New Equine Herpesvirus Treatment Option Studied, AAEP 2009

The typical veterinarian's arsenal against equine herpesvirus includes vaccination to prevent the disease and supportive care/antiviral medication when the disease strikes. However, one more weapon might eventually be added to that list, somewhere in between vaccination and treatment: siRNA administration during outbreak conditions.

Small interfering RNA, or siRNA for short, is a Nobel Prize-winning technology that Cornell University researchers have been studying for equine use. At the 2009 American Association of Equine Practitioners Convention, held Dec. 5-9 in Las Vegas, Nev., a study on the efficacy of siRNA treatment against equine herpesvirus-1 (EHV-1) infection was presented by Margaret M. Brosnahan, DVM, MS, Dipl. ACVIM, a post-DVM graduate student at Cornell.

"Complete prevention of EHV-1 infection is ideal, but unrealistic," she commented, noting that the virus can become dormant in the body (latent infection) and reactivate later. Also, immunity from vaccination is very short-lived, making it very hard to completely prevent disease. "So our goals are to reduce viral shedding, minimize disease transmission, reduce viremia (levels of virus in the bloodstream), and reduce the severity of clinical signs."

The treatment strategy used siRNAs targeted against two different EHV-1 genes involved in viral entry and replication. Equine herpesvirus can replicate (reproduce) very quickly, generating large amounts of virus to sicken the horse and spread to others. Thus, inhibiting its replication would logically reduce viral shedding into the environment (and thereby disease transmission), viremia, and the severity of clinical signs of disease.

For the study, 10 horses from 3-18 years of age were given siRNA targeted against EHV-1 intranasally (into the nose) 12 hours before and after the horses received a "dose" of EHV-1 intranasally. Four horses received siRNA directed against a firefly gene as controls.

Unexpectedly, Brosnahan reported, "There was no significant difference in viral shedding, viremia, or initial fever between the two groups." However, clinical signs of disease developed in fewer treated horses (2/10, 20%) compared to controls (3/4, 75%), and the number of horses requiring euthanasia due to intractable neurologic disease was significantly lower in treated horses than in control horses. No treated horses required euthanasia, while three of the controls did.

"We saw a reduction in mortality, which was the ultimate goal, but nasal shedding and viremia were not affected as hypothesized," Brosnahan concluded. "This suggests a much more complicated pathogenesis (disease process) than we currently understand. Research is ongoing."

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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