There are many strains of the virus causing blue tongue and immunity against one strain will not necessarily protect the animal against another. Lambs can get passive immunity suckling from an immune ewe. For this reason lambs of vaccinated ewes should not be vaccinated until they are weaned. The virus may persist in meat and offal for long periods and is resistant to many chemicals. It can be killed by a 3% sodium hydroxide solution. African sheep are less susceptible than British and Merino breeds and yearlings are more susceptible than lambs or adults. The virus is transmitted via biting midges and possibly mosquitos. Antelopes and possibly cattle are symptomless carriers. The incidence of the disease is higher in wet, low lying areas. Morbidity is 50% and mortality can be 90%.
Fever, reddening of the nostrils and mouth, frothy saliva, nasal discharge which turns bloody. Eventually the lips, ears, jaw and mouth swell. The tongue swells and turns purple and becomes ulcerated. Secondary bacterial infections are common. Respiration becomes laboured, the animal goes down and cannot eat. The feet become infected, the animal has wry neck and may vomit. Death occurs after 6 days. Most animals die. The few that survive take many months to recover and will probably never be economically viable.
There is no treatment against the blue tongue virus, but antibiotics will treat secondary bacterial infections.
• Don’t graze sheep in wet low-lying areas where the midge is present.
• If possible, keep sheep separate from cattle and wild game.
• Vaccination – 2 weeks after weaning, 3 weeks before breeding and then annually.
Ref: http://www.iah.bbsrc.ac.uk/bluetongue/BT_clinical.pdf Diseases and Parasites of Livestock in the Tropics. H.T.B. Hall