F U N D E D B Y U N D E R W R I T E R S AT L LOY D ' S , LO N D O N

JULY 2018
Volume 27, Number 3

C O M M E N TA RY
IN THIS ISSUE
Commentary
International
First Quarter2018  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 2Insect Bite .  .  .  .  .  .  .  .  .  .  . 3Theiler’s Disease .  .  .  .  . 3National
Protecting yourHorse .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 4Kentucky
Rabies .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 5

College of Agriculture,
Food and Environment
Department of
Veterinary Science

Immunotherapies on the Horizon for Insect Bite Hypersensitivity?

T

he season is upon us when approximately 10
percent of horses worldwide are faced with
one of the most frustrating skin diseases in theequine industry, insect bite hypersensitivity (IBH),
more commonly known as “sweet itch” or “summer
eczema.” IBH is an allergic, recurrent, seasonal dermatitis caused when the immune system overacts
or becomes ”hypersensitive” to allergens in thesaliva primarily of biting midges (Culicoides) andother biting insects.
Dr. Susan White in her article provides an
overview of current diagnostic and management
practices to combat signs of IBH. Although there
is no cure for IBH, there is ongoing research to assess immunotherapies as future treatment options
for horses suffering with IBH.
While IBH has a multifactorial cause, it is acknowledged that the immune system plays a rolein its development. Hypersensitivity reactions areclassified into four types, and IBH is a combination of Type I and Type IV. Type I hypersensitivity
reactions develop when antigen specific allergens
bind to IgE and cross-link receptors on mast cells,
basophils and also involve eosinophils. These cells
release vasoactive amines, such as histamine, that
affect blood vessel size and leakage and produce
inflammatory cytokines and other mediators,
which result in inflammation and pruritus. TypeIV reactions, often referred to as delayed-type
hypersensitivity reactions, involve cell-mediated responses particularly of activated Th-2 lymphocytes
which produce IL-5 cytokine and large numbersof eosinophils, both contribute to the clinicalmanifestations of IBH. Immunotherapy treatment
is based on a century-old concept that the immune
system can be desensitized to specific allergens
that trigger hypersensitivity reactions. Allergen

immunotherapy (AIT) involves inoculation with
gradually increasing doses of an allergen to which
the individual is allergic. The incremental increases
in exposure to the allergen cause the immune
system to become less sensitive to the substance,
which reduces the allergy signs when the substanceis encountered in the future. Data are conflicting
as to whether or not traditional AIT therapies are
successful in treating IBH. Research is now exploring whether IBH can be prevented by vaccination
with recombinant Culicoides antigens versus wholeantigen preparations to more successfully modulate
the immune response, in particular, IgE response.
Another interesting area under current investigation is oral immunotherapy, whereby transgenic
barely expressing allergens are administered orally
to prevent hypersensitivity to Culicoides.
Finally, given the important role of eosinophils
in the pathology of IBH and the fact that IL-5
drives eosinophil activity, a novel and allergenindependent vaccine has been evaluated thattargets IL-5 and limits eosinophil recruitment tothe affected skin. This is one of the few equine
studies to investigate whether a vaccine can induce
autoantibodies to cytokine IL-5 and result inclinical efficacy for IBH. This immunotherapeutic
approach may be the new generation for treating
chronic immune diseases, and may signal that new
therapies are on the horizon for horses sufferingfrom IBH.
CONTACT:
Amanda A. Adams, PhD
Amanda.adams@uky.edu(859) 218-1097
Maxwell H. Gluck Equine Research Center
University of Kentucky
Lexington, KY

2

I N T E R N AT I O N A L

First Quarter, 2018

T

Equine Disease Quarterly
Editors

Peter Timoney
Alan Loynachan
Cynthia Gaskill

Staff

Diane Furry
Tawana Brown
Dennis Duross
Anita Fleming
Correspondence should be
addressed to the editors,
Department of Veterinary
Science, Maxwell H. Gluck
Equine Research Center,
University of Kentucky,
Lexington, Kentucky USA,40546-0099
Telephone (859) 257-4757
Fax (859) 257-8542
Internet address:
http://gluck.ca.uky.edu/
equine-disease-quarterly
Material published in the
Quarterly is not subject tocopyright. Permission istherefore granted toreproduce articles, althoughacknowledgment of thesource and author is requested.
The University of Kentucky
is an Equal Opportunity
Organization.

