Johne’s disease is widespread in U.S. cattle, particularly in dairy herds. A USDA study found that 68.1 percent of U.S. dairy operations had the bacterium responsible for Johne’s disease present in at least one environmental sample. In beef herds, confirmed infection rates are lower but almost certainly underestimated. The disease is caused by a slow-growing bacterium called Mycobacterium avium subspecies paratuberculosis (MAP), which triggers chronic intestinal inflammation, progressive weight loss, and diarrhea that eventually becomes fatal.
Prevalence in Dairy Herds
The most comprehensive U.S. data comes from a 2007 USDA national survey, which isolated MAP from environmental samples on 68.1 percent of dairy operations. That means roughly two out of every three dairy farms in the country harbor the organism. Individual cow-level infection rates are harder to pin down because testing has significant blind spots (more on that below), but the sheer number of infected operations makes Johne’s one of the most common chronic diseases in the U.S. dairy industry.
The high herd-level prevalence reflects how easily the bacterium spreads in confined dairy environments. Calves typically pick up the infection in their first months of life by swallowing contaminated manure, milk, or colostrum. Once MAP is established on a farm, eliminating it is extremely difficult because infected animals shed the bacterium for years before showing any outward signs of illness.
Prevalence in Beef Herds
Johne’s disease appears far less common in beef cattle, though the true numbers are likely higher than what testing reveals. A USDA national survey found that 7.9 percent of tested beef herds had at least one positive animal, and only 0.4 percent of individual cows tested positive for antibodies. Those figures came from blood samples of over 10,000 cows across 380 herds in 21 states.
The USDA itself cautioned that this is a conservative estimate. The sampling protocol was designed to catch herds where at least 10 percent of animals were infected, meaning it likely missed many herds with lower-level infections. Beef cattle spend more time on pasture and have less direct contact with each other’s manure, which slows transmission compared to the tighter quarters of a dairy operation. Still, the disease circulates in the beef sector more than the headline numbers suggest.
Why Johne’s Is Hard to Detect
One of the biggest reasons Johne’s prevalence is difficult to measure accurately is the disease’s extraordinarily long incubation period. Calves become infected in their first weeks or months of life, but clinical signs (weight loss, chronic diarrhea, reduced milk production) typically don’t appear until adulthood, often two to five years later. During that silent phase, infected animals look perfectly healthy while shedding MAP into the environment and spreading it to other young stock.
The diagnostic tests themselves compound the problem. The most commonly used blood test, an ELISA antibody test, has a sensitivity somewhere between 75 and 82 percent when compared against fecal culture, depending on how borderline results are classified. That means it misses roughly one in five truly infected animals. Its specificity ranges from about 70 to 83 percent, so false positives also occur. Fecal PCR testing is highly specific (around 97 percent) but catches only about 27 percent of infected animals, making it useful for confirming infection but poor for screening. These limitations mean that for every cow that tests positive, others in the herd are likely infected but undetected.
Economic Cost to Producers
Johne’s disease costs the U.S. dairy industry an estimated $200 to $250 million per year. The bulk of that loss comes from reduced milk production. In herds where at least 10 percent of culled cows showed clinical signs consistent with Johne’s, the average cost worked out to roughly $245 per cow in the herd per year. That figure includes about $214 in lost milk value and $32 in higher replacement costs from premature culling. These losses accumulate quietly because infected cows produce less milk long before they show obvious symptoms, meaning many producers absorb financial damage without realizing Johne’s is the cause.
How the Disease Spreads
The fecal-oral route is the primary way MAP moves through a herd. Infected cows shed the bacterium in their manure, and calves ingest it from contaminated teats, bedding, water, or feed. Calves are most susceptible during their first months, though adult cattle can also become infected with heavy enough exposure. MAP has been isolated from the semen of bulls, raising the possibility of sexual transmission, and the bacterium can cross the placenta to infect calves before birth. Contaminated colostrum and milk are another significant route, particularly in dairy operations where pooled colostrum from multiple cows is common practice.
What makes MAP especially persistent is its ability to survive in the environment. The bacterium can remain viable in soil and water for months, meaning that even after infected animals are removed, the farm itself can serve as a reservoir of infection.
Management and Control Strategies
There is no cure for Johne’s disease, and no commercially available vaccine for cattle in the U.S. Control relies entirely on management practices that break the cycle of transmission from adult cows to young calves. Cornell University’s veterinary program recommends a layered approach: no single measure eliminates the risk, but stacking multiple precautions together gradually reduces herd prevalence over time as less-infected young animals replace older, more heavily infected ones.
The most important interventions target the calving environment and early calf care. Calves should be born in clean, dry areas used only for calving, separated from adult manure as quickly as possible. Colostrum and milk should come from cows that have tested negative on recent screening, or producers can use pasteurized milk or milk replacer instead. Identifying and removing the heaviest shedders through regular testing accelerates progress by reducing the bacterial load in the environment.
The USDA coordinates a Voluntary Bovine Johne’s Disease Control Program that provides guidance to states for managing the disease. Johne’s is a nationally reportable disease, meaning veterinarians and diagnostic labs report confirmed or suspected cases to state and federal animal health officials. Control measures typically include identifying and removing or separating test-positive animals, though participation in formal control programs remains voluntary.
A Possible Link to Human Disease
MAP causes gut inflammation in cattle that looks strikingly similar to Crohn’s disease in humans, and the bacterium can enter the food chain through infected milk and meat. Researchers at King’s College London developed a diagnostic test that detected MAP in the gut tissue of 100 percent of a sample of Crohn’s patients. In early 2025, the same team published results from a small trial of 28 Crohn’s patients who received an experimental MAP vaccine. The vaccine successfully activated immune cells that targeted MAP hiding in gut tissue, and patients on higher doses experienced symptom improvement and early signs of tissue healing, even though the trial wasn’t designed to measure clinical benefit.
This line of research is still evolving, and a direct causal link between MAP and Crohn’s has not been formally established. But the overlap between the two diseases has added a public health dimension to what was previously considered a purely agricultural problem, giving both producers and regulators additional motivation to reduce MAP prevalence in cattle herds.