Rat lungworm, a dangerous parasite, has been detected in San Diego County wildlife, marking its first presence in the western U.S.

SAN DIEGO COUNTY, Calif. — Editor’s note: A previous version of this article credited the research to the CDC. The study appeared in a CDC journal, but the research was conducted by the San Diego Zoo Wildlife Alliance and its partners. 

Brain-eating worms known as ‘rat lungworm’ have been found in San Diego County, the first time the parasitic worms have been found in any state west of Texas.

According to new research from the San Diego Zoo Wildlife Alliance, published by the CDC, the parasite Angiostrongylus cantonensis, better known as rat lungworm, was first detected in a 7-year-old male parma wallaby at the San Diego Zoo in December 2024. According to researchers, the wallaby was experiencing neurological issues, including head shaking, blindness, and limb rigidity.  The wallaby died 11 days following the diagnosis. 

Following the discovery, researchers began testing wild animals in and around the San Diego Zoo. From January  2025 through February 2025, researchers tested 64 dead wild rats at the Zoo. Three of the 64 rats tested positive for the lungworms. At the same time, wildlife rehabilitation programs contacted researchers, reporting illnesses in 10 Virginia opossums. The opossums were euthanized. Seven of the ten tested positive for the rat lungworm. 

The discovery in opossums and rats now leads researchers to believe the rat lungworms have a lasting foothold in San Diego County, with the potential to spread elsewhere. 

“Whereas [local infections] had not previously been documented in the United States west of Texas, identifying [rat lungworm] cases in wildlife in San Diego County provides support that A. cantonensis lungworm could now be considered endemic in this portion of southern California, with the potential to spread to other parts of the western continental United States,” reads the study from the San Diego Zoo Wildlife Alliance in conjunction with the San Diego Humane Society’s Project Wildlife, California Department of Fish and Wildlife, Universidad Cardenal Herrera-CEU, CEU Universities, California Animal Health and Food Safety Laboratory, University of California, Davis.  

In a statement to CBS 8, a spokesperson for the San Diego Zoo Wildlife Alliance said the research shows its dedication to investigating wildlife diseases.

“San Diego Zoo Wildlife Alliance is at the forefront of wildlife disease investigation. When the Alliance’s Disease Investigations team found the parasite, they set out to determine its source and have since detected the same agent in peridomestic rodents and wild opossums from various locations across the county. It is not surprising that San Diego Zoo Wildlife Alliance makes these discoveries, because it is looking in places few others are looking.”

Added the spokesperson, “As stewards of a One Health approach for people and wildlife, San Diego Zoo Wildlife Alliance shared the findings with the California Department of Public Health. Though additional research is needed, this study is crucial for understanding the parasite’s expansion and for protecting public health.”

According to researchers, while rat lungworm can be fatal to humans and animals, deaths are very uncommon.

Researchers say the lungworm is typically contracted by animals or people who eat or handle frogs, small slugs, lizards, rats, and freshwater crabs and shrimp.

The CDC recommends that residents wash fruits and vegetables thoroughly when rat lungworm is present.

In a statement, a spokesperson from the California Department of Public Health said the agency did not participate in the study and could not confirm the presence of rat lungworm in San Diego and throughout the state. 

The CDPH did, however, say the researchers’ findings show the presence of rat lungworm is possible.

“The San Diego study affirms that the parasite can be introduced to California through movement of infected animals from endemic areas. Because some species of snails and slugs present in California are capable of serving as hosts for rat lungworm, and the presence of the parasite in other parts of the state is unknown, it is advised to take certain food safety precautions. Persons should not consume any raw or undercooked wild snails or slugs, and should thoroughly wash all produce before consuming,” reads the CDPH statement to CBS 8.

CBS 8 reached out to the County of San Diego for more information on the presence of rat lungworms in San Diego County. The article will be updated with the County’s response. 

Watch: What is Rat Lungworm (Angiostrongylus) Disease?