New test can identify stallions at greatest risk of becoming carriers of troublesome virus

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The equine arteritis virus persists in horses outside the breeding season through carrier stallions.
The equine arteritis virus persists in horses between breeding seasons through carrier stallions.

American researchers have developed a new genetic test which can identify stallions at greatest risk of becoming long-term carriers of the equine arteritis virus.

Outbreaks of equine viral arteritis can result in significant economic losses to the equine industry due to high rates of foal loss in pregnant mares, the death of young foals, and establishment of the carrier state in stallions, who can spread the virus through their semen.

Most infections are subclinical. The virus was first isolated in 1953, during an outbreak of respiratory disease and abortion on a standardbred breeding farm near Bucyrus, Ohio, in the United States, although the disease had probably existed for many years before that.

In 1984 an outbreak in Kentucky saw the disease rise to global prominence. Infected horses have been recorded in many countries.

Clinical signs of infection are extremely variable. The typical picture is a fever, upper respiratory tract inflammation and discharges, weakness, depression, anorexia, limb swelling, sheath and scrotal swelling in stallions, and abortion in pregnant mares, usually from two months of gestation onwards. However, many cases present with little more than a loss of appetite.

It can also cause severe respiratory disease and inflammation of the small intestine in young foals.

Severity tends to be greater in old, young or debilitated animals.

Udeni Balasuriya.
Udeni Balasuriya. © Matt Barton, UK Ag Communications

While the virus is relatively fragile, it does survive chilling and freezing, so all semen from shedding stallions must be considered infective.

During outbreaks, transmission occurs by the respiratory route via aerosols. Mares are thought to spread the virus for about 14 days after infection and to then establish a lifelong immunity.

In experiments, the virus has been recovered from various sites in infected animals up to 19 days after infection. But it is carrier stallions that play the major role in dissemination and perpetuation of the virus by shedding it in their semen.

The virus is maintained in the equine population between breeding seasons by persisting in these carrier stallions, who may remain shedders for many years.

Now, researchers at the University of Kentucky Gluck Equine Research Center have developed a new test to determine the likelihood of stallions entering into a carrier state, if infected.

Gluck faculty members Professor Udeni Balasuriya, Professor Ernie Bailey and Peter Timoney developed the test to determine the genetic basis of a specific haplotype – a group of genes inherited from one parent.

Stallions possessing the susceptible haplotype, consisting of four specific nucleotide changes in the CXCL16 gene, are more likely to remain long-term carriers of the virus in their reproductive tract than horses that possess the resistant haplotype.

Stallions that are resistant initially shed the virus in their semen following infection but in most cases cleared the virus from their reproductive tract within months following infection. Stallions possessing even one copy of the susceptible haplotype are at greater risk of becoming long-term shedders of the virus.

“Since surgical castration can be resorted to in stallions that are confirmed carriers of equine arteritis virus, this test can help identify those horses that may spontaneously clear themselves of the virus, thus avoiding the loss of a valuable breeding animal,” said Kathryn Graves, director of genetic testing at Gluck Laboratory.

In addition, the test indicates which horses have the susceptible haplotype and therefore are at higher risk for becoming carriers if infected. In these cases, the risk of infection and becoming a carrier can be prevented through vaccination and implementation of appropriate management practices.

Despite the availability of the test, all colts and stallions negative for antibodies to EAV should be vaccinated against the virus in accordance with the manufacturer’s recommendations, irrespective of their genetic makeup for the CXCL16 gene.

The new test, available through Gluck’s genetic testing laboratory, costs $US100. It can be carried out on a mane or tail sample.

David Horohov, who chairs the Department of Veterinary Science and is director of the Gluck Equine Research Center, said it was to gratifying to see that the research had led not only to a better understanding of the nature of the persistence of the disease, but a test to identify those animals at risk for persistent infection.

The work was funded by an Agriculture and Food Research Initiative grant from the US Department of Agriculture.

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