United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture 

Veterinary Services 

Centers for Epidemiology and Animal Health December 2006 

_________________________________________________________________________________________________________________________ 

Vaccination Practices on 

U.S. Equine Operations 

Equine owners have several options for preventing 

and controlling infections in their equids. These 

options include reducing the likelihood of exposure 

to infectious agents and optimizing resistance to 

disease. Resistance to infectious diseases can be 

enhanced through vaccination and by improving 

overall health through multiple means, including 

meeting nutritional requirements and parasite 

control. Vaccination can reduce the likelihood of 

disease occurring in exposed animals. If exposure 

to infectious disease agents occurs, the degree of 

immunity, amount of exposure, and virulence of the 

disease agent all play a role in the outcome. The 

American Association of Equine Practitioners 

recommends that all equids receive vaccines to 

protect them against tetanus, eastern and western 

equine encephalitis (EEE/WEE), West Nile virus 

(WNV), and in most regions, rabies. These 

vaccines are considered “core” because they are 

thought to be safe and effective and because there 

is a real potential that equids can be exposed to the 

disease agents, which could lead to fatal illness. 

 One of the goals of the U.S. Department of 

Agriculture’s (USDA) National Animal Health 

Monitoring System (NAHMS) Equine 2005 study 

was to estimate equid vaccination practices. 

 For the Equine 2005 study, NAHMS collected 

data on equine health and management practices 

from a representative sample of operations with 5 

or more equids in 28 States divided into 4 regions.* 

The 28-State target population represented 78.0 

percent of equids and 78.6 percent of operations 

with 5 or more equids in the United States. 

Interviews were conducted from July 18 through 

August 12, 2005, and 2,893 equine operations 

provided data on equine health and management. 

*Regions: 

West: California, Colorado, Montana, New Mexico, Oregon, 

 Washington, and Wyoming 

Northeast: New Jersey, New York, Ohio, and Pennsylvania 

South: Alabama, Florida, Georgia, Kentucky, Louisiana, 

 Maryland, Oklahoma, Tennessee, Texas, and Virginia 

Central: Illinois, Indiana, Kansas, Michigan, Minnesota, Missouri, 

 and Wisconsin 

Of operations participating in the study, 40.3 

percent identified their primary function as 

“farm/ranch,” and 37.0 percent identified their 

primary function as “residence with equids for 

personal use.” Over 95 percent of operations 

had horses and 34.8 percent had equids other than 

horses, e.g., donkeys, burros, mules, ponies, and 

miniature horses. For this study, a resident equid 

was defined as an equid that spent or was 

expected to spend more time at the operation than 

at any other operation, whether or not it was 

present at the time of the interview. The operation 

was its home base. 

Vaccination 

Overall, 75.9 percent of operations gave some 

type of vaccine to resident equids during the 

previous 12 months. A higher percentage of 

operations in the West region gave at least some 

vaccines to resident equids compared to operations 

in the South and Northeast regions (figure 1). 

0

20

40

60

80

100

72.3 72.0

83.8

77.4 75.9

South Northeast West Central All Operations

Percent

Region

Figure 1. Percentage of Operations that Administered Any Vaccine to Resident 

Equids During the Previous 12 Months, by Region
United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture 

Operations with a primary function of 

farm/ranch or residence with equids for personal 

use were less likely to administer vaccines to 

equids than operations with a primary function of 

boarding/training, breeding farm, and “other” 

(table 1). 

 

Table 1. Percentage of operations that administered 

any vaccines to resident equids during the previous 

12 months, by primary function of operation: 

Percent Operation 

Primary Function 

Boarding/ 

Training 

Breeding 

Farm 

Farm/ 

Ranch 

Residence 

with 

Equids for 

Personal 

Use Other 

Percent 

96.8 89.7 67.8 74.9 91.2 

Overall, veterinarians were the primary source 

of vaccines for operations that administered any 

vaccine to resident equids during the previous 12 

months (76.0 percent of operations). On half the 

operations (50.3 percent) a veterinarian 

administered the majority of vaccines. As operation 

size increased so did the percentage of operations 

that used operation personnel to administer the 

majority of vaccines (table 2). 

