• Equisetum arvense [Cornell]
  • Pteridum aquilinum - Bracken [Cornell]
  • Marsilea drummondii - Nardoo [Cornell]
  • Raw flesh and viscera of certain fish and shellfish [Cornell]
  • Ferns & Some bacteria [Cornell]


Thiaminases are enzmyes found in a few plants and the raw flesh and viscera of certain fish and shellfish. When ingested these enzymes split thiamin (Vitamin B1), an important compound in energy metabolism, and render it inactive.[Cornell]


Thiaminases are enzymes that cleave the thiamin molecule and render it biologically inactive. Generally there are two types of thiaminases:
•Type I - the most common form, this type is found in fish, shellfish, ferns and some bacteria. It acts by displacing the pyrimidine methylene group with a nitrogenous base or SH-compound to eliminate the thiazole ring.
•Type II - found in certain bacteria, this type acts through the hydrolytic cleavage of the methylene-thiazole-N bond to yield pyrimidine and thiamin moieties.
Both types of thiaminase require a cosubstrate - usually an amine or sulfahydryl-containing compound such as proline or cysteine. Once the thiamin molecule is cleaved by a thiaminase the body is incapable of restoring it. Thus, the ingestion of significant amounts of thiaminases can induce thiamin deficiency even though there may be a sufficient amount of thiamin in the diet. [Cornell]

Many thiaminases are denatured by heat, but apparently vary in their heat stability.[Cornell]

Subjecting thiaminases found in some types of raw fish and fish entrails to cooking or other heat treatment will render those thiaminases inactive enough to prevent thiamin deficiency in carnivores. The original inhabitants of Australia soaked thiaminase-rich nardoo in water for a time and cooked them into bread and soup before ingesting them, thus avoiding the thiamin deficiency symptoms presented by people who eat nardoo raw. Fern thiaminases may not be completely destroyed by cooking, so until it is clearly established which combinations of cooking time, cooking pressure and cooking temperature is needed to eliminate pteridophyte thaminase, caution should be exercised in consuming ferns as human food. [Cornell]

Signs of Thiamine Deficiency in Humans

In humans thiamin deficiency leads to a disease termed "beri-beri". Symptoms of beri-beri are basically the same as thiamin deficiency in other non-ruminants - anorexia, cardiac enlargement, and muscular weakness leading to ataxia. However, the disease has been divided into the following two forms:
1.Dry beri-beri - usually without cardiac involvement, this form of the disease is typified by atrophy of the legs and peripheral neuritis. It occurs mainly in adults.
2.Wet beri-beri - the primary sign of this form of the disease is cardiac enlargement and edema. [Cornell]

Signs of Thiamine Deficiency in Ruminants

Thiamin deficiency in ruminants manifests itself as polioencephalomalacia. Signs of polioencephalomalacia include disorientation and wandering, blindness and opishotonus or retraction of the head. The brain of infected animals becomes inflamed and edematous. Ruminants will also show symptoms as seen in other animals - anorexia, poor feed utilization and weakness. [Cornell]

Normally ruminants are fairly resistant to thiamin deficiency since rumen microbes provide the animal with sufficient amounts of thiamin. However, the ingestion of thiaminases will lead to polioencephalomalacia. Additionally, young growing ruminants, especially cattle and sheep, fed high-grain diets are especially susceptable. Diets high in grains can encourage the growth of certain thiaminase-producing bacteria in the rumen. These bacteria, including Clostridium sporogenes and a few species of Bascillus can produce enough thiaminases to induce thiamin deficiency.[Cornell]


Bracken Fern (Pteridum aquilinum)

Bracken fern is widespread in humid temperate areas including the North American West Coast, Europe, Japan, Australia, and New Zealand. The highest thiaminase activity is in the rhizomes, but all areas of the plant contain some thiaminase and show seasonal variability as follows: [Cornell]


Non-ruminants consuming significant amounts of bracken fern in their diet will exhibit signs of thiamin deficiency. Horses fed hay with greater than twenty percent bracken fern will begin to be symptomatic within a month.[Cornell]

Horsetails (Equisetum arvense)

Horsetails are widespread in moist areas of the United States and Canada and contian significant thiamin activity. The most common instance of thiamase poisoning resulting from horsetails is the contamination of hay with horsetails. A horse consuming hay containing twenty percent or more of horsetail plants will show signs of thiamin deficiency in two to five weeks.[Cornell]

Historical Perspectives

The story of Australian explorers, or why you should cook your ferns.

Okay, so legend has it, and so does Moorehead's book Cooper's Creek (1963), that a couple of Australian explorers, Robert Burke and Willliam Wills, died in 1861 from thiaminase poisoning. They were wandering around the Australian continent, doing what explorers do, and their supply of pork, which was their main source of thiamin, ran out. So what did these guys start feasting on? Nardoo, of course. Now, if they had watched the Aborigines they would have known that one does not eat nardoo without cooking it first. But, being typical explorers, and thus thinking that they were smarter than the indigenous people that had lived on the continent for thousands of years, Burke and Wills ate their nardoo raw. They began complaining in their journals of increasing weakness and starvation, but "not so much from absulute want of food" because, Wills wrote, "I have a good appetite and relish the nardoo much but it seems to give us no nutriment." What they were suffering from then, was thiamin deficiency. Coupling a low-thiamin diet with a large intake of thiaminases from the nardoo, Burke and Wills developed beri-beri. They complained not only of weakness, but of edema too, and sensitivity to cold. They died. [Cornell]


  1. [Cornell] THIAMINASES, , Updated 05/22/2014, Accessed Feb 13, 2015,