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Antiviral Drugs for Equine HerpesvirusesBy Kentucky Equine Research Staff · May 8, 2017

You’ve vaccinated your horses, taken appropriate steps to minimize their chances of contracting an infectious disease, yet your desire to show your horse leads to many different venues. Alas, your horse develops a runny nose, depression, anorexia, and a high fever consistent with equine herpesvirus-1 (EHV-1) infection. After the barn has been quarantined, what options do you have in terms of treatment?

“Studies show that antiviral medications, such as those used in human HIV patients, have potential for EHV infections, including EHV-1 that causes upper respiratory tract infections as well as equine herpesvirus-associated myeloencephalitis (EHM),” relayed Kathleen Crandell, Ph.D., a nutritionist with Kentucky Equine Research (KER).

A recent review article* on EHV infections discussed the pros and cons of using antiviral medications in horses. Here is what you need to know about the popular antiviral drug, acyclovir, and other possible treatment options:

  • Acyclovir is an antiviral medication, not an antibiotic medication that fights infections caused by bacteria;
  • Antibiotics are not effective against EHVs;
  • Acyclovir and related antiviral drugs have been used both prophylactically (prior to exposure to an EHV) to prevent disease and after a horses has been diagnosed with an EHV infection;
  • Despite demonstrating effectiveness against EHVs in laboratory settings, acyclovir actually “behaves” differently in live horses. Acyclovir is poorly absorbed after administration by mouth prompting the researchers to write that “oral acyclovir is unlikely to effectively treat or prevent EHM in adult horses.”
  • Despite the lack of clinical efficacy of acyclovir demonstrated in multiple studies, the excellent safety profile and low cost associated with oral treatment often provides sufficient impetus for veterinarians and owners to treat affected horses; and
  • Intravenous administration appears more effective and successful but is much more expensive: approximately $250 per day when administered IV compared to $11 per day when administered orally.

Considering the potential for acyclovir to help horses infected with EHV, including EHM that currently available vaccines do not protect against, studies are ongoing.

“In addition to the traditional supportive care measures, such the anti-inflammatory drugs flunixin, dimethyl sulfoxide, and dexamethasone, don’t forget that omega-3 fatty acids also possess anti-inflammatory properties,” said Crandell.

She added, “KER’s palatable EO•3 contains marine-derived fish oils, including DHA and EPA, that is top-dressed onto the feed.”

For horses displaying neurologic signs, additional feed management may be necessary. Get tips on feeding horses with neurologic disease.

*Maxwell. L.K. Antiherpetic drugs in equine medicine. 2017. Veterinary Clinics of North America. Equine Practice. 33(1):99-125.