California’s monthslong spate of mushroom poisonings, in which four people have died and 43 others hospitalized, has become the largest known outbreak of its kind in U.S. history, experts say.

Three cases were reported earlier this week, long after the typical growing season for the mushrooms behind the illnesses, leaving public health officials and mycologists puzzled about why the poisonings have been so widespread and what is causing the trend.

The three newest cases were among a family that had picked mushrooms in Napa County on Saturday, became sick Sunday and was hospitalized at Stanford Medical Center, according to Dr. Christine Wu, Napa County’s public health officer.

Since the outbreak began in November, affected patients have ranged in age from as young as 19 months to as old as 84, according to the California Department of Public Health. Cases have come in clusters, with at least six involving families who likely shared the same batch of foraged mushrooms. Four people have received liver transplants, according to the state’s poison control system.

Poisoning cases have been reported in more than 10 counties in the San Francisco Bay Area and along California’s central coast. After cases began to spike in late fall, California health officials warned people not to eat any foraged mushrooms.

“This is definitely a much larger-than-usual outbreak. It is lasting longer and going much further into the year than we’d have anticipated,” said Heather Hallen-Adams, the toxicology chair of the North American Mycological Association. “ I don’t think we can really put a finger on why.”

The primary culprit behind the poisonings is the invasive death cap mushroom, which is also called Amanita phalloides. A second mushroom, called the western destroying angel, or Amanita ocreata, is responsible for a few cases. In an average year, about 50 cases of Amanita mushroom poisoning are reported nationwide — a total California has nearly hit on its own already. Five new cases have been reported this month.

“What’s unusual about that — the season for death caps would typically be December and January,” said Anne Pringle, a professor of mycology at the University of Wisconsin-Madison. “Why is this happening?”

The outbreak is exposing gaps in California’s public health system, as well as a lack of solid science about these toxin-producing mushrooms.

Health providers are not required to report amatoxin poisonings to the public health officials under state regulations, which means the California Department of Public Health is tracking cases through a more cumbersome and less structured process than it uses for many other conditions, such as E. coli, West Nile virus or shellfish poisoning.

“Right now, how we’re learning about these cases is actually through the California Poison Control System,” Wu said. “They’re trying to track these and then report them up to CDPH.”

Wu added that the state health department is working to add amatoxin poisoning to its list of reportable diseases. The department did not immediately respond to a request for comment.

The three family members sickened over the weekend were not Napa residents, Wu said, but had reportedly picked the mushrooms in a rural part of the county.

“These individuals were reported to me only because … they found the mushrooms in Napa County,” Wu said, adding that there was no requirement she be notified. “That’s information that I can act on to protect the other community members in Napa.”

The majority of people sickened by poisonous mushrooms in California since the outbreak began speak Spanish, though others speak Mandarin, Ukrainian, Russian, Mam and Mixteco (Indigenous languages in Central America).

Because of that, public health experts think some people may be relying on foraging practices developed outside the United States.

“We know that the death cap and the western destroying angel, when they’re growing, look a lot like some of the native mushrooms that are edible in their home countries,” Wu said.

She added that Napa County’s public health department has purchased radio ads to run in English, Spanish and Mixteco to warn of the poisonous mushrooms. California’s health department has produced flyers in nine languages.

“I’m really surprised that we haven’t been able to reach the Spanish-speaking community better, and that’s on public health,” Wu said. “So, we’re going to work harder for that.”

Death cap mushrooms are an invasive species that originated in Europe and came to California in the 1930s, most likely with imported nursery trees. Destroying angels are native to the state. In California, death caps typically grow near oak trees and occasionally pines. The mushroom is usually a few inches tall but can grow larger and has white gills, a pale yellow or green cap and a ring around its stalk.

The amatoxins the mushrooms produce can damage the kidneys, liver and gastrointestinal tract. Symptoms of amatoxin poisoning can take up to 24 hours to appear and include nausea, vomiting, diarrhea and stomach pain. More severe reactions, like fatal liver damage, can develop within two to three days. Amatoxin is responsible for the majority of all fatal mushroom poisonings. Ingesting as little as the volume of a sugar cube can be fatal.

Mike McCurdy, president of the Mycological Society of San Francisco, said death caps were prevalent over the weekend.

“This is a significant flush. It’s not just a fluky kind of thing, and it’s ranging all the way from Monterrey to Napa,” McCurdy said. “Nobody can remember a spring flush like this.”

McCurdy said he spent about 20 minutes looking for death cap mushrooms Saturday and found more than 20 “fruiting bodies” — the visible, aboveground part of the mushroom — around five coastal live oak trees.

Pringle, who has closely studied death cap mushrooms, said it’s a mystery why they are so common this year and lasting so deep into the season.

“It’s interesting and frightening,” she said.

In comparison to plants or animals, Pringle added, mushrooms are poorly studied and receive much less research funding. Scientists still don’t understand all the toxins they produce.

“Science needs funding to solve this problem,” she said. “If I could figure out a way to stop its invasion, that would enable me to stop the poisonings.”