Menu

    1. Home
    2. About
    3. Learning Hub
    4. Clinical Reference
    5. Resources
    6. Contact

Equine Influenza (Orthomyxovirus Infection) in Horses

Introduction

Equine Influenza is a highly contagious viral respiratory disease of horses caused by influenza A viruses. It is characterized by rapid spread within susceptible populations, especially in stabled horses, and causes significant economic losses in the equine industry due to disruption of training, competition, and transport activities.

Etiology
Causative Agent

Equine Influenza is caused by Influenza A virus, belonging to the family Orthomyxoviridae.

Virus Characteristics
  • Enveloped, segmented negative-sense RNA virus
  • High mutation rate (antigenic drift)
  • Surface glycoproteins: hemagglutinin (H) and neuraminidase (N)
  • Main equine strains: H7N7 (largely extinct) and H3N8 (currently dominant)
Epidemiology
Species Affected
  • Horses (primary host)
  • Donkeys and mules (less commonly reported)
Transmission
  • Aerosol transmission via coughing and sneezing
  • Direct contact between horses
  • Fomites (tack, equipment, handlers, transport vehicles)
Risk Factors
  • High-density housing (stables, racetracks, events)
  • Horse movement and transport
  • Poor ventilation
  • Inadequate vaccination coverage
Outbreak Characteristics

Equine influenza outbreaks spread rapidly and may affect a large proportion of susceptible horses within days.

Pathogenesis
Initial Infection

The virus enters via inhalation and infects epithelial cells of the upper respiratory tract.

Replication
  • Rapid viral replication in tracheal and bronchial epithelium
  • Destruction of ciliated epithelial cells
Effects on Respiratory Defense
  • Impaired mucociliary clearance
  • Increased susceptibility to secondary bacterial infections
Systemic Response

Inflammatory cytokines lead to fever and systemic illness.

Clinical Signs
Acute Respiratory Signs
  • Sudden onset of high fever (up to 41°C)
  • Dry, harsh cough
  • Serous nasal discharge progressing to mucopurulent
  • Depression and lethargy
Additional Signs
  • Loss of appetite
  • Enlarged submandibular lymph nodes
  • Exercise intolerance
Complications
  • Secondary bacterial pneumonia
  • Prolonged recovery in young or stressed horses
Summary for Practitioners

Equine influenza should be suspected in any rapidly spreading respiratory disease outbreak in horses, especially following movement or events. Strict isolation is essential during outbreaks.

Postmortem Findings
Gross Lesions
  • Tracheitis and bronchitis
  • Catarrhal exudate in airways
  • Pulmonary congestion in severe cases
Microscopic Lesions
  • Necrosis and loss of respiratory epithelium
  • Inflammatory infiltration
  • Loss of cilia in airway lining
Diagnosis
Clinical Diagnosis

Based on rapid spread, fever, and respiratory signs in a group of horses.

Laboratory Diagnosis
  • RT-PCR for viral RNA detection
  • Virus isolation from nasal swabs
  • Serology (paired serum samples for rising antibody titers)
Differential Diagnosis
  • Equine herpesvirus respiratory infection (EHV-1/EHV-4)
  • Equine arteritis virus
  • Strangles (Streptococcus equi)
  • Bacterial pneumonia
Summary for Practitioners

Laboratory confirmation is important, especially in outbreak situations where multiple respiratory pathogens may be involved.

Treatment
General Approach

No specific antiviral therapy exists for equine influenza.

Supportive Care
  • Rest (minimum 2–3 weeks depending on severity)
  • Anti-inflammatory drugs (NSAIDs)
  • Good ventilation and dust-free environment
Secondary Infection Control
  • Antibiotics if bacterial pneumonia develops
Summary for Practitioners

Rest is critical to prevent complications such as chronic respiratory disease. Premature return to work can prolong illness.

Control and Prevention
Vaccination
  • Routine vaccination is essential in performance and breeding horses
  • Requires regular boosters due to antigenic drift
Biosecurity
  • Isolation of new or returning horses
  • Quarantine during outbreaks
  • Disinfection of equipment and transport vehicles
Outbreak Management
  • Movement restrictions
  • Separation of affected and healthy horses
  • Monitoring of temperature and clinical signs
Summary for Practitioners

Vaccination combined with strict biosecurity is the most effective strategy for preventing outbreaks in high-density equine populations.

Zoonotic Importance

Equine influenza is not considered zoonotic and does not infect humans.

Economic Importance

The disease causes major economic losses due to disruption of equine sports, training downtime, treatment costs, and movement restrictions during outbreaks.

Summary

Equine influenza is a highly contagious viral respiratory disease of horses caused by an orthomyxovirus. It spreads rapidly in susceptible populations and requires strict biosecurity and vaccination programs for effective control.