African Horse Sickness

Antigenicity

• 9 serotypes

Hosts

• Clinical: horses, mules, donkeys
• Subclinical: african donkeys, zebras

Pathogenesis

• Virus oscillates between horses and midges
  • Replicates in the salivary glands of biting midges, and occurs where they breed
• Pantropic virus: infects endothelium and myocardium
• Peracute: causes sudden death with pyrexia before the onset of clinical signs
• Two forms:
  • Acute cardac-pneumonic form:
    • Interlobular pulmonary oedema
    • Pericarditis
    • Hemorrhage and oedema of the viscera
    • Death within 5 days
  • Subacute cardiac form:
    • Milder clinical signs but still a high mortality rate

Diagnosis

• Clinical signs
• Differential: Equine Arteritis Virus (EAV)
  • EAV causes oedema in the legs, which AHS does NOT
• Suckling mouse inoculation
• Virus serotype identification
• Serology
• RT-PCR will be used in future outbreaks

Control

Control measures are largely preventative:

• Slaughter or immediate isolation of sick animals
• Mandatory and immediate vaccination with appropriate serotype
• Restricted movement of susceptible animals
  • Protection zones and Surveillance zones set in place around outbreaks
• Control of vector: ectoparasiticides, etc.

Current Relevance

• Once only found in Africa, but since 1987 has recurred in Spain and Portugal
• Not present currently in the UK but global warming may expand midge breeding grounds

Lung oedema in African horse sickness (Image sourced from Bristol Biomed Image Archive with permission)

• Caused by orbivirus, family reoviridae
• Respiratory distress or cardiovascular failure
• Rapid death due to massive pulmonary oedema
• Hydrothorax may also develop
• Large amounts of froth present in airways