A parasitic worm capable of infecting the human brain has established local transmission in California for the first time, based on new evidence published by the Centers for Disease Control and Prevention. Researchers confirmed Angiostrongylus cantonensis, commonly known as rat lungworm, in a zoo-born parma wallaby that died in San Diego in December 2024, and in wild roof rats trapped on the same grounds weeks later. The findings signal that the parasite is no longer just a traveler’s disease or a Southeast Asian import but is now cycling through wildlife in the western continental United States.
A Wallaby’s Death Revealed a Local Parasite Cycle
The key breakthrough came from a parma wallaby born and raised at a San Diego zoo facility. The animal had never left the grounds, which ruled out the possibility that it picked up the infection elsewhere. It died on December 17, 2024, and subsequent PCR and genetic sequencing of its tissues confirmed Angiostrongylus cantonensis, as detailed in an Emerging Infectious Diseases report. Because the wallaby had no travel history, investigators concluded the infection was autochthonous, meaning it was acquired locally rather than imported, and therefore evidence that the parasite’s full life cycle was present on-site.
That conclusion prompted a targeted survey of rodents on the zoo property. Between January and February 2025, researchers trapped and tested 64 free-ranging roof rats. Two of those 64 animals carried the parasite, yielding a 3.1% infection prevalence. Separately, multiple Virginia opossums found dead on or near the same grounds between 2023 and 2025 also tested positive, with PCR sequencing verifying the parasite’s DNA. The opossum timeline is significant: it suggests the worm has been circulating in San Diego wildlife for at least two years before the wallaby case drew attention to it, and that more species than just rats are becoming involved in the local ecology of infection.
Why Earlier California Cases Looked Different
Rat lungworm is not entirely new to California. A decade ago, a captive African pygmy falcon at a San Diego Zoo–associated facility was found to have the parasite in its brain tissue, confirmed by both histology and PCR and described in a veterinary case report. But that detection and similar zoo events were attributed to imported paratenic hosts, animals such as frogs, lizards, or crustaceans that can harbor infective larvae without serving as the parasite’s primary host. In those scenarios, researchers believed the worm arrived inside an animal shipped from an endemic region, not that it was breeding or circulating in local rodent populations.
The 2024–2025 findings break that pattern. A zoo-born wallaby with no outside exposure, combined with infected wild rats living on the same property, points to a self-sustaining transmission cycle instead of an isolated import. Adult rat lungworms live in the lungs of their definitive rodent hosts, where they produce larvae that pass into the environment through feces, according to CDC surveillance data. Snails and slugs ingest those larvae, and other animals, including humans, become infected when they accidentally consume contaminated mollusks or produce. The presence of infected rats and multiple dead opossums over a multi-year window indicates that all major hosts and intermediate stages are now operating in San Diego without any need for repeated introductions from abroad.
The Parasite’s Expanding U.S. Footprint
Before the California confirmation, rat lungworm’s known U.S. territory was concentrated in the Southeast and in Hawaii. Between January 2011 and January 2017, the CDC confirmed Angiostrongylus cantonensis DNA-positive cases linked to central nervous system disease across eight U.S. states, with evidence of autochthonous transmission in the southern part of the continental country. Florida has been a particular hotspot, where field surveys have documented the parasite in both wild Rattus rattus populations and multiple terrestrial snail species, highlighting how quickly the worm can establish itself once it arrives in a suitable climate.
California’s confirmation extends the parasite’s known range roughly 2,000 miles west. That geographic shift matters because most U.S. public health messaging about rat lungworm has focused on Hawaii, where the disease is well-established, and on travelers returning from Southeast Asia, where the parasite is prevalent. Clinicians in western states may not routinely consider rat lungworm when evaluating patients with eosinophilic meningitis, the distinctive form of brain and spinal cord inflammation the parasite can cause. That diagnostic blind spot could mean human cases are already occurring but going unrecognized, particularly among people who garden, forage, or eat raw produce in areas where infected snails or slugs are present in yards, community gardens, and urban green spaces.
No Cure Exists, and Surveillance Gaps Persist
One of the more troubling aspects of rat lungworm is the absence of a targeted therapy. The CDC states there is no specific treatment for the infection. Most cases resolve on their own as the parasite eventually dies inside the human body, but the interim period can involve severe headaches, neck stiffness, tingling or painful sensations, and other neurological symptoms that last weeks or months. In rare instances, the infection is fatal. Supportive care, including pain management, control of nausea, and careful use of corticosteroids to reduce inflammation, remains the primary medical response, underscoring the importance of prevention and early suspicion rather than cure.
The California findings also expose a gap in wildlife and environmental surveillance. No publicly available data from state agencies had previously documented a sustained rat lungworm cycle in local rodents, even though the parasite had already been recognized elsewhere in the United States. The discovery in San Diego came through opportunistic testing of zoo animals and nearby wildlife, not through a systematic statewide program. That patchwork approach raises the possibility that other regions may already harbor the parasite without knowing it, particularly in coastal or subtropical microclimates that resemble those where the worm has thrived in the Southeast and Pacific islands.
What Experts Say Residents and Clinicians Should Do
Public health officials emphasize that individual risk can be reduced through practical food and environmental precautions. Because infection typically occurs when people inadvertently ingest tiny infected snails or slugs on raw produce, experts recommend thoroughly washing leafy greens, herbs, and other uncooked vegetables under running water, and discarding any produce that shows obvious snail or slug damage. Gardeners and homeowners in affected areas are also advised to wear gloves when working in soil or handling yard debris, and to supervise children who might play with snails or slugs that could carry larvae. These measures mirror broader food safety guidance promoted across the CDC website, but take on added urgency when a neurotropic parasite is circulating locally.
Clinicians, meanwhile, are being urged to add rat lungworm to their differential diagnosis for patients with meningitis-like symptoms who have relevant exposure histories, even if they have never left the mainland United States. Access to up-to-date reference materials can help: providers can search the CDC A–Z index for technical guidance on Angiostrongylus and other parasitic infections, and local health departments can work with federal partners to arrange specialized testing when cases are suspected. For communities where English is not the primary language, officials can draw on multilingual CDC resources to communicate prevention steps and symptom awareness, ensuring that information about the newly documented risk in California reaches the people most likely to encounter infected snails, slugs, or rats in their daily lives.
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*This article was researched with the help of AI, with human editors creating the final content.
