A parasitic tapeworm 10cm long lived in the brain of a British man for four years before doctors discovered it.
The 50-year-old first visited doctors in 2008 suffering from headaches, seizures, memory loss and a changed sense of smell.
An MRI scan showed a cluster of what appeared to be lesions, but specialists were baffled. They were more surprised when brain scans over four years showed the anomaly had travelled 5cm through tissue.
Doctors at St Thomas' Hospital in London took biopsies and tested the man for diseases including HIV, lyme disease and syphilis. However in 2012 they were amazed to find that amid his brain tissue was a "10cm ribbon-shaped larval worm". Once diagnosed, the man was treated easily with drugs and has recovered.
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The tapeworm Spirometra erinaceieuropaei has been reported only 300 times worldwide since 1953 and has never been seen before in Britain.
The worm causes sparganosis - inflammation of body tissues which can cause seizures, memory loss and headaches. The worm's rarity means that little is known about its lifecycle and biology.
It is thought that people may be infected by consuming tiny infected crustaceans from lakes, eating raw reptile and amphibian meat, or by using a raw frog poultice - a Chinese remedy to calm sore eyes.
It is unclear how the patient picked it up, but he is of Chinese origin and frequently returned to his home country.
Now scientists have sequenced the genome of the worm. Dr Effrossyni Gkrania-Klotsas, study author at the Department of Infectious Diseases, Addenbrooke's NHS Trust, said: "With only tiny amounts of DNA from clinical samples, we can find out all we need to identify and characterise the parasite."
Dr Matt Berriman, senior author and member of the Sanger Institute, said: "For this uncharted group of tapeworms, this is the first genome to be sequenced and has allowed us to make some predictions about the likely activity of known drugs".
The case was presented in the journal Genome Biology.