Bovine ephemeral fever

An arthropod–borne rhabdovirus known as ‘ephemeral fever virus’ or ‘bovine ephemeral fever virus'.

• occurs most years in northern Australia.
• usually spreads from north to south.
• governed by season and rainfall; yearly insect distribution
• can occur in herds just south of the usual distribution.
• herd immunity depends on
• • location relative to normal distribution
    • older animals with previous exposure will be immune
  • recent seasonal conditions
  • calves have a relative immunity up to approximately 6 months of age.

Unconfirmed. Thought to be mosquitoes and biting midges (sandflies).

There are typically 3 recognised stages of bovine ephemeral virus.

The acute febrile stage appears suddenly and is especially noticeable in dairy cattle. Affected cattle are likely to:

• show signs associated with a fever
• have a rectal temperature over 40°C
• shiver (in approximately 50% of cases)
• stand with their backs arched and heads held low, muzzles extended, drooling saliva
• have discharge from eyes and nostrils
• stop feeding and cud chewing
• have reduced milk production, especially in dairy cows.

The second stage is muscular stiffness and lameness in 1 or more limbs. Some secondary bloat may occur due to general inflammation of the abdominal cavity and ruminal stasis. Lameness may shift between limbs and joints may be visibly swollen.

During recovery, most affected animals resume eating and drinking. Animals may go down, with heavy animals in good condition being most affected. Some animals remain down due to muscle damage or damage to the spinal cord from constant struggling.

Post-mortem examination

Post-mortem examination is important to rule out other acute febrile diseases that often occur under the same conditions, such as tick fever. Post-mortem examination can either show small amounts of fibrin-rich fluid, or occasionally, an excess of pinkish to blood stained fluid in the heart sac, chest and abdominal cavities, and joint capsules.  Lesions in the lungs may be present in severe cases.

Economic

Significant losses can occur because BEF is most severe in more valuable classes of cattle, including:

• bulls
• pregnant and lactating cows
• fat, well-conditioned cattle.

Symptoms usually last only a few days, but the disease can significantly affect the herd's production in the following ways:

• dramatic drop in milk production - over 70% is not uncommon
• milk yield after recovery is often reduced by 15% or more
• lactating cows can dry up completely
• abortion can occur in heavily pregnant cows
• occasional deaths (3%) or prolonged recumbency leading to 'downer syndrome' can occur
• temporary or permanent paralysis may occur as a result of damage to the spinal cord
• most of the herd can be affected
• bulls may be temporarily infertile (up to 8 weeks).

The cost of treatment for affected animals can also be considerable.

While the vectors have not been confirmed, mosquitoes and biting midges (sandflies) are thought to be responsible.

The disease can also be spread by intravenous inoculation of small amounts of blood. BEF is not transmitted by close contact, bodily secretions, or aerosol droplets, nor does it seem to be transmitted in semen. Carriers are not known to occur and the virus is rapidly inactivated in meat.

The spread of the disease depends on the season and weather conditions - rain and prevailing easterly and southerly winds are necessary for the survival and dispersal of vectors. The National Arbovirus Monitoring Program (NAMP) monitors the spread of ephemeral fever virus within Australia.

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. It then spreads into southern Queensland, and central and coastal New South Wales.

It is less common in drought years, but can occur following an extended period of drought.

BEF epidemics are diagnosed on the presence of lameness, muscular stiffness, pain, short fever and rapid spread of the disease through herds.

The virus can often be cultured from a blood sample taken from animals in the early stages of the disease. A PCR test can also identify the presence of the virus, and is most successful when samples are collected in the first few days of clinical disease. Alternatively, 2 blood samples—the first taken during the fever stage and the second 14 days later—can be collected to detect the presence of antibodies to the virus.

Vaccination

There is a modified live vaccine for BEF that provides long-lasting protection. A 2-part vaccine - freeze-dried vaccine with chilled liquid adjuvant - that must be mixed prior to administering, it provides good levels of protection against BEF. While some cattle might still develop mild disease after vaccination, the severity and duration of illness tends to be much lower than in unvaccinated cattle.

The initial vaccine should be administered twice, 2–4 weeks apart, under the skin of the neck for long-lasting protection. To ensure that animals are fully immune before insect populations have the opportunity to breed, you should vaccinate before spring, especially in northern Australia.

An annual booster should be given 8–10 weeks before the BEF season.

Contact your veterinarian for advice on a vaccination program and use of the vaccine.

Treatment

• Anti-inflammatory drugs have been shown to reduce the course of the disease.
• Consult your veterinarian for an appropriate anti-inflammatory drug, considering the withholding period of the drug for meat and milk.
• Most animals will recover if given water, shade and food.