Thousands of mushroom species are studied and collected by amateur mushroom hunters, but only a handful cause death. False morel mushrooms (eg, Gyromitra esculenta, Gyromitra ambigua, Gyromitra infula) can cause fatal poisonings. These mushrooms are found on the ground or on rotten wood, are orange-brown to brown, have no gills, and have convoluted brainlike caps that are occasionally saddle-shaped. Gyromitra species fruit in the spring, and most poisonings occur during spring or early summer.
Some Gyromitra mushrooms contain hydrazones, including the toxin gyromitrin (N -methyl-N-formylhydrazone). Gyromitrin rapidly decomposes in the stomach to form acetaldehyde and N -methyl-N-formylhydrazine, which is converted to monomethylhydrazine (MMH) by slow hydrolysis. MMH is a water-soluble toxin that causes gastroenteritis, hemolysis, methemoglobinemia, hepatorenal failure, seizures, and coma. MMH is employed in rocket fuel and causes similar toxicity in aerospace industry workers. Cooking can render these mushrooms less toxic, although not reliably so. MMH is volatile and the fumes from cooking may cause toxicity.
MMH exposure is similar to that of isoniazid in that it generates functional pyridoxine deficiency by inhibition of pyridoxine kinase. Pyridoxine kinase inhibition interferes with production of pyridoxal phosphate, an essential cofactor for a number of enzymatic steps, including glutamic acid decarboxylase (GAD).
Gamma-aminobutyric acid (GABA), the primary inhibitory neurotransmitter of the brain, is produced from glutamate (an excitatory neurotransmitter) by the enzyme GAD. MMH also may inhibit GAD directly. The resultant GABA deficiency, with loss of inhibitory neurotransmission, may lead to seizures.
Inhibition of diamine oxidase in intestinal mucosa may be responsible for GI effects. Association of individual variability in acetylation rates (eg, slow vs fast acetylators) in hepatotoxicity is not well established.
Hemolysis and methemoglobinemia can occur. Resultant hemoglobinuria may cause renal failure.
In 2004, a total of 2,438,644 toxic substance exposures and 1,183 deaths were reported to the American Association of Poison Control Centers (AAPCC) Toxic Exposure Surveillance System. Mushroom exposures accounted for 8,601 cases and 5 fatalities. 
In the same 2004 report, known gyromitrin-containing mushrooms accounted for 52 exposures and no fatalities. Only 1 fatality was in an unknown mushroom type. 
Unknown mushroom type makes up the majority of exposures each year, usually accounting for well over 80% of mushroom exposures, but deaths in this group remain remarkably low (0-2 per year since 1996).
In 2003, known gyromitrin-containing mushrooms accounted for 71 exposures and only 1 fatality. 
In 2002, gyromitrin-containing mushrooms accounted for 66 exposures and 1 death. 
In 2001, gyromitrin-containing mushrooms accounted for 44 exposures and no deaths. 
In 2000, gyromitrin-containing mushrooms accounted for 36 exposures and no deaths. 
In 1999, gyromitrin-containing mushrooms accounted for 36 exposures but no recognized fatalities. 
In 1998, gyromitrin-containing mushrooms accounted for 77 exposures but no deaths. 
In 1997, gyromitrin-containing mushrooms accounted for 86 exposures, 3 with major outcomes, but no deaths. 
In 1996, gyromitrin-containing mushrooms accounted for 62 exposures but no deaths. 
Another study published in May 2000 used data from the National Center for Health Statistics and found no difference from the AAPCC annual report in numbers of deaths caused by mushroom exposures. 
No adequate international database exists. In the past, gyromitrin-containing mushrooms have been associated with significant mortality in Eastern Europe.
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Mortality rates from 10-40% have been reported; however, death from gyromitrin-containing mushrooms in North America is exceedingly rare.
Toxicity of gyromitrin-containing mushrooms varies by region and season.
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Children are more sensitive to gyromitrin toxicity than adults.
Overall about 60% of mushroom exposures are in children younger than 6 years. Although with Gyromitra species specifically, children younger than 6 years account for only 3% of exposures.
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