Caseous Lymphadenitis in Sheep and Goats
Caseous lymphadenitis (or CLA) has been called the plague of small ruminant producers. It occasionally occurs in other species such as horses, cattle, fowl and hedgehogs. It is also potentially transmitted to humans by infected animals. CLA occurs in two forms: in the skin and in organs.
CLA is caused by a bacterium called Corynebacterium pseudotuberculosis, which is often found in some of the same sites favoured by the tuberculosis bacteria, and often causes a wasting syndrome in infected animals. Once one animal on a premise is infected, open draining wounds associated with infected lymph nodes result in environmental contamination. Other sheep or goats are then infected through routine procedures that result in broken skin, such as shearing, tail docking, horn removal and castration. The bacteria is quite resistant to commonly used sheep dips and the use of these may even spread the disease.
Abscesses usually take one to three months to form from the point of entry in the skin or lymph node, and then slowly spread by blood or lymph to organs or other lymph nodes. CLA abscesses are characteristically thick walled. When fresh they are filled with soft pasty white to green exudate, but older abscesses display a very typical onion like appearance of rings of thick dry pus. While some animals waste away as organs are affected, other animals may appear fat and healthy and only on post-mortem exam is the extent of involvement recognised. Slaughtered sheep and goats are condemned when multiple abscesses are found as this indicates the immune system has not contained the disease well and that bacteria are circulating throughout the body.
Figure 1: External CLA abscesses under the jaw, and in the parotid lymph node. This may rupture and drain repeatedly into the environment, infecting a herd.
The incidence of abscesses increases with age, and up to 40 per cent of animals in a flock may be affected. Both forms are considered impossible to completely cure, are very slow to progress and often wax and wane as abscesses rupture and drain. A small ruminant producer should consult a veterinarian if they have concerns over CLA in their flock, but herd culling and environmental decontamination is often advised to remove infection from a premise. Antibiotic treatment is long-term and does not cure disease. Surgical removal of abscesses remains the most effective treatment but does not cure infection. There are vaccines for CLA. They do not completely prevent disease in a flock but do reduce prevalence and severity. The vaccine is not approved for use in goats.
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