Fatal error

What makes the world's most dangerous mushroom so deadly? (14 minute read)

I still remember the fear and fascination I felt when I thought I’d found death cap mushrooms. It was some years ago, when I was walking on one of the less-used tracks in Wellington’s town belt. I caught a glimpse of some large, yellowish mushrooms among the pine needles which carpeted the ground, so I crouched down and brushed the pine needles away to take a closer look.

At first, it didn’t occur to me that they might be death caps. But once I looked more closely, I could see certain features, so I began comparing them to the description I carried in my mind. Photos of death caps in published field guides usually show a yellow-green cap, but they do vary in colour and can be brown, tan, yellow or white. The mushrooms I found were pale yellow, within the range of possibilities. Typically, all other parts of the death cap are white, although some part of the underside can be yellow. I picked one of the mushrooms to look underneath, and observed that it was completely white apart from the upper surface of the cap. The base of the death cap stem (known as the stipe in mushrooms), is bulbous and surrounded by the membranous remnants of the sac which encases the mushroom when young. I could see the sac at the base, but there were also remnants of it attached to the cap of my mushrooms. That’s not typical of death caps, but not unknown. Death caps also have a skirt around the stipe, which is sometimes obvious but sometimes not, particularly in older mushrooms. My mushroom didn’t have an obvious skirt, but there were signs of one.

Death cap, Amanita phalloides, growing under European beech near Melbourne (image credit: Jeremy Hegge)

I looked up at the trees around me. I didn’t see any oaks, and I knew that death caps are most often found under oaks. But they can be found under other trees too. Even if there aren’t any oak trees around, death caps can’t be ruled out.

One of the challenges with identifying mushrooms, and in fact any living thing, is that they are variable. Some look exactly like the pictures and diagrams in books or online. Others don’t look quite right. Maybe they are growing in different conditions – warmer or colder, wetter or drier, sunny or shady. Maybe they look slightly different in different countries. Maybe there are several different species contained under one name and scientists haven’t yet untangled exactly what is what. Sometimes, though, a specimen which doesn’t match the images is actually something else. I later learned that the mushrooms I found weren’t death caps. My identification was incorrect. I’d found a related mushroom, less dangerous, although still toxic.¹

But I didn’t know that at the time. I remember walking away, feeling very aware of the hand which had held the mushroom. I knew touching death caps wasn’t dangerous, but I still felt as if my hand was contaminated.

I see dangerously poisonous plants every day, whether it’s the foxglove in my garden, the karaka trees on the roadside or the ngaio trees in the park across the road. But poisonous plants don’t usually evoke the same feelings of dread as deadly mushrooms. I’ve observed this through my involvement in the group which identifies unknown plants and mushrooms in emergency situations, when they have been eaten by a person or pet. People are more fearful of fungi, and more likely to blame a mushroom than a plant in a suspected poisoning. This is largely because of the terrifying reputation of the death cap.

Life-threatening poisonings by mushrooms aren’t common, but they are much more common than plant poisonings, despite the prevalence of poisonous plants². The vast majority of these poisonings involve either the death cap or its close relatives.

Right now, death caps are in the news because of the trial of Erin Patterson, who is accused of deliberately poisoning her lunch guests with death caps, resulting in the deaths of three people and the near-fatal illness of a fourth in 2023. However, these are far from the only fatal or near-fatal death cap poisonings in Australia over the last 20 years. It’s hard to be sure how many there have been, but the most recent was in May 2024, when a Melbourne woman died and her son was lucky to survive after eating death caps. Nor is Australia alone in this problem. There are reports of fatal or near-fatal poisonings attributed to the death cap and related species from countries around the world. The greatest number of reports are from parts of Europe, India and China, although there are also cases from the USA, Great Britain, Thailand and New Zealand.

What makes these mushrooms so deadly? To answer that, I spoke with Debbie Viess, who is co-founder of the Bay Area Mycological Society in California. She has particular expertise in the mushroom group that the death cap belongs to, and the toxins these mushrooms contain. She also has many years of experience identifying mushrooms associated with poisoning.

Death caps belong to a group – in scientific terms, a genus – known as Amanita. But not all amanitas are deadly. Debbie tells me about the first type of amanita she encountered. “I came upon a beautiful little mushroom in the middle of my favourite hiking trail, and I had no idea what it was. I used to do a lot of sketching, so I dug through my pouch and found an old bank slip and a pencil, and I sketched it. I carried it around with me for years and eventually discovered it was a grisette, one of the little edible grey amanitas. That spurred me on to learn more.”

Yes, that’s right – an edible amanita. In fact, there are many edible amanitas. The genus is divided into sections³, and the sections are quite different in whether or not they are toxic, and in the toxins they contain. “There’s the grisettes⁴ and another section with choice edibles, known commonly as blushers⁵. These actually have a toxin, but it doesn’t seem to be toxic orally. If you injected it, then it’s bad news. But the toxin’s deactivated by heat and acid, so if you eat it, especially if you cook it, it's not a big deal. Then there’s the Caesar’s mushroom section⁶. All the members are edible. I've never had the Italian Caesars but I’ve eaten four of the North American Caesars. Most of them, I’m not crazy about the flavour, they’re not something I’d want to eat again. My favourite to eat is the coloured grisette, Amanita velosa.”

