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Equine Herpesvirus (EHV) Infections in Horses

Introduction

Equine Herpesvirus (EHV) infections are a group of highly significant viral diseases affecting horses worldwide. The most important forms include respiratory disease, abortion (commonly called “equine viral abortion”), neonatal death, and neurological disease (Equine Herpesvirus Myeloencephalopathy, EHM). The disease is particularly important in breeding and performance horses due to its contagious nature and potential for outbreaks.

Etiology
Causative Agents

Equine herpesviruses belong to the family Herpesviridae, subfamily Alphaherpesvirinae.

  • Equine Herpesvirus-1 (EHV-1): causes respiratory disease, abortion, neonatal disease, and neurological disease
  • Equine Herpesvirus-4 (EHV-4): mainly causes respiratory disease
Virus Characteristics
  • Enveloped double-stranded DNA viruses
  • Capable of establishing latency in nervous and lymphoid tissues
  • Reactivation occurs under stress
Epidemiology
Species Affected
  • Horses (primary host)
  • Donkeys and zebras (can be infected but less commonly studied)
Transmission
  • Direct contact with nasal secretions
  • Aerosol transmission between horses
  • Contact with aborted fetuses, placental tissues, or contaminated equipment
Risk Factors
  • Stress (transport, training, competition)
  • Mixing of horses from different sources
  • Poor biosecurity in stables and breeding facilities
  • High-density housing
Pathogenesis
Initial Infection

The virus enters via the respiratory tract and replicates in epithelial cells of the upper respiratory mucosa.

Viremia and Spread
  • Virus spreads to lymph nodes and bloodstream
  • Infection of endothelial cells plays a key role in neurological disease
Latency

EHV establishes lifelong latency in trigeminal ganglia and lymphoid tissues, with potential for reactivation and shedding.

Pathogenesis of Disease Forms
  • Respiratory disease: epithelial damage and inflammation of upper respiratory tract
  • Abortion: vascular damage in placenta leading to fetal death
  • Neurological disease (EHM): vasculitis and thromboischemic damage to spinal cord
Clinical Signs
Respiratory Form (EHV-4 and EHV-1)
  • Fever
  • Nasal discharge
  • Coughing
  • Depression
  • Enlarged lymph nodes
Reproductive Form (EHV-1)
  • Late-term abortion (often without prior signs)
  • Stillbirths
  • Weak foals
Neurological Form (EHM)
  • Ataxia (incoordination)
  • Hind limb weakness
  • Urinary incontinence
  • Recumbency in severe cases
Neonatal Disease
  • Severe pneumonia
  • Septicemia-like presentation
  • High mortality
Summary for Practitioners

EHV should be suspected in any outbreak of respiratory disease in stables, sudden abortion storms, or acute neurological disease in horses, particularly following stress or movement events.

Postmortem Findings
Gross Lesions
  • Respiratory tract inflammation
  • Pulmonary congestion in foals
  • Placental lesions (necrosis and edema)
  • Spinal cord hemorrhages in neurological cases
Microscopic Lesions
  • Vasculitis and thrombosis (especially in EHM)
  • Necrosis of endothelial cells
  • Non-suppurative inflammation
Diagnosis
Clinical Diagnosis

Based on clinical signs and herd history (especially recent stress or horse movement).

Laboratory Diagnosis
  • PCR testing (nasal swabs, blood, tissues)
  • Virus isolation
  • Serology (paired samples for rising antibody titers)
Differential Diagnosis
  • Equine influenza
  • Equine arteritis virus
  • Neurological diseases (West Nile virus, rabies)
  • Bacterial abortions (e.g., Leptospira)
Summary for Practitioners

Accurate diagnosis requires laboratory confirmation, especially for neurological and abortion cases, due to overlap with other equine diseases.

Treatment
General Management
  • No specific antiviral cure
  • Supportive care is the main approach
Neurological Cases
  • Anti-inflammatory therapy (NSAIDs, corticosteroids in selected cases)
  • Fluid therapy and bladder management
  • Support for recumbent horses
Infected Horses
  • Isolation
  • Rest and stress reduction
Summary for Practitioners

Early supportive treatment improves outcomes in neurological cases, but prognosis depends on severity of spinal cord damage.

Control and Prevention
Vaccination
  • Vaccines available for respiratory and abortion forms
  • Do not fully prevent infection but reduce severity and shedding
Biosecurity
  • Isolation of new or returning horses
  • Segregation of pregnant mares
  • Disinfection of equipment and stables
Outbreak Management
  • Quarantine of affected premises
  • Restriction of horse movement
  • Monitoring of exposed horses (temperature checks)
Summary for Practitioners

Control depends on strict biosecurity, rapid identification of cases, and vaccination strategies in high-risk populations such as breeding farms and competition stables.

Zoonotic Importance

Equine herpesvirus infections are not zoonotic and pose no direct risk to humans.

Economic Importance

EHV causes significant economic losses due to abortion storms, reduced performance, treatment costs, movement restrictions, and mortality in foals and neurological cases.

Summary

Equine herpesvirus infections are important multi-system diseases of horses, causing respiratory, reproductive, neurological, and neonatal disease. Latency and reactivation make control challenging, requiring strong biosecurity and vaccination programs.