Printed on recycled paper

he International Collating Centre, Newmarket, United Kingdom, and other sources
reported the following equine disease outbreaks.
The Republic of South Africa (RSA) reported
an expected seasonal increase in cases of African
Horse Sickness. A total of 88 cases were recorded
in eight of the nine provinces. No cases occurred in
the Western Cape Province, including the Disease
Controlled Area.
The UK and the USA reported outbreaks ofequine influenza. Two cases in recently imported
non-vaccinated horses were recorded in the UK.
The disease can be considered endemic in the USA
in which it was confirmed in three states, withmultiple outbreaks in one of the states.
France, Germany, Ireland, and the USA reported multiple outbreaks of strangles. The number ofoutbreaks included 21 in France, two in Germany,
numerous cases in Ireland, and 65 outbreaks in
22 states in the USA, 10 involving multiple casesof the disease.
Equine herpesvirus 1 (EHV-1) infection wasrecorded in France, Germany, Ireland, Japan,
RSA, Switzerland, the UK, and the USA. Infection associated with fever was reported by France(two outbreaks), Ireland (seven outbreaks), andSwitzerland (one outbreak of combined EHV-1
and equine herpesvirus 4 infection). Respiratory
disease was diagnosed in Germany (25 cases), theUK (eight outbreaks, majority in non-vaccinated,
non-Thoroughbred horses), and the USA (widespread in various states). Cases of EHV-1 abortion
were recorded in France (eight outbreaks, three
involving multiple cases), Germany (four cases),
Japan (single cases on 13 premises, all but four invaccinated mares), the UK (five outbreaks involving vaccinated or non-vaccinated Thoroughbred
and non-Thoroughbred mares), and the USA(two cases).
EHV-1 associated neurologic disease was reported from France (one case), RSA (two cases onsame premises), Switzerland (one case), the UK(a single case on two premises), and the USA (23outbreaks involving 27 horses and multiple states).

EHV-4 infection was recorded by France (fouroutbreaks) and Switzerland (two cases). France(10 outbreaks) and Germany (four cases) reported
occurrences of EHV-4 related respiratory disease.
Also, France confirmed one case of EHV-4 abortion.
The USA recorded multiple cases of EHV-2/5
infection in several states, some associated withclinical evidence of respiratory disease.
Equine infectious anemia was confirmed by
Canada (three cases, one of which was clinicaland died), France (one clinical case), and the USA(one case).
France reported equine piroplasmosis was endemic in the country. The RSA recorded several
cases of the disease in four provinces.
Taylorella equigenitalis, the causal agent of contagious equine metritis was detected in an aged
stallion in France, and in six stallions and one mareon a total of six premises in Germany.
A single case of equine coital exanthema caused
by EHV-3 was diagnosed in a mare in Kentucky.
A number of cases of nocardioform placentitis
were reported by the USA, several involving infection with Amycolatopsis spp and an additionalgroup associated with Crossiella equi infection, all
but one in Kentucky.
The USA confirmed 11 cases of salmonellosis
during the review period, all isolates belonging toserogroup B.
Three cases of Clostridium perfringens Type A
toxin genotype were diagnosed in foals in Kentucky.
Germany confirmed rotavirus infection in two
foals on the same premises.
A case of Lawsonia intracellularis infection was
diagnosed in a foal in Kentucky.
The USA recorded four cases of Eastern Equine
Encephalomyelitis (EEE) in the first quarter of
2018, all in Florida.
Equine encephalosis was reported by the RSA,
with numerous cases in one province and isolatedcases in three other provinces.
Rhodococcus equi infection is endemic in theUSA, with many cases going unreported.

Insect Bite Hypersensitivity in Horses

Alarge percentage of horses affected with seasonal pruritic dermatitis are hypersensitive(allergic) to the bites of insects. Culicoides spp(gnats), black flies, stable flies, and horn flies are
the most commonly implicated insects although
any biting insect may contribute to insect biteinduced hypersensitivity (IBH). IBH is characterized by intense pruritus (itching) that often leads
to excoriation (abrasive skin damage), extensive
hair loss, secondary infections, and chronicallyto hyperkeratosis and lichenification (thickenedskin). Many horses develop IBH in middle age
or later, although horses with atopy may exhibit
clinical signs as early as 1 year of age. Clinical signs
often progress in each subsequent year.
Diagnosis is made from the signalment, history,
clinical signs, and ruling out of other possible diagnoses. The distribution of lesions on an affectedhorse with IBH is dependent on the biting characteristics of the insect(s) responsible. Since there
is still much to be learned about the identificationand feeding habits of many insects implicatedin allergic dermatoses, it may not be possible toidentify the exact etiological agent(s). IBH typically improves and exacerbates seasonally, whereas
clinical expression of allergies in atopic horses may
occur at any time of the year. Urticaria (hives),
commonly found in atopy, is not a diagnosis but
a cutaneous reaction pattern that may be induced
by a wide variety of causes, both immunologic andnon-immunologic. Rule outs for urticaria include
drug and vaccine reactions; stinging and biting
insects (such as wasps) and arachnids; infections;contact irritation; vasculitis; and cold, stress, or
exercise-induced lesions. Other diseases that exhibit pruritus are Oxyuris (pin worm) infestation,
onchocerciasis, and mite (Chorioptes) and tick infestations. Diseases that occasionally are associatedwith pruritus include ringworm and pemphigus,
an autoimmune disease.
In all cases of pruritic dermatitis, managerial
procedures to decrease exposure to exciting agents
as well as symptomatic therapy to reduce pruritus
is warranted. In IBH procedures to decrease expo-

