It's also a symptom of several other diseases - many of which are just as fatal and just as contagious as IBD.
If you're up on on your IBD reading then you will notice what most of us have noticed - even the foremost researchers have not determined all of the ways in which it spreads, people are fairly lax about performing necropsies - so it's often implicated without verification and it's symptoms mimic countless other ailments.
Only when the inclusion bodies are in the brain do animals exhibit neurological symptoms. Most animals present with ongoing subclinical infections that most people never associate with IBD.