A parasitic worm that can wreak havoc on a person's brain could be infecting more Hawaiians than once thought, according to new research. And some of these people may be picking up the so-called rat lungworm parasite after eating (knowingly or otherwise) an invasive creature that's part slug, part snail, the scientists said.

Health officials in the Aloha State were interested in discovering the prevalence of this brain-infecting parasite and the cause of the infection in humans. By looking back at reports beginning in 2007, when health workers began tracking the disease in Hawaii, they found 82 cases (two of which were fatal) through 2017. These cases involved residents as well as tourists and visitors.

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But researchers, including David Johnston, an epidemiologist in the disease outbreak control division of the Hawaii State Department of Health, suspect that this is likely an underestimate of the actual number of people sickened by the parasitic infection. That's because some infected people have no symptoms or only mild ones, and so they don't seek medical attention. In fact, since the study concluded, news reports have suggested that 10 such cases were reported in Hawaii in 2018 and five so far this year. [8 Awful Parasite Infections That Will Make Your Skin Crawl]

Though most people infected with the parasite recover without treatment, in rare cases, they can develop neurological problems or even die from it, according to the Centers for Disease Control and Prevention (CDC). Humans can't transmit the infection to other humans.

Nearly 80% of the people whose cases were reported needed to be hospitalized, and the illness struck people on the Big Island, Maui, Kauai and Oahu.

Slugs, snails and semislugs

Cases of rat lungworm disease are primarily found in tropical parts of Asia, such as Thailand and Taiwan, with some cases popping up in Australia, Africa and the Caribbean, according to the CDC. Very few cases have turned up in the continental U.S., the CDC reports.

The infection first made its way to Hawaii around 1959, according to the researchers.

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But since 2007, health professionals have been required to report the infection to the state's Department of Health to track its spread, making the new study the largest one to date.

People may get sick from rat lungworm disease when exposed to a parasitic worm called Angiostrongylus cantonensis. The worm carries out part of its life cycle inside slugs and snails, which can become carriers of the parasite when they pick up the worm's larvae after eating infected rat feces.

The researchers found that people in Hawaii could catch the disease when they mistakenly or purposely ingest these infected slugs or snails on raw, unrinsed fruits or vegetables.

Some of the baby slugs and snails are so small they are barely noticeable on produce, Johnston said. That's why it's important to fully inspect and wash raw produce before eating it, he noted.

The new study also revealed that some adults may have gotten sick when they swallowed a slug on a dare, consumed a raw (or undercooked) snail or drank a contaminated beverage (infected slugs can crawl into garden hoses or water catchment tanks). A few cases in young children occurred when they unknowingly put the critters in their mouths.

But for many of the cases in the study, it was difficult to identify the specific exposure responsible for the rat lungworm infection, Johnston told Live Science.

He also explained that there are a number of potential sources of the infection present in Hawaii, such as the Cuban slug, a giant African snail and a marsh snail, which are known to carry the parasite and pose a risk of infecting humans.

Also of interest is an invasive type of semislug (Parmarion martensi). This part slug, part snail could be increasing human exposure in the state because they tend to be fast climbers, so they can get into things around the home, such as outdoor sinks and dishware, or water tanks; they’ve also been found to carry high numbers of the parasite, Johnston said.

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Once people become infected, researchers found that the most common symptoms in kids under age 9 were fever, vomiting and irritability; older children and adults were more likely to complain of headaches, muscle or joint pain, tingling sensations in the skin and stiff neck.

Sometimes the parasitic infection can lead to a rare form of meningitis (eosinophilic meningitis), which causes increased levels of white blood cells in the fluid around the brain and spinal cord.

The study was published online yesterday (July 8) in The American Journal of Tropical Medicine and Hygiene.

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Originally published on Live Science.