EQUINE | African Horse Sickness
Myocardial Dysfunction in Horses with
AHS – should we be reconsidering our
treatment strategy?
Hewetson, M.a 1 , Shaik, Tb 2
1. Deptartment of Clinical Science and Services, The Royal Veterinary College, North Mymms, Herts, AL9 7TA,
United Kingdom
2. Department of Companion Animal Clinical Studies, Faculty of Veterinary Science, University of Pretoria, Private
Bag X04, Onderstepoort, 0110, South Africa
Background
African horse sickness virus (AHSV) is a lethal non-
contagious insect-borne arbovirus affecting all equids.
AHSV is a double stranded RNA virus that belongs
to the genus Orbivirus within the Reoviridae family.
Nine immunologically different serotypes have been
identified 1 , and the virus shares properties with other
Orbiviruses e.g. bluetongue and equine encephalosis
virus 2 . AHSV is transmitted by Culicoides midges, with
C. imicola and C. bolitinos playing an important role
in Africa 3 .
Of all equids, horses are most susceptible to the disease
with a mortality rate of between 70-95 per cent 2 . The
disease is enzootic throughout sub-Saharan Africa 4 ;
however, it has been reported in North Africa and has
occasionally extended as far North as Spain 2 . African
horse sickness is an OIE notifiable disease, and is
currently considered to pose a significant risk to horse
populations in Europe 5 . Epizootics outside the African
continent are thought to be related to movement of
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infected animals; however, climate change and the
propagation of infected vectors via wind over long
distances cannot be ruled out 6-8 . Clinical signs of the
disease typically appear 5 to 7 days after infection,
and include pyrexia, inappetence, tachycardia,
tachypnoea, haemorrhages and oedema of varying
intensity and location 2 . Four forms of AHS have been
described 4 : acute (pulmonary) form, subacute (cardiac)
form, mixed (pulmonary and cardiac) form and a febrile
form. Horses with the pulmonary form present with per
acute pulmonary oedema, hydrothorax and respiratory
distress; whereas horses with the cardiac form present
with subcutaneous and intramuscular oedema,
hydropericardium and myocardial haemorrhage 9,10 .
The mixed form produces changes characteristic
of both the pulmonary and the cardiac form and is
considered to be the most common form of AHS 2 .
The febrile form of AHS is a mild form of the disease
seen in horses with immunity to one or more serotypes
of AHSV, and is characterised by a pyrexia with no
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