Imagine the following telephone conversation. "Yes, I'd like to order some disease, please."

"Yes, ma'am. We can get some disease right out to you. When do you want your disease delivered?"

"The sooner, the better!"

"We'll overnight that disease for you to get tomorrow. Is there anything else?"

"No, that order of disease will take care of me for now. I'll probably be calling for more disease in a few weeks for some other horses. Thanks!"

Seem a bit far-fetched? Not really. Just substitute the word semen every place you see the word disease and you are hearing a conversation that takes place hundreds of times a day during the breeding season.

One of the most problematic diseases, with the most potential to be shipped along with semen from anywhere in the world, is equine viral arteritis (EVA). Arteritis is a highly contagious viral disease that can cause fever, respiratory illness, ocular inflammation, edema (swelling; especially of the limbs), birth of weak or sick foals, and abortion. It can be transmitted through the respiratory route or venereal route (through natural cover or artificial insemination). That means it can cause a flu-like disease in competition or non-breeding animals, and that same virus can cause abortion in mares. The arteritis virus also can cause some horses to develop an immune response (have antibodies), but they never have any obvious clinical signs.

One of the scariest problems for breeders is that the virus can become established in sexually mature males, causing them to shed the virus in their semen for years. It is estimated that the carrier rate among seropositive stallions is nearly 44%, based on screening done at the Gluck Center of all breeds of stallions.

EVA is a very manageable disease, noted Peter Timoney, FRCVS, PhD, head of the Gluck Equine Research Center in Kentucky and one of the world leaders in arteritis research.

Timoney also noted that the carrier state has never been detected in a seronegative stallion or in a stallion vaccinated with the modified live virus vaccine (produced by Ft. Dodge Animal Health).

Outbreaks of EVA have occurred at racetracks, sales, and horse shows, as well as on breeding farms. In fact, venereal transmission by the carrier stallion, either by natural service or AI, has been shown to be the most important means of persistence and disemination of the virus in various horse populations throughout the world. (See information on EVA from the AAEP Infectious Diseases Committee, AAEP Wrap-up.)

The risk of spread of equine arteritis virus on a national and international level has been enhanced in recent years by the widespread acceptance and use of artificial insemination and by the increasing number of stallions used for dual-hemisphere breeding. If uncontrolled, either practice could lead to the indavertent introduction of this virus into susceptible populations of horses. Measures to circumvent this risk currently are being included in the import policies of nearly all of the major horse breeding countries of the world.

However, the United States has no regulations regarding testing of imported semen--or semen shipped from state to state--for any diseases, including EVA. Documented incidents of abortion from arteritis virus have occurred on farms in the United States traced back to infected imported semen.

According to Timoney,"We know a great deal about the biological characteristics of the causal virus and the epidemiology of the disease, and we have had a safe and effective modified live virus vaccine against the disease available for a significant number of years," said Timoney. "Collectively, these factors have permitted the formulation of effective programs for prevention and control of EVA."

Basically, the vaccine given to mares 21 days prior to breeding allows them to develop antibodies to the arteritis virus and prevents them from getting the disease. Vaccinating seronegative stallions (those which have not had the disease or developed antibodies to the virus) will prevent them from getting the disease and becoming shedders of the virus.

Guidelines Established

Timoney headed a group established by the American Horse Council (AHC), and backed by the AAEP, that developed guidelines to offer breeders and others an approach to preventing EVA (see guidelines page 42). The goal of the group was to develop a voluntary, industry-driven protocol to assist stallion owners in preventing establishment of the carrier state in stallions and minimize the risk of EVA-related abortion in mares. The guidelines also could serve as a basis for establishing controls over the importation of EAV-infected carrier stallions and infective semen.

"Because of the potential economic threat posed by EVA, it is highly advisable that breed registries recommend that this protocol be adopted by breeders as part of good breeding practice," noted the AHC in an address on this issue. "The protocol provides a practical, realistic, and unified approach that permits the continued use of carrier stallions or infective semen. Some breed registries already have taken such an initiative."

The North American Department of the Warmblood Studbook of the Netherlands instituted a requirement in 1996 that in order for foals to be eligible for registration, each stallion must be tested for EVA and the results published in the annual directory. Owners are provided with a protocol that allows for the continued use of carrier stallions and effectively protects mares against the disease. Although the protocol was initially implemented on a voluntary basis, membership response for the initiative was so favorable that it became a mandatory program.

As an additional protection, the Registry requires frozen semen imported into the United States for resale in this country be tested for EAV at the Gluck Center.

