Bluetongue is a non-contagious, viral disease spread by biting insect particularly biting Midges of the Culicoidesspecies. It affects domestic and wild ruminants, particularly (sheep, cattle, goats, buffalo, antelope, deer, elk and camels). Sheep are most severely affected by bluetongue. Cattle, although infected more frequently than sheep, do not always show signs of disease.
The disease occurs worldwide. Cattle often have a higher infection rate than sheep and demonstration and severity of clinical signs varies depending on the strain of virus. In countries where BT is endemic the impact is largely on loss of trade due to restrictions and the costs of surveillance, health testing and vaccination. BT has a significant global distribution in regions where the insect vector (i.e. biting midges species Culicoides) is present, including Africa, Asia, Australia, Europe, North America and several islands in the tropics and subtropics. The virus is maintained in areas where the climate will allow biting midges to survive over winter.
There are more than 1000 species of Culicoides species but less than 20 are considered competent vectors of BT virus. The geographical distribution of the insect vector species therefore generally limits the distribution of the disease. Generally, sheep found in areas where the disease is endemic are naturally resistant to BT. Outbreaks occur when susceptible sheep are introduced to endemic areas, or when the virus is introduced to a region by windborne movement of infected Culicoides. Occurrence of BT generally parallels vector activity surging during periods of high temperature and rainfall and subsiding with the first severe cold weather.
Bluetongue does not affect humans so there are no human or public health implications.
- Bluetongue virus is spread by insects or vectors called biting midges (Culicoides spp.). Other biting insects, such as ticks or sheep keds, may also transfer the virus. Vectors are infected with BT virus after ingesting blood from infected animals. Without the vector, the disease cannot spread from animal to animal. BT virus transmission can occur throughout the year, particularly during rainy periods.
- Bluetongue is not contagious and is not spread by contact between animals.
- However, the virus may be spread by contaminated objects such as surgical equipment and needles.
- The virus can be found in semen, but sexual transmission does not appear to be a major route of infection.
- Infected cattle play a significant role in maintaining the virus in a region. Cattle may serve as a source of virus for several weeks while displaying little or no clinical signs of disease and are often the preferred host for insect vectors.
- Virus can also be transferred through the placenta to the fetus.
- The BT virus is not transmitted through contact with wool or consumption of milk.
Many animals infected with the bluetongue virus do not show signs of disease
- High temperatures (40oC or greater)
- Excessive salivation
- Swollen muzzles, lips, tongue and jaws neck and/or ears, reddening and ulcers in the mouth (particularly on the dental pad, gingiva and lips).
- Conjunctivitis, and nasal discharge with crusting around the nostrils.
- Some animals will have hyperemia and swelling around the coronary band of the hoof with the affected hooves becoming painful
- Lameness is common and some animals may slough affected hooves.
- Occasionally animals die acutely with no clinical presentation seen.
- Pregnant animals may abort, give birth to weak nonviable lambs or have lambs that show nervous signs.
- Some sheep can also lose their hair coat several weeks after recovery from the disease.
- Depending on the susceptibility of the flock to this agent, up to 100% of the animals can be affected with mortality approaching 30%.
- Hemorrhages and ulcerations of the oral and nasal tissue
- Weakness, depression, weight loss
- Profuse diarrhea, vomiting, pneumonia
- Difficulty breathing, causing the tongue to stick out from the mouth; the tongue is often bluish in color, giving the disease its name.
Treatment and control
- Identification, surveillance and tracing of susceptible and potentially infected animals
- Quarantine and/or movement restrictions during insect activity period
- Identification of specified zones;vaccination; and insect control measures.
- Vaccination is used as the most effective and practical measure to minimize losses related to the disease and to potentially interrupt the cycle from infected animal to vector. It is essential to use a vaccine designed to provide protection against the specific strain (or strains) of virus of concern in a particular area. Vaccines are available for animals deemed to be at high risk of contracting bluetongue, however, these vaccines are not without risk.
- Implement insect control and prevention measures to reduce the spread of disease by the vector. This may include destroying insect habitat, use of insecticides, or moving animals into barns during the vector’s peak activity time (dusk until dawn).
- Any needles and surgical equipment used with animals suspected of having bluetongue should be considered contaminated and disposed of properly.
Vaccination is the only effective tool to protect susceptible animals from bluetongue. Vaccine for bluetongue (serotype 8) is available, livestock keepers should order BTV 8 vaccine through their vet as they would other veterinary medicines.
Vaccination will: Allow keepers to protect their animals from bluetongue. Reduce the economic and welfare impact of bluetongue. Animals should be vaccinated as early as possible each year, particularly before the onset of warm weather, when the risk of infection increases