my research said that Trypanosomes could be killed by three cycles of freezing and thawing. Digenea is killed by freezing, as are haemonregarines. To kill some other sporocysts that leeches may carry, ( that I do not know if snakes can catch) require freezing to -5 C. There are sporocysts that are in leeches, and snakes could represent the second host. But I do not know for sure, I merely strongly suspect. If it is the case, they would cause the snake great pain as they create cysts in the muscle tissue after the secondary host is ingested. (okay this may be greater detail then anyone wishes.) This also does not guarentee that freezing kills all sporocysts, after all trichinosis is a sporocyst and freezing does not kill all forms of that either...

that all being the same, I'm not sure I would trust a leech after being frozen and thawed three times. the research I did into the subject suggested cooking well as the only safe way to use them as pet food.

Medical leeches are not guaranteed to be free of these parasites, since non are transferable to humans (unless you eat one) they are not considered a danger to humans. My research sources all stated clearly that there is no risk to humans of contracting anything from a leech unless eaten. ( all of the illness is specific to fish, amphibians and reptiles)