sure to insects should be fully implemented. Farmsanitation to compost manure properly, eliminate
wet areas with decomposing vegetation, and theuse of fly predators or feed through fly inhibitors
can greatly reduce fly populations. Heat, humidity,
and solar radiation exacerbate pruritus, thus theprovision of shade and wind currents by fans provide relief. Stabling to avoid the most significant
insects (Table 1) and the use of fly sheets and facemasks are helpful. Because some horses develop
contact allergies to topical products, judicious andcautious use of fly repellants is advisable. Simple
feeds and whole grains are better than mixed,
multiple-grain sweet feeds. Cool-water rinses
and shampoos can rehydrate and sooth dry skin
as well as reduce the amount of allergens on theskin. Colloidal oatmeal, pramoxine, and 1 percent
hydrocortisone shampoos or leave-on hydrocortisone rinses may reduce pruritus and minimize or
reduce the amount of systemic antipruritic medication needed. Systemic medications (steroids andantihistamines) may help interrupt the itch-scratch
cycle, however long term use, particularly of steroids, should be avoided. A complete and detailed
investigation into the etiology of the disease should
occur simultaneously with general symptomaticcare. Intradermal skin testing to determine antigens for allergen specific immunotherapy may be
helpful, particularly in young animals with atopy.
Clients should understand that hypersensitivities
and atopy are lifelong. Affected horses will need
continuous management and/or therapy. Often apatient may be symptom-free with low exposure toinciting antigens and symptomatic as the antigen
load increases. Antigen exposure is additive, thus
comprehensive management is needed to best
control clinical disease.
CONTACT:
Susan L. White, DVM, MS, DACVIM
slwhite@uga.edu(706) 296-8607
Department of Large Animal Medicine
College of Veterinary Medicine
University of Georgia
Athens, GA

Theiler’s Disease

Theiler’s disease, or equine serum hepatitis, isan infrequent but sometimes life-threatening
liver disease of adult horses. Sir Arnold Theiler
first described the disease in the early 1900s as
a form of highly fatal acute liver failure that occurred 4-12 weeks after administration of equine

antiserum, which was used as part of a vaccination
strategy for African Horse Sickness. Since then,many additional cases of serum hepatitis havebeen reported, and a variety of blood productsof equine origin have been associated with thedisease. In North America, the majority of recent

34Insect Bite Hypersensitivity in Horses (left):
Table 1. Breeding location and time of feeding for insects associatedwith IBH.
InsectBreeding Area

Feeding Times

Stable flies

Manure and rotting
vegetation

DaytimeHorn flies

Cattle manure

DaytimeHorse and deer flies

Vegetation and water

Daytime

Culicoides spp

Standing water andmanureTwilight to dawn

Running water

Morning and evening

Simulium (black) flies

Rabies in the Horse and Beyond in Kentucky (page 5):
Table 2. Rabies virus test results broken down by wildlife, domestic
species (excluding horses), and horses.

Results

Wildlife

Domestic
Species(excluding
horses)HorsesTOTAL

Negative

12,37816,733

1,034

30,145

Positive

680

28

25

733

Unsatisfactory
specimen

823

675

11

1,509

TOTAL

13,881

17,436

1,070

32,387

Figure 1. Geographic locationof the 25 horses testing positive
for rabies virus in Kentucky from1989 to 2017.

