You cannot tell whether a snail carries rat lungworm by looking at it. Infected snails look, move, and behave identically to uninfected ones. There is no visible sign, no change in shell color, no unusual slime, and no behavioral clue that distinguishes a carrier from a clean snail. The only way to confirm infection is through laboratory testing, which means the practical question shifts from “how do I spot an infected snail?” to “how likely is this snail to be infected, and how do I protect myself?”
Why You Can’t See the Infection
Rat lungworm (Angiostrongylus cantonensis) larvae live inside a snail’s tissues, not on its surface. The parasite’s life cycle starts when rats shed larvae in their feces, snails or slugs pick up those larvae while feeding, and the larvae develop inside the snail over several weeks. At no point does this process produce visible symptoms in the snail. The larvae are microscopic, and the snail continues eating, moving, and reproducing normally.
Even researchers who study these parasites full-time cannot identify an infected snail without bringing it into a lab. The standard method involves digesting the snail’s tissue with an enzyme solution and then examining the results under a microscope to count larvae, or running a PCR test that detects the parasite’s DNA. These are not tools available to someone standing in their garden.
Which Snails and Slugs Carry It
Almost any land snail or slug can serve as a host, but some species carry the parasite far more often than others. Giant African land snails are among the most common carriers in tropical regions. In one study from the Canary Islands, over 70% of a native snail species tested positive using sensitive molecular methods. Common garden snails and banded snails in the same area also tested positive, though at lower rates.
In the southeastern United States, two invasive freshwater species have tested positive in recent surveys: apple snails and Japanese mystery snails. Researchers collecting snails from Georgia waterways in 2024 found infection rates that varied dramatically by location. Apple snails from one site in Camden County, Georgia had a prevalence of roughly 189 per 1,000 snails, nearly one in five. Other sites tested at much lower rates, and some had no infected snails at all. This patchiness is typical. Infection depends entirely on whether infected rats are present in the area and defecating near where snails feed.
Rat lungworm has now been detected in snails or slugs across a growing list of U.S. states: Florida, Texas, Mississippi, Alabama, California, Oklahoma, South Carolina, and Georgia. Hawaii has the longest-established risk. The parasite’s range is expanding as both the rats that carry it and the invasive snails that serve as intermediate hosts spread into new territory.
The Slime Trail Risk
One detail that surprises most people: the larvae can be present in snail slime, not just in the snail’s body. Research published in 2023 found that stressed snails, those exposed to heat, pesticides, or physical disturbance, released larvae into their slime trails. About 13% of infected snails under stress shed larvae this way, with individual snails releasing anywhere from roughly 45 to over 4,000 larvae in a single 24-hour period. Unstressed snails released none.
This matters because it means you don’t necessarily need to eat a snail to be exposed. Produce grown in a garden where infected snails crawl could carry larvae in residual slime. A child picking up a snail and then touching their mouth is another plausible route. The larvae shed in slime have been shown to be infectious in animal studies.
How to Protect Yourself
Since you can’t identify infected snails visually, protection comes down to assuming any snail or slug in a risk area could be carrying the parasite and acting accordingly.
- Wear gloves when handling snails, slugs, or anything they’ve crawled on. Wash your hands thoroughly with soap and water afterward, even if you wore gloves.
- Wash produce carefully. Leafy greens, herbs, and anything grown close to the ground in areas where snails are active should be washed under running water. Inspect leaves for tiny slugs or snail fragments that can hide in folds.
- Remove snails and slugs from around your home and garden. Reducing their numbers reduces the chance that the parasite’s life cycle continues near your living space. Removing rats, the parasite’s definitive host, is equally important.
- Never eat raw or undercooked snails or slugs. Cooking to an internal temperature that kills the larvae eliminates the risk from consumption. This includes freshwater snails and prawns, which can also carry the parasite.
- Watch children in areas where snails are common. Young children are more likely to put snails in their mouths or handle them without washing up afterward.
What Rat Lungworm Does to Humans
If someone does ingest the larvae, the parasite migrates to the brain and spinal cord, causing a type of meningitis called eosinophilic meningitis. Symptoms typically appear one to three weeks after exposure and can include severe headache, neck stiffness, tingling or pain in the skin, nausea, and in some cases temporary paralysis of the face or limbs. Most people recover over weeks to months as the larvae die off (the parasite cannot complete its life cycle in humans), but severe cases can cause lasting neurological damage.
The infection is diagnosed through a spinal fluid sample. The CDC operates a specialized PCR test that can detect the parasite’s DNA in cerebrospinal fluid, requiring at least 2 milliliters of fluid kept cold during shipping. There is no widely available blood test, and standard imaging often looks normal in early stages, which is why the infection is frequently misdiagnosed initially.
Assessing Your Local Risk
Your risk depends primarily on geography. If you live in Hawaii, the Gulf Coast states, or the southeastern U.S., the parasite is established in local snail and rat populations. Parts of the Caribbean, Southeast Asia, the Pacific Islands, and Australia also have well-documented transmission. In cooler northern climates, the risk is currently very low, though not zero as invasive snail species continue to spread.
If you find large invasive snails on your property, particularly apple snails near freshwater or giant African land snails (which are illegal to keep in the U.S.), your local agricultural extension office can help identify them and may be able to arrange testing. Some state health departments and university parasitology labs also accept snail samples for surveillance purposes, especially if you’re in a region where the parasite has recently been detected.