Frequently Asked Questions (FAQs)

EQUINE VIRAL ARTERITIS (EVA) -- Questions & Answers
       by Benjamin R. Buchanan, DVM, DACVIM, DACVECC
      (click here to access as a MS Word document)

What is EVA?
Is this a new disease?
What are the symptoms?
How does a horse get the disease?
Have there been any recent outbreaks?
Who is at risk?
How do I protect my horses?
Should I test every horse?
What do I do if my horse tests positive?
Should I vaccinate?
Who should I vaccinate?
How do I vaccinate?
What if I want to breed my mare to an infected stallion?
What is the risk of EVA to the horse industry?
Other considerations: TEXAS
Other considerations: OKLAHOMA
Recommended readings

What is EVA?
Equine Viral Arteritis (EVA) is a contagious disease of horses, donkeys and mules. It is caused by an enveloped single stranded RNA virus in the family Arteriviridae. The actual virus is the Equine Arteritis Virus. The virus is found in horse populations world wide (except Japan and Iceland) and is included on the B list of the Office International des Epizooties (O.I.E.), which are diseases of economic significance and should be reported annually from endemic countries.  The virus only infects horses and is not transmissible to humans. Approximately 72 hours after infection from inhaling the virus, the virus is reproducing in the blood vessels, white blood cells, and lymph nodes in the lungs. The virus then spreads to other organs. By 6-8 days post exposure, the virus causes inflammation in the muscular layer of small arteries which causes edema. From 7-21 days after exposure horses will develop antibodies to the virus and will test positive on blood for exposure. Although it is not life threatening to most horses, some strains can causes late term abortion in pregnant mares and fatal pneumonia in neonatal foals. Because of its potential impact to the breeding industry and international regulations concerning exporting horses, EVA is a potential threat to the horse industry.

Is this a new disease?
EVA is not a new disease. It was first reported over a century ago in Europe and was first isolated in the United States in the mid 1950s. It received national attention in the Thoroughbred industry after an epidemic in Kentucky in 1984 even though no abortions were reported in this outbreak. In 2006 two additional outbreaks that caused abortions in New Mexico and Utah have again raised the level of attention concerning EVA, especially in the Quarter Horse and related breeds.

Although this is an old disease, a recent report found that close to 60% of horse owners in the USA have no knowledge of the disease. EVA has a worldwide distribution with diseases reported on almost every continent. A 2001 study found that 1.9% of the horses in California tested positive, while 16.1% of horses imported into the country tested positive. While it has been found in almost every country with a large horse industry, there are still significant importation policies that will bar horses that test positive from being exported out of the United States. However some countries will allow importation of horses that test positive IF there is proper documentation of negative status at the time of vaccination.

Previous studies have found that the prevalence of positive blood tests and infected stallions was highest in the Standardbred and some Warmblood breeds. While the Quarter Horse and Thoroughbred have been shown to have the lowest incidence, there is increasing evidence that the disease is increasing in the Quarter Horse breed over the last two years.

 What are the symptoms?
Horses show a range of symptoms 3-14 days after exposure to the virus depending on the route of exposure. Clinical signs occur sooner when infection occurs via the respiratory route. The disease typically causes fever and mild respiratory disease with occasional limb swelling that resolves without treatment. A low white blood cell count is also a consistent finding.

Many horses which are infected do not show any signs of disease. Common symptoms include: fever, nasal discharge, loss of appetite, depression, conjunctivitis, ocular discharge, and swollen limbs. Other signs of disease include pneumonia and difficulty breathing, muscle soreness, skin rash, depression, swelling around the eyes, swollen genitals in stallions, and swollen mammary glands in mares.

Abortion in pregnant mares may be the first recognized clinical sign. Abortion rates in infected mares may be as high as 70% on a given farm during an outbreak and depend on the strain of virus. Although a mare will develop an infection from breeding to an infected stallion, this will not cause her to abort. She will shed infectious virus particles via her respiratory secretions for up to 3-4 weeks after breeding, and these particles can infect other pregnant mares in later stages of gestation causing them to abort. A mare will typically abort from 10 - 34 days following exposure to the virus during her third to tenth month of pregnancy.

