Equine Herpesvirus: Valtrex Treatment Appears Successful

Can antiviral drugs be used to successfully treat horses with equine herpesvirus? Recent research into the drug Valacyclovir (trade name Valtrex) seems to indicate it has a place in treating horses exposed to--and diagnosed with--equine herpesvirus.

Numerous outbreaks of neurological disease due to a variant strain of equine herpesvirus-1 (EHV-1) in the past few years has prompted an increase in related research in equine herpesvirus (EHV). Unfortunately, the outbreaks have also led to increased misinformation regarding the findings and conclusions that have been reached. Some of this research has entailed new approaches to the management and treatment of disease caused by EHV.

Veterinarians discussed acyclovir as a possible treatment option, however the therapy was unsubstantiated for use in horses. Two abstracts presented at the 2005 Forum for the American College of Veterinary Internal Medicine, and subsequently published in veterinary journals, have sought to evaluate the utility of the use of acyclovir for equine herpesvirus by defining the drug's behavior in the horse through pharmacokinetic analysis. Both of these papers concluded that horses could not absorb enough of the drug to be effective against EHV,

Researchers then turned their attention to valacyclovir (Valtrex®), a prodrug (altered to be absorbed intestinally, where it is acted upon by enzymes to produce the active drug, acyclovir).

The initial investigations of valacyclovir in the horse are strongly supportive of much better absorption, and therefore, the ability to attain high enough concentrations in the body to be effective against EHV.

Valtrex has now been utilized successfully and appears to have been helpful in containing and treating many of the recent outbreaks of EHV-1 encephalomyelitis. According to reports, the drug has successfully been used to reduce fever and keep exposed horses from developing neurologic signs, as well as treating neurologic horses, which subsequently improved significantly. Some of these horses had been down and were able to stand again after treatment.

The same investigators are in the planning phases of a challenge study in which the clinical changes in response to various antiviral drugs are compared in horses that have been challenged with herpes virus. More specific information regarding dosages, dosage intervals, herpesvirus sensitivity to the drug, and other antiviral approaches are on the horizon.

About the Author

Bradford G. Bentz, VMD, MS, Dipl. ACVIM, ABVP (equine)

Brad Bentz, VMD, Dipl. ACVIM, ABVP, ACVECC, owns Bluegrass Equine Performance and Internal Medicine in Lexington, Ky., where he specializes in advanced internal medicine and critical care focused on helping equine patients recuperate at home. He’s authored numerous books, articles, and papers about horse health and currently serves as commission veterinarian for the Kentucky State Racing Commission.

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