Scientists Study 'Silent' EIA

In the study, blood tests revealed that 26 of apparently healthy horses were EIA-positive. However, 18 of the 33 horses that tested negative on blood tests were EIA-positive via molecular testing.

Photo: Erica Larson, News Editor

Why do horses that are seropositive (have a positive blood test) for equine infectious anemia virus (EIAV) pass the virus on to some nearby horses but not others?

Actually, they might be passing it on more than we think. Recent study results suggest that horses can carry the virus for up to two years without being seropositive on a Coggins test, ELISA, or Western Blot blood test.

This “silent” form of EIAV might sound terrifying, but researchers found silent cases (horses infected without seroconversion, or positive blood samples) only on farms where seropositive horses lived together with seronegative horses over a long period.

“I do not believe we would find the same situation if there were only seronegative horses,” said researcher Adriana Soutullo, PhD, of the Laboratory of Agricultural Diagnostics and Research in the Ministry of Production of the Santa Fe Province, and of the Immunology Laboratory at the Faculty of Biochemistry and Biological Sciences at Litoral National University, both in Argentina.

Soutullo collaborated with a team of North and South American researchers, including Sheila Cook, PhD, and Frank Cook, PhD, of the University of Kentucky Gluck Equine Research Center, in Lexington.

Seropositive horses seem to be able to transmit the virus to other horses without making those horses’ blood samples turn positive in standard tests, she said. Those “seronegative” horses might eventually seroconvert into seropositive horses—as long as two years later.

In their study, Soutullo and colleagues conducted EIAV testing on 59 apparently clinically healthy horses of various breeds from five stables in Argentina’s Santa Fe area. They used two kinds of tests: blood tests and molecular tests.

The blood tests (AGID and ELISA tests) revealed that 26 of these apparently healthy horses (44%) were positive for the EIA virus, Soutullo said.

However—and to the researchers’ surprise—18 of the 33 remaining, seronegative horses (55%) tested positive in molecular testing (PCR), she added. Over the next two years, the scientists continued to test these seronegative horses regularly. Some seroconverted during this time, but others did not. More in-depth testing confirmed that these horses did, in fact, harbor EIAV DNA sequences in their bodies. In other words, they carried the EIA virus without developing the changes in the blood that would create the markers that cause positive blood tests.

“What we were really looking for was an explanation about why some horses didn’t become infected with EIAV after living with seropositive horses—and what we found was that, in fact, they were infected, but just silent,” Soutullo said.

“But our study results shouldn’t cause owners to worry,” she explained. “We don’t know if these horses have sufficient virus loads to allow transmission of the infectious disease. I think that the horses that are most likely to transmit the disease are those in the acute stage.”

Soutullo said the most common situation is that horses might show up as seronegative by the AGID blood test, but only during the first two months or so of infection. Afterward, they should be seropositive by AGID.

Owners and practitioners can limit risks of EIAV spread mainly by good hygiene during veterinary care, Soutullo added.

“The principal recommendation I can make is to always change the needle and syringe when performing injections,” she said.

The study, “Serologically silent, occult equine infectious anemia virus (EIAV) infections in horses,” was published in Veterinary Microbiology

About the Author

Christa Lesté-Lasserre, MA

Christa Lesté-Lasserre is a freelance writer based in France. A native of Dallas, Texas, Lesté-Lasserre grew up riding Quarter Horses, Appaloosas, and Shetland Ponies. She holds a master’s degree in English, specializing in creative writing, from the University of Mississippi in Oxford and earned a bachelor's in journalism and creative writing with a minor in sciences from Baylor University in Waco, Texas. She currently keeps her two Trakehners at home near Paris. Follow Lesté-Lasserre on Twitter @christalestelas.

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