Amanita pallidobrunnea, an Australian species of grisette (image credit: Jeremy Hegge)

As well as the three edible sections of amanita, there are other sections which are more or less toxic. One⁷ includes the famous fly agaric, the distinctive red mushroom with white spots, which contains neurotoxins. Eating it causes nausea as well as neurological symptoms such as confusion, poor muscle control and visual distortions. In severe cases it can cause muscle spasms, seizures and coma-like sleep. On the whole, mushrooms in this section aren’t usually fatal, but they can be. Debbie explains that we aren’t sure about the toxicity of everything in this section. “The amount, intensity and exact mix of the toxins varies not only between species, but within individuals in a species. Some of them, like the panther amanitas, are definitely toxic. Others are maybe not so toxic, but I certainly don't recommend people try eating them.”

Another section⁸ has even more variation and uncertainty in the toxins. Some European and North American species in this section have dangerous kidney toxins. An Australian species⁹ has toxins which mimic certain brain chemicals, resulting in an array of symptoms including excessive sweating, salivation, blurred vision, vomiting and convulsion. Many others in this section, and others, have simply not been studied.

Amanita pyramidifera (section Roanokensis). Toxicity not reported (image credit: Jeremny Hegge)

But it’s the section which contains the death cap, known as section Phalloideae, which is by far the most dangerous. Mushrooms from this section are found worldwide, and, as I mentioned earlier, they are responsible for almost all fatal mushroom poisonings. At least half are attributed to the death cap itself, which is known scientifically as Amanita phalloides. However, in some areas, such as the USA, other species in section Phalloideae also cause fatalities. Debbie tells me that it’s hard to know exactly how many species there are, because analysis of the genes is teasing out new species all the time. However, there are two common names which are widely used in English speaking countries. “The white ones are known as destroying angels. And then coloured ones are known as death caps. I don't use a lot of common names. But destroying angel and death cap? These names are evocative of powerful, frightening mushrooms. I want people to be a little taken aback.”

One of the death cap’s cousins in section Phalloideae, Amanita marmorata, native to Australia and recently found in New Zealand, where it has been spreading (image credit: Jeremy Hegge)

These mushrooms really do deserve their chilling names. The course of the illness they produce is particularly horrible. Most toxic mushrooms produce symptoms within 1-2 hours of eating them, but with death caps and destroying angels, the symptoms take at least 6 hours to develop, often longer. By that time, the toxins have already been absorbed and circulated through the body. The next phase of the illness is known as the gastro-intestinal phase, with nausea, vomiting, cramping and diarrhoea, which can be severe. This phase lasts 12-24 hours, but then seems to resolve. If doctors haven’t correctly determined what has caused the illness, people can be discharged from hospital at this point. However, the toxins are still causing harm in the body, particularly the liver and kidneys. The apparent recovery can last for a day or two, but then the liver and kidneys begin to deteriorate and organ failure can occur. The prognosis depends to some extent on the amount of mushroom eaten, but without treatment around 50% of people who are poisoned die.

What is it that makes death caps and destroying angels so deadly? Debbie tells me that they contain three groups of toxins, the amatoxins, phallotoxins and virotoxins¹⁰. “It's the amatoxin that sickens us and that destroys our organs. Phallotoxins are also highly toxic, in fact, maybe even more so than amatoxins, but they're not toxic orally. Virotoxins aren’t toxic orally either. But amatoxins are easily absorbed from the digestive tract, and they’re not broken down by heat, acid or digestive enzymes.”

Amatoxins bind to a critically important enzyme¹¹ in our cells, which stops protein synthesis (that is, it stops cells from making proteins) and causes cell death. “This affects the areas in the body that have the most rapid cell replacement. So that's your gut lining and your liver. Eventually, it can affect all your organ systems, but it starts with your gut, so you have the violent vomiting and all of that. Your gut is ravaged but once the amatoxin is gone from there, then it will heal. Most of the amatoxin is excreted by your kidneys quite rapidly, but in cases of severe dehydration it can affect the kidneys. This is dangerous, because your kidneys are the only way that you excrete the toxin out of your body. So that's why aggressive intravenous rehydration has been the number one way to save people that have been poisoned.”

The most dangerous part of amatoxin poisoning is the way it affects the liver. “Within the liver, it gets recirculated, and it continues to kill cells there unless you can manage to flush it out. Amatoxin is bound to bile salts, and is shunted to the gall bladder. Then amatoxin-contaminated bile is released from the gall bladder to the duodenum and used to digest food in the intestines. But 95% of bile salts are reabsorbed from the intestine and returned to the liver, so the amatoxin is also returned to the liver and causes secondary poisoning. It just continues cycling, harming the liver, then harming the liver again, and again. It can completely destroy the liver, and you can’t live without your liver. But, the thing about the liver is that you only need a little piece left, and it can regenerate. So people who have been poisoned but survive generally have no after effects unless they've had to have a liver transplant. That's its own trouble, but at least you're alive.”

