After four years of headaches, a British man was found to have a rare guest living inside his skull: a tapeworm. The case is reported in the journal Genome Biology.

Does your head hurt yet? Give it a minute.

The nearly half-inch-long Spirometra erinaceieuropaei worm was found during a brain biopsy to investigate strange patterns doctors had seen moving slowly across the host's brain scans over the years. The worm doesn't have a mouth, so it wasn't literally eating the man's brain tissue -- just absorbing nutrients from it as it slowly wriggled across the organ. During the man's four years of getting his brain scanned, the worm moved two inches from the right side of the brain to the left.

This worm is rare, with this being the first reported case in Britain of the parasite infecting a human and only the second such case in Europe. Even in the parasite's homeland of China, the Guardian reports, only 1,000 cases have been reported in humans since 1882. The tapeworm is usually found in amphibians and crustaceans, though it can infect the guts of cats and dogs later in its life cycle -- growing nearly five feet long.

Doctors believe that the British patient picked up the unwelcome traveling companion on a trip to China. It's believed that most human infections are caused by accidentally swallowing tiny crustaceans in lakes, eating raw meat from reptiles or amphibians, or even by using raw frog poultices intended to reduce headaches.

So, maybe stop putting raw frog on your eyes.

Luckily, scientists will learn something from this man's squirmy predicament. For the first time, they now have this rare worm's complete genome sequence. Its genome was found to be 10 times longer than other tapeworm genomes (though still only one-third the size of a human genome). The worm carries a lot of repetitive genes, but it also shows an increase in the number of genes that help it break up proteins. Breaking down proteins helps a parasite invade a host, so it's clear this grub has adapted to be the ultimate body snatcher.

But don't worry too much: By sequencing the tapeworm's genome, the researchers have figured out which existing drugs it may be susceptible to. So if your frog poultices come back to haunt you, this Brit's plight may make your own treatment a bit less invasive than multiple brain biopsies.