New research from the University of Hawaii at Manoa shows rat lungworm disease can be transmitted by many species, not just slugs and snails.
The joint study between UH-Manoa and the University of London analyzed 138 reports published between 1962 and 2022, finding 32 different species can act as carriers of the disease. These include freshwater prawns/shrimp, crayfish, crabs, flatworms, fish, sea snakes, frogs, toads, lizards, centipedes, cattle, pigs and snails among others.
At least 13 species were directly linked to causing rat lungworm disease in humans, including prawns/shrimp, crabs, flatworms, fish, frogs, toads, lizards and centipedes.
“There’s no reason to think the others wouldn’t also be involved in human transmission,” said author of the study Robert Cowie of UH-Manoa’s School of Ocean and Earth Science and Technology. “It’s just that no one has ever documented that yet.”
Cowie confirmed coqui frogs in Hawaii carry the disease.
“That was kind of a surprise,” he said of the report. “It was the first time that particular species of frog had been demonstrated to be able to carry rat lungworm.”
Roughly 182 cases of the rat lungworm disease were recorded in Hawaii from 1959 to 2021, averaging five cases annually the last several years.
“That’s the total number of known cases,” Cowie clarified, estimating the number to be much higher due to undiagnosed cases. “There’s a diversity of symptoms, which is why sometimes it’s very difficult to diagnose.”
The state Department of Health’s list of symptoms include nausea, vomiting, fever, tingling skin sensations, followed by neck stiffness, unremitting headaches and eosinophilic meningitis, which can cause paralysis.
Symptoms last anywhere between two to eight weeks, although headaches and other long-term effects can last years.
“In a really serious case, you could go into a coma, and in some cases, die,” said Cowie, adding it is rare for people in Hawaii to die from the disease. From 2009 to 2019, there were two reported fatalities in the state.
The disease has a complex life cycle, with slugs and snails serving as “intermediate” hosts, and rats as “definitive” hosts, where the worms can reach maturity and reproduce.
Just as with rats, people are infected when they eat an infected species.
“People can also get infected if they eat so-called paratenic hosts, which are known as carrier hosts,” said Cowie. “These are animals that become infected by eating infected snails or slugs, but in which the worms cannot develop to maturity as they do in a rat.”
In a person’s brain, the worms move around and feed and grow until they die, which causes damage and severe inflammation resulting in extreme headaches and other issues with the brain.
The severity of symptoms is often connected to the number of parasites consumed.
“The more worms you eat, the worse your case is going to be,” said Cowie. “If you eat a snail that has thousands of worms in it as opposed to a few, you’re going to be worse off.”
Recorded cases are most common in the southeast portion of the Big Island.
In 2019, for example, all confirmed cases of the disease were in Hawaii County.
The presence of semi-slugs, hosts known for their smaller shells, are a possible cause of the Big Island cases.
Semi-slugs can carry at least twice as many rat lungworm larvae as other mollusk species, and are more mobile.
When semi-slugs drown in water, rat lungworm larvae can escape the host and survive for at least 21 days, enabling larvae to contaminate rain-catchment tanks or find their way into shrimp and land crabs.
Precautions people can take include cooking potential contaminants, washing all fruits and vegetables under running water, and inspecting produce for slugs, snails and other possible hosts.
“If you’re growing any food at home, try and control snails and slugs by putting out bait, for instance,” Cowie said. “Or making sure your yard is nice and tidy, minimizing the number of hiding places for snails and slugs.”
UH-Manoa’s SOEST is conducting additional research on whether slime trails from slugs and snails can spread the disease.
“One of the things we’re doing in the lab right now is a study of the release of infectious larvae of rat lungworm in slime,” he said. “The jury’s still out. We haven’t finished the experiments yet, and are still analyzing some of the results.”
Cowie hopes to have the report published early next year.
“That’s a really important study from a local perspective,” he said. “Awareness of which species may harbor the parasite is critically important, both in Hawaii and more widely.”
Email Grant Phillips at gphillips@hawaiitribune-herald.com.