The world's deadliest mushroom could be in your backyard

Researchers are warning that the world's most poisonous mushroon, Amanita phalloides—commonly known as the death cap—is expanding its range in southern coastal B.C. since it was first discovered here more than two decades ago.

In a paper published in the latest issue of the B.C. Medical Journal (BCMJ), UBC medical student Maxwell Moor-Smith, pharmacist and poison specialist Raymond Li, and UBC associate professor and physician Omar Ahmad write that mushroom foragers and health-care providers need to be aware of both the risks involved in consuming wild mushrooms and the symptoms of A. phalloides poisoning.

The toxic compound found in death caps, known as an amatoxin, is thought to be reponsible for about 90 percent of all mushroom fatalities worldwide. It attacks the liver and kidneys, can be lethal even in small doses, and is not rendered harmless by cooking, freezing, or drying.

After ingestion and swift intestinal absorption, initial severe symptoms of abdominal pain, vomiting, and diarrhea appear after six to 24 hours; within the next day or two, a "false recovery" phase occurs, with a lessening of the first symptoms. The third phase, four to nine days after eating the mushroom, the article notes, "is characterized by acute liver and multisystem organ failure that can lead to convulsions, hemorrhage, coma, and death".

There is no definitive antidote for A. phalloides poisoning, and if acute liver failure takes place, organ transplantation is the only cure. According to European studies, up until the 1950s more than half (60 to 70 percent) of those who consumed death caps died; advances in emergency treatments have since reduced that toll to about half of poisoned children under 10 and around 17 percent of those older (for an overall mortality rate of about 22 percent).

Because the death cap can be easily mistaken for harmless edible mushrooms like puff balls and straw mushrooms, the paper's authors warn amateur pickers to be vigilant, especially between June and November, its typical B.C. growing season. They also note that A. phalloides looks very different during its various stages of maturity, and relate local examples of misidentification, one with tragic results.

"In 2003 a 43-year-old man in Victoria consumed an immature death cap he thought was a puffball mushroom. In 2008 a 63-year-old woman in Vancouver consumed a mature death cap she assumed was a paddy straw mushroom, a common variety in Asia but one not native to North America. Both patients recovered following hospitalization. In 2016 a 3-year-old boy died after consuming a death cap foraged from a residential street in Victoria."

The deadly mushroom's range expansion is noted in detail in the paper:

"A. phalloides specimens were first collected in BC in 1997 from under European chestnut trees at Lake Errock in the upper Fraser Valley. The first identification of A. phalloides in Victoria was in 1998 from under a large European beech tree on the landscaped grounds of Government House, the residence of BC’s lieutenant governor. A. phalloides was detected in Vancouver in 2008 under European hornbeam trees that had been planted by the city in the 1960s...Since these first specimens were collected, there have been numerous reports of A. phalloides found in Vancouver and the Fraser Valley, on Southern Vancouver Island, and on the Gulf Islands.

"As an ectomycorrhizal fungus, A. phalloides forms an obligate symbiotic relationship with the roots of trees, which have been mostly nonnative, broad-leaf trees...This association may have limited the mushroom’s spread so far. However, in Victoria A. phalloides has now been found in association with native BC Garry oak trees, which may allow the mushroom to expand its range even further."

The researchers write that a California study showed A. phalloides can spread by as much as nine kilometres per year in the state's native coastal oak population, and they relate that both the Canadian Forest Service and the Oak Bay parks department announced in 2017 that death caps appeared earlier in the season and in greater abundance than in years before.

Last year, the B.C. Centre for Disease Control (BCCDC), in partnership with the Vancouver Mycological Society (VMS), launched a public-awareness campaign to educate the public about A. phalloides (see poster below).

The BCCDC, in a bulletin posted in October 2018, noted that the VMS identified more than 100 locations in the Vancouver area where death caps have been found, noting that the mushroom "is found in urban environments. It was brought in on the roots of trees that line our streets and parks, like hornbeam, European beech, hazelnut, lindens, English and red oaks, and sweet chestnut trees."

The BCMJ researchers note that "prompt recognition" of the symptoms of A. phalloides poisoning is crucial to recovery chances: "Clinicians should be particularly alert if the patient reports consuming foraged puffball mushrooms or paddy straw mushrooms, as both of these mushrooms are known to resemble the death cap. With the expanded range of A. phalloides in BC, physicians should include amatoxin toxicity in the differential diagnosis of a patient presenting with gastroenteritis or hepatotoxicity and a history of ingesting foraged mushrooms."