This is a bacterial disease (Corynebacterium pseudotuberculosis) featuring abscessation of lymph nodes and sometimes other organs. Bacteria enter the body through cuts and abrasions and spread via the lymph and blood to establish chronic infections in lymph nodes.
Infection spreads by direct contact between animals and by susceptible animals coming into contact with bacteria in contaminated yards or pastures.
The number and size of abscesses in affected animals will increase over time, and clinical disease is often more noticeable in older animals. The disease may be fatal.
In the export process, the abscesses are a common incidental finding at necropsy in animals dying of other causes. It is a significant potential cause of partial and whole carcass condemnation in overseas abattoirs.
External abscesses may be detected as firm to fluctuant, non-painful rounded lumps, usually under the jaw or in front of the shoulder and thigh. Sometimes they will have ruptured and be discharging pus. Emaciation occurs when internal abscesses are numerous or large.
At necropsy, abscesses may be found anywhere in the body including brain, bone and mammary gland, and may be the explanation for unusual clinical signs. The most common sites are in the submandibular, prefemoral and prescapular lymph nodes (visible as subcutaneous abscesses), and internally in the lung, and bronchial and mediastinal lymph nodes. In sheep, the pus is pale green and becomes paler as it hardens with age. In goats, pus remains soft and pasty.
Laboratory confirmation requires submission of a swab of pus submitted chilled for bacteriology.
Differential diagnoses include melioidosis, internal parasites, poor dentition, undernutrition, liver abscessation and Johne's disease. An additional differential diagnosis in goats is infection with caprine arthritis-encephalitis virus.
It is untreatable. Antibiotics are unable to penetrate the abscesses. Superficial abscesses tend to recur after surgical drainage or excision.
A vaccine is available for sheep and goats but will not prevent all new infections nor cure existing infections, and effective control requires a range of biosecurity measures in addition to vaccination. Sheep for export should be sourced from flocks that have implemented effective control programs.