Johne’s Disease (Paratuberculosis) in Sheep

Johne’s disease, also called paratuberculosis, is a chronic contagious intestinal disease of sheep caused by Mycobacterium avium subspecies paratuberculosis (MAP). This bacterium infects the lower small intestine and nearby lymph nodes, leading to long-term inflammation that interferes with nutrient absorption. Over time, affected sheep may become thin, weak, and unthrifty.

One of the challenging parts of Johne’s disease is its slow course. Sheep are usually infected when they are very young, often soon after birth, but they may not show visible illness until they are adults. During that silent period, some infected animals can still shed MAP in manure and contaminate the environment.

For pet parents and flock managers, this means a sheep can look normal for a long time while the infection spreads quietly within the flock. Because there is no dependable cure, the main goals are early recognition, accurate testing, and practical flock-level control planning with your vet.

Johne’s disease is caused by MAP, a hardy bacterium shed mainly in the manure of infected animals. It can also be present in lower numbers in colostrum and milk. Young lambs are the most vulnerable because their intestinal tissues are more easily infected, especially in the first months of life.

The usual route of infection is fecal-oral exposure. Lambs may ingest MAP while nursing on contaminated udders, eating feed or drinking water contaminated with manure, or living in heavily contaminated lambing and nursery areas. Introducing an infected but outwardly healthy sheep into the flock is a common way the disease starts or persists.

MAP can survive in the environment for long periods, which makes sanitation and lamb management especially important. Stress, crowding, and repeated exposure do not directly cause Johne’s disease, but they can make control harder by increasing contamination pressure within the flock.

Diagnosis usually starts with your vet reviewing the sheep’s age, body condition, weight-loss history, parasite control, nutrition, and any flock pattern. Because many diseases can cause chronic wasting, Johne’s disease should be considered alongside internal parasites, chronic pneumonia, ovine progressive pneumonia, dental problems, liver fluke in some regions, and other long-term illnesses.

Testing in sheep often involves a combination approach. Blood antibody testing may help identify later-stage infection, but it can miss many infected animals, especially early or subclinical cases. Fecal culture, tissue culture, histopathology, acid-fast staining, and sometimes PCR-based methods may be used depending on the lab and the case. In sheep, tissue testing from the ileocecal area and mesenteric lymph nodes at necropsy is often one of the most useful ways to confirm infection.

A negative result does not always rule out Johne’s disease in an individual sheep. That is why your vet may recommend repeat testing, testing multiple flock mates, or submitting a deceased or euthanized suspect sheep for necropsy. In many flocks, the most practical and accurate diagnosis is made at the flock level, not from a single test on one animal.

Prevention focuses on protecting lambs from MAP exposure. The most important step is keeping birthing and nursery areas as clean and manure-free as possible. Lambs should not be raised in heavily contaminated spaces, and feed and water sources should be positioned to reduce fecal contamination.

Work with your vet on a closed-flock or low-risk purchasing plan whenever possible. New sheep can introduce Johne’s disease even if they appear healthy, so source animals carefully and ask about flock history and testing. In flocks with known infection, your vet may recommend testing adults regularly and removing or segregating positive animals.

Good prevention also includes practical recordkeeping. Track chronic weight loss, poor thrift, unexplained deaths, and test results over time. If a suspect sheep dies or is euthanized, necropsy can provide valuable information for the rest of the flock. In some regions or heavily affected operations, vaccination may be considered, but it does not replace sanitation, testing, and careful lamb management.