Akabane virus

Nearly all cattle breeders in the northern part of Australia, including north-eastern New South Wales, have experienced congenital defects in calves, and more specifically, those congenital defects that give rise to calves known as curly, imbecile, silly or dummy calves. A small percentage of these are, no doubt, the result of non-infectious causes, perhaps inherited or the result of some effect during gestation.

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THE VIRUS Two viruses, both of the arbovirus group, have been implicated in the cause of these congenital defects. Akabane virus, named after the district in Japan where it was first found, and Aino virus. Generally, both these viruses are referred to as Akabane virus.

THE DISEASE Akabane virus does not, as far as is known, cause any overt sign of disease, in either young or adult animals. Its effect is on the foetus and the conditions which result in the disease in the foetus may be summarised as follows- 1. The pregnant dam has not previously had exposure to the virus and therefore has no specific immunity and becomes infected, and 2. The virus passes through the placenta and infects the calf, specifically the calf's central nervous system (CNS) where it may cause varying amounts of damage. silly calves which may have difficulty in suckling, walking or learning herd behaviour, etc. Other species may become infected. These include buffalo, camels, horses, sheep, goats and dogs. The disease in these hosts is not commonly observed and, apart from in buffalo, is probably not a particularly important infection.

INFECTION As Akabane is an infectious disease, it is necessary to know how the virus is transmitted if the infection is to be combatted. It has been shown that animals that have been infected with Akabane virus are found mostly in those parts of Australia which have the blood sucking midge "Culicoides brevitarsis." This, and the fact that the Akabane virus has been isolated from this midge, has led to the supposition that it is C. brevitarsis which is responsible for the spread of the disease. Normally, in areas where the midge is prevalent, no signs of the disease is seen because the dams are immune. When suitable weather conditions (such as warm humid summers) prevail, the virus-carrying midge can extend its normal range into areas with susceptible animals. If these cattle have not previously been infected (often the younger animals) then it can be expected that the disease will make an appearance in the calving season.

DIAGNOSIS AND TREATMENT Diagnosis can often be made on the basis of clinical signs and can be confirmed by demonstration of antibody in the blood of the calf or in the blood of the dam. Treatment of Akabane disease, like most viral diseases, is impractical. In mild cases, the calf may be saved by assisting it in suckling, but it is rarely worth the trouble. Normal practice is to destroy the affected calf. There is no vaccine currently available to carry out preventative therapy.

CONCLUSION Akabane disease is an important disease in those areas which are subject to only an occasional influx of C. brevitarsis. It is a useful method for obtaining information about the spread and persistence of C. brevitarsis which may be a possible vector for the insect borne viral disease "Bluetongue" - should Bluetongue ever enter Australia.

References

Merck veterinary manual

Couch A.J. (1984) Akabane Virus. Australian Red Poll Society Newsletter 92: 22-23