[Lori P]

James said it well. In the wild, animals tend to wander large areas and are able to pick and choose not only thier diet but are able to move away from where they defecate. In captivity, they are confined and live in the same area that they eat and defecate in.

Consider wild horses; they live their lives without ever being dewormed. Try that with a captive horse, and he will succumb to parasite overload in no time. Because he is pastured and forced to graze the same land over and over, the same land that he passes worms onto, he will constantly re-infecet himself. And as soon as his system is stressed, his immune system drops and the parasites thrive.