Background

Bluetongue (BT) is a viral disease transmitted by arthropod Culicoides infecting ruminants and a few other animals. It was first recognized more than 100 years ago in South Africa. Bluetongue virus (BTV) infection of ruminants is enzootic in temperate and tropical regions. However, recently drastic changes in the global distribution of bluetongue disease have been seen, mostly in European countries since 1998. Several BTV serotypes have been identified in south-east America, the Caribbean ecosystem, and other parts of the world, including Australia and the Middle East. It is proposed that global climate change is accountable for the spread of infection globally. Mostly, BTV infection is subclinical, but severe disease outbreaks occur in the summer and autumn season. The bluetongue lesions in sheep are well defined, which include ulcers, erosions, lameness, coronitis, depression, weakness, and facial edema. However, post-mortem lesions include hemorrhage and necrosis of gastrointestinal tract, edema and haemorrhage of lymph nodes, pleural and pericardial effusion, etc . The epidemiology of bluetongue in India is complex. Around 26 BTV serotypes have been identified in India. Seven different species of Culicoides are responsible for the transmission of the arbovirus to other countries. These species are found in a wide range of habitats, including animal dung and enriched moist soil. Even though bovines and goats have shown viremia equivalent to sheep and thus can infect Culicoides responsible for onward transmission of BTV, making them potential reservoir hosts. Currently, both live and killed vaccines are available for bluetongue. Live attenuated vaccines are serotype-specific and can be transmitted to unvaccinated animals. Recombinant vaccines for bluetongue are under development. In India, bluetongue is controlled by the use of attenuated pentavalent vaccines.

Cause

A virus disease caused by an arthropod borne Orbivirus.

Host Species

Cattle, Sheep, Goat, Deer, Buffalo

• The disease is transmitted biologically by certain species of Culicoides
• Other vectors: The argasid tick Ornithodoros coriaceus , the sheep ked (Melophagus ovinus) and mosquitoes (Aedes lineatopennis and Anopheles vagus) ingest the virus when sucking the blood of infected sheep and can transmit the infection in a mechanical way.
• Venereal transmission: Bluetongue virus has been found in the semen of infected bulls during the initial viremic period, and infection has been transmitted through bull semen to susceptible cows

Sheep

• Naturally occurring, florid bluetongue in sheep has the following clinical characteristics.
• Initially, there is rise of body tempararture followed by discharge from nose, and salivation, with reddening of the buccal and nasal mucosae are apparent.
• Naturally occurring, red and rosy tongue in sheep is the most characteristic sign.
• The mucopurulent nasal discharge is occasionally blood stained
• The saliva is frothy in nature.
• Some affected sheep show severe conjunctivitis, accompanied by profuse lacrimation.
• Swelling and edema of the lips, gums, dental pad and tongue with involuntary movement of the lips are also found.
• Excoriation of buccal mucosa followed by blood stained saliva and mouth with offensive order are found.
• Lenticular, necrotic ulcers develop, particularly on the lateral aspects of the tongue. The tongue becomes swollen and purple in color after development of necrotic ulcers.
• Hyperemia and ulceration are also common at the commissures of the lips, on the buccal papillae and around the anus and vulva.
• Animal feels difficulty in swallowing.
• Increase rate of respiration is also found because of obstruction on respiration.
• Foot lesions, including laminitis and coronitis, and manifested by lameness and recumbency appear only in some animals.
• The appearance of a dark red to purple band in the skin just above the coronet, due to coronitis, is an important diagnostic sign.
• The head and neck become twisted to one side in some affected animals due to direct action of the virus on muscle tissue.
• The animal cannot take food due to muscle stiffness and weakness. There is a marked, rapid loss of condition.
• There is facial swelling with extensive swelling and drooping of the ears and hyperemia of the non-woolen skin may be present.
• Vomition, dysentery and secondary aspiration pneumonia may also occur.
• Death in most fatal cases occurs about 6 days after the appearance of signs.
• The recovered animals, after several months, remain separated from flock and show cracking of the hooves as well as wrinkling and cracking of the skin around the lips and muzzle.
• The lambs borne from recovered animals exhibits porencephaly and cerebral necrosis.
• Pregnant animals may abort in enzootic area without showing any characteristic signs.

Cattle

The disease is inapparent in nature and few animals may show following clinical signs:

• High rise of body temperature (104-106°F)
• Inappetance
• Stiffness and laminitis in all four limbs
• Excessive salivation
• Edema of the lips
• Serosanguineous exudate may appear in the nostrils and a discharge from the eyes.
• Foul breath
• The ulcerative lesions on the tongue, lips, dental pad, and muzzle are seen in some cattle.
• Sometimes coronitis followed by sloughing of the hoof may occur.
• Some cows develop photodermatitis and teat lesions.
• The infection at early stage of pregnancy leads abortion or congenital deformities of calves including hydranencephaly, microcephaly, curvature of the limbs, blindness and deformity of the jaw.

No specific treatment is there for bluetongue. Only following supportive treatment will help.

• Local irrigations with mild disinfectant solutions may cause some relief.
• Fluid and electrolyte therapy is delmnisted to correct dehydration.
• Antibiotic is prescribed to prevent secondary bacterial infections.
• Lesions around the eyes, nostrils and mouth should be cleaned and dressed with antiseptics.
• Good nursing is essential.

Preventive measures

• Infected animals exhibiting symptoms should be isolated from healthy animals.
• Vector control: Reduction of infection consist of reducing the risk of exposure to infected Culicoides and reduction in Culicoides numbers. But, it is very difficult.
• Spraying of fly repellents and insecticides on cattle and sheep help to reduce the risk of exposure of infection.
• The sheep and cattle are to be housed separately at night.
• Sheep are allowed to graze at higher altitudes with avoidance of low & marshy areas
• In transmission period, the cattle may be administered ivermectin injection, as high mortality is seen in Culicoides that fed on cattle.

Control

Vaccination is the only effective way to control the disease.

Type of vaccines

Polyvalent inactivated vaccine (serotype 1,2,10,16 & 23)

Vaccination schedule

Vaccination against bluetongue is recommended to sheep at the age of 3 months followed by booster after 1 month and repeat annually.

Dose

2mL S/C route or following manufacturer's instructions

Note: Deworming is must at least 15 days before vaccination.

Source/manufacturer of vaccine in India

Indian Immunologicals, Hyderabad (Telangana)