Evaluation of the prophylactic use of valacyclovir in horses to ameliorate the effects of neurologic equine herpesvirus type 1 (equine herpesvirus myeloencephalopathy or EHM) was undertaken in a study featuring a number of equine clinicians around the country [Maxwell, L.K.; Bentz, B.; Gilliam, L.L.; Ritchey, J.W.; Pusterla, N.; Eberle, R.; Holbrook, T.C.; McFarlane, D.; Rezabek, G.B.; Whitfield, C.; Goad, C.L.; and Allen, G.P. Efficacy of the early administration of valacyclovir hydrochloride for the treatment of neuropathogenic equine herpesvirus type 1 infection in horses. American Journal of Veterinary Research, Oct 2017, Vol 78, no. 10].
Three different groups (six each) of horses were assigned to the study:
- a control group receiving a placebo;
- a treatment group given valacyclovir at the detection of fever; and
- a prophylactic treatment group given valacyclovir to begin one day prior to inoculation with the neuropathogenic strain of herpesvirus.
The horses, all mares more than 20 years of age, underwent two separate experiments separated by a six-week interval. The first group of nine (three in each group) received oral valacyclovir or placebo for one week after viral inoculation; a second round of the other nine horses then received treatment or placebo for two weeks. Those receiving valacyclovir were given an oral loading dose of 27 mg/kg every eight hours for two days, followed by a maintenance dose of 18 mg/kg orally every 12 hours.
The study reported that horses receiving acyclovir ended up with less viral shedding and viremia as compared to the control group. Similarly, horses receiving prophylactic valacyclovir for a two-week period had lower rectal temperatures, improved clinical scores (respiratory rate, heart rate and nasal discharge), and more decreased viremia than control horses.
The authors further stated that while the risk of ataxia wasn’t affected by treatment, the severity of ataxia was decreased in valacyclovir-treated horses compared to control horses.
Conclusions of the study are encouraging for managing herpesvirus, particularly in a barn where horses have been exposed, but are not yet sick. The study results stated: “Valacyclovir treatment significantly decreased viral replication and signs of disease in EHV-1-infected horses; effects were greatest when treatment was initiated before viral inoculation, but treatment was also effective when initiated as late as two days after inoculation.”