Combined treatment with valacyclovir and heparin reduced the incidence of equine herpesvirus myeloencephalopathy and improved survival in naturally occurring equine herpesvirus-1 outbreaks.

Overview

• This study evaluated the effects of combined valacyclovir and heparin treatment on horses naturally exposed to equine herpesvirus-1 (EHV-1), focusing on the incidence of neurological disease (equine herpesvirus myeloencephalopathy, EHM) and survival outcomes.
• The findings suggest that early and combined administration of these medications reduced the rate of EHM and increased survival in affected horses.

Background and Study Design

• This was a multicenter retrospective study covering August 2017 to March 2018 involving 111 horses exposed to EHV-1.
• EHV-1 infection was confirmed through PCR testing of blood or nasal secretions.
• Horses exhibiting neurological signs were diagnosed with EHM.
• The study recorded clinical signs, treatments administered, and survival outcomes.
• A subset of these horses was treated with valacyclovir (an antiviral) and heparin (an anticoagulant), with administration categorized as either prophylactic or metaphylactic:
  • Prophylactic: medication given before any EHV-1 clinical signs appear (e.g., fever > 38.6 °C, limb edema, nasal discharge, neurological deficits).
  • Metaphylactic: medication given after clinical signs onset but before severe disease progression.

Treatment Groups and Outcomes

• Total horses diagnosed with EHV-1: 22
• Number developing EHM (neurological complications): 9
• Treatment breakdown:
  • 14 horses received metaphylactic valacyclovir combined with heparin; 5 of them developed EHM.
  • 5 horses received prophylactic valacyclovir and metaphylactic heparin; only 1 developed EHM.
  • 4 horses received no valacyclovir or heparin; all developed severe neurological symptoms (grade 5/5 ataxia) and were euthanized.

Key Findings

• All horses treated with valacyclovir and heparin survived, indicating a clear survival benefit of the combined treatment.
• The incidence of EHM was notably lower in horses receiving the combination therapy compared to untreated horses.
• Early administration of valacyclovir (before clinical sign onset) combined with heparin administration after symptom development seems to be most effective in reducing neurological disease risk and mortality.

Conclusions and Implications

• Combined treatment with valacyclovir and heparin reduces EHM incidence and improves survival outcomes in horses naturally exposed to EHV-1.
• Timing of medication administration is crucial: prophylactic valacyclovir may help prevent symptom onset, and metaphylactic heparin can stabilize or improve outcomes after clinical signs appear.
• This treatment approach can be considered during EHV-1 outbreaks to reduce severe neurological disease and death in horses.
• Further controlled studies may help refine dosing and administration protocols for optimal clinical use.