EVA is also reported to cause a severe respiratory condition sometimes accompanied by colic or diarrhea in neonatal foals. It is thought that the foals are infected in the uterus when the non-vaccinated mare is exposed to another horse shedding the virus. The foals die of a severe interstitial pneumonia within the first few weeks of life. No successful treatment has been reported. Older foals (2-3 months) have been shown to become fatally infected after birth. The severity of the infection is dependent on the strain, but does raise the potential for complications with the foal when a vaccinated mare is exposed to a known positive stallion for the first time. When a foal is infected they will produce very large numbers of the virus and contaminate the environment.

How does a horse get the disease?
A horse will contract EVA when it is exposed to another infected horse shedding the virus. Because the virus does not survive very long outside of the body, the horses must have very close contact to transmit the infection. A horse can shed the virus even if it is not showing any signs of disease. It is this horse-to-horse spread, and lack of significant clinical signs, combined with the natural desire of horses to mingle in a herd and ability of horses to be rapidly moved around the country that contributes to disease spread and outbreaks.

            There are five major routes of infection to horses including

1. Respiratory: This is the primary route of infection in breeding horses, show horses, sales horses, and race horses. The virus is shed in the respiratory secretions of an infected horse and infects a healthy horse. The newly infected horse then sheds the virus and can infect other horses.

2. Venereal: This is a major route of infection in breeding operations as the virus can be recovered from the semen of infected stallions. The infected semen will infect the mare and any other horse it comes in contact with. A non-vaccinated mare will then shed virus into the environment for 3-4 weeks.

3. Other body secretions: The virus has been recovered from the urine, feces, and other body secretions of infected horses. In mares it has also been recovered from vaginal secretions, ovaries, oocytes, and uterine secretions.

4. In the uterus: Although uncommon the virus may pass through the placenta and infect a fetus.

5. Human spread: Without proper precautions, the virus can be spread on shared tack or equipment and by personnel working on infected and non-infected horses.

 Once a horse has been infected, their immune system will protect them against future infections. If they are exposed again, they will not develop any clinical signs or shed the virus. A mare that has previously been infected with EVA (by breeding or respiratory) and bred with infected semen may shed the virus from vaginal secretions after insemination for 2-3 days, but should not shed the virus in any respiratory secretions or other body fluids.

Once a stallion is infected he may become a chronic carrier. The virus will live in the accessory sex glands and the stallion will not show any signs of disease. These stallions often have a very strong positive blood test for EVA. Other than the semen, an infected stallion should not secrete EVA in any other bodily fluid. An infected stallion will not shed the virus in his respiratory secretions if he is exposed again. However a chronic carrier is likely infected for life and will always shed the virus in his semen.

Have there been any recent outbreaks?
EVA is not a reportable disease in all states and there is not a good national source that reports outbreaks of EVA. However there have been two known outbreaks this past year in both New Mexico and Utah.

Who is at risk?
While all ages of horses may become infected with the virus, the two groups the horse industry is most concerned with are the pregnant mares and stallions. Because infection of non-vaccinated mares may cause loss of a large percentage of a foal crop, preventing infection in pregnant mares is important. This can be accomplished with appropriate isolation and farm biosecurity.

Persistently infected stallions are the reservoir for the virus in the horse population. Prevention of young colts from becoming infected is important in controlling the disease. This can be accomplished by annual testing and vaccination of breeding stallions and young colts before they are breeding age.

How do I protect my horses?
The most critical aspect of protecting a farm is to practice biosecurity. All new arrivals should be isolated from the herd for 30 days. Keep pregnant mares separate from horses in training. Observe strict hygiene when breeding or collecting semen from carrier stallions.  In addition to strict biosecurity, appropriate vaccination is an adjunct to protecting your horses and farm. The following are specific measures recommended in the USDA’s Equine Viral Arteritis Uniform Methods and Rules (UM&R).