Liver transplants have been the last-ditch treatment for amatoxin poisoning for some years. They aren’t used often, but have saved lives in a number of cases, including a man in New Zealand who ate death caps in 2005. There are also various drugs which have been used with some success, and others are being investigated. Debbie tells me that even with prompt treatment around 10% of poisonings in the US result in death. But she still notes that it’s a very painful poisoning and that treatment is expensive.

Although the vast majority of amatoxin poisonings result from eating death cap or its close relatives in section Phalloideae, Debbie mentions that there are a few other mushrooms which have amatoxins too. “But they’re small, so they're not the sort of things people eat. Well, people do eat them, but they don't eat them as frequently as the big, showy, delicious, meaty amanitas. But if people are eating little brown mushrooms, they might end up eating something like Galerina, which also has amatoxins.”

Although such poisonings are rare, there was a recent case in Utah where a man died with symptoms of amatoxin poisoning after eating what he believed were psychedelic mushrooms, but was probably Galerina. These mushrooms, with the evocative common name ‘funeral bells’, are found in many places around the world. Although there’s considerable uncertainty about how much amatoxin different mushrooms in this group contain, it’s certainly a warning to anyone who consumes little brown mushrooms.

But I have to come back to the true death cap, Amanita phalloides, because there’s something else about this mushroom which makes it the most dangerous of the amatoxin-containing mushrooms. As well as being the deadliest amanita, it is also one of the most invasive. It’s showing up in places where it has never been seen before, and where people may be unaware of its dangers.

The death cap is originally from Europe, but it has now spread to many other parts of the world. Amanitas are fungi which form associations with tree roots, and the death cap has a particular association with the oak and beech families of trees. It has spread with the movement of these trees, and in new areas is usually found under the species it was introduced with, at least at first. However, in new areas it is moving onto different kinds of tree. In eastern North America, it is mostly found under pine trees. In western North America, it is often found with their native oak species. In Australia and New Zealand, it is mostly found with European oak trees, but the possibility it could be found under other trees shouldn’t be ignored. The death cap has also reached South Africa, South America and parts of India, although I don’t have information on which trees it is associated with there.

In California, the death cap seems to be particularly successful. It has been expanding its range of associated trees, producing mushrooms year-round instead of just autumn, and growing larger than anywhere else in the world. In New Zealand, the death cap has been moving steadily southwards. Although it hasn’t yet been found in Wellington, it has now crossed Cook Strait, and was found in the South Island in 2022. It’s also spreading in Australia. A survivor of death cap poisoning in Canberra recounted that he was poisoned by mushrooms he harvested from an area where he’d foraged many times when he was younger. The death caps were a new arrival, and he was completely unaware of them.

While we are getting better at treating amatoxin poisoning, more people are likely to encounter death caps in future, and we need to be aware of the danger. The tragic poisonings which are now before the Australian courts were exceptional, and exactly what happened is up to the jury to decide. Putting that case aside, poisoning by death caps and other amatoxin-containing mushrooms is entirely avoidable. While there are those who suggest never eating a mushroom which wasn’t bought at the supermarket, I think that this is overly cautious advice. However, anyone collecting wild mushrooms to eat must have sufficient knowledge of both edible and poisonous species if they are to avoid a fatal error.

Reliable identification is a skill which takes time and effort to develop. It requires practice and feedback from more knowledgeable people, it’s not something which can be learned from a book or replaced with an app. As it says in one of the articles Debbie has written about edible amanitas, visiting a website or reading a book is insufficient information upon which to risk your life. If you make an ID mistake, it could be your last.

If you’d like more information, Debbie has written a number of articles on amanitas, death caps and amatoxin poisoning.

Amanita phalloides: Invasion of the Death Cap

Amatoxin: The Deadliest of All Mushroom Poisonings

A list of articles about amanitas from the Bay Area Mycological Society

Next week, I’ll be following up with an article about identification and foraging safety for both mushrooms and plants.

1. For those who are curious, it was Amanita junquillea. It’s a member of the same section (Amanita section Amanita) as the familiar red and white fly agaric, with similar neurotoxins. It’s another variable species and becoming common around Wellington.

2. In humans, at least. It’s different for animals. Anecdotally, through my work with the identification group, there are a reasonable number of dog poisonings associated with mushrooms, including some fatalities from death caps. Cats, however, are most likely to suffer life-threatening poisoning from lilies.

3. I have mostly put the scientific names for the sections as footnotes for those who want more detail.

4. Section Vaginatae

5. Section Validae

6. Section Caesareae

7. Section Amanita

8. Section Roanokenses

9. Amanita preisii

10. These toxins are all known as cyclopeptides. Peptides are made of the same basic molecules as proteins, but are smaller, and cyclopeptides are peptides where part of the structure is in a ring.

11. The enzyme is RNA polymerase II, and it’s responsible for using DNA as the template to synthesise the precursor molecules for messenger RNA (mRNA) and some other kinds of RNA.