cases have been associated witof tetanus antitoxin, although commer
products also have been incriminated. The numberof horses that become ill following administrationof a specific lot of incriminated blood product isestimated at 1 to 2 percent, although mor
might have sub-clinical disease. An infectious
agent, such as a virus, has long been suspected tocause this condition, and a ne
was recently discoverof serum hepatitis. The par
the diseased horse an
it had received nine weeks earlier
experimental horses
resulted in transmission of this nevirus and in liver disease.
immune responses tolow percentage of clinically affected horses.
An identical disease, both in terms of clinicalsigns and pathologic findings, is sporadicallyin adult horses without recent administration of anequine origin biologic. These “non-biologic
tend to mostly occur between J
and can occur in small outbr
weeks. Non-biologic
commonly on broodincidence of the non-biologic cases suggests thepossibility of insect transmission of parthese “non-biologic” cases.
Clinically affected horses with Theiler’frequently have both neurologic signs (hepatic
encephalopathy: head pressing, stumbling, blindness) and jaundice (yImportance of Prot
Encephalomy
0

2

4

6

0

2

4

6

Figure 2. Geographic locationof skunks in Kentucky testing
positive for rabies virus in the last
29 years.

The two most fr
of equine encephalitis or equine encepha
lomyelitis in North America are Eastern E
Encephalomyelitis (EEE) and W
litis (WNE) viruses. Both are mosquito-borneand neurotropic. The respectiv
restricted to equids in terms of their host range;
each can be transmitted
other mammalian and avian species. Both diseases
are a source of concern for the equine industr
not only from theconsequences of either infection, but also fromthe economic losses involv

Insect Bite Hypersensitivity in Horses

Alarge percentage of horses affected with seasonal pruritic dermatitis are hypersensitive(allergic) to the bites of insects. Culicoides spp(gnats), black flies, stable flies, and horn flies are
the most commonly implicated insects although
any biting insect may contribute to insect biteinduced hypersensitivity (IBH). IBH is characterized by intense pruritus (itching) that often leads
to excoriation (abrasive skin damage), extensive
hair loss, secondary infections, and chronicallyto hyperkeratosis and lichenification (thickenedskin). Many horses develop IBH in middle age
or later, although horses with atopy may exhibit
clinical signs as early as 1 year of age. Clinical signs
often progress in each subsequent year.
Diagnosis is made from the signalment, history,
clinical signs, and ruling out of other possible diagnoses. The distribution of lesions on an affectedhorse with IBH is dependent on the biting characteristics of the insect(s) responsible. Since there
is still much to be learned about the identificationand feeding habits of many insects implicatedin allergic dermatoses, it may not be possible toidentify the exact etiological agent(s). IBH typically improves and exacerbates seasonally, whereas
clinical expression of allergies in atopic horses may
occur at any time of the year. Urticaria (hives),
commonly found in atopy, is not a diagnosis but
a cutaneous reaction pattern that may be induced
by a wide variety of causes, both immunologic andnon-immunologic. Rule outs for urticaria include
drug and vaccine reactions; stinging and biting
insects (such as wasps) and arachnids; infections;contact irritation; vasculitis; and cold, stress, or
exercise-induced lesions. Other diseases that exhibit pruritus are Oxyuris (pin worm) infestation,
onchocerciasis, and mite (Chorioptes) and tick infestations. Diseases that occasionally are associatedwith pruritus include ringworm and pemphigus,
an autoimmune disease.
In all cases of pruritic dermatitis, managerial
procedures to decrease exposure to exciting agents
as well as symptomatic therapy to reduce pruritus
is warranted. In IBH procedures to decrease expo-

sure to insects should be fully implemented. Farmsanitation to compost manure properly, eliminate
wet areas with decomposing vegetation, and theuse of fly predators or feed through fly inhibitors
can greatly reduce fly populations. Heat, humidity,
and solar radiation exacerbate pruritus, thus theprovision of shade and wind currents by fans provide relief. Stabling to avoid the most significant
insects (Table 1) and the use of fly sheets and facemasks are helpful. Because some horses develop
contact allergies to topical products, judicious andcautious use of fly repellants is advisable. Simple
feeds and whole grains are better than mixed,
multiple-grain sweet feeds. Cool-water rinses
and shampoos can rehydrate and sooth dry skin
as well as reduce the amount of allergens on theskin. Colloidal oatmeal, pramoxine, and 1 percent
hydrocortisone shampoos or leave-on hydrocortisone rinses may reduce pruritus and minimize or
reduce the amount of systemic antipruritic medication needed. Systemic medications (steroids andantihistamines) may help interrupt the itch-scratch
cycle, however long term use, particularly of steroids, should be avoided. A complete and detailed
investigation into the etiology of the disease should
occur simultaneously with general symptomaticcare. Intradermal skin testing to determine antigens for allergen specific immunotherapy may be
helpful, particularly in young animals with atopy.
Clients should understand that hypersensitivities
and atopy are lifelong. Affected horses will need
continuous management and/or therapy. Often apatient may be symptom-free with low exposure toinciting antigens and symptomatic as the antigen
load increases. Antigen exposure is additive, thus
comprehensive management is needed to best
control clinical disease.
CONTACT:
Susan L. White, DVM, MS, DACVIM
slwhite@uga.edu(706) 296-8607
Department of Large Animal Medicine
College of Veterinary Medicine
University of Georgia
Athens, GA

