What is Blue Tongue?

Bluetongue (BT) is a non-contagious, viral disease that affects domestic and wild ruminants, such as sheep, cattle, goats, buffalo, antelope, deer, elk and camels. It is transmitted by insects, particularly biting midges of the Culicoides species.

Blue Tongue is also refered to as Sore Muzzle, Muzzle Disease, Pseudo Foot-n-Mouth Disease.

The virus which causes BT is a member of the Reoviridae family. Twenty-four (24) different serotypes have been identified and the ability of each strain to cause disease varies considerably.

The severity of disease also varies among different species with symptoms being most severe in sheep resulting in deaths, weight loss and disruption in wool growth. In highly susceptible sheep, morbidity can be as high as 100%. The average mortality is from 2-30% but can be as high as 70%.

BT is a disease listed under the OIE Terrestrial Animal Health Code and must be reported to the World Organisation for Animal Health (as per the OIE Terrestrial Animal Health Code).

Transmission and spread

Transmission between animals is via insects. The insects become infected with BT virus after ingesting blood from infected animals and then transmits it to another ruminant. Uninfected insects cannot spread the disease from animal to animal.

BT virus transmission can occur throughout the year and particularly during rainy periods.

The virus has been found in semen from infected rams and can be transmitted to susceptible ewes, but this risk is not significant. The virus can also be transferred through the placenta to the fetus.

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

Clinical Signs of Blue Tongue

Clinical signs vary and can include:

• Fever

• Hemorrhages and ulcerations of the oral and nasal tissue

• Excessive salivation and nasal discharge

• Swelling of lips, tongue, and jaw

• Inflammation of the coronary band (above the hoof)

• Lameness

• Weakness

• Depression

• Weight loss

• Profuse diarrhea and vomiting

• Pneumonia

• ‘Blue’ tongue as a result of cyanosis (rare)

• Abortion

• 'Wool break' in wool growth in recovering sheep. This can result in partial or complete loss of wool

There are foci of bulbar and palpebral conjunctival hemorrhage. 

Swelling and sores around mouth and tongue 

Inflammation and bruising of the coronet band, causing lameness 

Diagnosis

BT may be suspected based on typical clinical signs, prevalence of possible infected insects, and areas where the disease is endemic.

Prevention and control

In endemic areas, there are monitoring programs that actively sample animals for presence of the virus. This is in combination with active surveillance programs to identify 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 the most effective and practical way to minimize losses related to the disease and to interrupt the cycle from infected animal to insect. The vaccine must be designed to provide protection against the specific strain (or strains) of virus of concern in a particular area.

Geographical distribution

BT is present worldwide, 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. Currently, most of Canada is disease-free of Blue Tongue, but due to climate changes, there is a risk of this changing.

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, particularly European breeds 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 frost or severe cold weather.

References

The Center for Food Security and Public Health, Iowa State University

Merck Veterinary Manual