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The first cases of caseous lymphadenitis (CLA) in Scotland were diagnosed by SAC Veterinary Services in 1996 in a flock in the Scottish Borders. Since then SAC VS has been at the forefront of research into the disease in the UK. This work, which has been supported by SEERAD throughout, led to the establishment of close research links with staff at the Moredun Research Institute. This joint research ultimately led to the development of a blood test for CLA, which is now commercially available in the UK, and initial studies of vaccination as a means of controlling the disease.

Description

Caseous lymphadenitis has now been established within the UK for more than 15 years. In that time the bacterial disease caused by Corynebacterium pseudotuberculosis and known more commonly as CLA, has spread widely through both the sheep and goat industries. The disease is now present in all parts of the country, with the condition still being most common within the terminal sire breeds of sheep. A survey carried out in 2001 suggested that as many as 18% of flocks in that sector were affected. Latterly other sheep breeds including hill and upland have been affected and further spread of the condition seems inevitable.

In sheep and goats the infection is characterised by the formation of abscesses within the lymph nodes and other internal organs, most notably the lungs. Whilst the majority of infected animal remains clinically healthy, most go on to develop visible external swellings. These chronic abscesses often discharge thick pus on to the skin and leave characteristic scars. In the last two years there have been a number of reports of an association between the internal or visceral form of CLA and chronic wasting in affected flocks. This is an established form of the disease in other parts of the world, and the emergence of such a syndrome in the UK is a worrying development that must be closely monitored.

The widespread use of commercial vaccines to control CLA is still not possible, since none is yet licensed for use in the UK. However, the production of an autogenous CLA vaccine and the emergency importation of commercial vaccines is possible in affected flocks, although this requires a special license from the Veterinary Medicines Directorate, a division of DEFRA.

An encouraging development in the fight against CLA is a new blood test for the condition in sheep. This test detects antibodies to Corynebacterium pseudotuberculosis, the bacterium that causes the condition. It can identify infected animals before they develop the characteristic external abscesses, and is effective in finding sheep with only the internal form of the disease. Such animals would otherwise carry the infection without detection and represent the greatest threat in spreading the disease from one flock to another.

Experience with the blood test suggests that it can be very effective in reducing the number of infected animals within a flock, with the ultimate aim being that of total disease eradication. Another use for the test is in screening purchased animals before they are introduced to a new flock.

In the following web pages important factors in the spread of CLA will be discussed and progress made in controlling the disease will be described.

Partners

• SEERAD AA Programme – 100 Series AA112
• Moredun Research Institute
• Vets in Practice