A 50-year-old man reported to British doctors in 2008 that he had been suffering from headaches, memory problems, strange olfactory hallucinations and seizures. For four years, his medical team struggled to find an answer before discovering that his brain was hosting a rare parasite.

According to the Guardian, after a series of initial tests, doctors found an unidentified mass in the patient’s brain that appeared — after repeated scans — to be moving from one side of his head to the other.

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The mass was biopsied in a surgical procedure at Addenbrooke Hospital in Cambridge. Geneticists at the Wellcome Trust Sanger Institute identified the sample as a rare species of tapeworm that the man had most likely contracted in China.

The worm Spirometra erinaceieuropaei is so rare that only 300 human patients have been identified as victims since 1953, and only two of those victims were in Europe.

Spirometra erinaceieuropaei normally infects amphibians and crustaceans in China, spending one mature phase of its development as worms up to 1.5 meters long in the intestines of dogs and cats. How the UK patient contracted the worm remains a mystery, whether it was ingested in contaminated food or water or burrowed in through the skin.

Scientists believe that a traditional Chinese remedy may have been to blame. In what is called a frog poultice, Chinese healers place raw frog meat on the eyes of people suffering from eyestrain headaches. It is possible that the meat from an infected frog could have passed a larval worm into the patient’s eyes and from there to the brain. Anything that enters the human body through the eyes has a direct path into a person’s bloodstream.

Doctors tracked the worm’s movements for four years before choosing to biopsy the mass. When they realized what it was, they removed it in 2012. The patient had been tested for syphilis, HIV and tuberculosis in an attempt to identify what was causing his symptoms, which included seizures, intense memory flashbacks, and strange smells.

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Dr. Hayley Bennett told the Guardian, “This worm is quite mysterious and we don’t know everything about what species it can infect or how. Humans are a rare and accidental host for this particular worm. It remains as a larva throughout the infection. We know from the genome that the worm has fatty acid binding proteins that might help it scavenge fatty acids and energy from its environment, which may be one the mechanisms for how it gets its food.”

Findings in the case and the medical team’s discoveries about the worm were published Friday in the journal Genome Biology.