Theiler’s Disease

Theiler’s disease, or equine serum hepatitis, isan infrequent but sometimes life-threatening
liver disease of adult horses. Sir Arnold Theiler
first described the disease in the early 1900s as
a form of highly fatal acute liver failure that occurred 4-12 weeks after administration of equine

antiserum, which was used as part of a vaccination
strategy for African Horse Sickness. Since then,many additional cases of serum hepatitis havebeen reported, and a variety of blood productsof equine origin have been associated with thedisease. In North America, the majority of recent

34Insect Bite Hypersensitivity in Horses (left):
Table 1. Breeding location and time of feeding for insects associatedwith IBH.
InsectBreeding Area

Feeding Times

Stable flies

Manure and rotting
vegetation

DaytimeHorn flies

Cattle manure

DaytimeHorse and deer flies

Vegetation and water

Daytime

Culicoides spp

Standing water andmanureTwilight to dawn

Running water

Morning and evening

Simulium (black) flies

Rabies in the Horse and Beyond in Kentucky (page 5):
Table 2. Rabies virus test results broken down by wildlife, domestic
species (excluding horses), and horses.

Results

Wildlife

Domestic
Species(excluding
horses)HorsesTOTAL

Negative

12,37816,733

1,034

30,145

Positive

680

28

25

733

Unsatisfactory
specimen

823

675

11

1,509

TOTAL

13,881

17,436

1,070

32,387

Figure 1. Geographic locationof the 25 horses testing positive
for rabies virus in Kentucky from1989 to 2017.

cases have been associated with the administrationof tetanus antitoxin, although commercial plasma
products also have been incriminated. The numberof horses that become ill following administrationof a specific lot of incriminated blood product isestimated at 1 to 2 percent, although more horses
might have sub-clinical disease. An infectious
agent, such as a virus, has long been suspected tocause this condition, and a new equine parvovirus
was recently discovered in a horse with a fatal caseof serum hepatitis. The parvovirus was present in
the diseased horse and in the biologic product thatit had received nine weeks earlier. Inoculation ofexperimental horses with the commercial productresulted in transmission of this newly discovered
virus and in liver disease. Variation in individual
immune responses to the virus could explain the
low percentage of clinically affected horses.
An identical disease, both in terms of clinicalsigns and pathologic findings, is sporadically seenin adult horses without recent administration of anequine origin biologic. These “non-biologic” cases
tend to mostly occur between June and November
and can occur in small outbreaks that span a few
weeks. Non-biologic cases seem to occur mostcommonly on broodmare farms. The seasonal
incidence of the non-biologic cases suggests thepossibility of insect transmission of parvovirus inthese “non-biologic” cases.
Clinically affected horses with Theiler’s diseasefrequently have both neurologic signs (hepatic
encephalopathy: head pressing, stumbling, blindness) and jaundice (yellow gums and eyes). Oncethe neurologic signs are noted, there is a rapid
progression to death in approximately 70 percent of the cases. Horses that receive supportive
therapy and survive for five days after the onset offulminant disease generally recover and have nolong-term effects.
Another recently discovered virus that causes
liver disease in horses is non-primate hepacivirus(NPHV). NPHV is genetically the closest homologof human hepatitis C virus (HCV) discovered todate. Experimental horse infections consistently result in biochemical and histopathologic evidence ofhepatitis, but the disease is mild and clinical signs
are either absent or very mild in recently infectedhorses. NPHV and equine parvovirus are present
in some healthy horses, indicating that horses canbecome healthy carriers of these viruses.
The USDA Center for Veterinary Biologics
has issued a notice that all licensed equine blood
products have to test free of equine parvovirus
and NPHV. This should improve horse health byeliminating most of the blood product-associatedcases of hepatitis. Non-biologic associated cases
will likely continue to occur until natural meansof virus transmission are determined and necessarycontrol methods implemented.
CONTACT:
Thomas J. Divers, Joy E. Tomlinson andGerlinde R. Van de Walle
tjd1@cornell.edu(607) 253-3100
College of Veterinary Medicine
Cornell University
Ithaca, NY

N AT I O N A L

Importance of Protecting your Horse against Eastern Equine
Encephalomyelitis and West Nile Encephalitis
0

2

4

6

0

2

4

6

Figure 2. Geographic locationof skunks in Kentucky testing
positive for rabies virus in the last
29 years.

