Johne’s Disease in Sheep: Chronic Wasting and Diarrhea Risks

Johne’s disease, also called paratuberculosis, is a chronic contagious intestinal disease of sheep caused by Mycobacterium avium subspecies paratuberculosis (MAP). The bacteria slowly damage the lower small intestine and nearby lymph nodes, making it harder for the animal to absorb nutrients. Over time, that leads to wasting, weakness, and eventual death.

This infection usually starts early in life, often when lambs swallow MAP from manure-contaminated udders, feed, water, bedding, milk, or colostrum. The difficult part is that infected sheep may look normal for months or years while still contributing to flock spread. By the time a ewe or ram looks thin, the infection may already be established in the group.

In sheep, Johne’s disease often looks a little different than it does in cattle. Progressive weight loss is the hallmark sign, while diarrhea is less consistent. Some sheep also develop a rough fleece or wool that pulls out more easily in advanced disease. Because the course is slow and signs overlap with parasites, poor nutrition, dental problems, and other chronic diseases, your vet usually needs testing to sort it out.

Johne’s disease is caused by MAP, a hardy bacterium shed in the manure of infected animals. It can also be present in lower numbers in milk and colostrum. MAP survives well in the environment, including on pasture and in water, so contaminated lambing areas, feeders, waterers, and high-traffic pens can keep the cycle going.

Most sheep are infected when they are young lambs, even though they may not look sick until adulthood. Infection usually happens through the fecal-oral route. A lamb may nurse on a contaminated teat, eat feed or bedding with manure on it, or drink contaminated water. In some cases, infection can occur before birth.

Flocks often become infected when a subclinically infected sheep is introduced. That means the animal looks healthy but is already carrying MAP. Once Johne’s is present, crowding, manure buildup, shared lambing spaces, and keeping lambs close to adult manure increase risk. Good management can lower exposure, but it does not erase infection overnight.

Diagnosis usually starts with your vet looking at the whole picture: age, body condition, flock history, parasite control, nutrition, and whether other sheep have had chronic wasting. Because Johne’s disease can mimic internal parasites, poor teeth, chronic pneumonia, ovine progressive pneumonia, caseous lymphadenitis, and other causes of weight loss, testing is important before making major flock decisions.

Common tests include fecal PCR to look for MAP genetic material and blood ELISA to look for antibodies. PCR is generally more sensitive and specific than serology, while ELISA is often used as a lower-cost screening tool at the flock level. A negative test does not fully rule out infection, especially in early or silent stages, so your vet may recommend repeat testing or testing multiple adults.

If a sheep dies or is euthanized, necropsy with histopathology and tissue PCR or culture can provide the clearest answer. In sheep, samples from the ileum and mesenteric or ileocecal lymph nodes are especially useful. For many flocks, the most practical approach is not one test on one sheep, but a flock plan that combines testing, culling decisions, and management changes over time.

Prevention centers on keeping young lambs away from MAP. That means reducing manure exposure in lambing areas, keeping bedding clean and dry, raising feed and water off the ground when possible, and avoiding overcrowding. If your flock has known or suspected Johne’s, your vet may recommend separating lambs from high-risk adults and managing age groups more carefully.

Closed flocks are safer than frequently purchasing replacements. If you do bring in sheep, buy from sources with a strong health history and ask about Johne’s status and testing. A healthy-looking sheep can still be infected, so quarantine and pre-purchase planning with your vet matter.

Routine adult testing can help identify higher-risk animals, especially those that are thin, older, or from lines linked to previous cases. Positive animals are often culled or strictly segregated to reduce contamination. Because MAP can persist in the environment and because early infection is hard to detect, control usually takes years of steady management, not one round of testing.

Vaccination against MAP exists in some countries and settings, but it is not a stand-alone solution and may be restricted or uncommon depending on local regulations and program goals. For most US flocks, the practical foundation is still the same: biosecurity, lamb protection, manure control, and a flock plan built with your vet.