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As for drinking the milk from an individual known Johne's cow... personally, I wouldn't. But given that most dairy herds in this area, and probably in your area too, have some Johnes cows and the farmers and their families often drink the raw milk without any ill effects, I couldn't say it's not safe.
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It's a 'higher' bacterium... has a very thick 'waxy' glycolic acid cell wall and is extremely slow-growing - hence the fact that most animals are infected in utero or in infancy, but are adults - 2, 4, 8, 10 years of age before they 'break' with clinical disease. In the laboratory, when we're growing it on conventional HEY agar slants, the earliest that we anticipate seeing detectible growth is at 12-16 weeks post-inoculation... it's REALLY slow-growing! With some of the new liquid-growth culture techniques and detection methods, we can call a positive culture as early as 42-45 days!
MAP bacteria are engulfed by macrophages(a type of white blood cell), but because of their cell wall structure, the macrophages cannot effectively kill them, and they continue to grow and reproduce (albeit slowly) inside those phagocytic vacuoles - until the cells rupture, releasing them to be ingested by the next wave of macrophages, and the cycle begins again.
There are some antibiotics... not commonly used in, and probably not approved for use in, food animals... that can kill MAP... and have been used - apparently effectively - in treating MAP infection in other species (rhesus macaques, laboratory animals such as hamsters, etc. ) But... there are no approved or effective antibiotics which could be used to treat the MASSIVE numbers of MAP bacteria in a clinicall-affected Johne's cow... and just because an antibiotic is effective in the Petri dish, it's not necessarily going to be able to be delivered to the target tissue in concentrations high enough to effect killing of these bacteria... and even if it could, the cost might be so incredibly high to treat a mature cow or bull as to be unrealistic.
A clinically-affected Johne's cow, with what we consider to be 'high-shedder' numbers of bacteria have been shown to shed approximately 60 million MAP organisms in their feces every day - leading to some pretty substantial environmental contamination. Just another reason not to keep that Johne's seropositive cow, even though she may not be 'clinical' yet... seroconversion usually occurs right around the time that they begin fecal shedding and onset of clinical signs.
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