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African horse sickness in south-east Asia

African horse sickness is exotic to Australia – but was diagnosed for the first time in southeast Asia in 2020. If it reaches our shores it has the potential to cause high mortality rates in horses as well as having trade implications.

AHS is not a zoonoses but Hendra is one of the differential diagnoses.

Above: AHS diagnosed for the first time in Thailand in 2020 and confirmed in Malaysia in September 2020 (Source: UN FAO)

African horse sickness (AHS) is a serious disease in horses with a mortality rate of 50-95%. It can present as an undulating fever, as an acute respiratory disease and/or subacute cardiac disease.

The first case of AHS was confirmed in Thailand in March 2020 followed by another 14 cases in domestic horses in several different localities over the next four months. The first case in Malaysia was confirmed in September 2020.

AHS is transmitted by Culicoides species midges and is not contagious between horses. Transmission depends on infected horses developing a sufficient viraemia to infect insect vectors. Viraemia in horses can extend for 21 days but 4 to 8 days is more usual.

If AHS gets to Australia it will most likely be via infected midges blowing in monsoonal winds. (Exotic Culicoides species have been identified on several occasions in various locations across northern Australia in the past). It is not known whether biting insects endemic to Australia, such as Culicoides brevitarsis or mosquitoes, are able to transmit the AHS virus.

If you think AHS is a possibility, ring your state or territory government veterinarians directly or the Emergency Animal Disease Watch Hotline 1800 675 888.

Hendra virus infection (endemic) and Nipah virus (exotic) are potential differential diagnoses for horses that have an elevated temperature, respiratory or circulatory issues.  These viruses are significant zoonoses so be sure to factor this into your response. Other differentials are listed in the CSIRO EAD Field Guide.

Above: An oedematous lung from an AHS case (Source: CSIRO EAD Field Guide)