Equine viral arteritis: assessing the threat, causes and prevention

LEXINGTON, KY. – The ongoing risk that equine viral arteritis (EVA) poses for American horses, its detection, carriers, possible new strains and the various means of prevention were the subject of a recent DVM Newsmagazine interview with expert Peter Timoney, MVB, PhD, FRCVS, chairman of the Department of Veterinary Science at the University of Kentucky in Lexington. He has spent 24 years studying EVA. We asked him the following questions:

DVM: How significant of a threat do you view equine viral arteritis (EVA) this year? Why?

Dr. Timoney: The primary threat from this virus and the disease it can cause is abortion in the pregnant mare. Specifically, equine arteritis virus (EAV) can cause widespread outbreaks of abortion in unprotected populations of pregnant mares and – unlike outbreaks of equine herpesvirus 1 (EHV-1) – abortion may occur over a very wide range of gestational age of the fetus in utero, from two months through to term.

In mares exposed to EAV very late in gestation, the virus may not cause abortion but rather congenitally infect the fetus, which will be born suffering from a fulminant and rapidly progressive interstitial pneumonia. Such affected foals invariably succumb during the first few days of life.

A second major threat posed by EAV is the ability of the virus to establish persistent infection, or the carrier state, in a variable percentage of infected stallions or sexually mature colts. This carrier rate can vary from less than 10 percent to greater than 70 percent.

The threat of EVA is always present, reflective, in part, of the increased trade in horses and semen imported into the United States without restriction or testing to meet the demand for access to specific bloodlines that they represent. Against this backdrop it's easy to identify why the threat of EAV will continue and even grow.

DVM: How difficult of a diagnosis is EVA in relation to other respiratory pathogens?

Dr. Timoney: EVA is one of the three major viral respiratory pathogens of the horse, together with equine influenza virus, EHV-1 and EHV-4. The clinical signs of EVA are not sufficiently characteristic on which to base a diagnosis of the disease. EVA mimics the clinical syndromes caused by other equine respiratory pathogens. Furthermore, certain other non-respiratory infectious and non-infectious diseases can clinically resemble EVA, such as equine infectious anemia, purpura hemorrhagica and toxicosis due to the plant hoary alyssum (Berteroa incana). Getah virus, a mosquito-borne viral infection exotic to the United States, closely resembles EVA on clinical grounds.

A provisional diagnosis of EVA must be confirmed by testing appropriate specimens by a laboratory proficient and experienced in the diagnosis of this disease. More widespread diagnostic capability exists today than ever before – a significant number (20) of laboratories in the United States are currently approved to carry out the virus neutralization test for this infection.

DVM: Are new strains emerging?

Dr. Timoney: EVA is an RNA virus, so spontaneous mutation occurs much more frequently with it than with DNA viruses. A key factor in the emergence of new strains appears to be long-term viral persistence in the carrier stallion.

Studies have been performed on stallions where the virus sequentially isolated over a significant number of years was found to continue to change over time in those individuals. It is believed that they – carrier stallions – are the primary source of genetic diversity of this virus and of the emergence of more pathogenic strains of EAV that appear from time to time.

DVM: What are your diagnostic recommendations?

Dr. Timoney: In horses exhibiting suggestive clinical signs or in cases of suspect subclinical EAV infection, a non-clotted blood sample should be obtained for virus detection and, if possible, a nasal or nasopharyngeal swab also should be obtained. A clotted blood sample also should be submitted for serologic testing for antibodies to the virus. This acute-phase serum sample should be followed up in 14 days to 21 days with a second (convalescent) sample to demonstrate either seroconversion or a significant (4x or greater) increase in the level of antibodies between the initial acute and subsequent convalescent samples.

In cases of abortion, the fetus and placenta should be sent in a leak-proof container to a qualified laboratory for examination and testing for the presence of EAV. Such materials are highly infectious, so if the fetus is aborted at pasture, any unprotected horses that investigate the abortion will stand a significant risk of becoming infected.

In the case of a stallion being evaluated for presence of the carrier state, a clotted blood sample should be obtained for antibody testing and submitted together with any documentation as to whether the horse was ever vaccinated against EVA. If the blood sample is positive for antibodies to EAV and there is no certified history of vaccination against EVA, there is the potential that the horse could be a carrier.

Semen from the stallion should then be collected, making sure that the sample contains the sperm-rich faction of the ejaculate. That sample should either be frozen or refrigerated and submitted to a competent laboratory for virus detection.

There is evidence that some carrier stallions will cease shedding the virus and cease being carriers. We're not yet sure how this comes about; all carrier stallions should be screened before the start of each breeding season and during the interbreeding season, i.e., approximately every six months, to see if they have ceased to shed virus.

DVM: Could you comment on subclinical infection and identifying carriers?

Dr. Timoney: It's important to appreciate that the majority of naturally acquired cases of EAV infection are subclinical. Clinical cases of EVA – those displaying any or all of a range of signs typically considered characteristic of the disease – represent only the tip of the iceberg of the actual number of horses infected.

Carrier stallions are clinically normal but serologically positive. Being constant semen shedders of the virus, they have been shown to transmit the infection to 85 percent to 100 percent of the mares they are exposed to. If a mare is exposed to EAV, she may exhibit no clinical signs, yet be acutely infected, shedding the virus principally via the respiratory route. Any unprotected, in-contact, pregnant mares run a significant risk of becoming infected and, if so, aborting within 30 days after exposure to the virus.

DVM: With the exception of vaccination, are there other prevention strategies veterinarians should employ?

Dr. Timoney: Outbreaks of EVA invariably are associated with movement of horses and with the use of carrier stallions or virus-infective semen. Perhaps more than any other country in the world, the present volume of horse movement in the United States is enormous.

Ideally, wherever there is considerable movement on or off a premises, horses should be kept isolated for three weeks before they are allowed to commingle with the resident population.

The risk of spread of EVA or other infectious diseases through movement is not unique to this country. The European Union currently comprises more than 25 member states and horses can move freely between member countries consistent with EU policy. Against such a backdrop, it is easy to see how diseases can be spread easily from one member state to another. The time has come to implement a program providing greater prevention and control over EVA.

Observance of sound management practices with an appropriate program of vaccination are the keys to preventing and controlling the disease.

Dr. Peter Timoney

Ms. Wetzel is a freelance writer in Cleveland, Ohio.