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Brain-Invading Rat Lungworms Are Lurking in California

Officials say the risk to the general public remains low, but the discovery suggests the worm’s geographic range in the U.S. may be expanding.

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A parasitic worm capable of invading the human brain has been detected in wildlife in Southern California. It’s not just the brain worm that you have to worry about if you’re in Southern California, but also the disease it carries: rat lungworm infection.

Researchers reported the findings in the journal Emerging Infectious Diseases, published by the U.S. Centers for Disease Control and Prevention. The parasite, Angiostrongylus cantonensis, was identified in rats, opossums, and a zoo animal in San Diego County.

Officials say the risk to the general public remains low, but the discovery suggests the worm’s geographic range in the United States may be expanding.

A Parasite That Can Infect Humans Has Been Found in Southern California Wildlife

An investigation was launched shortly after a seven-year-old Parma wallaby at the San Diego zoo developed severe neurological symptoms back in December 2024. The wallaby suffered blindness, headshaking, loss of coordination, and hind leg paralysis. It was treated for 11 days before being euthanized. A necropsy revealed six rat lungworms embedded in its brain.

The incredibly rare diagnosis sparked an investigation. Among 64 roof rats collected around zoo grounds, two were infected. At the same time, wildlife rehabilitators examining deceased opossums from across San Diego County found that seven of 10 animals carried the parasite, many with signs of eosinophilic meningitis.

Rat lungworm is usually found in Southeast Asia, though it has established itself in areas like Hawaii and parts of the southeastern United States. Scientists believe it originally spread around the world through rats traveling on ships.

If lifecycle shifts between two or three animals: rats and slugs/snails. Infected rats shed larvae and their feces, then snails or slugs come along and consume the larvae, and then rats come along again to be reinfected when they eat the snails/slugs. It’s the disgusting circle of life.

Humans or other animals can become hosts if they ingest infected snails, slugs, or contaminated produce. Once inside a human, the larvae can migrate to the brain, potentially causing meningitis. Symptoms include severe headaches, stiff neck, tingling skin sensations, nausea, fever, and, in rare cases, coma or death.

No human cases have been reported in San Diego County, but researchers are urging healthcare officials across Southern California to be on the lookout for rat lungworm infection when evaluating patients with sudden, unexplained neurological issues.

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