Bluetongue Virus and Caseous Lymphadenitis

Bluetongue in Sheep

·         Viral disease affecting sheep, cattle, deer, goats and camelids.

·         Sheep are most affected but cattle are the main reservoir for the virus.

·         Distribution is dependent on presence on Culicoides (midge) host which was originally restricted to Africa; however recently (1999) eastern Europe cases were noted, as well a dramatic increase in cases in 2007 across northern Europe. DEFRA highly recommend responsible import of cattle from across Europe; including immunity to BTV before becoming pregnant and no positive result after double testing. Recently, (2009) cases have reduced.

·         BTV cannot be naturally transferred directly between animals. Biting midges act as vectors. Peak midge populations occur late summer and autumn.

·         Clinical signs include changes to the mucus membranes around the mouth, nose and coronary band of the foot. Generally the signs are more severe in sheep, than cattle.  Symptoms notifiable are large numbers of sheep with lameness, high rectal temperatures, salivation, and lacrimation, ocular and nasal discharge. Incubation period of the virus is 4-12 days, followed by a fever (42 degrees C) and appear stiff or reluctant to move. Adopt arched back stance, neck extended, head lowered. Swelling on the face and ears, along with pulmonary oedema, sometimes causing breathing difficulties. Erosions may appear on lips, turning to ulcers. Profuse salivation and serous to mucopurulent nasal discharge. Reddening of the coronary band, and around the muzzle and mouth. Tongue may be swollen.
KEY SIGNS: High rectal temp, eye and nasal discharge, drooling (as a result of ulceration), facial swelling, lameness (as inflammation of coronary band).

·         Differential diagnosis includes foot and mouth disease; however F&M will show higher percentage of flock affected, along with high temps and erosions of mouth and interdigital skin and coronary band.

Caseous Lymphadenitis (CLA)

·         Chronic contagious skin disease mainly of sheep and goat. Incidence of disease increasing with age. Transmission within flock is lower in UK compared to AUS & USA.

·         Caused by bacterium Corynebacterium pseudotuberculosis. Transmission occurs via close contact or contaminated shearing equipment; rams butting is often a cause of transmission between rams. Prevalence of infection increases with age and intensity of management conditions.

·         Characterised by abscessation of superficial lymph nodes; particularly parotid (base of ear), submandibular (below the jaw), popliteal (hindleg), precrural (hindleg) and precapular (foreleg) lymph nodes.

·         This form of the disease is referred to as cutaneous or superficial form of CLA. Spread of infection to viscera is visceral or internal CLA.

·         Differential diagnosis should include actinobacillosis, TB and local abscess formation.

·         Diagnosis can be made by positive blood test which will show exposure to organism. Confirmed by culture of C. pseuodotuberculosis.

·         Treatment is difficult due to intracellular site of bacteria and fibrous capsule surrounding lesions. Lancing abscesses only increases the chance of spread.