You have asked some very good questions, that really needs to be addressed every once in awhile. A very good reference book for you would be "understanding reptile parasites, a basic manual for herpetoculturists and veterinarians" by Dr Roger Klingenberg, it is under $10 at beanfarm.com.
According to the book, the top three causes of death in captive reptiles and amphibians are, 1) nutrition, 2) bacterial infections, 3) parasites. Parasites positively and absolutely without a doubt contribute to diseases in reptiles, so why do so many people refuse to believe it is an issue in their collection?
The author specifically addresses the differences between a wild and captive reptile with parasites. Most importantly, wild reptiles are fairly active. Rarely will an animal sit in an area the size of a 10 gallon for more than a day at a time. When a snake sheds in the wild, it can immediately move away from its own skin that still has all of its ectoparasites attached to it. If it defecates in one place, it probably will not crawl through it again. The point here is that a wild reptile can move away from the parasites it just eliminated, and reduce significantly the chances of direct reinfection. It does not take much for you to see that a captive reptile, especially the way some people don't take care of them, can easily reinfect themselves with their own parasites if they are not cleaned very frequently. This is especially complicated for directs cycle parasites. Many parasites require an intermediate host, like a frog or a snail, but some can be eliminated in the stool, and directly reinfect the hosts by a simple tongue flick, drinking the same water they have crawled through with fecal residue on their bodies, or even burrowing directly through the skin. Because of this simple feature of actively moving and getting away from their waste products, many parasitic problems in nature are completely self-limiting.
Another important feature for captive reptiles is stress. There are many stresses that a captive reptile must endure that would never affect the wild reptile. In captivity we give them an extremely small space to move about, we either give them an improper temperature, and / or an improper thermogradient, a lack of hiding spaces for security, improper or substrate, and handling/noises/disturbances. All of these things ultimately will result in a immunosuppression and provide parasites and subsequent secondary infections with the opportunity to root. You can be assured that for a newly captured wild hognose, the tremendous amount of stress involved with capture, transport, and then being stuck in a 10 gallon, can easily overwhelm an animal that is already burdened by a parasite load. For these reasons, it is unfair for reptile owners to expect their animals to just "deal with it" as you mentioned.
The author emphasizes the need for complete cleanliness in captivity. He lists the most problematic reasons for parasite problems and reptile collections: fecal accumulation/cleanliness, contaminated water, "musical food", a lack of effective quarantine, and the owner's personal hygiene (handwashing).
Parasites are relatively easy to diagnose, the chemotherapeutic drugs for treatment are relatively safe. Snake are the easiest group of reptiles to administer drugs to, while turtles are definitely the hardest. You must assume that all wild caught animals are loaded with parasites. All of these reasons provide very little justification for not being proactive and treating your entire collection for parasites annually.
Here are a couple of things I would like to point out:
Eastern hognoses do not eat exclusively toads, up to 50% of their diet can be invertebrates and small mammals.
ANY food source, not just amphibians, are a source of parasites. There are a few parasites that are transmissible from feeder mice. Freezing does apparently help eliminate some of the chances of contamination.
Serious reactions from a hognose bite are very rare but can happen to people with sensitivities to be stings and the such. Worst case scenario: www.herpnet.net/bite. In general, hognoses should be considered completely harmless and any bites from a hognose are directly a result of handler mistakes. I consider it very important to wash your hands before handling hognoses, especially when handling other animals in your collection. I have firsthand accounts of people getting bitten by hognoses after having their hands in a leather glove for holding raptors (this was an elderly lady that is allergic to everything, and she had no problems from the bite), and one man who didn't wash his hands after eating a bacon, egg, and cheese biscuit!
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