Bovine Ephemeral Fever (BEF)

Bovine ephemeral fever (BEF), commonly known as three-day sickness, is a disease of cattle and buffaloes, marked by short fever, shivering, lameness and muscular stiffness. Caused by an insect borne virus, the disease is widespread in northern Australia with seasonal spread into southern regions.

It causes serious economic losses through deaths, loss of condition, decreased milk production, lowered fertility of bulls, mismothering of calves, delays in marketing and restrictions on the export of live cattle.

Cause

Rain and prevailing easterly and southerly winds are necessary for the survival and dispersal of the vectors which spread BEF virus. The National Arbovirus Monitoring Program (NAMP) monitors the spread of BEF virus within Australia. An annual report for this program with further information and maps showing the previous extent of BEF in Australia is available to download.

In most years, BEF cases start at the beginning of the wet season in northern Australia and then spread south and east down the east coast, into southern Queensland and inland and coastal regions of central and occasionally southern New South Wales. Occasionally, outbreaks will occur in northern Victoria. In the Northern Territory, incursions occasionally reach the Alice Springs region, and the virus occasionally reaches as far south as the Pilbara region of Western Australia.

The Bovine Ephemeral Fever virus is the cause of BEF. The BEF virus is transmitted between cattle by flying insects. The main vector of BEF virus in Australia is the mosquito Culex annulirostris (Figure 1).

Figure 1. The banded mosquito, Culex annulirostris, is a vector of BEF virus. Image courtesy of Jess Morgan

Impact and Incidence

In northern Australia, older cattle generally have immunity from previous exposure to BEF, with introduced or young growing cattle being most susceptible to infection. Once exposed to the virus, cattle develop a long-lasting immunity. Calves are relatively immune to ephemeral fever until they are about six months old.

Levels of ‘herd immunity’ will vary depending on location of the herd. Outbreaks of disease affecting cattle of all ages can occur at the end of a series of drought years or in herds on the southern fringes of the normal BEF distribution.

There is no known breed difference in susceptibility. Heavy cattle, such as bulls, steers approaching slaughter weights and high producing dairy cattle are most at risk of exhibiting serious signs of disease. This is a function of their size and weight more than breed.

Clinical signs

Typically, three stages of the disease are recognised.

1. The acute febrile stage appears suddenly, especially noticeable in dairy cattle. Cattle show signs associated with a fever. Their rectal temperature will generally be over 40^oC, however, this can vary in the early stages of the illness, they shiver, stand with backs arched and heads held low with muzzles extended, drooling saliva. Often there is a discharge from the eyes and nostrils. Feeding and cud chewing stop and milk production, especially in dairy cows, is reduced.
2. The second stage is muscular stiffness and lameness in one or more limbs associated with an intense inflammatory response to the BEF virus. Some degree of secondary bloat may also occur due to cessation of ruminal activity. The lameness may shift between limbs. Joints may be visibly swollen.
3. During the recovery stage, the great majority of affected animals resume eating and drinking and most recover uneventfully. However, some animals may go down, with heavy animals in good condition being most affected and dairy cattle often do not return to full production resulting in substantial production losses.

Some animals remain down due to muscle damage or develop damage to the spinal cord. Generally, about 1% die or are destroyed because they cannot get up, although this number may be as high as 10% in some mobs.

However, recent research has shown that the BEF virus has the ability to damage the nervous system in some infected cattle. This expression of the virus varies from outbreak to outbreak and this manifestation of the virus explains the ‘bad’ BEF that some producers experience. Some animals that recover from BEF will have the staggers due to spinal cord damage and some that cannot rise will also have permanent damage to the brain and spinal cord with or without muscle damage associated with being down. Pregnant cows may abort due to the high fever and heifers with young calves may mismother their calves.