1. Isolate all new arrivals and horse returning from other farms, sales, or race tracks for 3-4 weeks

2. Segregate pregnant mares from other horses and maintain mares in small groups based on anticipated foaling dates until they have foaled

3. Before each breeding season, blood-test all new breeding stallions for the presence of antibodies to EVA

4. Have the semen of antibody positive, non-vaccinated stallion laboratory tested to identify any carrier animals

5. Annually vaccinate all noncarrier breeding stallions at least 4 weeks before the start of each breeding season.

6. Physically isolate any carrier stallions

7. Observe strict hygienic precautions when breeding or collecting semen from carrier stallions to avoid the risk of inadvertent transfer of infection through indirect contact with virus-contaminated objects.

8. Restrict breeding carrier stallions to vaccinated mares or mares that have previously tested positive for naturally acquired antibodies to the virus

9. Vaccinate antibody negative mares against EVA at least 3 weeks before breeding to a known carrier stallion or with virus-infective semen

10. For 3 weeks after they have been bred to a carrier stallion, isolate mares vaccinated for the first time against EVA from all but know EVA antibody positive horses. It is especially important to avoid contact between such mares and other pregnant mares to which the virus can be spread by the respiratory route.

11. In breeds or areas in which there is a high prevalence of EVA infections, vaccinate all immature foals between 6 and 12 months of age against EVA as advised by your veterinarian. If implemented over a period of years, such a program of targeted vaccination would greatly reduce the number of carrier stallions and largely eliminate the primary reservoir of EVA.

12. Determine the infectivity status of semen used for AI, especially if imported from abroad. When breeding mares with EVA-infected semen, adopt the same precautions as for mares bred by natural cover to a carrier stallion.

The equine arteritis virus is not a very resistant virus. It is sensitive to sunlight, high temperatures, low humidity, drying and various disinfectants and lipid solvents.  The virus can live in refrigerated or frozen temperatures. This allows the virus to remain infective in cooled shipped and frozen semen.  Removing organic material and cleaning with a dilute bleach solution should kill the virus.

Should I test every horse?
Not every horse should be tested, unless there is an outbreak concern, or another non-vaccinated horse on the farm has tested positive. To help control the disease and track infected horses during an outbreak, every horse that is being vaccinated should be confirmed negative by testing before vaccination.

Stallions to be used for breeding should be tested annually. If a stallion tests negative and might be exposed to the disease during the breeding season he should be vaccinated. A stallion can be exposed by transport of an inapparently infected mare or young horse that is shedding the virus. EVA-vaccinated stallions should be isolated and they should not be exposed to infected horses, non-vaccinated horses, and should not be used for breeding for 28 days after vaccination.

What do I do if my horse tests positive?
Once a horse is infected, it will test positive by a serum neutralizing antibody test (SNA or SN) of the blood for over a year, even though it has cleared the disease. EVA is a manageable disease if proper control programs aimed at minimizing and eliminating direct or indirect contact of susceptible horses with the secretions / excretions of infected animals are followed. Stopping the spread of EVA in breeding populations and the prevention of the establishment of the carrier state in stallions and colts is the goal of any control program. An industry policy to test and vaccinate breeding stallions, isolate and vaccinate sero-negative mares being bred to positive stallions, and vaccinating young colts to prevent possible carrier states may help to reduce or even eliminate the disease in the horse industry.

Stallions: When a mature stallion is exposed and becomes infected, there is the potential that he may become a chronic shedder of the virus. In 30 – 60% of infected stallions, the virus will continue to live in the accessory sex glands and be excreted into the semen. This semen is then infectious to other horses. Because the survival of the virus in the accessory sex glands is under the control of testosterone, castration may eliminate the carrier state. Because castration is unacceptable in some blood lines, protecting the mare by vaccination combined with isolation may prevent any spread of the disease after breeding. It is possible to isolate the virus from the semen of infected stallions and differentiate an infected stallion from one that only has a positive blood test but is not infected.

