Study links valacyclovir-heparin use to lower EHM risk
Bottom line
A new JAVMA study suggests that combining valacyclovir with heparin during equine herpesvirus-1 outbreaks may lower the risk of equine herpesvirus myeloencephalopathy, or EHM, and improve survival in exposed horses. The multicenter retrospective study reviewed 111 horses naturally exposed to EHV-1 between August 2017 and March 2018, with infection confirmed by PCR on blood or nasal secretions. The paper adds to a limited treatment evidence base around EHV-1, a virus that can trigger respiratory disease, abortion, neonatal death, and, in some cases, neurologic disease with serious outbreak consequences. (madbarn.com)
Why it matters: For equine veterinarians, the finding is notable because treatment options for preventing EHM during active outbreaks remain limited, and existing consensus guidance has described only moderate evidence for preexposure valacyclovir use. Prior research has suggested valacyclovir can reduce viral replication and lessen neurologic severity, while heparin has been explored because EHV-1–associated vascular injury and thrombosis are thought to contribute to neurologic disease. Together, that makes the combination clinically relevant, especially for outbreak protocols, triage decisions, and discussions with pet parents and facility managers about early intervention. (pmc.ncbi.nlm.nih.gov)
What to watch: Whether these retrospective findings change outbreak guidance, and whether prospective studies confirm which horses benefit most, when treatment should start, and how protocols can be applied in the field. (pmc.ncbi.nlm.nih.gov)
Key facts
- Study type
- Multicenter retrospective study
- Journal
- Journal of the American Veterinary Medical Association
- Population
- 111 horses naturally exposed to EHV-1
- Study period
- August 2017 to March 2018
- Exposure confirmation
- PCR on blood or nasal secretions
- Treatment studied
- Valacyclovir plus heparin
- Main finding
- Lower incidence of equine herpesvirus myeloencephalopathy and better survival
- Disease context
- EHV-1 can cause respiratory disease, abortion, neonatal death, and neurologic disease
A newly published JAVMA study reports that combined treatment with valacyclovir and heparin was associated with a lower incidence of equine herpesvirus myeloencephalopathy and better survival during naturally occurring EHV-1 outbreaks. The study followed 111 horses with PCR-confirmed exposure or infection across outbreaks from August 2017 through March 2018, adding fresh clinical data to a treatment area where evidence has been thin and outbreak management stakes are high. (madbarn.com)
That matters because EHV-1 remains one of the most disruptive infectious threats in equine practice. The virus is widespread in horse populations and can present as respiratory disease, abortion, neonatal death, or neurologic disease. When neurologic cases emerge, facilities can face quarantine, cancellations, transport restrictions, and difficult treatment decisions. AAEP guidance and the updated ACVIM consensus statement both emphasize how quickly EHM concerns can escalate once neurologic signs appear in a group setting. (aaep.org)
The new paper builds on earlier work rather than appearing out of nowhere. Previous experimental and observational studies have suggested that valacyclovir may reduce viral replication and lessen the severity of neurologic signs, especially when started early, though evidence that it prevents EHM outright has been less consistent. Heparin has drawn interest for a different reason: EHV-1 pathogenesis involves endothelial injury, platelet activation, and thrombosis, which may contribute to spinal cord and neurologic damage. Earlier reports and laboratory work raised the possibility that anticoagulation could help limit that cascade. (pmc.ncbi.nlm.nih.gov)
The study itself was retrospective and multicenter, so it doesn't settle causality, but it does offer real-world outbreak data. According to the study summary, horses receiving the combination therapy had a lower incidence of EHM than untreated horses, and early valacyclovir administration before clinical signs, paired with heparin after symptom development, appeared to be the most effective pattern. That aligns with the broader literature suggesting timing matters: antiviral therapy may be most useful before or very early in disease progression, when limiting viremia and viral spread is still possible. (madbarn.com)
Industry and expert guidance have been cautious on this topic, which is part of why the study stands out. The ACVIM consensus statement says there is moderate evidence that valacyclovir given in advance of exposure can limit development of EHM, but it also notes that the treatment literature overall is sparse. A recent systematic review reached a similar conclusion, finding only a small number of pharmacologic studies in horses and limited high-quality evidence across agents, including heparin. In other words, this JAVMA report lands in a field that has been actively looking for practical, evidence-informed outbreak tools. (pmc.ncbi.nlm.nih.gov)
Why it matters: For veterinary professionals, the practical takeaway isn't that valacyclovir plus heparin is now a settled standard of care. It's that the combination may offer a more structured therapeutic approach during confirmed outbreaks, particularly for exposed horses at higher risk of progression. In equine hospitals and ambulatory practice, that could influence how clinicians think about early PCR testing, fever surveillance, case stratification, and when to initiate treatment rather than waiting for neurologic signs to declare themselves. It may also shape conversations with barns, trainers, and pet parents about the value of rapid detection and the limits of current evidence. (aaep.org)
There are still important caveats. Because the study was retrospective, treatment assignment was not randomized, and supportive care, outbreak conditions, and case mix may have varied across sites. That means the results are best viewed as encouraging, practice-relevant evidence, not a final answer. Still, in a disease where neurologic progression can be devastating and where prospective outbreak trials are difficult to conduct, even retrospective multicenter data can carry weight for clinicians making time-sensitive decisions. (madbarn.com)
What to watch: The next questions are whether professional guidance begins citing this study, whether equine referral centers adopt more formal combination-treatment protocols during outbreaks, and whether future prospective studies can clarify dosing, timing, adverse-effect monitoring, and which exposed horses are most likely to benefit. (pmc.ncbi.nlm.nih.gov)