10/10/2026
My latest column from the Delmarva Farmer
THE BIG THREE
The Big Three — chronic, contagious, incurable diseases that affect sheep and goats — are small ruminant lentivirus, caseous lymphadenitis, and Johne’s disease. While the economic impact of the Big Three is unknown, they are considered major production-limiting diseases and should not be ignored.
Caseous lymphadenitis (CL) is probably the most common, though prevalence varies. CL is a bacterial disease that causes internal and external abscesses (lumps), mostly in lymph nodes. CL can be a cause of wasting. Human cases of CL are rare.
The bacteria enter via cuts or breaks in the skin and less commonly mucous membranes. Pus from draining abscess(es) is the primary means of spread. Respiratory secretions and milk are less common vectors. CL is diagnosed by finding bacteria in the pus from an abscess. It is worth mentioning that two-thirds of abscesses tested (in sheep/goats) are not CL.
Caprine arthritic encephalitis (CAE) and ovine progressive pneumonia (OPP) were recently merged into a single disease called small ruminant lentivirus. Small ruminant lentivirus infects white blood cells and impacts multiple organ systems.
While cross infection is possible, CAE affects mostly goats and OPP, mostly sheep. Arthritis (lameness and swollen knees) is the most common symptom in goats, while sheep are more likely to experience chronic weight loss, pneumonia, and hard bag (mastitis).
It usually takes two to four years for symptoms to appear. Disease transmission is via body secretions. In goats the primary vector is milk (colostrum). In sheep, transmission is more lateral (respiratory). There is no evidence that small ruminant lentiviruses can infect people.
While considered common in U.S. dairy cattle, Johne’s disease (paratuberculosis) is less common in small ruminants. It is a bacterial disease that affects the small intestines, causing chronic wasting and death. Johne’s is considered to be a hidden killer because it has a very long incubation period (two to five years).
Johne’s is spread the oral-f***l route. While few animals show signs of the disease, all infected animals shed the bacteria. A diagnosis of Johne’s is confirmed via f***l testing and post-mortem. There is disagreement (and no proof) that Johne’s is zoonotic.
The best protection against the Big Three is strict biosecurity, as all are introduced to a farm primarily via infected animals. Keeping your flock/herd closed to outside additions helps to prevent the introduction to these as well as other contagious diseases. If the flock/herd cannot be kept closed, new animals should ideally be sourced from farms that have verified test-negative flocks/herds. Quarantining new animals is always important.
If you want to find out if any of these diseases are already present on your farm, you can submit blood samples to an accredited lab. Serological tests measure antibodies against the disease-causing bacteria or virus. However, tests of individual animals should be interpreted cautiously.
Blood testing is more useful for herd-level diagnosis (screening): In other words, is the disease present in my flock/herd and what is the prevalence? With that said, test results can be used to make culling decisions and guide management decisions. Your veterinarian can help you undertake a testing program and interpret the test results for you.
Eradication of the Big Three is possible and recommended — especially for farms selling breeding stock.
Originally published in the Delmarva Farmer, Oct. 2, 2026
https://americanfarm.com/articles/delmarva-farmer/the-big-three-shepherds-notebook/
