The rat lungworm, which can infect certain animals and humans and prove fatal, was found in wild opossums, rats and a wallaby at the San Diego Zoo, according to a new study published by the U.S. Centers for Disease Control and Prevention.
Not previously believed to be established in California, the rat lungworm cases were identified in multiple animals primarily last year, and as early as 2023, according to the study published this month in the CDC’s scientific journal, Emerging Infectious Diseases.
The researchers said that while the risk of transmission to humans is low, it can lead to neurological illness, meningitis or even death. They characterized the parasite as “an emerging veterinary and public health concern.”
The parasite’s usual life cycle involves infected rodents passing the larvae through their feces, which is then eaten by gastropods, such as snails and slugs, according to the study. Other rodents then eat the gastropods and become infected with the disease. Frogs, lizards and crustaceans have also been known to transfer lungworm when ingested by animals. And a University of Sydney study reported that both humans and dogs could be susceptible to the parasite.
Originating in southern China in 1935, the parasite has spread across the world over the last century, including in Hawaii and the southeastern United States. Human cases have been sporadically reported in people who have recently traveled outside of the United States.
The infection is often spread by people either eating undercooked snails or slugs, or through people eating raw produce that has whole or parts of infected snails or slugs, according to the report.
The San Diego study began after a 7-year-old parma wallaby that was born and raised at the zoo began showing symptoms of infection in December 2024. It was euthanized 11 days later.
Following the animal’s death and diagnosis, 64 rats that were killed through extermination or found dead on the San Diego Zoo property were examined, and two adult rats were found to be infected with lungworms and associated pneumonia.
Sho Nakagun, veterinary pathologist at San Diego Zoo Wildlife Alliance, is the lead author on the study. Additional authors on the study include researchers from San Diego Humane Society’s Project Wildlife; California Department of Fish and Wildlife; Universidad Cardenal Herrera-CEU, CEU Universities; and California Animal Health and Food Safety Laboratory, University of California, Davis.
“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,” zoo officials said in a statement. “It is not surprising that San Diego Zoo Wildlife Alliance makes these discoveries, because it is looking in places few others are looking.”
Also, of seven juvenile Virginia opossums received by Project Wildlife, a Ramona-based wildlife rehabilitation program, six were diagnosed with the parasite.
The study concludes by saying the parasite poses a “substantial risk to humans” and now “could be considered endemic to this portion of southern California.”
The researchers said doctors and veterinarians should be considering the parasite when diagnosing central nervous system disease in humans and animals in the southern U.S.