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.

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.