The rat lungworm: a parasite that migrates to the human brain

Health · Published 18 October 2026 Swipe more like this →

In short: A tiny nematode, less than a millimeter long, can cause severe neurological disease by traveling from a snail to a human brain. Known as the rat lungworm, Angiostrongylus cantonensis is the leading cause of eosinophilic meningitis in parts of Asia and the Pacific, turning a common garden pest into a significant public health threat.

Angiostrongylus cantonensis is a neurotropic nematode that primarily inhabits the pulmonary arteries and right ventricle of rats, which serve as its definitive hosts. The parasite's life cycle involves a complex journey through different species. After females lay eggs in the rat's pulmonary arteries, first-stage larvae (L1) hatch, migrate to the pharynx, and are passed in feces. From there, they penetrate or are ingested by gastropod intermediate hosts, such as snails and slugs. Inside these mollusks, the larvae undergo two molts to become third-stage (L3) larvae, which are the infective stage for mammals.

Humans become incidental hosts by ingesting these L3 larvae, often through raw or undercooked snails, slugs, or produce contaminated with their slime. Upon entering a person, these larvae move toward the brain to mature into young adults. While they may occasionally reach the lungs, they typically die in these tissues because they are incapable of fully maturing in humans. The presence of these larvae in the brain triggers a local host reaction known as eosinophilic meningitis, characterized by elevated pressure, proteins, and leukocytes in the cerebrospinal fluid. In some cases, the infection can be fatal or lead to permanent neurological damage.

The disease has a significant geographic footprint, originating in Southeast Asia and the Pacific Islands, but has recently been reported in the southern United States, Australia, and Hawaii. Historically, the parasite was first described by Hsin-Tao Chen in 1935 and identified in human cerebrospinal fluid in 1944. While research into treatments is ongoing—including studies on the diterpene lactone andrographolide, which showed activity against various life stages—the infection remains a serious concern due to globalization and the increasing spread of the parasite across different regions.

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