Scientists Believe Snail-Slug Hybrid Is Spreading Potentially Fatal Brain Parasite In Hawaii

Investigators have noted an invasive half-snail, half-slug creature called a semi-slug that is capable of carrying a high concentration of rat lungworm parasites. Hawaii Department of Health

A half-snail, half-slug creature is likely responsible for the spread of a brain-invading parasite in Hawaii, according to a study in the American Journal of Tropical Medicine and Hygiene.

This “semi-slug” is capable of carrying a high concentration of rat lungworm parasites, a small roundworm that affects the brain and spinal cord to produce meningitis-like symptoms. Already this year, five cases of the infection have been reported in both visitors and residents of the state, half that of 2018's cases.

But to understand how people come into contact with the parasite, we first need to understand its lifecycle. The adult form of the parasitic nematode can only be found in rodents, but snails and slugs that eat rodent feces can become infected by the parasite in its juvenile phases. People who consume these snails, slugs, and “semi-slugs” – whether intentional or accidental – can become infected with the rat lungworm.

Researchers analyzed cases diagnosed between 2007 and 2017 and tallied up a total of 82 infections, two of which were fatal. In the 10-year study, annual infection rates ranged from a low of one case to a high of 21. How people came into contact with the parasite, formerly known as Angiostrongyliasis, varied. One patient reported eating a slug, while others noticed a snail or slug at the bottom of their serving bowl. The pathway for infection was not found in most cases, though it’s believed that eating unwashed produce could up a person’s risk of exposure.

Infection is difficult to diagnose as there is no readily available blood test. Adult patients often report headache, a stiff neck, fatigue, and symptoms consistent with meningitis and other neurological issues. In rarer cases, people may experience temporary paralysis of the face and light sensitivity. These symptoms usually start between one and three weeks after exposure but have been known to range between one day and six weeks, according to the Hawaii Department of Health. To diagnose infection, doctors will sometimes use a polymerase chain reaction test to detect the DNA of the parasite in a patient’s cerebrospinal fluid or other tissue, but more often than not they will diagnose the condition based on a patient’s exposure history.

There’s no specific treatment other than waiting it out; the parasite cannot grow or reproduce in humans and will eventually die in their host, causing potentially dangerous but treatable inflammation. That’s why health officials say the best medicine is prevention. Don’t eat raw or undercooked snails or slugs – no matter how many times someone double dares you – and always wash and inspect produce. Eliminating rodents, slugs, and snails from around the house may also help.

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