Can they be? I don't mean meme-contagious, I mean in the traditional microbe or virus sense. To be clear, I probably have a number of pathogenic organisms living in and on me now, but without some opportunity or stress they won't do much.
Clearly some of the symptoms are culturally shaped, but swelling and cold (testified to by others)? This wendigo-ness isn't something shared by other tribes, as far as I can find, so either the physical symptoms are culturally shaped too, or the tribe is more susceptible genetically than other tribes. So suppose one had an endemic infection that susceptible individuals succombed to in times of starvation or panic, that left the brain in a confused state that the culture gave a name and shape to--cannibal wendigo. I assume it's possible--I don't know if it is either remotely likely or possible to prove or disprove.
Alternatively, the disease isn't very contagious except when the victim is heavily afflicted, so the victim becomes doubly dangerous, as an attacker and as a vector.
On the "disprove" side, there are other sources of stress besides starvation and panic--the infection should come forward at other times as well, though possibly the madness would be given a different cultural shape. I haven't heard of anything. AmerIndians have high rates of mental illness, but given the general stresses involved you can't conclude anything about that except that feeling poor and useless is really bad for your health.