Clinical signs associated with Ivermectin intoxication are primarily related to general neuromuscular weakness. Death is a function of paralysis of the respiratory muscles. Ivermectin binds irreversibly and can take at least 7 days before reversal of clinical signs is observed. Signs and Symptoms Parasites are quite common in wild caught, and yes even captive bred and born reptiles. In the wild many reptiles live with few ill effects caused by the internal parasites that they might be harbouring.
Reptiles can also pick up some parasites from eating food items that contain parasites.
I'm quite sure they eat food items in the wild that have parasites in them, but again the conditions of captivity can create an environment that causes the parasites to multiply much more easily. I say qualified reptile vet because vets who have not specialized in the treatment of reptiles and amphibians may not recognize some of the parasites in the stool samples. It is important to have the stool tested because the different medications listed above work on very different types of parasitic infection. I hope this answers many of your questions. Most of the information about the drugs, dosages and toxic effects were found in Dr. Doug Mader's Reptile Medicine and Surgery.