Bluetongue

The severity of disease varies among different species, with clinical signs being most severe in sheep, resulting in death, weight loss and disruption in wool growth. In highly susceptible sheep, morbidity can be as high as 100%. Mortality averages are 2–30% but can be as high as 70%.

Cattle often have a higher infection rate than sheep, and the demonstration and severity of clinical signs varies depending on the strain of virus. However, production losses, particularly milk yield, can remain for a long time and are significant. Infected animals can abort or give birth to weak offspring.

If you have further questions or if you would like to speak to someone, call the bluetongue hotline: 024 7771 0386. 

Bluetongue is a notifiable animal disease. If you suspect it you must report it immediately by calling the Defra Rural Services Helpline on 03000 200 301. In Wales, contact 0300 303 8268. In Scotland, contact your local Field Services Office. Failure to do so is an offence.

Where is the disease found?

Bluetongue has a significant global distribution in regions where the insect vector (i.e. biting midges species Culicoides) is present, including Africa, Asia, Australia, Europe and North America. The virus is maintained in areas where the climate will allow biting midges to survive over winter. The geographical distribution of the insect vector species generally limits the distribution of the disease.

Outbreaks occur when susceptible sheep, particularly European breeds, are introduced to endemic areas or when the virus is introduced to a region by windborne movement of infected Culicoides.  It can also be introduced through the import of an infected animal or germinal products (semen or embryos). The occurrence of bluetongue generally parallels vector activity, surging during periods of high temperature and rainfall and subsiding with the first frost or severe cold weather.

How is the disease transmitted and spread?

The insect vector is the key to transmission of bluetongue virus between animals. Vectors are infected with bluetongue virus after ingesting blood from infected animals. Without the vector, the disease cannot spread from animal to animal.

Bluetongue virus transmission can occur throughout the year, particularly during rainy periods. Infected cattle play a significant role in maintaining the virus in a region. Cattle may serve as a source of the virus for several weeks while displaying little or no clinical signs of disease and are often the preferred host for insect vectors.

The virus has been found in semen from infected bulls and rams and can be transmitted to susceptible cows and ewes, but this is not a significant mechanism of transmission. Virus can also be transferred through the placenta to the foetus.

The bluetongue virus is not transmitted through contact with animals, wool or consumption of milk. There is no public health risk associated with bluetongue.

What are the clinical signs of the disease?

The appearance of clinical signs in cattle will depend on the strain of virus, and it is possible that cattle will show no signs of illness; however, clinical signs have included:

• Nasal discharge
• Swelling of the head and neck
• Conjunctivitis (red and runny eyes)
• Swelling inside and ulceration of mouth
• Swollen teats
• Tiredness
• Saliva drooling out of mouth
• Fever
• Milk drop in dairy cows

Prevention and control measures

In endemic areas, sentinel-monitoring programs actively sample animals in sentinel herds to monitor for presence of the virus. In combination with active surveillance programs to identify the location, distribution and prevalence of insect vectors in an area, control measures can be implemented in a timely fashion such as:

• Identification, surveillance and tracing of susceptible and potentially infected animals
• Quarantine and/or movement restrictions during insect activity period
• Vaccination
• Insect control measures

Vaccination is available for some strains, but there is currently no licenced vaccine for BTV3. There is considered to be no cross protection between strains, i.e. a vaccine for BTV4 will not protect an animal against BTV3 or any other strain. 

Where a vaccine is available, it is used as the most effective and practical measure to minimise losses related to the disease and to potentially interrupt the cycle from an 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.

Source: World Organisation for Animal Health

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