What is thiaminase?
Thiaminase is an enzyme, a chemical compound that destroys or inactivates thiamine. Thiamine is an important vitamin also known as Vitamin B1. There is not just one type of thiaminase, but several different ones, some of which can be produced by bacteria, fungi, plants and potentially animals.
The lack of Vitamin B1 in humans is called beriberi.
Thiamine Deficiency Syndrome and its symptoms
Vitamin B1 (also known as thiamine, thiamine hydrochloride, and in older text books, as aneurine hydrochloride) is an essential nutrient for most animals. It is a colorless and water soluble chemical that helps to convert carbohydrates into glucose. It is particularly important for the correct functioning of the nervous system. A lack of Vitamin B1 is called a Thiamine Deficiency Syndrome.
Symptoms of this syndrome are well known from several commercially important fish groups and can be confirmed using appropriate biochemical tests. Flatfish fed exclusively with thiaminase-rich clams suffer and die from paralysis and related physical shocks. Eels show a trunk-winding syndrome and hemorrhages along the base of the fins (similar symptoms have been reported from moray husbandry, too). Salmonids show nervous disorders, poor appetite, poor growth and jumpiness (again, similar things have observed among a variety of ornamental fish species). Skin congestion and haemorrhage have been reported from carp and other cyprinids. In general then, excessive amounts of thiaminase are connected with symptoms of sickness that include poor growth, loss of appetite, abdominal swelling and hemorrhage, loss of equilibrum, convulsions, muscle atrophy and a weak immune system.
While it has not yet been scientifically proven that pet fish suffering from the above mentioned symptoms have Thiamine Deficiency Syndrome, the parallels with their food fish relatives are striking. The problems of thiaminase are now well known in the professional fields of animal nutrition (e.g. fish farms), but so far this information has not been widely taken up by aquarists and pet owners. But it is clear that those hobbyists keeping large predatory fish and other carnivorous animals need to be familiar with the problem of Thiamine Deficiency Syndrome, and use that information to make sensible choices when selecting food for their livestock.
Aren't thiaminase containing fish eaten in the wild?
Yes, they are, and this can cause predatory fish massive problems. 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.
However, most of the time predatory fishes maintain a kind of balance between those prey fish rich in thiaminase and those fish rich in Vitamin B1. As long as the predator has a reasonably varied diet, it should get enough Vitamin B1 to stay healthy. It should be mentioned that in the examples of the sick salmonids from the Baltic, the key problem was that they were not eating a varied diet, but mostly consuming just one type of prey.
The big problem for captive fish is that they are fed frozen fish. Thiaminase is not destroyed by freezing, and over time will break down whatever Vitamin B1 is present in the frozen fish. The longer the fish is stored, the less Vitamin B1 it will contain. Furthermore, any fish fed such frozen fish will be consuming the thiaminase, and that will destroy some of the Vitamin B1 it already has. Making things even worse, freezing and thawing both break down some of the Vitamin B1 content of food as well.
While freezing does not destroy thiaminase, heating it will. This is why cooked fish is not dangerous with regard to thiaminase for human or animal nutrition. From the perspective of a fishkeeper, the big drawback to cooking food is that heating destroys a lot of the useful nutrients as well. While omnivorous humans compensate for that by eating a varied diet containing both raw and cooked plant and animal foods, piscivorous fish have no such option. They cannot be fed cooked fish and expected to stay healthy.
At least some types of live feeder fish will contain more Vitamin B1 than frozen foods, but the downside here is that the convenience of live foods is accompanied by a major risk of introducing pathogenic microorganisms such as mycobacteria and endoparasites. Feeder fish are also expensive compared with frozen foods, and as will be described shortly, many of the types of feeder fish widely sold contain a great deal of thiaminase anyway, dramatically reducing their usefulness.
