Equine Viral Arteritis (EVA) is a contagious disease of equids caused by equine arteritis virus (EAV), an RNA virus that is found in horse populations in many countries. While typically not life-threatening to otherwise healthy adult horses, EAV can cause abortion in pregnant mares (and uncommonly cause death in young foals) and establish a long-term carrier state in breeding stallions.
The current licensed vaccine in North America is a highly attenuated, modified live virus product. It has been shown to be safe and effective in stallions and non-pregnant mares. Mild post-vaccinal febrile reactions with transient lymphopenia have been observed in a small percentage of first-time vaccinated horses.
Vaccination is an effective means of protecting against EVA. Like most vaccinations, it does not necessarily prevent viral re-infection and limited viral replication when the horse is exposed to a challenge virus.
However, in first-time vaccinates, the frequency, duration, and amount of vaccine virus that is shed via the respiratory tract is significantly less than that observed with natural infection. Should shedding occur, it takes place during the first week following vaccination.
The occasional stallion may shed very low concentrations of vaccine virus in
its semen for one to three days following first time vaccination.
In planning a vaccination program against EVA, it is important to consult with state and/or federal animal health officials to ensure that any such program is in compliance with the state's control program for EVA, if one exists.
It is not possible to differentiate the vaccine-induced antibody response from that due to natural infection. Negative certification is of importance should a vaccinated stallion be considered for export later. It is strongly recommended therefore that prior to vaccination, serum from all first-time vaccinates be tested and confirmed negative for antibodies to EAV by a USDA-approved laboratory using virus neutralization. Testing should be performed shortly prior to, or preferably at, the time of vaccination. Mares intended for export should be similarly tested.
The indications for vaccination against EVA have been:
1. To protect stallions against infection and the potential development of the carrier state.
2. To protect colt foals against infection and the potential to become carriers after reaching sexual maturity.
3.To prevent outbreaks of abortion by immunizing seronegative mares before being bred to a carrier stallion or with EAV-infective semen.
4. To curtail outbreaks in non-breeding populations.
The purpose of the isolation period is twofold:
1. To enable the vaccinated mare adequate time to develop immunity against the disease before being exposed to EAV infection at time of breeding.
2. To afford ample opportunity for cessation of possible post-vaccinal viral shedding via the respiratory tract.
- Should such mares fail to become pregnant, they can be bred back to a carrier stallion or with infective semen without the need for revaccination or an additional 3-week isolation period post-insemination.
- After being re-bred, such mares should be physically separated from unvaccinated or unprotected horses for 24 hours to avoid possible risk of respiratory transmission from contact with infectious virus in voided semen.
The manufacturer does not recommend use of this vaccine in foals less than 6 weeks of age unless under circumstances of high risk of natural exposure to infection.
Colt (male) Foals
Vaccination and Exporting of Horses: In instances where there is uncertainty or concern over whether vaccination against EVA could prevent the export of a horse to a particular country, it is advisable to consult one of the USDA APHIS VS EXPORT SERVICE CENTERS to determine the specific import requirements of that country. There are several countries that bar entry of any equid that is serologically positive for antibodies to EAV, regardless of vaccination history. Countries that do accept EVA vaccinated horses, regardless of gender, typically require stallions or colts to have a certified vaccination history including confirmation of pre-vaccination negative serological status and annual revaccination at 12-month intervals.