The two most frequently encountered causesof equine encephalitis or equine encephalomyelitis in North America are Eastern Equine
Encephalomyelitis (EEE) and West Nile Encephalitis (WNE) viruses. Both are mosquito-borneand neurotropic. The respective viruses are not
restricted to equids in terms of their host range;
each can be transmitted to humans and certain
other mammalian and avian species. Both diseases
are a source of concern for the equine industry
not only from the potentially life-threatening
consequences of either infection, but also fromthe economic losses involved.

Eastern equine encephalomyelitis poses anannual threat to equids in the Gulf and Atlantic
coastal states and the Great Lakes region, extendingin certain years as far north as eastern Canada. It
is occasionally recorded in some inland states suchas Arkansas, Oklahoma, Tennessee, Kentucky,
and Iowa. Evidence of EEE activity is most often
reported in Florida, in which it has been confirmed
as early as January, as recorded in 2018. Over the
past 15 years, the yearly incidence of EEE cases inequids has ranged from 60 (2011) to 712 (2003),
with an annual average of 206 cases. In 2017, 86
cases of the disease were reported in 13 states.

3
arme properly, eliminate
eas with decomposing vegetation, and theough fly inhibitors
educe fly populations. Heat, humidity,
and solar radiation exacerbate pruritus, thus theshade and wind currents by fans prooid the most significant
ecause some horses develop
contact allergies to topical products, judicious andimple
feeds and whole grains are better than mixed,
drate and sooth dry skineduce the amount of allergens on theskin. Colloidal oatmeal, pramoxine, and 1 percent
tisone shampoos or leave-on hydrocortiuritus and minimize or
uritic medicaoids andantihistamines) may help interrupt the itch-scratch
er long term use, particularly of steoided. A complete and detailed
occur simultaneously with general symptomaticto determine antigens for allergen specific immunotherapy may beticularly in young animals with atopy.
Clients should understand that hypersensitivities
continuous management and/or therapy. Often aee with low exposure toinciting antigens and symptomatic as the antigen
e is additive, thus
ment is needed to best4

Insect Bite Hypersensitivity in Horses (left):
Table 1. Breeding location and time of feeding for insects associatedwith IBH.
InsectBreeding Area

Feeding Times

Stable flies

Manure and rotting
vegetation

DaytimeHorn flies

Cattle manure

DaytimeHorse and deer flies

Vegetation and water

Daytime

Culicoides spp

Standing water andmanureTwilight to dawn

Running water

Morning and evening

Simulium (black) flies

Rabies in the Horse and Beyond in Kentucky (page 5):
Table 2. Rabies virus test results broken down by wildlife, domestic
species (excluding horses), and horses.

Results

Wildlife

Domestic
Species(excluding
horses)HorsesTOTAL

Negative

12,37816,733

1,034

30,145

Positive

680

28

25

733

Unsatisfactory
specimen

823

675

11

1,509

TOTAL

13,881

17,436

1,070

32,387

Figure 1. Geographic locationof the 25 horses testing positive
for rabies virus in Kentucky from1989 to 2017.

cases have been associated with the administrationof tetanus antitoxin, although commercial plasma
products also have been incriminated. The numberof horses that become ill following administrationof a specific lot of incriminated blood product isestimated at 1 to 2 percent, although more horses
might have sub-clinical disease. An infectious
agent, such as a virus, has long been suspected tocause this condition, and a new equine parvovirus
was recently discovered in a horse with a fatal caseof serum hepatitis. The parvovirus was present in
the diseased horse and in the biologic product thatit had received nine weeks earlier. Inoculation ofexperimental horses with the commercial productresulted in transmission of this newly discovered
virus and in liver disease. Variation in individual
immune responses to the virus could explain the
low percentage of clinically affected horses.
An identical disease, both in terms of clinicalsigns and pathologic findings, is sporadically seenin adult horses without recent administration of anequine origin biologic. These “non-biologic” cases
tend to mostly occur between June and November
and can occur in small outbreaks that span a few
weeks. Non-biologic cases seem to occur mostcommonly on broodmare farms. The seasonal
incidence of the non-biologic cases suggests thepossibility of insect transmission of parvovirus inthese “non-biologic” cases.
Clinically affected horses with Theiler’s diseasefrequently have both neurologic signs (hepatic
encephalopathy: head pressing, stumbling, blindness) and jaundice (yellow gums and eyes). Once