A stallion that tests negative for the virus is classified as a sero-negative stallion. Stallions that test positive and do not have documentation of their negative status on the date of vaccination will be classified as a sero-positive stallion and have two options.

Option 1. Breed the stallion to two negative mares and retest the mares again in 28 days. If the mares test positive, the stallion is infected. If the mares test negative the stallion can be classified as a sero-positive nonshedding stallion.

Option 2. Collect semen and submit it for virus isolation. A negative test will classify the stallion as a sero-positive nonshedding stallion.

 A stallion who is documented to be a sero-positive infected shedder should notify the owner of all mares bred and booked to that stallion immediately. If the stallion is to continue breeding mares, he should be kept in a separate facility away from any non-infected and non-vaccinated horses.

Young colts that have not reached sexual maturity may become infected without developing a carrier state. However any breeding age stallion with out proper documentation of vaccination that tests positive should have his semen tested for EVA.

Mare that has aborted: Immediately quarantine the farm and prevent any horse from leaving. Isolate any affected horse. Test other exposed horses. Vaccinate all at-risk horses. Notify the state veterinary office of a possible outbreak. Save any aborted fetus and placenta for testing. Any mare with a confirmed EVA abortion needs to be isolated from other horses for 30 days. All parts of the pregnancy (the fetus, the placenta, the amniotic and chroio-allontoic fluid) have tremendously high levels of the virus.

Non-pregnant mare or gelding: Immediately isolate and quarantine the positive individual. All traffic onto and off the farm should be stopped and every other horse should be tested. Contacting a state veterinarian is necessary to determine the date and source of the infection.

Should I vaccinate?

Pros
Vaccination of the horse carries little risk and is effective at preventing both infections and carrier states in stallions. Over several years, vaccinating young colts and breeding stallions will eliminate the carrier population of stallions and significantly reduce the threat of EVA to the horse industry. A single vaccination should protect a horse for 1-3 years. How long they will continue to test positive is not known.

Cons
Testing issues – Testing of horses prior to vaccination to confirm their negative status is highly recommended. An additional test the day of vaccination is also recommended to adequately document a stallion’s positive blood test is from vaccination.

Legal issues – The EVA vaccine is a controlled vaccine in the state of Texas. To acquire the vaccine in the state of Texas a veterinarian must have a client-patient relationship and submit a formal request to the state. This request needs to identify the horses that are to be vaccinated. Fort Dodge Animal Health is not supposed to release any vaccine to a veterinarian in the state who has not previously acquired permission.

EVA is not a reportable disease. However for the health and safety of horses in the state, anytime a horse is diagnosed with EVA the veterinarian is should alert the state veterinary office. The state veterinarian will then investigate and try to determine if there is the possibility of an outbreak and the need for any quarantining of horses and facilities.

Importation/Exportation issues – Some countries require testing for EVA before importation and will not accept a horse for import that has a positive blood test for any reason. As it is not possible to determine if a positive blood test is from the vaccine-strain or a real infection, vaccinating a horse today may prevent you from exporting him or his semen many years later. The length of time a horse will test positive to the virus is over 3 years in most cases after a single vaccination or infection.

Who should I vaccinate?
A stallion that may be at risk of exposure to EVA should be properly tested, documented negative and vaccinated. This is a decision that should be made on a farm to farm basis depending on the level of risk. A stallion should be vaccinated >30 days before the start of breeding season.

An open or maiden mare that is going to be bred to a known positive stallion should be vaccinated >30 days before exposure to the stallion. Any open mare may be at risk of exposure to EVA should be properly tested, documented negative and vaccinated >30 days before the breeding season. She should be kept in isolation from other horses for 3 weeks post breeding the first time she is bred to a known positive stallion.

