All About Thiaminase

Setting the record straight about thiaminase, the enzyme that causes vitamin deficiencies in fish-eating snakes.


Anyone who keeps garter snakes has probably heard about thiaminase, and how its presence in fish can be harmful to the garter snakes that are fed that fish. A lot has been said and written about thiaminase. Trouble is, a lot of what has been said and written isn’t exactly right. Some of what has been said and written is flat-out wrong.

What I want to do here is set out, one step at a time, what thiaminase is, what it does, why it’s a problem, and how to avoid it. Let’s start at the most basic level.

What is thiaminase?

Thiaminase is an enzyme found in some fish species, as well as some plants, bacteria and an African species of silk worm.

What does it do?

Enzymes catalyze chemical reactions in living cells, and play a role in a body’s biochemistry. We don’t actually know what specific role thiaminase plays in the species that contain it. But we do know that it breaks down the thiamin (vitamin B1) molecule.

Is that bad?

Oh yes. If you eat something that contains thiaminase, you won’t get any thiamin out of it. And all living things need thiamin for their bodies to function properly: it helps convert carbohydrates into glucose and is necessary for the nervous system to work properly. While plants, fungi and bacteria can synthesize their own thiamin, animals must get it from their diet.

Animals whose diet is deficient in thiamin suffer from neuropathic and cardiovascular disorders. Among human beings, these disorders include Korsakoff’s syndrome, a neurological disorder whose symptoms include amnesia, and beriberi, a nasty and sometimes fatal sickness with multiple symptoms that affected those with poor diets in southeast Asia. Vitamin deficiencies are not fun.

What does this have to do with garter snakes?

Like every other animal, snakes need thiamin. But snakes that eat fish are at a higher risk of thiamin deficiency because quite a few fish species contain thiaminase. If a snake’s diet is mostly or completely made up of fish that contain thiaminase, that snake is going to develop a thiamin deficiency.

Since garter snakes are often fed fish in captivity — and it’s frequently the same kind of fish over and over again — they often develop thiamin deficiencies. (So do water snakes.)

What happens to a snake with a thiamin deficiency?

I saw it happen myself when I was a child. When I was eight years old, I was given a Red-sided Garter Snake (Thamnophis sirtalis parietalis). The pet store gave us frozen bait minnows (salted!) to feed the snake, which I did, dutifully, a couple of times a week.

A few months later the symptoms of thiamine deficiency turned up. My snake repeatedly went into convulsions, thrashing around the cage, gaping, and generally looking like it was having seizures. (For an eight-year-old kid this was a frightening sight!)

A call to the local herpetologist — Ken Stewart at the University of Manitoba — revealed the problem. It was the first I’d heard of vitamin B1 deficiency; none of the books I’d read mentioned it. (Then again, this may not have been widely known: this was around 1980. At about the same time, Mike Rankin, working at the Canadian Museum of Nature, lost more than 60 garter snakes and water snakes because of thiamin deficiency, he later told me.)

What should I do if that happens to my snake?

The treatment is simple: get some vitamin B1 into the snake. Once that happens, the snake will recover. There are a couple of ways of doing this.

One is to get a vet to administer a vitamin B1 injection.

Another is to feed the snake — or force-feed, if needed — a diet rich in vitamin B1. In my case, that’s what was done: my parents force-fed my snake liver coated with thiamin. (For the record, my snake recovered and lived two or three more years.)

Thiamin is a water-soluble vitamin and as such it’s hard to overdose.

How do I prevent that from happening again?

Thiamin deficiency happens when a snake eats nothing but fish that contains thiaminase. You need to remove the thiaminase from the snake’s diet, or at least limit it.

One way is to remove the thiaminase from the fish in question. You can do that by denaturing the thiaminase, which involves heating the fish to about 80°C (175°F) for a few minutes, but that strikes me as just nuts. It’s far easier simply to feed the snake something that doesn’t have thiaminase in the first place.

You could offer the snake a more varied diet, so that the snake can get its thiamin from the other things it eats, like worms, other kinds of fish, and mice.

You could switch to a species of fish that doesn’t contain thiaminase. There are quite a few of them. More on that below.

But best of all, in my opinion, is to feed the snake an all-mouse diet, if it will take it.

Which fish contain thiaminase?

