A Brain-Invading Worm Spread By Rats and Snails Has Reached California

A brain-invading worm spread by rats and snails is making inroads into the United States. A study this week seems to show the nematode Angiostrongylus cantonensis, a.k.a. the rat lungworm, is now making itself cozy in California.

Veterinary and zoo experts in San Diego detailed their discovery Thursday in the journal Emerging Infectious Diseases (disclaimer: there are graphic images from the report included below). They identified lungworm infections in a zoo-dwelling Parma wallaby as well as in nearby rat and opossum populations. The worm has likely become an endemic threat to both animals and people living in the state, the researchers say.

“Our findings indicate [locally acquired] infections, which pose a substantial risk to humans and accidental vertebrate hosts,†they wrote.

Accidental brain hosts

Like many parasites, the rat lungworm has a complicated life cycle.

Adult worms mature, mate, and lay eggs in a rodent’s lungs. These eggs hatch into larvae and are coughed out into the rodent’s throat, then swallowed back in through the digestive tract, finally ending up in poop. A snail or slug then eats the contaminated poop (or gets infected through close contact). Once inside the snail, the larvae enter their infectious stage of life, ready to be eaten by another rat and start the cycle anew.

Sometimes, though, the larvae infect other hosts. In some animals, the worms can bide their time until they’re eaten by rats, but in humans, they reach a dead end. The larvae still try to carry out their life cycle in people, though, which includes migrating to the brain. Though some infected with these brain worms never become sick, others can develop a potentially life-threatening condition called eosinophilic meningitis. People usually catch these infections by eating raw or undercooked infected snails or by eating produce contaminated with snails.

The rat lungworm is endemic to parts of Southeast Asia and the Pacific Islands, including Hawaii. But more recently, it’s started to establish a foothold in other places, including within the continental U.S. Local cases have been reported so far in Louisiana, Florida, Oklahoma, Texas, and Georgia.

Officials at the San Diego Zoo first detected these worms inside the brain of a 7-year-old male Parma wallaby born and raised there in December 2024. The wallaby developed severe neurological illness and was euthanized 11 days into being hospitalized. Given the unusual discovery, they decided to examine the bodies of roof rats exterminated or found dead near the zoo between January and February 2025. Sixty-four rats were examined, two of which (3.1%) had lungworm.

Right around the same time, officials at a local wildlife rehabilitation program began to receive opossums with neurological illness possibly caused by rat lungworm infection. Ten of these opossums collected between 2023 and 2025 were examined after death, seven of which had eosinophilic meningitis and were infected with worms. Of these seven cases, six were infected by the lungworm.

The spread of rat lungworm

The researchers couldn’t find lungworm infections among the snails they collected in the area (at least one snail had worm larvae, but they couldn’t clearly identify the species). Still, the findings overall point to A. cantonensis having become endemic in Southern California. If so, it would reflect a disturbing expansion of the worm’s range. Until now, local cases haven’t been found in any continental state west of Texas, the researchers note.

“Further studies are needed to analyze the effect of this geographic expansion and associated risks in California,†they wrote. “Nevertheless, [rat lungworm disease] should be part of the differential diagnosis for central nervous system disease in humans and animals in the wider southern United States.â€

Human rat lungworm infections are rare, and even most cases of eosinophilic meningitis will resolve without antiparasitic treatment (the worms tend to die off on their own). But it’s one of many emerging diseases that could become more common in the U.S. and other countries as the climate warms, experts have warned. It’s also, as this report shows, a potential danger to animals in zoos and the wild.