A pregnant mare should not be vaccinated if possible because there is no available safety data or label for the vaccine. However, there are anecdotal reports of safely vaccinating mares and there have been no reports of abortions or neonatal infections associated with the current vaccine. If the mare has never been vaccinated, exposed to a positive stallion, or infected with the virus and is going to be exposed to a known positive stallion she should be vaccinated during the pregnancy and again thirty days before exposure to the stallion. This will allow her to pass some antibodies to her foal and protect the foal when it is exposed to the virus during the breeding process. The foal can be exposed to the virus through other horses, leaking of infected semen out of the uterus, or replication and shedding of the virus by the mare after breeding. If the mare has been exposed to the virus previously (through breeding or an infection) she should be vaccinated 30 days before exposure to the stallion. A pregnant mare should be vaccinated if she is on a farm having an outbreak.

A recipient mare that is going to receive an embryo from a donor mare being bred to a known positive stallion should be vaccinated >30 days prior to receiving the embryo and then kept in isolation from other horses for 3 weeks post transfer.

A gelding is only a candidate for vaccination if he is going to be exposed to a horse that may be shedding the virus, is on a farm where many horses are being vaccinated and may be exposed to a horse shedding the vaccine strain, or is on a farm that is having an outbreak. Exportation of geldings should also be considered when vaccinating. See Pros and Cons of vaccination above.

A colt/filly born to an unvaccinated mare can be vaccinated at a young age. If they are vaccinated before 8 months, a booster at one year of age is recommended. If a colt/filly is born to a vaccinated mare, they will get antibodies to EVA in the mare’s colostrum. While this will protect them from the virus it will cause them to test positive for up to 8 months of age. Because of immature immune responses a vaccine before 8 months of age should not be considered protective. Appropriate vaccination of young stallion prospects in the horse industry over several years will help to reduce or eliminate EVA in the population.

How do I vaccinate?
Please review information above concerning risks of vaccinating before deciding to vaccinate any of your horses. Horses must be isolated in a facility > 25 feet from any non-vaccinated horse. The vaccine is a live virus and although it will not cause a disease it will replicate in the horse. This replication is necessary for the horse to develop a good immune response and be adequately protected. During this replication stage, the horse will excrete live vaccine strain virus into the air. This vaccine strain of the virus can infect a non-vaccinated horse. Although it will not cause a disease in this horse, it will cause the horse to test positive on a blood test for the disease. At this time there is no way to determine if a positive blood test is from a vaccine-strain of the virus or a real infection. Because the virus lives in the accessory sex glands of stallions, proper documentation is necessary before vaccination to protect the owner from any claims that a stallion is an infected carrier of the disease.

The veterinarian administering the vaccine must keep records of the horse, date, and location of vaccination for the state veterinarian.

Stallions-Confirm negative status before vaccinating and again at the time of first vaccination. Vaccinate annually. Confirming a negative status on the same days as vaccination may protect the owner if any questions arise about the status of the stallion. Additionally saving an extra sample on the day of vaccination is advisable for retesting should any abnormal result be encountered.

Geldings-Confirm negative status before vaccinating. Vaccinate annually.

Performance mares and fillies- Confirm negative status before vaccinating. Vaccinate annually.

Broodmares-Confirm negative status before vaccinating. Vaccinate at least 30 days prior to breeding. There is no published data on the safety or risk of adverse effects on the use of this vaccine in pregnant mares.

Any colt or filly raised on a vaccinated mare- Consider drawing blood for testing before the foal nurses. Confirm negative status before vaccinating and again at the time of the first vaccination in colts. Do not vaccinate before 6 months of age as the antibodies the foal receives in the colostrum may inactivate the vaccine. Vaccinate between 6 months and 12 months of age. Booster 4-6 months later. Vaccinate annually.

Any colt or filly raised on a non-vaccinated mare- These foals can be vaccinated at any age. However the immune system of younger horses is unpredictable and they should be boostered 4-6 months later. Vaccinate annually.

What if I want to breed my mare to an infected stallion?
If you want to breed your mare to an infected stallion, she must be vaccinated at least 30 days prior to exposure to the stallion.

1. This is the first time she has been exposed to an infected stallion. The mare may shed infectious virus particles into the environment for up-to 21 days after breeding from live cover or with shipped, frozen, or fresh extended semen. She must be kept in strict isolation during this time.

