Slugs, snails are not alone in causing rat lungworm disease in humans

A review of decades of research revealed more than a dozen kinds of animals in addition to slugs and snails have caused rat lungworm disease in people around the world.

Researchers from the University of Hawai’i at Mānoa and the University of London (UK) combed through nearly 140 scientific studies published between 1962 and 2022 and found 32 species of freshwater prawns/shrimp, crayfish, crabs, flatworms, fish, sea snakes, frogs, toads, lizards, centipedes, cattle, pigs, and snails can act as carriers of the rat lungworm parasite (Angiostrongylus cantonensis). Of these, at least 13 species of prawns/shrimp, crabs, flatworms, fish, frogs, toads, lizards, and centipedes have been associated with causing rat lungworm disease in humans.

This work was the master’s degree thesis research of the first author, Helena Turck, as part of a graduate program in One Health jointly run by the Royal Veterinary College and the London School of Hygiene and Tropical Medicine, both part of the University of London, UK. Robert Cowie, senior author on the study and faculty member in the UH Mānoa School of Ocean and Earth Science and Technology (SOEST), supervised Turck’s research remotely during the pandemic and co-authored the new publication. Professor Mark Fox of the Royal Veterinary College is also a co-author of the study.

Cowie explained that the rat lungworm has a complex life cycle that involves slugs and snails as so-called “intermediate” hosts and rats as “definitive” hosts in which the worms reach maturity and reproduce. Rats become infected when they eat an infected snail or slug. People also become infected when they eat an infected snail or slug, and this can lead to serious illness and occasionally death.

Humans, accidental hosts

“But people can also get infected if they eat so-called paratenic hosts, which are also known as carrier hosts,” said Cowie, who is a research professor in the Pacific Biosciences Research Center at SOEST. “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. However, in such hosts the worms become dormant, but still infective. And if one of these hosts, or part of one, is then eaten raw by a person — an accidental host — development can continue, but only up to a point.”

That point is when they are in the person’s brain, where they are moving around, feeding, and growing. But then the worms die. The damage to the brain and the massive inflammation that results when they die is primarily what causes the symptoms of rat lungworm disease.

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