The following fish have been reported to contain thiaminase:

A number of clams, mussels, shrimp and other aquatic invertebrates have also been reported to contain thiaminase, but I don’t expect you to feed those to garter snakes.

Which fish don’t contain thiaminase?

The following fish have been reported safe — or at least, free from thiaminase:

Obviously you’re not going to be feeding barracuda to a garter snake: some of these are moot in a practical sense. But you get the idea.

Certainly there are several fish that are free from thiaminase that are relatively easy to acquire; in most cases you’ll be going to the supermarket instead of the bait store. But in most cases this means fish fillet rather than whole fish, and that means you’ll have to supplement the diet. But at least frozen fish fillet doesn’t have parasites.

Even though rosy reds and goldfish have thiaminase, there are still some options left if your snake will only eat live fish. Feeder guppies and platies, though not mentioned above, are generally considered safe from a thiaminase perspective (though, like all live fish, hardly risk-free in terms of internal parasites). These are widely available in pet stores.

Is it true that ocean fish are safe and freshwater fish are not safe?

No. There are freshwater fish that have thiaminase and freshwater fish that don’t, and there are oceangoing fish that have thiaminase and oceangoing fish that don’t. You can’t generalize it that way.

Is it true that frozen fish contains thiaminase and fresh fish doesn’t?

No. Some species have it, some species don’t. That doesn’t change whether they’re alive or dead, fresh or frozen.

But all the books say that frozen fish is thiamin deficient, that thiamin deficiency is a result of feeding frozen fish to snakes.

They’re all wrong — or likely one authority was wrong, and the rest are repeating the advice without confirming it for themselves.

Several books argue that the thiaminase enzyme forms after death, which makes live fish safe and frozen/thawed fish the problem. These are the authorities who argue in favour of heating the fish to denature the thiaminase, because in their view thiaminase can’t be avoided when using frozen fish. (And of course live fish is unsafe because of parasites. They’ve argued themselves into quite the corner.)

But freezing or dying will not create thiaminase in a fish that didn’t have it in the first place; if it did, my childhood garter snake would have come down with thiamin deficiency again after the following winter’s diet of ocean perch.

And if the species does contain thiaminase, it will have it when it’s alive. Wild salmon who feed on fish rich in thiaminase have developed thiamin deficiency, as Marco Lichtenberger points out.

The offspring of salmon from the Baltic Sea — which apparently feed mostly on thiaminase-rich herring and relatively little food that contains high levels of Vitamin B1 — were found to suffer from a condition called Reproduction Disorder M74. This was later identified as being simply one particular form of Thiamine Deficiency Syndrome. The eggs produced by adult salmon were provided with very little thiamin, and the fry that emerged almost all died soon after hatching. Comparable problems have been found among salmonids in the Great Lakes of North America, and this has been hypothesized to be related to a diet containing a large proportion of alewives, another type of thiaminase-rich fish.

So freezing doesn’t have anything to do with thiaminase. It’s better to say that freezing doesn’t solve the thiaminase problem, since freezing does eliminate the internal parasites that make live fish such a risky food item.

This sounds awfully complicated.

It isn’t, really. Let me sum up:

  1. Some fish contain thiaminase, which destroys thiamin (vitamin B1), an essential nutrient.
  2. It doesn’t matter whether the fish is frozen or fresh. Some species have thiaminase, some species don’t.
  3. Snakes fed nothing but fish with thiaminase develop thiamin deficiency.
  4. Thiamin deficiency is potentially fatal but can be treated with massive doses of thiamin.
  5. Don’t feed fish with thiaminase to your snake. There are lots of alternatives.


Thiaminase is not just a concern of people who keep fish-eating snakes, and in a way that’s a good thing, or we wouldn’t know as much about it. Aquarium hobbyists know about thiaminase too, because turtles and carnivorous fish can also develop thiamin deficiencies when they’re fed fish with thiaminase. So do fisheries biologists and fox breeders, for similar reasons.

The lists of fish with and without thiaminase were compiled from the following sources: Nutrient Requirements of Mink and Foxes, 2nd revised edition (1982), pages 64 and 65; Austin’s Turtle Page; The Aquarium Wiki; and Marco Lichtenberger’s article on Wet Web Media.

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