Doctors in Australia had screened, scanned and tested a woman to find out why she was sick after being hospitalized with abdominal pains and diarrhea. They were not prepared for what they found.

A three-inch red worm was living in the woman’s brain.

The worm was removed last year after doctors spent more than a year trying to find the cause of the woman’s distress.

The hunt for the answer, and the alarming discovery, was described this month in Emerging Infectious Diseases, a monthly journal published by the Centers for Disease Control and Prevention.

The woman, whom the article identifies as a 64-year-old resident of southeastern New South Wales, Australia, was admitted to a hospital in January 2021 after complaining of diarrhea and abdominal pain for three weeks. She had a dry cough and night sweats.

Scientists and doctors from Canberra, Sydney and Melbourne said in the journal article that the woman was initially told she had a rare lung infection, but the cause was unknown.

Her symptoms improved with treatment, but weeks later, she was hospitalized again, this time with a fever and cough. Doctors then treated her for a group of blood disorders known as hypereosinophilic syndrome, and the medicine they used suppressed her immune system.

Over a three-month period in 2022, she experienced forgetfulness and worsening depression. An MRI showed that she had a brain lesion and, in June 2022, doctors performed a biopsy.

Inside the lesion, doctors found a “stringlike structure” and removed it. The structure was a red, live parasitic worm, about 3.15 inches long and .04 inches in diameter.

They determined that it was an Ophidascaris robertsi, a type of roundworm that is native to Australia and reproduces in a large snake, the carpet python, which takes its name from its intricate markings. The pythons shed the worm’s eggs in their feces. The eggs are then ingested by small mammals, and the worms can grow inside them.

Roundworms infect hundreds of millions of people globally, according to the Cleveland Clinic, but the researchers in Australia said this was the first report of the Ophidascaris worm species infecting a human.

The woman may have been infected by the worm the same way small animals typically are: by accidentally consuming worm eggs.

Carpet pythons were at a lake area near where the woman lived, the article said. She had no direct contact with the snakes but often gathered warrigal greens, which are similar to spinach, from around the lake to cook. The article said that she could have inadvertently consumed worm eggs by eating the greens or because her hands or her kitchen were contaminated with them.

Scott Gardner, a professor of biological sciences and the curator of the Manter Laboratory of Parasitology at the University of Nebraska-Lincoln, said that people do not need to panic about being infected by an Ophidascaris from snakes and should use good hygiene to avoid being infected by parasites.

“A lot of the parasites that can affect people do so because we get in the wrong place at the wrong time,” Dr. Gardner, who was not involved with the Australia study, said in an interview. “So we ingest some eggs that aren’t supposed to come into us, and if we’re immunocompromised, then we can have a pretty serious infection.”

Karina Kennedy, the director of microbiology at Canberra Hospital and an author of the article, said in a news release that the woman’s initial symptoms “were likely due to migration of roundworm larvae from the bowel and into other organs, such as the liver and the lungs.”

In the first stages of the woman’s illness, however, doctors were not able to find evidence of the parasite, Dr. Kennedy said.

“At that time, trying to identify the microscopic larvae, which had never previously been identified as causing human infection, was a bit like trying to find a needle in a haystack,” she said.

Six months after the brain surgery, the woman’s psychiatric symptoms remained, but had improved, the article said. She was also treated with medicine to kill worm larvae that may have been in her other organs. She is still being monitored by infectious disease and brain specialists.

Dr. Kennedy, who is also an associate professor at the Australian National University medical school, advised people to wash their hands after gardening and touching foraged products, and to thoroughly wash foods and surfaces used for cooking.

In the article, the scientists and doctors involved with the woman’s case said that her experience emphasized the risk of diseases spreading from animals to humans. Outbreaks of these kinds of diseases have become more frequent in recent decades and account for about 60 percent of all known infectious diseases and 75 percent of new and emerging ones, according to the C.D.C.

Though the type of worm that infected the woman is endemic to Australia, the Ophidascaris species infects snakes in other parts of the world. Scientists said in the article that this case showed “that additional human cases may emerge globally.”

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