A range of insect/arthropod borne viruses (arboviruses) occur in Australia. They are noncontagious viruses and are transmitted by their insect vectors. Climatic factors (rainfall and temperature) determine the distribution of potential vectors and therefore the diseases they transmit.
Arboviruses of significance that are present in Australia include Bovine Ephemeral Fever virus (BEF), also known as three-day sickness, Bluetongue virus (BTV) and Akabane viruses. While enzootic bovine leucosis (EBL) virus can also theoretically be transmitted by biting flies, this is not known to be a significant means of transmission of this virus in Australian cattle herds. A range of other arboviruses also occur in Australia but are not usually of economic significance to livestock producers.
Distribution and insect vectors
The arboviruses BEF, Bluetongue and Akabane viruses are transmitted only if their insect vectors are present in sufficient density.
The biting midge Culicoides brevitarsis is the main vector of both BTV and Akabane virus. A close correlation exists between the southern limits of C. brevitarsis and the distribution of the two viruses, although the viruses are less widely distributed than their vectors. Other biting midge vectors of BTV in Australia, which are less widely distributed than C. brevitarsis, are C. actoni, C. dumdumi, C. fulvus and C. wadai.
Many regions in Australia have never recorded the presence of the biting midge C. brevitarsis and they are therefore free from BTV and Akabane virus. BEF virus, which is primarily spread by the mosquito Culex annulirostris has a more variable distribution, particularly in southern Australia, because this insect has greater tolerance to lower temperatures. Climatic conditions have a significant effect on insect vector distribution and partly account for the changes that occur at the boundaries between areas where viral transmission occurs and areas free of transmission.
Research in Australia since the mid-1970s has provided a detailed understanding of the epidemiology of Australian BTV strains and their Culicoides midge vectors. The important vector species in Australia are likely to have all originally arrived on air currents from neighbouring countries.
Bluetongue virus (BTV)
A limited number of BTV strains occur in Australia and they do not impact livestock production. They do however impact on export of live animals and some animal products, especially genetic material.
The limits of BTV transmission in Australia are shown on the interactive BTV zone map, accessible at https://namp.animalhealthaustralia.com.au/public.php?page=namp_zone_map&raw=1&aha_program=2 which defines areas in which no viral transmission has been detected for the past 2 years.
BTV is endemic in northern and north-eastern Australia (New South Wales, Northern Territory, Queensland and Western Australia), and remains undetected in South Australia, Tasmania and Victoria.
Akabane virus
A number of closely related viruses cause Akabane disease in cattle. The distribution of Akabane viruses varies within the limits of its presumed vector, C. brevitarsis, occurring endemically in northern Australia (northern Queensland, Northern Territory and Western Australia) and showing a distinct seasonal spread in New South Wales and southern parts of Queensland.
Akabane disease mostly impacts breeding female cattle causing a range of clinical syndromes with the end result being foetal and calf losses. As with BEF virus, where Akabane virus infection occurs commonly, young cattle are exposed to the virus and become immune so very few signs of illness occur. It is mostly on the southern fringes of its distribution, such as the northern tablelands of NSW, where incursions of the virus are less common that the clinical manifestations of the disease are seen. This is because young, naïve replacements that have been retained since the last outbreak of disease become exposed to the virus as breeding females.
Clinical signs
Akabane virus infection does not produce any obvious clinical signs in infected adult cattle. However, if a naïve pregnant heifer or cow is infected, the virus can pass from the blood into the developing foetus and attacks rapidly dividing cells, especially in the developing brain and spinal cord, resulting in foetal malformations in 30-40% of infected foetuses. The extent of damage depends on the stage of gestation when infection occurs with most damage occurring at 3-6 months of gestation.
Abortions may occur associated with two clinical syndromes – hydranencephaly (fluid accumulation in the brain cavity) and arthrogryposis (bent up limbs, fixation of joints and secondary muscle wastage). The stage of pregnancy when infection occurs determines which of these conditions occur and some calves may have both syndromes. The spread of pregnancies in a herd will impact severity – herds with tight seasonal calving patterns may be more impacted than herds where pregnancies are at a range of stages when infection occurs.
Calves with hydranencephaly present typically as ‘dummy calves’ – they may appear blind with no sucking reflex. Calves with arthrogryposis have damaged nerve supply to their muscles which prevents joints from being straightened. This can result in calving difficulties and often requires affected calves to be cut up or born by caesarean section.
Detection of antibody to Akabane virus in a foetus or calf that has not yet suckled can also be diagnostic, but the presence of antibodies will depend on the stage of pregnancy when infection occurred.
Infection of adult cattle results in the development of antibody within 4-6 days, and this may be used to monitor the occurrence of the virus in a herd and make decisions aimed at the management of the condition. However, unless the dams are from an area where Akabane is not known to occur, the mere presence of antibody may not be diagnostic of a likely disease problem.
Most affected calves are not viable and humane euthanasia is often the best treatment option.
Control and prevention
A vaccine for Akabane disease was available in the early 1990’s but was not commercially viable and is no longer available. Awareness of the disease and its distribution assists in its management. Avoid introducing pregnant cattle from disease free zones into the endemic zones.
Tools