2. This is the second time she has been bred to an infected stallion. The mare may shed infectious virus particles into the environment for 48-72 hours as semen leaks out of the uterus/vagina. She needs to be isolated during this time.

What is the risk of EVA to the horse industry?
EVA can have significant impact in the horse industry in both the breeding and performance horses.

1. Performance horses: Because very few horses in the US have been exposed to the virus, outbreaks are possible. Respiratory signs associated with EVA can interfere with training programs similar to influenza and herpes (or rhino). This costs money in time lost, and cost of treatment. Additionally positive blood tests may prevent a horse from being exported out of the country, and local outbreaks may prevent a horse from moving within the US.

2. Breeding horses: The biggest impact from EVA is likely in the breeding industry. Infection with certain strains may cause loss of a large percentage of a foal crop due to abortion and neonatal death. Breeding mares to infected stallions also costs money in appropriate isolation during vaccination and post breeding. Infection in stallions may lead to reduced demand due to the trouble of preparing the mare, as well as, loss of international markets due to importation restrictions.

Other considerations:

TEXAS: EVA is not a reportable disease in the state of Texas. As such it is up to a veterinarian or owner to notify the state of any positive test. There is currently no policy developed by the state veterinarian or horse industry concerning EVA. Many breeding facilities are requiring a negative test to EVA before receiving any horse.

OKLAHOMA: A meeting was held October 27 to discuss guidelines concerning EVA for the 2007 breeding season. The following proposal was made

In 2007 any broodmare entering breeding facilities will need:

1.      Up to date Health Certificate

2.      A negative Coggins test

3.      Proof of vaccination with ARVAC > 30 days ago OR a negative serologic test < 30 days before presentation.

Please check with any facility you are shipping horses to before arrival to check for any specific requirements for competition horses and sales horses. A follow-up meeting is to be held in mid-January so please realize that these recommendations/requirements may change.

Other recommended reading:

The above information is based upon research of published literature and conversations with several veterinarians who have extensive experience with this disease. While much of it can be supported with peer reviewed literature, some of it is based entirely on experience and common sense. Additional links of resources (some used in this paper) are presented below to allow you to view the opinions and recommendations of others to make the best informed choice for your horses.

USDA Uniform Methods and Rules (Excellent Detailed Resource) http://www.aphis.usda.gov/vs/nahps/equine/eva/eva-umr.pdf

USDA EVA Fact Sheet www.aphis.usda.gov/lpa/pubs/fsheet_faq_notice/fs_ahequineva.html

EVA: Is the disease a cause for industry concerns: by Dr. Peter Timmoney published in Impulsion, Spring and Summer 2005 (Excellent Resource) http://www.ca.uky.edu/gluck/images/info%20pages/EVA%20Is%20the%20Disease%20a%20Cause%20for%20Industry%20Concern.pdf

Link to University of Kentucky Gluck Center www.ca.uky.edu/gluck/index.htm

Texas Animal Health Commission http://www.tahc.state.tx.us/animal_health/diseases/eva/eva.shtml

Utah Department of Agriculture and Food Update November 2006 http://www.ag.state.ut.us/pressrel/EVAUpdate-100606.html

Promed Notice of Outbreak July 2006

http://www.promedmail.org/pls/promed/fp=2400:1202:14095524957900134967::NO::F2400_P
1202_CHECK_DISPLAY,F2400_P1202_PUB_MAIL_ID:X,35264

Promed Update on Outbreak November 2006 http://www.promedmail.org/pls/promed/f?p=2400:1202:14095524957900134967::NO::F2400_P1202_CHECK_DISPLAY,F2400_P1202_PUB_MAIL_ID:X,33526

AQHA Racing Journal, Question and Answer with Dr. Todd Lenz, http://www.aqha.com/aqharacing.com/news/06stories/06evaqandawdrlenz.html

AAEP Handout

http://www.xcodesign.com/aaep/displayArticles.cfm?ID=285

AAEP Recommendations for EVA Vaccination http://www.xcodesign.com/aaep/displayArticles.cfm?ID=36