The literature on this subject is pretty sparse, but there is no question
that the practice is risky to the wild populations, even if the captive
has been held for just a short time. In 1972 a California zoo that was
(illegally) holding 28 San Francisco garter snakes experienced an outbreak
of amoebiasis that spread throughout the collection in the water pipes.
By the time it was "controlled" the epidemic had wiped out most of the
snake collection, including the garter snakes (control was largely by
death/cull of the affected animals). The particular form of amoeba was an
*** that apparently originated in a newly-acquired wild boid, so the
potential for disaster would have been complete had, for example, the San
Francisco garter snakes been released back to the wild before the epidemic
was manifested. Fortunately, federal and state wildlife people
understood that potential and thus let the snakes remain for the time
being at the zoo. This problem presents a real conservation dilemma,
because many populations of endangered fauna (including nearly all of
the remaining SF garter snake populations) can ill-afford the removal
of even one or two reproductive ***s, but they can even less afford the
return of the captives.
In my opinion, once an animal is removed from a wild population, it should
remain a captive forevermore. This should give us some guidance on the
unnecessary removal of wild specimens.