Rat lungworm disease is an infection caused by a parasitic worm that normally lives in rats but can accidentally infect humans, most often through contaminated food. When the larvae reach the human brain, they can trigger a form of meningitis characterized by severe headaches, nausea, and neck stiffness. Most people recover, but serious neurological complications are possible.
How the Parasite Moves Between Rats and Snails
The parasite behind this disease, Angiostrongylus cantonensis, has a two-host life cycle that depends on both rats and snails. Adult worms live inside the pulmonary arteries and heart of rats, including black rats, brown rats, and cotton rats. Female worms lay eggs that hatch in the rat’s lungs. The newly hatched larvae travel up to the throat, get swallowed, and pass out in the rat’s feces.
Once in the environment, the larvae are picked up by snails or slugs, where they develop through two additional stages until they become infective. When a rat eats an infected snail, the larvae migrate to the rat’s brain, mature into young adults, then travel back to the pulmonary arteries to reproduce. The cycle starts over.
Humans are accidental dead-end hosts. The larvae can reach the brain, but they cannot complete their life cycle. Instead, they die there, and the immune response to the dying worms is what causes most of the damage.
How Humans Get Infected
The most common route is accidentally swallowing infected snails or slugs, often in tiny pieces chopped up in vegetables, salads, or vegetable juices. You don’t need to knowingly eat a slug. Small slugs or snail fragments hiding in leafy greens can be enough. Contamination of hands during food preparation is another possible route.
Other animals can also carry the larvae without being part of the parasite’s normal life cycle. Freshwater shrimp, land crabs, and frogs can harbor infective larvae, and eating these raw or undercooked may transmit the infection, though this pathway is less well established. Slug slime on produce may also contain larvae.
Symptoms and What They Feel Like
Many infections are mild or produce no symptoms at all. When symptoms do appear, they closely mimic bacterial meningitis: severe headaches (often described as affecting the whole head), neck stiffness, nausea, and vomiting. Abnormal sensations in the arms and legs, like tingling or numbness, can also occur. In some cases, the eyes are affected.
The distinguishing feature of rat lungworm meningitis is what doctors find when they test the fluid surrounding the brain. Instead of the immune cells typical of a bacterial infection, they find elevated levels of a different cell type called eosinophils. This pattern, called eosinophilic meningitis, is a hallmark of the disease, though these cells may not show up in very early or very late stages.
Serious complications are rare but real. A CDC review of 12 confirmed U.S. cases found that two months after diagnosis, all patients were alive and 11 had improved, but four still had ongoing neurological problems including facial nerve issues and lower-extremity weakness. One patient developed seizures five months after diagnosis. In rare cases, the infection can lead to coma or death.
Where It Occurs
The parasite was first identified in China in 1935 and is now endemic across Southeast Asia, East Asia, parts of North and South America, and many Pacific and Caribbean islands. Hawaii is the best-known hotspot in the United States, but cases have been reported in multiple mainland states as well.
The geographic range is expanding. Over the past two decades, the parasite has been detected in the Canary Islands, the Balearic Islands, mainland Spain, and most recently in southern Italy. This spread through the Mediterranean basin means the disease is no longer confined to tropical regions. Warmer temperatures and the global movement of rats and snails likely contribute to its widening range.
How It’s Diagnosed
Diagnosis is challenging because there is no widely available commercial test. Doctors strongly suspect rat lungworm when a patient has meningitis symptoms, elevated eosinophils in either the blood (above 5%) or spinal fluid (above 10%), and a history of recent travel to an area where the parasite is known to exist.
Spinal fluid analysis typically shows elevated protein, normal or low glucose, and increased pressure. Finding the actual worm in spinal fluid would confirm the diagnosis, but this is rare because the larvae tend to cling to the membranes surrounding the brain. Some research laboratories have developed molecular (PCR) tests, and serologic blood tests exist in specialty settings, but neither is routinely available.
Treatment and Recovery
There is no antiparasitic drug proven to kill the worms in humans. Treatment focuses on managing pain and controlling the intense inflammatory response in the brain. Corticosteroids are the primary tool. In one controlled trial, a two-week course of corticosteroids cut the median duration of headache from 13 days to 5 days. Only about 9% of treated patients still had headaches at two weeks, compared to nearly 46% of those who received a placebo.
Anti-parasitic medications have been tried alongside corticosteroids in small studies, but adding them doesn’t appear to provide clear additional benefit. There is also concern that killing the worms too quickly could worsen brain inflammation as the immune system reacts to large amounts of dying parasite tissue.
For patients with dangerously elevated pressure in the brain, doctors can relieve symptoms by carefully draining small amounts of spinal fluid at regular intervals. This procedure directly reduces headache intensity.
Most people recover fully, though the timeline varies. Mild cases may resolve on their own within weeks. More severe infections can leave lingering neurological effects for months, and a small number of patients experience lasting nerve damage.
Preventing Infection
Prevention comes down to avoiding accidental ingestion of snails, slugs, or their slime. The most important step is thoroughly washing raw vegetables, especially leafy greens and herbs grown in areas where slugs and snails are common. Inspect produce carefully, since tiny slugs can hide in lettuce leaves and herb bunches.
If you’re in an endemic area, avoid eating raw or undercooked snails, slugs, freshwater shrimp, land crabs, or frogs. Wash your hands thoroughly after handling any of these animals or after gardening in areas with slug activity. Removing snails, slugs, and rats from around your home and garden also reduces the risk by breaking the parasite’s life cycle at the local level.