The B.C. Centre for Disease Control has issued an alert for residents of southern British Columbia to watch for the world's deadliest mushroom this fall, and especially this Thanksgiving long weekend.

In an October 7 alert, the BCCDC warned British Columbians "to keep an eye out for poisonous mushrooms, especially around young children and pets as we head into the long weekend....With fall’s cooler and wetter weather, mushrooms can now be found in both urban areas and forests."

Specifically, the centre wants people to be especially alert for the death cap mushroom, which it describes as "the most poisonous mushroom in the world".

The deadly fungus attacks the liver and kidneys, sometimes resulting in death or the need for organ transplantation. In 2016, a three-year-old in Victoria died of death cap poisoning.

The death cap (scientific name Amanita phalloides) is an invasive species first discovered in Vancouver more than two decades ago. It is thought to have come here from Europe on the roots of imported trees.

The poisonous fungus has been found growing on urban lawns in and around Victoria on Vancouver Island, on Galiano Island, all over Vancouver's streets and boulevards, and up the Fraser Valley.

The BCCDC release said the Vancouver Mycological Society (VMS) had identified more than 100 death cap sites in Vancouver alone after  a 2008 survey. A VMS death cap time line shows 1997 as the year of the first discovery of death caps in B.C., near Mission, with an initial Victoria sighting coming the next year. Vancouver's first death cap appearance was in 2008, beneath hornbeam street trees.

By 2017, the death cap had announced itself in suburban and agricultural aeas of Surrey, Mission, and Langley, mostly under old sweet chestnut and hazelnut trees. Other trees favoured by A phalloides are hornbeam, beech, linden, and oak.

Vancouver mycologist Paul Kroeger, who has studied B.C. mushrooms for more than three decades, helped with an investigation after a sighting near Victoria in 2016 of A. phalloides specimens that had associated with the roots of Garry oak trees, which inhabit unique grassy meadows and woodlands in the southern part of Vancouver Island. Kroeger, along with Shannon M. Berch and Terrie Finston, published a paper about the discovery, the first with a native tree, in November 2016 in the.journal Botany.

Kroeger, who is also a consultant for the B.C. Drug and Poison Information Centre (DPIC), told the Straight by phone that commercial importation almost certainly is responsible for the death cap spread in the Lower Mainland and Vancouver Island. "Most of the Vancouver appearances were associated with street-tree plantings in the 1960s," he said.

"We think there's a strong association with the recent spread [of A. phalloides] and the tree-nursery and horticulture industries."

Approximately 90 percent of fatalities caused by mushrooms worldwide are thought to be due to A. phalloides. The mortality rate after ingestion of death caps is about 22 percent overall, with children under 10 only having a 50-50 chance at surviving (those over the age of 10 have about a 17 percent mortality rate).

The fungus's toxic compound, called amatoxin, is fatal in small doses and cannot be neutralized by cooking, drying, or freezing. Once eaten, symptoms such as vomiting, diarrhea, and severe abdominal pain can appear within a half-dozen hours and up to a full day or more. Those warning indications often subside after a day or two, only to be replaced by severe signs of liver, kidney, or multisystem organ failure within four to nine days. These can lead to hemorrhaging, convulsions, coma, and death.

The BCCDC warns that pets are at grave risk as well.

One of the contributing factors to the accidental eating of death caps is the fact that at two stages of its fruiting development, it resembles edible mushrooms: an early aboveground stage resembles the popular puffball mushroom, and a later stage bears a likeness to paddy straw mushrooms, which are sometimes used in Asian cooking.

Although death caps typically appear in the fall's wetter weather, they can appear on summer lawns and boulevards after heavy rain.

DPIC pharmacist Raymond Li, quoted in the BCCDC release, said that mushroom exposures involving children are usually responsible for a majority of calls to the poison centre. "Typically, around two-thirds of our mushroom calls involve children five and under, so parents and caregivers need to be mindful of what's on the ground where their kids are playing. But this year we have noticed an increase in mushroom calls involving adults."

Kroeger told the Straight that if you have handled death caps, it's best to take a simple precaution. "You don't absorb much toxin through the skin, but you want to wash your hands afterwards."