EQUINE | CPD Article
African Horse Sickness
Contributor(s): Christopher Brown, Rachael Conwell, Melissa Kennedy, Timothy Mair, Graham Munroe,
Veronica Fowler
Introduction
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A non-contagious infection caused by African Horse
Sickness (AHS) virus (AHSV) usually causing acute
and often fatal disease in horses.
Notifiable in the USA and the EU, OIE-listed disease.
Incidence: transmitted by night-flying insect
vectors; primarily seen in sub-Saharan Africa. The
disease has both seasonal (late summer/autumn)
and cyclical incidence (warm-phase events).
Signs: there are four classical clinical forms of AHS;
○ ○ Pulmonary: the pulmonary form is periacute
and occurs in fully susceptible animals with
death usually occurring in a few hours. Clinical
signs include: respiratory distress, sweating,
extension of head and neck, froth exudes from
the nostrils.
○ ○ Cardiac: the cardiac form is subacute with
mortality around 50%. Clinical signs include:
swelling of neck, suborbital fossae and
conjunctiva. Paralysis of the esophagus can
occur resulting in aspiration pneumonia.
○ ○ Mixed: clinical signs of both the cardiac and
pulmonary forms are present with mortality
around 70%.
○ ○ Horse sickness fever: clinical signs are mild,
often only involving transient fever. Most often
observed in zebra and donkeys
Diagnosis: virus isolation, serology, RT-PCR, RT-
LAMP. AHS can be confused with encephalosis,
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equine infectious anemia, equine morbillivirus
pneumonia, equine viral arteritis, babesiosis and
purpura hemorrhagica so laboratory confirmation
is essential.
Treatment: no specific therapy. There are
attenuated (monovalent and polyvalent) vaccines
commercially available.
Prognosis: poor to grave - mortality is related to
the species of equidae affected and the serotype/
strain of the virus. Horses are most susceptible (50-
95% mortality), followed by mules/hinnys (50%
mortality). Donkeys and zebra are very resistant
in enzootic regions but can be susceptible (10%
mortality) in European and Asian countries.
Presenting signs
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High fever.
Extensive edema - especially of head and neck and
orbital fossae.
Progressive lung edema.
Respiratory distress.
Froth from nostrils.
Paralysis of the esophagus.
Pneumonia.
High mortality rates (depending on species of
equidae).
• Equine Health Update •