 

Table 2. For operations that administered any 

vaccines to resident equids during the previous 12 

months, percentage of operations by person who 

administered the majority of vaccines and by size of 

operation: 

 Percent Operation 

 Size of Operation (Number of Equids)

Small 

(5-9) 

Medium 

(10-19) 

Large 

(20 or 

More) 

All 

Ops. 

Person Pct. Pct. Pct. Pct. 

Veterinarian 54.1 44.4 39.2 50.3 

Operation 

personnel 

(including 

operator) 29.6 39.1 43.4 33.3 

Equid 

owner 

(not 

operator) 15.8 16.4 17.4 16.1 

Other 0.5 0.1 0.0 0.3 

Total 100.0 100.0 100.0 100.0 

Operations vaccinating one or more equids 

Overall, 94.4 percent of operations that 

administered one or more vaccines to resident 

horses during the previous 12 months knew which 

vaccines were given. Of these operations, 44.5 

percent vaccinated against rabies, 72.5 percent 

against influenza, 75.6 percent against EEE/WEE, 

81.3 percent against tetanus, and 85.3 percent 

against WNV. Before 1999, WNV was not 

recognized in the United States. Subsequent to the 

recognition of WNV, several vaccines became 

available for use in equids, the first under a 

conditional license in summer 2001. This killed 

vaccine with an adjuvant (Fort Dodge) was fully 

licensed in 2003. Since then, several other WNV 

vaccines have been licensed for use in equids, 

including one in 2004 (Merial canary pox WNV 

vaccine) and one in 2006 (Intervet chimera vaccine 

combining WNV and yellow fever vaccine virus). 

There were regional differences in vaccination 

for several diseases. For example, 48.6 percent of 

operations in the Northeast region, 38.0 percent in 

the South region, 28.8 percent in the Central 

region, and 18.4 percent in the West region 

vaccinated one or more resident equids against 

rabies. A higher percentage of operations in the 

Northeast and Central regions (17.1 and 17.0 

percent, respectively) vaccinated against Potomac 

horse fever compared to operations in the South 

and West regions (7.2 and 6.1 percent, 

respectively). The difference in vaccine use across 

regions may be due to a perceived difference in the 

likelihood of exposure to causative agents or a 

difference in vaccination recommendations from 

veterinarians in the various regions.

APHIS photo by Charles Kerlee 
United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture 

Reasons for Not Vaccinating

All operations were queried about the use of 

eight specific vaccines: influenza, strangles, 

rhinopneumonitis (herpesvirus), rabies, WNV, 

EEE/WEE, tetanus, and equine viral arteritis (EVA). 

Operations reported which of these eight vaccines 

were administered to equids less than 1 year of 

age, equids more than 1 year of age, and 

broodmares. When the operation reported that a 

specific vaccine was not administered, a follow-up 

question offered eight alternatives as to why the 

vaccine was not used: concern of adverse reaction 

to vaccine, vaccine considered ineffective, little risk 

of disease exposure, not recommended by 

veterinarian, financial constraints on horse 

expenditures, thought important but did not get 

around to it, effort and cost of vaccination 

outweighed financial and other benefits of 

vaccination, or reasons other than those listed 

above. 

Operations that gave vaccines but not the specified 

vaccines 

Reasons given by operations that gave some 

vaccines but not the specified vaccines followed 

similar patterns across the eight types of vaccines. 

The highest percentages (ranging from 39.9 to 58.5 

percent) reported that little risk of disease exposure 

was the reason for not giving each of the eight 

vaccines. For all but the WNV vaccine, the second 

highest percentage of operations (18.2 to 31.8 

percent) reported not recommended by the 

veterinarian as the reason for not giving the 

specified vaccines, followed by effort and cost of 

vaccination outweighed financial and other benefits 

of vaccination (7.0 to 11.2 percent of operations). 

