FieldDxAfrican horse sicknessClinical signs, differential diagnosis, treatment and prevention in horses.
Overview
A life-threatening hemorrhagic disease of horses and other equids, endemic to sub-Saharan Africa. African horse sickness virus (AHSV), an Orbivirus with nine serotypes, transmitted by Culicoides midges.
Clinical signs
- Incubation period averages 5–7 days; viraemia begins with the fever and persists 2–14 days or longer.
- PERACUTE PULMONARY FORM ("dunkop" = thin head): pyrexia, congestion, severe dyspnoea, and death from fulminant pulmonary oedema. Large volumes of frothy, serofibrinous, blood-tinged fluid discharge from the nostrils.
- SUBACUTE CARDIAC FORM ("dikkop" = thick head): fever and congested mucous membranes initially, followed some days later by marked subcutaneous oedema and swelling of the supraorbital fossae, head and neck, producing a…
- MIXED (ACUTE) FORM: combination of pulmonary and cardiac forms with both lung involvement and visible subcutaneous/supraorbital oedema. Probably the most common form seen in South Africa.
- HORSE SICKNESS FEVER: mild form with fever for a few days plus depression, followed by full recovery. Seen in immunized animals, in resistant species and in some donkeys/zebra.
- General features across forms: pyrexia, oedema of lungs, pleura and subcutaneous tissues, petechiation and widespread haemorrhage. Postmortem findings include large volumes of pleural cavity fluid and pulmonary oedema.
Predisposing factors
- Residence in or movement into endemic areas — sub-Saharan Africa from Senegal east to Ethiopia/Somalia and south to South Africa
- Abundance of the Culicoides spp. biting-midge vector; incidence peaks in late summer rainfall periods. Heavy rainfall followed by hot, dry weather favours outbreaks.
- Species susceptibility: horses are most susceptible (case fatality up to 90% in naive populations); donkeys and mules develop milder disease
- Naive/unvaccinated status — the peracute pulmonary form is seen predominantly in naive animals, including foals that have lost colostral immunity.
Look-alikes to rule out
Treatment
- There is NO specific curative or antiviral treatment for African horse sickness.
- Management is supportive only, directed at cardiac and pulmonary support — strict rest, minimal handling and stress, and good general husbandry.
- Supportive measures may include careful fluid therapy, anti-inflammatory therapy and oxygen supplementation; over-hydration must be avoided because pulmonary oedema is the principal lethal lesion.
- Complete rest is essential; exertion during the febrile/viraemic period markedly worsens the prognosis.
- Prognosis depends on form: pulmonary ("dunkop") approximately 95% fatal, cardiac ("dikkop") approximately 50% fatal, horse sickness fever usually recovers fully.
- Because there is no treatment, effort is directed almost entirely at prevention — vaccination, vector control and movement restriction.
Prevention
1. VACCINATION is the mainstay of control in endemic countries. Commercial live-attenuated (cell-culture attenuated) polyvalent vaccines are available and give good but not absolute protection; annual revaccination is recommended.
Source
A Textbook of the Diseases of Cattle, Horses, Sheep, Pigs and Goat by ELSEVIER Cols D, E, F, G, H added from: Merck Veterinary Manual, "African Horse Sickness", https://www.merckvetmanual.com/generalized-conditions/african-horse-sickness/african-horse-sickness
MSD Veterinary Manual — African Horse Sickness (accessed 2026-07-31)
Condensed and attributed, never reproduced whole. Open the full entry in the app.