Caseous lymphadenitis (CLA) is a contagious disease of sheep and goats, caused by an infection with Corynebacterium pseudotuberculosis. In sheep the infection commonly manifests as abscessation of the lymph nodes, following entering through skin lesions. A survey in 2001 found that 18% of terminal sire breed flocks in the UK were infected. In the UK, the most common lymph nodes are the parotid and submandibular. Abscessation of these superficial lymph nodes is usually quite easily recognised, particularly in shorn sheep. More insidious is the visceral form of the condition, which follows inhalation of the bacteria and may have no involvement of superficial lymph nodes. Instead abscesses form in the bronchial and mediastinal lymph nodes (or others throughout the abdomen). These animals would not be picked up on a visual or physical examination and may remain in the flock, shedding bacteria and spreading the disease to in-contact animals. The pathological impact of the visceral form is unknown, with some authors reporting it as subclinical, while others suggest it may contribute to the 'catch-all' diagnosis of Thin Ewe Syndrome. Following infection the lymph nodes are infiltrated by neutrophils. A cycle of necrosis and encapsulation occurs which gives the characteristic onion ring structure to the abscesses. The content of the abscesses often has a green tinge. The abscesses can be as large as 15cm diameter and may rupture, discharging either temporarily or chronically.
Diagnosis
Culture of abscess content can demonstrate presence of the bacterium in suspicious cases. An antibody ELISA against Phospholipase D (an exotoxin of C. pseudotuberculosis) can identify animals which have seroconverted. CLA which is found at the abattoir may lead to carcase trimming or, in a generalised case, to total condemnation.
Control
Control is a long process, requiring identification of positive individuals and isolation until they can be removed from the flock. The best method of identification is the ELISA mentioned above. Temporal separation of repeated tests by 6-8 weeks increases the sensitivity of the testing. Antibiotic therapy is of no practical use due to the protective capsule of the abscess. Proprietary vaccines are not currently available in the UK. An autogenous CLA vaccine could be produced on a farm by farm basis under an Emergency Licence from the VMD.
Sensible precautions
- Quarantine and screening of imported animals
- Removal of infected animals
- Shearing equipment should be dipped in strong disinfectant or chlorine bleach before and after use
- Older animals should be handled last during routine procedures
C. pseudotuberculosis is zoonotic, although it would appear not to be easily transmitted. Those who may come into contact with discharging abscesses (eg. shearers) should take suitable precautions. If infection in humans does occur, the most common pathology is axillary lymphadenopathy. This would present as painful swellings in the armpit regions.
The CLA monitoring scheme from SAC
Moredun Institute and SAC have developed the ELISA test described above. This test is implemented into a monitoring scheme to award certificates for groups of sheep and goats where no evidence of CLA has been seen. The scheme rules calls for a group of sale sheep to isolated from contact with the rest of the flock, and both physical examination and blood sample testing are completed at the start and end of the isolation period. Please contact your veterinary surgeon if you wish to obtain a certificate for a group of sheep or goats which you are going to sell.