Importance of Protecting your Horse against Eastern Equine
Encephalomyelitis and West Nile Encephalitis

edicine

0

2

4

6

0

2

4

6

Figure 2. Geographic locationof skunks in Kentucky testing
positive for rabies virus in the last
29 years.

sed as part of a vaccination
ickness. Since then,of serum hepatitis haveted, and a variety of blood products
been associated with theth America, the majority of recent

CONTACT:
Thomas J. Divers, Joy E. Tomlinson andGerlinde R. Van de Walle
tjd1@cornell.edu(607) 253-3100
College of Veterinary Medicine
Cornell University
Ithaca, NY

N AT I O N A L

ACVIM

edicinethe neurologic signs are noted, there is a rapid
progression to death in approximately 70 percent of the cases. Horses that receive supportive
therapy and survive for five days after the onset offulminant disease generally recover and have nolong-term effects.
Another recently discovered virus that causes
liver disease in horses is non-primate hepacivirus(NPHV). NPHV is genetically the closest homologof human hepatitis C virus (HCV) discovered todate. Experimental horse infections consistently result in biochemical and histopathologic evidence ofhepatitis, but the disease is mild and clinical signs
are either absent or very mild in recently infectedhorses. NPHV and equine parvovirus are present
in some healthy horses, indicating that horses canbecome healthy carriers of these viruses.
The USDA Center for Veterinary Biologics
has issued a notice that all licensed equine blood
products have to test free of equine parvovirus
and NPHV. This should improve horse health byeliminating most of the blood product-associatedcases of hepatitis. Non-biologic associated cases
will likely continue to occur until natural meansof virus transmission are determined and necessarycontrol methods implemented.

T

he two most frequently encountered causesof equine encephalitis or equine encephalomyelitis in North America are Eastern Equine
Encephalomyelitis (EEE) and West Nile Encephalitis (WNE) viruses. Both are mosquito-borneand neurotropic. The respective viruses are not
restricted to equids in terms of their host range;
each can be transmitted to humans and certain
other mammalian and avian species. Both diseases
are a source of concern for the equine industry
not only from the potentially life-threatening
consequences of either infection, but also fromthe economic losses involved.

Eastern equine encephalomyelitis poses anannual threat to equids in the Gulf and Atlantic
coastal states and the Great Lakes region, extendingin certain years as far north as eastern Canada. It
is occasionally recorded in some inland states suchas Arkansas, Oklahoma, Tennessee, Kentucky,
and Iowa. Evidence of EEE activity is most often
reported in Florida, in which it has been confirmed
as early as January, as recorded in 2018. Over the
past 15 years, the yearly incidence of EEE cases inequids has ranged from 60 (2011) to 712 (2003),
with an annual average of 206 cases. In 2017, 86
cases of the disease were reported in 13 states.

5

In temperate regions, transmission of EEE virus
is seasonal, occurring in the summer and the fall.
In sub-tropical regions such as Florida, there is ayear-round risk of EEE, with virus transmission
peaking in the summer months.
Equids and humans are tangential or dead-end
hosts of EEE virus and neither plays a role in thenatural life-cycle of the virus. Infections in horses,
mules, and donkeys are frequently life-threatening;case fatality rates can be as high as 90 percent.
West Nile encephalitis (WNE) is also a cause ofsignificant concern to veterinary practitioners andmembers of the equine industry. Within four years
following initial introduction of the causal virus in
New York State in 1999, the virus had spread to
48 states and several provinces in Canada. Since
1999, the yearly incidence of WNE cases in equids
has ranged from 60 (2000) to 15,257 (2002). The
annual average number of cases over the past 10
years was 272. In 2017, 307 equine cases werereported in 39 states.
Similar to EEE, transmission of WNE virus isseasonal, occurring in the summer and extending
well into the fall. Neither equids nor humans serveas amplifying hosts for WNE virus insofar as viremias are insufficient in magnitude and duration to
infect mosquitoes. Unlike EEE, only about 10%of WNE virus-exposed horses will develop clinicalinfections. Reported case-fatality rates in affectedhorses can reach 30-40 percent, less than half thatencountered in cases of EEE.
The American Association of Equine Practitioners (AAEP), in accordance with criteria definedby the American Veterinary Medical Association
with respect to “core vaccines”–namely those that
protect against diseases that are endemic, of potential public health significance, and represent a riskof causing severe disease—strongly recommends
that horses be immunized against EEE and WNE.
Available inactivated whole-virus vaccines against
EEE (including Western Equine Encephalomyelitis) have been shown to be safe and effective inprotecting against this disease. Two inactivatedwhole-virus vaccines, a live canary pox vector vaccine and an inactivated flavivirus chimera vaccine
are available against WNE. All have been confirmed safe and effective in preventing the disease.
Despite the AAEP recommendations to horseowners to vaccinate their horses against EEE
and WNE, regrettably many fail to do so. The
vast majority of equine cases of EEE and WNE
either have no history of vaccination against theparticular virus or else the vaccination history is
incomplete. There is need for an ongoing concerted
effort, utilizing all avenues of communication including social media, to alert horse owners of thedangers of these two vector-borne diseases and of
the importance of vaccination as an effective meansof prevention and averting the losses that continue
to occur every year in unprotected horses.
CONTACT:
Peter J. Timoney, MVB, MS, PhD, FRCVS
ptimoney@uky.edu(859) 218-1094
Maxwell H. Gluck Equine Research Center
University of Kentucky
Lexington, KY

