Answer: Caseous lymphadenitis (CL) is somewhat uncommon, but very contagious. It is caused by the bacterium, Corynebacterium pseudotuberculosis. The incidence seems to vary by region to an extent. It can lead to eventual death, and economic loss in condemned carcasses, fleece and pelts, loss of purebred stock sales, and shortened productive life. The disease has no known cure.
CL is characterized by external and internal abscesses. The disease is spread by actively draining external abscesses, and by coughing and nasal discharges. A break in the skin is prerequisite to subsequent infection. However, it can also be transmitted through pulmonary and oral mucous membranes. The incubation period varies from one to three months, and it has been known to exist in the environment for up to eight months, according to the Merck Veterinary Manual. External abscesses can be bio-assayed to determine if the disease agent is in fact C. pseudotuberculosis. Internal forms of the disease are much harder to diagnose and more open to interpretation.
Management of CL most often takes the form of culling all infected animals, which is a difficult reality. The Merck Veterinary Manual mentions the use of tulathromycin either subcutaneously or by injection into the abscesses. It noted a positive effect, at least for the short term in drying up the abscesses. However, it also noted that abscesses would normally re-occur. It’s a good idea to ask your vet about this antibiotic therapy, or any others that he might know of.
Your vet is a good source of professional advice. If she is not very familiar with CL, ask for additional help from some of her colleagues in regions of the United States where CL is common. Some additional questions for your vet include:
1. Is culling of the entire flock necessary?
2. What are appropriate treatment regimens?
3. What is an appropriate isolation and biosecurity protocol?
4. Is there a good diagnostic lab test available with few false positives or false negatives? What is the cost?
5. Can the animals be run through the sale barn if external abscesses are not present? If treated with an antibiotic, how long will it be before the abscesses involute? Since the antibiotic regimen will be with prescription and off label, what is the withholding time for slaughter?
6. How long must you wait before repopulating your farm with more sheep?
7. When purchasing sheep, are "CL-free flocks" to be trusted?
You can find much more information on topics related to sheep production in the Livestock section of the ATTRA website.
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