Officials sound alarm after detecting parasite in region for first time: 'Additional surveillance and testing will be necessary'

As humans have moved around the globe, countless species have hitched a ride. Unfortunately, this has sometimes included parasites capable of spreading deadly diseases.

The small, parasitic rat lungworm has recently been detected in animals in San Diego County, California, an area previously outside its range, the Los Angeles Times reported in early March.

What's happening?

Rat lungworm, as the name suggests, is a parasite that has been found primarily in rats. It prefers to live in rat lungs but spreads to new hosts through rat feces.

While rats are its preferred host, it can also infect a range of other species, including invertebrates such as freshwater crabs, prawns, snails, and slugs; frogs; and mammals such as dogs, raccoons, and even humans.

Multiple wild animals and one zoo animal in San Diego County have tested positive for the parasite since 2024. Researchers wrote about the outbreak in the journal Emerging Infectious Diseases, which is published by the U.S. Centers for Disease Control and Prevention.

The recent discovery represents "a notable expansion of the range of this parasite in North America," according to the study.

The Los Angeles Times reported that, in 2024, a 7-year-old male parma wallaby, born and raised at the San Diego Zoo, began displaying neurological symptoms. After 11 days, the animal was euthanized. The discovery of the parasite during an autopsy led to testing of other animals, revealing infected rats and opossums.

"Additional surveillance and testing will be necessary to determine whether the detections of rat lungworm in the animals evaluated in San Diego County represent an isolated introduction of the parasite or ongoing local transmission," spokeswoman Elizabeth Manzo of the California Department of Public Health told the Times in a statement. 

Why is this concerning?

Unfortunately, humans are on the menu for rat lungworm, and it's because of what we eat. Anyone who eats crabs, prawns, snails, slugs, or frogs, or any fresh produce that has been touched by an infected snail or slug, could end up infected.

Potential symptoms include severe headaches, a stiff neck, skin tingling or pain, low fever, nausea, vomiting, coma, and, in some cases, even death.

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Spreading disease in this way is one of the worst-case scenarios when a displaced species enters a new territory.

What's being done about rat lungworm?

According to officials, the disease is not yet considered to be endemic, or established, in California. It is possible that it is not spreading between animals in the area and that it has only been introduced to a small population from outside areas.

"However, the San Diego study affirms that the parasite can be introduced to California through movement of infected animals from endemic areas," the statement from the state's Department of Public Health said. 

It continued: "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."

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