Caveat: most information on M. paratuberculosis infection and Johne’s disease has been obtained through studies on domestic species (for example cattle, sheep, goats). While it is believed that this information likely is applicable to bison as well, there may be some facets that differ for which we do not yet have a good understanding. Please bear this in mind as you use this web-site.
It
is generally believed that the majority of infections occur by the animal's consuming
the organism through contaminated water or feed. The organism crosses from the
lumen of the small intestine via a specialized portal cell ("M cell"). The animal's
response to this infection begins when the living organism is then taken up by
a cell that is part of the immune system, the macrophage. The organism can remain
and replicate in this cell for long periods of time, during which the animal shows
no sign of infection.
There is a range
of pathologic lesions that can be seen in bison. This variation is due both to
the stage of infection at necropsy and the species being examined. For example,
in some cases, there can be a complete lack of gross lesions - the gastrointestinal
tract may appear completely normal. 
In
cases at the other extreme, the ileum appears thickened and corrugated and the
mesenteric lymph nodes may be enlarged and edematous. This thickening and enlargement
results from the extensive number of immune cells infiltrating the tissue to combat
the innumerable organisms swamping the tissue. Bison may be in good body condition
or be emaciated with a total absence or necrosis of abdominal fat stores. Cases
may fall between these two extremes of (1) nonexistent and (2) marked gross indications
of infection. To assess subtle tissue changes, it can be helpful to compare a
segment of ileum directly with a segment of duodenum and check for relative differences
in lymph vessel distention, mucosal color and thickness, etc.
When the gastrointestinal
tissues (ileum, ileo-cecal lymph node, mesenteric lymph nodes are the best sites)
are examined microscopically, the animal may present any one of a number of pathologic
pictures. Again this is a reflection of the stage of infection and elements of
the immune response. In addition, the statistics of sampling are against lesion
detection for early-stage cases: it is much more likely to collect a sample of
uninfected vs. infected tissue for histopathology
from 3 feet of ileum. Diligent scrutiny of numerous fields may be necessary before
happening upon a hint of acid-fast staining material or a giant cell. Even after
an extensive survey, the tissues may appear normal in virtually every field. At
the other extreme, florid signs of infection may be seen throughout with massive
numbers of giant cells packed with acid-fast staining rods.
The majority of cases will fall somewhere between these two scenarios.
(While not directly pertinent to bison in some aspects, see the web pages under "Dairy" for a broader description of this topic.)
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