This paper makes a lot of good points that MAP is the cause of
both diseases.
Old Mike
http://www.gutpathogens.com/content/2/1/21
"3) The age at which an individual is infected. It takes a smaller dose of MAP to cause either ulcerative colitis or Crohn's disease in a child as compared to an adult"
I think the reason why people get crohn's disease at young age is related to the activity of the peyer's patches at that age, which open a route for intracellular pathogens to enter into deeper tissue. I don't think it has anything to do with "doses of MAP"
The issue with suggesting MAP for UC is that the genetic predisposition for UC isn't related to macrophage handling of mycobacteria.
In crohn's disease many genes overlap with other mycobacteria diseases, specifically leprosy, in UC they do not.
It is a "reasonable assumption" that someone with crohn's disease would be vulnerable to a mycobacteria, in UC this is unexpected. There's no indication that someone with UC would be vulnerable to a mycobacteria, in crohn's disease, genetic mutations dictate that someone would be very vulnerable to intracellular pathogens.
In ruminants, MAP is very specific for the ileum. The terminal ileum is the place MAP and AIEC thrive, they're extremely rare in the colon.
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Also, don't forget that many people carry MAP in their body and don't have crohn's disease, they seem perfectly capable of ridding themselves of this bacteria, which is extremely pathogenic in nature and causes a deadly disease in many many animals.
If MAP causes disease in some individuals, it's reasonable to assume that their genetic make-up predisposes them to infection, which is the case in crohn's disease, not in UC, there's no indication that someone with UC would be vulnerable to a mycobacteria.
Farmers are exposed to MAP all the time, and rates of crohn's disease aren't any higher than the general population.
IF MAP is a zoonotic pathogen causing disease in humans, it would need to be individuals that are vulnerable to specific mycobacteria, which would include people with crohn's disease, but not UC, and not the general population.