Mycobacterium avium subspecies paratuberculosis, or MAP, is a slow-growing bacterium that establishes itself inside the cells of an infected host. As an obligate parasite, it can only reproduce within an animal’s immune cells, called macrophages. The organism belongs to the same family as the bacteria that cause tuberculosis but is a distinct subspecies with unique characteristics.
MAP in Animals (Johne’s Disease)
In animals, MAP infection is known as Johne’s disease and primarily affects ruminants like cattle, goats, and sheep. This chronic, progressive inflammation of the intestines is found worldwide and causes considerable economic losses for the livestock industry due to decreased productivity and animal death.
Symptoms develop slowly, often taking years to appear. Signs in cattle include persistent diarrhea, gradual but severe weight loss despite a normal appetite, and reduced milk production in dairy cows. This progression leads to a wasting condition where the animal becomes emaciated and weak.
After an animal ingests MAP, the bacteria are taken up by macrophages in the intestinal lining. The bacteria survive and multiply within these immune cells, triggering a chronic inflammatory response. This response causes the intestinal wall to thicken, impairing its ability to absorb nutrients and leading to weight loss and diarrhea.
Transmission and Environmental Persistence
The primary transmission pathway for MAP among livestock is the fecal-oral route. Infected animals, even those without symptoms, shed large quantities of the bacteria in their manure. Young animals are the most susceptible and can become infected by ingesting contaminated colostrum, milk, feed, or water.
The bacterium spreads slowly from one animal to another, making it difficult for farmers to detect a problem before it becomes widespread in a herd. This gradual increase of infected individuals can take place over several years.
MAP is exceptionally hardy and can survive in the environment for extended periods. It has been found to persist in soil for over a year and can also survive in water sources like ponds and streams. This resilience means a pasture or barn can remain a source of infection long after an infected animal is removed.
The Link to Human Disease
A potential link between MAP and Crohn’s disease in humans is a subject of scientific debate. Crohn’s is a chronic inflammatory bowel disease that shares some symptoms with Johne’s disease.
Evidence supporting a link includes the detection of MAP genetic material, and sometimes live bacteria, in the tissues and blood of people with Crohn’s disease more often than in healthy individuals. Proponents suggest the bacterium could trigger the inflammation. Potential human exposure routes include contaminated food products like undercooked meat, pasteurized milk, or municipal water.
Conversely, many researchers argue there is not enough evidence to prove causation. A counterargument is that MAP may be a harmless colonizer that takes advantage of pre-existing inflammation rather than causing it. The scientific community has not reached a consensus, as studies have not consistently found MAP in all Crohn’s patients, and the connection remains an active area of research.
Diagnosis and Control Measures
Diagnosing Johne’s disease is challenging due to its long incubation period. Veterinarians detect MAP through blood tests that screen for antibodies or by testing fecal samples for the bacteria’s DNA. Another method is attempting to grow the organism in a laboratory culture.
Since there is no cure, controlling Johne’s disease on farms relies on management strategies. This includes testing the herd to identify and remove infected animals and practicing strict hygiene to protect young animals. Managing manure to prevent contamination of feed, water, and bedding is also a key practice.
For public health, the focus is on reducing potential human exposure through the food supply. Pasteurization uses heat to kill pathogens in milk and reduces MAP organisms. However, some studies suggest the bacterium’s hardiness may allow a small fraction to survive, prompting ongoing research into the effectiveness of current pasteurization methods.