I think this is another good point too...

Pythons, although their symptoms may be somewhat less, are just as affected as boas. There are asymptomatic carriers, so the fact that a boa or python within an infected collection does not show signs of the illness should not be taken to mean that it is immune to it. Boas are most associated with being asymptomatic carriers.
Most associated, not only associated
Signs of infection in boas include central nervous system disorders such as paralysis, being unable to right itself when turned over, "star-gazing", inability to strike or constrict. Other signs include chronic regurgitation, extreme weight loss, respiratory infections, and dysecdysis due to the inability to control body movements enough to rub off the old skin. The disease is rapidly fatal in young and juvenile boas, typified by rapid onset of flaccid paralysis.
Weight loss is a factor, but not the only factor. I've also heard of mouth rot being a good first indicator.