Study links combo therapy to lower EHM risk in EHV-1 outbreaks

A new JAVMA study suggests that combining valacyclovir with heparin may reduce the incidence of equine herpesvirus myeloencephalopathy and improve survival during naturally occurring EHV-1 outbreaks. The retrospective, multicenter analysis included 111 horses exposed to EHV-1 from August 2017 through March 2018, adding practice-level data to a clinical question that has long mattered to equine veterinarians but has had relatively little field evidence behind it. (eurekamag.com)

That matters because EHM is the most feared neurologic manifestation of EHV-1, and outbreak management still depends heavily on early diagnosis, isolation, movement restrictions, temperature monitoring, PCR testing, and supportive care. In its updated consensus statement, ACVIM says treatment of horses with EHM is largely empiric and supportive, and concludes that evidence-based studies on drug treatment remain sparse. The same statement says valacyclovir has better bioavailability than acyclovir, and that preexposure use of valacyclovir may help limit EHM, but it stops short of endorsing post-sign treatment as clearly effective. (aaep.org)

The new report stands out in part because it examines naturally occurring outbreaks rather than only experimental infection models. Earlier work had shown that prophylactic valacyclovir could reduce viral replication, dissemination, and severity of clinical signs in experimentally infected horses, but not fully prevent infection. Separately, heparin has drawn interest because EHV-1-associated neurologic disease is linked to vascular injury and thrombosis, and prior laboratory work showed low-molecular-weight heparin could inhibit EHV-1-induced platelet activation ex vivo. (pmc.ncbi.nlm.nih.gov)

That biologic rationale has been building for years. A Cornell research summary described anticoagulants, including unfractionated heparin and low-molecular-weight heparin, as promising thromboprophylactic candidates after preliminary work suggested they could block EHV-1-induced platelet activation, while antiplatelet drugs such as aspirin and clopidogrel did not. A 2016 case report also raised the possibility of metaphylactic heparin use during an acute outbreak involving a neuropathogenic strain. (vet.cornell.edu)

Even so, the broader literature has stayed cautious. A recent systematic review of pharmacologic interventions for EHV-1 found that most studied treatments showed no benefit or only minimal efficacy, and rated the confidence in evidence supporting heparin’s benefit in naturally occurring infection as very low despite a large observed effect. The 2024 ACVIM consensus statement similarly concluded that there is no evidence that pharmacologic treatments given after onset of clinical signs prevent or alter the course of EHM, while acknowledging moderate evidence for valacyclovir used in advance of exposure. In other words, the new JAVMA study is encouraging, but it lands in a field where recommendations have historically outpaced robust clinical proof. (pmc.ncbi.nlm.nih.gov)

Why it matters: For veterinary professionals, this study may be most useful as a decision-support signal rather than a definitive practice changer. In referral hospitals and outbreak settings, clinicians often have to choose whether to treat exposed horses aggressively before neurologic signs develop. These findings support the idea that targeting both viral replication with valacyclovir and thrombotic or vascular sequelae with heparin could be more effective than relying on antiviral therapy alone. But because the study was retrospective, treatment assignment was not randomized, and outbreak conditions can vary widely, clinicians will still need to weigh timing, case selection, bleeding risk, cost, monitoring burden, and biosecurity logistics before adopting a combination protocol broadly. (eurekamag.com)

The study also arrives at a time when equine practices are still balancing client expectations with imperfect evidence. Pet parents and trainers often want a concrete preventive treatment during EHV-1 scares, yet professional guidance continues to emphasize that no pharmacologic intervention replaces rigorous outbreak control. The ACVIM statement underscores that nasal PCR testing is often the most productive single sampling strategy in horses with fever or suspected EHM when only one sample can be collected, and that outbreak response still starts with surveillance and containment. That means the practical impact of this research may be greatest in structured protocols where testing, isolation, and early therapeutic decision-making happen together. (aaep.org)

What to watch: The next step is likely prospective validation. If investigators can reproduce these results in controlled, ideally randomized studies, combined valacyclovir-heparin therapy could move from promising outbreak strategy to guideline-level recommendation; until then, expect it to remain a closely watched option for high-risk EHV-1 exposure scenarios rather than settled standard of care. (aaep.org)