whether or not it's causing harm to your snake is unknown- 13 years is a long time. How big is your garter snake? at 13 years, he/she must be rather big! I noticed when I used to feed goldfish that my garters did not ever grow as fast, or as large as the ones I keep now.

But as always, since I am well aware of the thiaminase issue, have witnessed it's killings first hand, and have had people come rushing into my store with a seizuring, prematurely dieing young snake, I will steer far, far away from any fish known to contain thiaminase. I am fully convinced that NONE of the garter snakes I owned a few years ago are still alive because they were fed goldfish. They all showed the "symptoms" before they passed. Odd body movements, shudders, seizures even...It's just not worth the trouble. I also don't like how if I look hard enough I can find flatworms embedded in the skin and flesh of the feeders at my store. They stick out like little pimples and when cut out they can be up to 1/2 inch long. Does anyone know if these are transmissible to snakes, like, if they are burrowed in the fish in hopes of being eaten, or are they already at the end of their life cycle when they are burrowed in a fish as a cyst? They freak me the heck out.

I switched to frozen thawed silversides as the primary fish source for my snakes, along with cuts of trout and salmon, and have not looked back. My question about the frozen thawed aspect of thiaminase is - if my silversides didn't have it in them when they were alive, surely I don't see it spontaneously appearing when they are frozen and stored, since it's already documented of being present in many fish species that are alive and well? I can see how maybe freezing might make it more potent if it was already present in that particular species though.