Blue-tongue in Sheep

Blue tongue virus is a member of the genus Orbivirus and family Reoviridae. It is transmitted by Culicoides midges. In Africa, it is transmitted by Culicoides imicola. It causes disease in ruminants, with sheep being the most susceptible.  Goats are affected occasionally, and cattle rarely. In sheep, it may result in significant morbidity and variable mortality. Symptoms include erosions and ulceration on mucous membranes, lameness as a result of coronary band inflammation and necrosis, and dyspnea. Affected sheep may loose their wool after infection, and occasionally they may slough their hooves.  

Wild ruminants are infected by the virus, but do not show symptoms. In Africa, some large carnivores have antibodies to bluetongue. The virus was first recognised in South Africa and has since been found in Africa, Europe, the Middle East, Australia, the South Pacific, North and South America and parts of Asia. Geographic distribution is limited by the distribution of the Culicoides midges. Twenty four serotypes have been identified world-wide. Serotypes identified by the Onderstepoort Veterinary Institute in recent years are as follows:  
a). 2008 and 2009 =  1;3;4;6;8;16;18;24
b). 2009 and 2010 =  1;3;4;6

The main vectors are various species of biting Culicoides midges. Ticks or sheep keds may be mechanical vectors and the disease may be spread on veterinary equipment. Cattle are the preferred hosts of many Culicoides species. They develop a prolonged viraemia and often amplify the virus, therefore playing a major role in transmission of the virus.
Infected animals remain infectious to the insect vector for several weeks. The incubation period in sheep is between 5 and 10 days. Cattle show viraemia at 4 days post-infection. They rarely develop symptoms.

Clinical signs seen in sheep include fever, depression, salivation, dyspnea and panting. The lips, ears  and eyelids may be swollen and hyperaemic. Dyspnea and panting may be seen. The tongue is often very swollen, may be cyanotic and protrude from the mouth. The mouth often has erosions and ulcerations on mucous membranes. This may be sufficiently severe to lead to necrosis and sloughing of mucous membranes. Hooves are often painful and the coronary band hyperaemic, leading to lameness. Such animals should not be moved long distances, as this may contribute to sloughing of hooves. Abortions may occur, or so-called dummy lambs may be born to affected ewes. Other signs may be seen, such as torticollis, pneumonia and conjunctivitis.  Some sheep may shed their wool three to four weeks after recovery. Severity of disease and death rate is influenced by the strain of virus.

At post mortem, the following signs may be seen:
Oedema of face and ears
Crusty nasal exudate
Hyperaemia of coronary bands
Petechia, erosions and ulcers on  tongue and dental pad and rest of oral cavity.
Necrotic or cyanotic oral cavity
Trachea may be hyperaemic with foam.
Hydrothorax  is seen occasionally
The heart may have petechiae, ecchymoses and necrotic foci.
The reticulum and omasum may show hyperaemia and erosions.
Characteristically haemorrhage is seen at the base of the pulmonary artery.
Oedema may be seen in intermuscular fascial planes and skeletal muscles may have focal haemorrhages or necrosis.

Cattle rarely show symptoms, although clinical signs have been seen and have included nasal discharge, swelling of head and neck, conjunctivitis, swelling and ulceration of the mouth, swollen teats, lethargy and salivation.

Clinical diagnosis is based on symptoms and insect activity. In cattle the disease must be confirmed by laboratory testing. Differential diagnoses include vesicular stomatitis, foot-and-mouth disease, rinderpest, photosensitivity due to plant poisoning, malignant catarrhal fever, infectious bovine rhinotracheitis, bovine virus diarrhoea, foot rot, oestrus ovis infection.

Laboratory diagnosis includes PCR. This allows for rapid diagnosis and can identify the serogroup and serotype.  Several EDTA blood samples should be collected from several live febrile animals as early in the course of disease as possible.  
Serology may also be used for diagnosis. Competitive ELISA identifies IgG antibodies and will be be positive from 7 to 14 days onwards. These antibodies are persistent. Complement fixation test is positive as early as 1 to 2 days after symptoms appear and measures IgM antibodies. These antibodies start to disappear by 7 days. (Personal communication: Dr. Alison Lubisi).
Serum neutralisation tests will identify the serotype involved.

Control is based on control of the insect vector. Peak populations of Culicoides occur in late summer and autumn.  Synthetic pyrethroids or organophosphates are effective against Culicoides. Housing animals from late afternoon to early morning reduces the risk of infection. Animals should also be kept away from low-lying areas near open water.
Although the virus does not affect horses, Culicoides will feed on them and breed in manure piles, so this should be considered in control of an outbreak.
A freeze dried polyvalent live attenuated  vaccine is available in South Africa.  The vaccine is given as a series of three separate injections with different serotypes of bluetongue virus in each bottle. The vaccine will only stimulate immunity to all serotypes after a number of annual innoculations.
(See vaccine insert for full instructions. )