Kentucky and New York also have regulations in place for Thoroughbreds regarding protocols of breeding shedding stallions.


EVA Guidelines

In June of 1996, the American Horse Council (AHC) was asked to address the issue of equine viral arteritis (EVA) because of its continuing economic impact on U. S. horse owners. The AHC established a working group to develop an industry-driven control program for EVA that would include a protocol for identifying and managing sources of equine arteritis virus (EAV) infection by way of carrier stallions or virus infective semen.

Representatives of the following breed organizations were appointed to the AHC working Group on EVA: the American Quarter Horse Association, the U. S. Trotting Association, the Tennessee Walking Horse Breeders & Exhibitors Association, the International Arabian Horse Association, the Appaloosa Horse Club, the Federation of North American Sport Horse Registries, and The Jockey Club.

In addition, the group included Peter Timoney, FRCVS, PhD, head of the Gluck Equine Research Center in Kentucky; Ralph Knowles, DVM, from the Maryland Department of Agriculture, chairman of the U.S. Animal Health Association's Committee on Infectious Diseases of Horses; Dr. Don Lein, Director of the Veterinary Diagnostic Laboratory at Cornell University; Don L. Notter, DVM, Kentucky State Veterinarian; and Amy Mann of the American Horse Council.

The goal of the working group was to develop a voluntary, industry-driven protocol to assist stallion owners in preventing establishment of the carrier state of this disease in stallions and minimize the risk of EVA-related abortion in mares. The protocol also serves to limit the liability associated with the use of stallions which shed equine arteritis virus or infective semen. The protocol is a basis for establishing controls over the importation of EAV carrier stallions and infective semen.

EVA is a contagious viral disease of members of the horse family that is primarily spread by direct contact with horses acutely infected with equine arteritis virus. While it is considered a respiratory infection of the horse, the disease can be spread through breeding a carrier stallion to a susceptible mare.

The relative increase in the number of confirmed outbreaks of EVA in horses in the United States in recent years has begun to concern horse owners, notwithstanding the fact that from the medical perspective, EVA is not regarded as a disease of major significance. Horse owners need to be provided with a protocol that will help ensure protection of their horses from EVA and prevent problems that could result from the use of carrier stallions or infective semen.

Other matters that need to be addressed include the liability of owners whose stallions shed EVA, and the problems associated with the movement of carrier stallions or virus-positive semen at the national and international level.

Following is the protocol developed by the working group to help horse owners identify carrier stallions and EVA infective semen so that either can be used without attendant risk of the occurrence of EVA. Adoption of the protocol would greatly lessen the likelihood that the disease would be driven underground, which, were it to occur, would greatly hinder horse owners being able to protect their horses effectively against this disease.

Because of the potential economic threat posed by EVA, it is highly advisable that breed registries recommend that the protocol be adopted by breeders as part of good breeding practice. The protocol provides a practical, realistic, and unified approach that permits the continued use of carrier stallions or infective semen.

Questions regarding the protocol should be directed to Amy W. Mann at the AHC offices, 202/296-4031.


 Guidelines For Breeding A Mare To An Equine Arteritis Virus Shedding Stallion

At least 30 days prior to breeding, the mare should be tested for serum neutralizing antibodies to equine arteritis virus. A blood sample should be submitted to a veterinary medical diagnostic laboratory approved by the USDA to conduct this serological test. Based on that result the following procedures are recommended.

NON-PREGNANT MARES WHICH ARE ANTIBODY NEGATIVE (TITER OF LESS THAN 1:4)

If the mare is found to be serologically negative, she should be vaccinated as soon as possible with the licensed modified live virus vaccine against EVA. After vaccination, the mare should be isolated for 21 days to allow her time to develop adequate protective immunity against subsequent exposure to the virus and to prevent the minimal risk of spread of the vaccine virus to any susceptible horses with which she might come into contact.

Twenty-one days following vaccination, the mare may be bred to a shedding stallion. She should not be bred to a shedding stallion during that period.

After being bred for the first time to a shedding stallion, the mare should be isolated for 21 days from any horses on the premises serologically negative for antibodies to the virus. Subsequent breedings do not require an additional period of isolation.

Occasionally a mare may be vaccinated against EVA, but for some reason, is not bred that year to a shedding stallion. If this should happen, the mare should be vaccinated again before being bred to a shedding stallion. No isolation is necessary following re-vaccination.