For the WNV vaccine, the second highest 

percentage of operations (13.3 percent) cited the 

reason for not giving the vaccine was effort and 

cost outweighed financial and other benefits of 

vaccination, followed by concern of adverse 

reaction (10.9 percent of operations). 

 In June 2003, the Centers for Veterinary 

Biologics (CVB)—under the auspices of the USDAAnimal and Plant Health Inspection ServiceVeterinary Services—conducted an investigation 

into reported concerns that the only WNV vaccine 

available for equids at the time (the killed vaccine 

with an adjuvant) was associated with pregnancy 

loss. On June 25, 2003, CVB released their 

findings from the investigation, which concluded 

that there was no clear evidence the vaccine posed 

a risk to pregnancy in equids. Subsequently, a 

study conducted at Texas A&M and published in 

the “Journal of the American Veterinary Medical 

Association” in December 2004 concluded that 

pregnancy rates and fetal outcome among 

vaccinated mares were similar to unvaccinated 

mares. It is possible that some equine owners still 

have concerns about the WNV vaccine and its 

effect on pregnant mares despite the fact that 

subsequent investigations appeared to find no 

correlation between the vaccine and pregnancy 

rates. 

Except for WNV, strangles, and EVA vaccines, 

the remaining rankings of reasons for not giving 

specific vaccines were financial constraints on 

horse expenditures, thought important but did not 

get around to it, concern of adverse reaction, 

vaccine considered ineffective, and “other.” These 

reasons accounted for 15 percent or less of 

responses. For strangles vaccine, there was a 

somewhat higher concern about an adverse 

reaction than for the other vaccines, while for EVA 

vaccine the concern about an adverse reaction was 

lower. 

Operations that gave no vaccines 

For operations that did not vaccinate, almost twothirds (58.9 to 65.0 percent) reported that little risk 

of disease was the reason for not vaccinating. The 

next most common reasons for not giving 

vaccinations were consistent across vaccines: 

effort and cost of vaccination outweighed financial 

and other benefits of vaccination (12.3 to 13.3 

percent of operations), thought important but did 

not get around to it (7.8 to 12.3 percent of 

operations), and financial constraints on horse 

expenditures (5.2 to 5.8 percent of operations). 

None of the other reasons for not vaccinating 

exceeded 3.2 percent of operations. 

Movement patterns of equids on 

nonvaccinating and vaccinating operations

For operations that did not vaccinate any 

equids, 14.9 percent had nonresident equids come 

onto the operation and stay for fewer than 30 

consecutive days. For operations that vaccinated 

one or more equids for at least one disease, 20.3 

percent had nonresident equids come onto 

operation and stay for fewer than 30 consecutive 

days. 

Approximately 4 of 10 operations (40.5 percent) 

that did not vaccinate any equids had resident 
United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture 

equids leave the operation and return. Twenty-nine 

percent of operations that did not vaccinate any 

equids transported equids off the operation by 

vehicle. For these operations, travel by vehicle was 

within the respective State on 94.5 percent of 

operations, to adjacent States on 13.3 percent of 

operations, and beyond adjacent States on 5.7 

percent of operations. In comparison, on operations 

that did vaccinate, 94.8 percent of operations 

transported equids within State, 37.1 percent to 

adjacent States, and 12.7 beyond adjacent States 

(table 3). 

Table 3. For operations that transported resident 

equids by vehicle off the home operation and 

returned during the previous 12 months, percentage 

of operations by destination and by vaccination-use 

status of operations: 

Percent Operations 

 Vaccination Status 

Vaccinated 

One or More 

Equids 

Did Not 

Vaccinate Any 

Equids 

Destination Percent Percent 

Within State 94.8 94.5 

Adjacent States 37.1 13.3 

Beyond 

adjacent States 12.7 5.7 

 

________________________________ 

For more information, contact: 

USDA:APHIS:VS:CEAH 

NRRC Building B, M.S. 2E7 

2150 Centre Avenue 

Fort Collins, CO 80526-8117 

970.494.7000 

E-mail: NAHMS@aphis.usda.gov 

http://nahms.aphis.usda.gov 

#N454.1206 

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