KENTUCKY

Rabies in the Horse and Beyond in Kentucky:
A Look at the Last 29 Years

R

abies virus exposure typically occurs following
the bite of an infected animal. Dependingon the anatomic site of exposure, an incubation
period of variable duration follows as the virus
evades the immune response by hiding in thecentral nervous system. Amplification of the virus
occurs in the dorsal root ganglion after which it
travels towards the brain via the spinal cord. At
this point, clinical signs of rabies become manifest
and therapy is almost invariably futile. Without
early treatment, rabies is nearly 100 percent fatal.
Rabies is a zoonotic (capable of being transmitted
from animals to humans) disease that is distrib-

uted nearly worldwide. Attention to the diseaseis primarily focused on preventive and control
strategies. Many countries are considered rabiesfree for the purposes of importing dogs into theUnited States (https://www.cdc.gov/importation/
rabies-free-countries.html).
Rabies can be prevented by pre-exposure vaccination of humans and animals. A human diploidcell vaccine and a purified chick embryo vaccine
are available for humans (the latter is mainly usedoutside of the USA) for pre- and post-exposure torabies, with rabies immune globulin available only
for post-exposure treatment in exposed humans.

Equine Disease Quarterly Newsletter
Department of Veterinary Science
Maxwell H. Gluck Equine Research Center
University of Kentucky
Lexington, Kentucky 40546-0099
Address Service Requested

Pre-exposure vaccination involves administrationof three doses of vaccine given over a one month
period. In unvaccinated humans, post-exposure
treatment consists of the administration of fivedoses of vaccine. Vaccines for multiple species ofdomestic animals are available to licensed veterinarians. Wildlife vaccines may be available from veterinarians, but are typically used in targeted locations
by the Kentucky Department of Fish and Wildlife
and the United States Department of Agriculture.
From January 1989 through December 2017,
Kentucky tested 32,387 animals for rabies virus.
Of this total, 2.3 percent (733 animals) tested positive, 93.1 percent (30,145 animals) tested negative,
and 4.6 percent (1509 animals) were unsatisfactory
for testing (i.e. the sample was untestable due tomaceration, degradation, or insufficient material). Of note: Animals suspected of having rabies
should not be euthanized by traumatic insult (e.g.
gunshot) to the brain, because trauma frequently
renders the sample unsatisfactory for testing. Of
the 733 positive animals, only 7.2 percent (53animals) were from domestic species (i.e. pets andfarm animals), and the other 92.8 percent (680
animals) were wildlife. Twenty-five of the positivedomestic animals were horses (Table 2), whichmeans that horses accounted for less than 1 percent of the total positive rabies cases in Kentucky
over the last 29 years. Rabies-positive horses wereprimarily located in Central Kentucky (Figure 1).
The terrestrial reservoir for rabies in the state is the
striped skunk, and skunks positive for rabies virus
have been located in all counties where infectedhorses have resided (Figure 2).
As a closing comment, a robust surveillance
program involving the UK Veterinary Diagnostic
Lab, Breathitt Veterinary Center, the Kentucky
Cabinet for Health, Kentucky Fish and Wildlife,and USDA is in place to thoroughly monitor rabies
in Kentucky. The vast majority of animals testing
positive for rabies are wildlife, with little or no
exposure to humans, pets and domestic animals.
More importantly for horse enthusiasts within
the state, the number of horses testing positive forrabies is extremely low.
CONTACT:
Jacqueline Smith, PhD
jsmit8@uky.edu(859) 257-7559
University of Kentucky Veterinary Diagnostic Laboratory
Lexington , KY