Three people in Hawaii have come down with what appears to be a rare parasitic disease called rat lungworm disease in recent weeks. Two of the victims (friends who had a meal together) told the Honolulu Star Bulletin that they experienced "agonizing pain" after eating raw vegetables – and physicians fear they may have accidentally swallowed slug larvae hidden inside folds of raw peppers.

Physicians at Hilo Medical Center on the Big Island of Hawaii reportedly discharged the patients several times before finally admitting them in mid-December, because they could not find anything wrong with them; one of the pals is now in a coma, the newspaper reports.

Rat lungworm is a tropical disease found in warm, moist climes that is caused by Angiostrongylus cantonensis, a parasitic worm carried by rats (the parasites live in the pulmonary arteries of rats, hence the name "rat lungworm"). The rats excrete worm larvae in their feces, which are sometimes eaten by small snails and slugs that often nestle in the folds of lettuce, peppers and other produce.

"Many different species of slugs and snails are known to be carriers of this particular nematode [rat lungworm parasite]," Robert Hollingsworth, an entomologist at the U.S. Pacific Basin Agricultural Research Center in Hilo told ScientificAmerican.com. When people ingest the worm, it travels from the gastrointestinal tract to the central nervous system. Most people experience no symptoms or only mild ones such as muscle aches and sensitivity to light and recover without treatment, in most cases without ever suspecting a parasite (which typically dies off in a few weeks).

In rare instances, the worm causes potentially deadly meningitis, an infection of the fluid that bathes the spinal cord and brain. Symptoms include severe headache, stiff neck, tingling or painful feelings in the skin, low-grade fever, nausea, and vomiting, according to the Centers for Disease Control and Prevention (CDC). "There have been documented deaths but they are very rare," says Sarah Park, the state epidemiologist for Hawaii. The severity of the illness seems to depend on how many worms are ingested, how strong a person's immune system happens to be, and how long the worm stays in the central nervous system, she notes, adding that it in some cases worms have survived for up to several months.

"We don't know about all the cases out there," because there is no diagnostic test for the disease, Park told ScientificAmerican.com, saying that diagnoses are made based on clinical observations, suspicion of exposure to the parasite, and the presence of elevated levels of eosinophils, white blood cells in the cerebrospinal fluid that might indicate a parasitic infection.

A 2007 study published in the journal Pacific Science suggests that the prevalence of rat lungworm disease may be on the rise. According to the paper (co-authored by Hollingsworth), an invasive slug species from Southeast Asia, Parmarion martensi, arrived in Hawaii in 2004 and began out-competing the Cuban slug, Veronicella cubensis, one of the most common large slug species in Hawaii. Researchers found that 77.5 percent of the invasive species were carrying the rat lungworm parasite compared to 24.3 percent of the Cuban slugs.

"The transmission potential of this species may be higher than that of other slugs and snails in Hawaii," the researchers wrote.

The best way to avoid rat lungworm disease? Don't eat raw snails or slugs and wash your vegetables and fruit very well, Park advises, noting that they are small [as short as 2 mm in length] and can easily escape notice if hiding in creases of produce.

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