PREGNANT MARES WHICH ARE ANTIBODY NEGATIVE (TITER OF LESS THAN 1:4)

The current licensed modified live virus vaccine against equine viral arteritis is not approved for use in pregnant mares. While a mare that is in good health might be vaccinated following parturition, a mare that has had a complicated foaling, or is otherwise not in good health, should not be vaccinated until she has regained her health. The foal should also be in good health and be at least two weeks old before its dam is vaccinated.

There is minimal risk that suckling foals out of serologically negative mares might be exposed to the vaccine virus when the mare is vaccinated against EVA.

RE-VACCINATION

Mares that will be bred to shedding stallions should receive an annual booster vaccination against EVA 21 days prior to being used for breeding purposes. No isolation is necessary following re-vaccination.

MARES WHICH ARE ANTIBODY POSITIVE (TITER OF 1:4 OR GREATER)

Mares that test serologically positive for antibodies to equine arteritis virus can be bred to a shedding stallion without the need for prior vaccination against EVA. Antibody positive mares that are bred to a shedding stallion by natural cover should be kept separate from other susceptible horses for 24 hours to avoid possible mechanical transmission of virus from voided semen. Any vehicle used to transport such mares immediately following breeding to a shedding stallion should be thoroughly cleaned and disinfected prior to transport of susceptible horses.


Guidelines For Breeding Stallions

Prior to the breeding season (at least 60 days is recommended), the stallion should be blood tested for neutralizing antibodies to equine arteritis virus.

ANTIBODY NEGATIVE (TITER OF LESS THAN 1:4)

If serologically negative, the stallion should be vaccinated with a licensed modified live vaccine against EVA and isolated for 30 days after vaccination. An annual booster vaccination against EVA should be given on a regular basis every 12 months, but no sooner than 30 days prior to being used for breeding.

ANTIBODY POSITIVE (TITER OF 1:4 OR GREATER)

If the stallion is found serologically positive for serum neutralizing antibodies to equine arteritis virus, without written evidence certifying his negative serological status prior to vaccination, he needs to be tested for presence of the carrier (shedding) state. This can be determined by either one of the following methods:

  • Attempted isolation of equine arteritis virus from two separate ejaculates collected and submitted by an accredited veterinarian to a laboratory approved by the USDA to conduct this test; or
  • Test breeding the stallion to two mares serologically negative for antibodies to equine arteritis virus at least twice on each of two consecutive days (four covers) and the mares checked for the development of serum antibodies to the virus 28 days after breeding.

NON-SHEDDING STALLIONS WHICH ARE ANTIBODY POSITIVE

Serologically positive stallions with written certification of negative antibody status prior to vaccination against EVA by a USDA-approved laboratory need not be tested for virus shedding.

Stallions serologically positive for antibodies to equine arteritis virus from natural exposure that have previously been tested and found to be non-shedders (non-carriers) of the virus should have written confirmation of their non-shedder status and receive an annual booster vaccination against EVA.

SHEDDING STALLIONS WHICH ARE ANTIBODY POSITIVE

Shedding stallions can be used for commercial breeding provided they are managed in accordance with the above guidelines. Stallion owners and stallion managers should disclose the shedding status of their stallions to mare owners, breed associations, and, where required, state authorities. Shedding stallions can be bred safely to adequately immunized mares or to mares that have tested serologically positive for neutralizing antibodies to equine arteritis virus.

Occasionally, shedding stallions will spontaneously stop shedding equine arteritis virus. Owners may wish to retest the semen of shedding stallions from time to time to determine if the stallion is still shedding virus.


 Other Recommendations

TEASER STALLIONS

Teaser stallions should be vaccinated against EVA on an annual basis in accordance with this protocol.

IDENTIFICATION OF CARRIER (SHEDDING) STALLIONS

It is recommended that breed associations publicly disclose the names of those stallions registered with their breed association that are confirmed shedders of equine arteritis virus.

PREVENTION OF THE CARRIER STATE

Breeding stallions that are found serologically negative for antibodies to equine arteritis virus should be vaccinated against EVA to prevent development of the carrier state.

In order to prevent the carrier (shedding) state, especially in those breeds in which the infection is widely prevalent, as well as to prevent equine arteritis virus infection, colts under 270 days of age that are serologically negative for antibodies to equine arteritis virus should be vaccinated against EVA. Written certification of their negative serological status to equine artertitis virus should be obtained before vaccination.

USE OF MODIFIED LIVE VACCINE AGAINST EVA

It is essential to have written official certification of a horse's negative serological status to equine arteritis virus prior to initial vaccination against this disease.

Stallions, and mares that will be bred to shedding stallions, should receive an annual booster vaccination against equine arteritis virus prior to being used for breeding purposes.

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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