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The secret of the four-chambered stomach: Why are ruminants the main victims of John's disease?
John's disease, a chronic infection caused by Mycobacterium avium subspecies paratuberculosis (MAP), primarily affects the small intestine of ruminants. This contagious disease affects not only cattle but also a variety of other species, such as sheep, goats, and even a few non-ruminant animals such as rabbits and birds. The disease has been reported around the world, but only some states in Australia have been certified disease-free.
Ruminants are the main victims of John's disease because their physiological structure is closely related to the reproduction of the pathogen.
MAP is a bacterium similar to human Mycobacterium tuberculosis, but it grows more slowly and often requires specific chemicals to promote its growth in the laboratory. This bacterium specifically infects mammals, not birds, which puts ruminants, especially cattle, at greater risk if they become infected.
Key signs and symptoms
Ruminants such as cattle often show symptoms such as subtle weight loss, reduced milk volume, and rough hair in the early stages of John's disease. As the disease worsens, typical symptoms such as diarrhea and "bottleneck edema" appear, demonstrating the devouring power of AIDS.
If cattle have already developed diarrhea, this is almost in the final stages of John's disease and subsequent death seems inevitable.
These symptoms usually appear more than two years after the animal is infected. However, newborn animals are most susceptible to infection when they come into contact with infected feces or mother's milk. In fact, pathogens in breast milk and the environment are major sources of infection in newborn calves.
Pathophysiology
The main site of invasion of John's disease is the ileum of the small intestine, which is rich in lymphoid tissue. When MAP enters these lymphoid tissues, it would normally be engulfed by macrophages, but the bacteria are able to survive and multiply within the macrophages, thus forming an infection state that cannot be effectively eliminated.
Although macrophages try to phagocytose MAP, they are unable to perform their task effectively for some reason, which is the main reason why the disease continues to worsen.
Human risks and prevention and control measures
Although John's disease has not yet been established as infectious to humans, MAP causes clinical symptoms in some populations that are similar to enteritis in humans, which raises alarm. To date, the World Health Organization has not classified it as a zoonotic disease. Regular annual testing of cattle health is considered one of the effective control measures.
One of the most effective ways to protect against John's disease is to pasteurize milk, which kills the pathogen after it is heated.
Conclusion
John's disease affects a variety of ruminants worldwide, and with continued research, our understanding of this disease is gradually deepening. However, why certain animals are particularly susceptible to falling victim to the disease remains an unexplored question.
