AFRICAN HORSE SICKNESS
Camilla T. Weyer
Alan J. Guthrie
Equine Research Centre, Onderstepoort, South Africa
African horse sickness (AHS) is an infectious but non-contagious viral disease
of equines transmitted by Culicoides midges. AHS has an extremely high
mortality (>70%) in horses. The disease is manifested by pyrexia and clinical
signs and lesions compatible with impaired circulatory and respiratory function
that are characterized by subcutaneous, intermuscular and lung edema, fluid
leakage into the body cavities, and hemorrhages of serosal and mucosal
surfaces.
AHSV is classified in the genus Orbivirus in the family Reoviridae. It has
similar morphology and shares many properties with other Culicoidestransmitted orbiviruses such as bluetongue (BTV), equine encephalosis (EEV)
and epizootic hemorrhagic disease (EHDV) viruses. There are 9 serotypes of
AHSV.
EPIDEMIOLOGY
AHS is endemic in sub-Saharan Africa. In subtropical regions, including South
Africa, AHS is strictly seasonal with the first cases historically occurring in late
summer and an abrupt disappearance following the onset of colder weather in
the fall. Recently a change in this pattern has been seen, with cases starting
in the endemic areas as early as December and January. There is still an
abrupt decrease in the number of cases in the winter months, but certain
provinces such as the Eastern Cape have isolated cases throughout the
winter months as well. This is most likely due to the milder winters and the
lack of heavy frosts, especially in the coastal regions.
The area around the Cape of Good Hope in South Africa has historically been
free from AHS, with outbreaks due to the introduction of AHSV-positive horses
from other provinces. In an effort to maintain this area as an AHS free zone
for export purposes, movement control of equids into the area was
implemented by the South African State Veterinary Authorities since 1997.
The Western Cape is currently divided into 3 zones; the Metropolitan Cape
Town AHS free zone, the AHS surveillance zone, and the AHS protection
zone (Figure 1). Horses moving into the AHS Controlled Area in the Western
Cape Province from other provinces must have been vaccinated with the
Onderstepoort Biological Products (OBP) polyvalent vaccine a minimum of 40
days prior to movement, a veterinarian must certify them healthy a minimum
of 48 hours prior to movement, and a movement permit supplied by the State
Veterinarian of their area must accompany them. Even with the current
movement controls in place, AHS outbreaks have occurred in the AHS
Controlled Area.
15-‐18
February
2016
East
London
Convention
Centre,
East
London,
South
Africa
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