’s house/farm

Property 

line

Barn A

Barn B

Barn C

Office

Outdoor Riding Arena

Office

Isolation Barn

Indoor Riding Arena

The following equine numbers correspond to the “Schematic of an 

Interior Equine Barn” and Form B : All equines use 

Outdoor and 

Indoor Riding 

Arenas.

Barn A houses equines 1–16

Barn B houses equines 17–30

Barn C houses equines 30–50

Isolation barn houses equine 31, the index case

Neighbor’s 

farm contains 

only beef 

cattle, no 

equines.

Index case was 

moved from Barn 

C to the Isolation

Barn on 1/1/08. 

No other equines

are using the 

isolation building.
USDA APHIS VS / 73

Appendices

Key: Barn A

S/numbers= assigned numbers for equine stall location on 

operation (corresponds to Form B )

s9

s13 s14 s15 s16

s10 s11 s12

s5 s6 s7 s8

s1 s2 s3 s4

Indoor Exercise

Arena

Barn A

Example of Schematic for Interior Equine Barn

Tack 

room

Feed 

storage

stocks

stocks

Sliding door Swinging door, 

pivot on left

Interior Barn or Housing Schematic

Indicate where nose-to-nose contact can occur (e.g., bars between stalls).

Use dotted lines for nose-to-nose contact areas.
Appendices

74 / EHM 2008

Appendix D: List of Selected Sources of Information on EHM

The following Web sites have information related to EHV, EHM, and biosecurity.

http://www.vetmed.ucdavis.edu/ceh/topics.htm

http://www.aaep.org/control_guidelines_nonmember.htm

http://www.aaep.org/vaccination_guidelines.htm

http://www.aphis.usda.gov/lpa/pubs/HorseBioSecurity_final.pdf

http://www.cfsph.iastate.edu/BRM/equineresources.htm