From time to time, the media reports that someone has acquired a serious parasitic infection from eating raw vegetables. The patient suffers from severe headache, fever, tingling sensations, stiff neck, and possibly vomiting. Sometimes the illness progresses to coma and, rarely, death. Usually, medical specialists suggest that the patient has accidentally swallowed a slug. Meet the rat lungworm, Angiostrongylus cantonensis.
Angiostrongylus cantonensis
The rat lungworm is a nematode (roundworm) normally inhabiting the pulmonary artery (the artery that carries blood from the heart to the lungs) and heart of rats. Adult worms are delicate and slender, attaining a length of just 1.5 to 2.5 centimetres (females are larger than males). Immature adults that typically cause problems in humans are smaller — about 2mm long.
This parasite is found in warm climates and most reported human cases have been in the eastern hemisphere: China and Southeast Asia, islands in the Pacific, and Australia. A few reports come from Central and South America, the southern United States, and Islands in the Caribbean.
The Life Cycle of Rat Lungworm
Angiostrongylus cantonensis requires two hosts:
- Female worms in the rat pulmonary artery produce eggs (15,000 per day, per female), which are carried to the lungs in the bloodstream.
- Eggs hatch in the lungs and larvae travel up the airways, or are coughed up, and are swallowed. Larvae are deposited in the environment with rat droppings.
- Slugs and snails feeding on rat droppings become infected.
- When the snail or slug is accidentally swallowed by a rat, or other potential host, infective stage larvae migrate from the intestine to the brain. Infective larvae may also escape from the mollusk in its slime trail and survive on damp vegetation such as salad greens.
- Larvae mature in about a month. Young adults move to the brain’s surface and migrate to the pulmonary artery.
- Worms continue to mature in the pulmonary artery, producing eggs in about another two weeks.
When a human becomes infected with rat lungworm, the life cycle apparently doesn’t complete itself, and no larvae are passed in the feces. Immature worms, however are recovered from spinal fluid, brain tissue, and eyes.
Rat Lungworm Infection – Angiostrongylosis, or Eosinophilic Meningitis
Most cases of rat lungworm infection are mild and the victim recovers completely. Some cases, however, cause permanent disability, and some end with death. The symptoms of angiostrongylosis in humans are caused by both maturing worms, and by the body’s immune response.
- Maturing worms moving about in brain tissue can do considerable damage. Worms also migrate into the spinal fluid, the tissue lining the skull, and the eyes.
- Dead worms cause an inflammatory immune response that is more destructive to host tissues than the immune response to live worms — because of this, drug treatment to destroy the parasites can be risky.
How Do People Catch Rat Lungworm?
Humans are accidental hosts of A. cantonensis, They acquire the parasite in a variety of ways:
- Eating raw snails is an important source of infection in Asia and on Pacific islands. Other animals that are sometimes eaten raw, such as shellfish and crabs, have also been implicated.
- Accidentally eating slugs or snails with raw vegetation that has not been thoroughly washed, or eating these animals “on a dare” explains some infections.
- Swallowing infectious larvae that have been deposited on edible plants, or drinking water that the larvae have been washed into probably accounts for the rest.
Rat Lungworm and Invasive Species
Angiostrongylus cantonensis probably originated in East Asia: it was discovered in rats in Canton, China in 1935. It has spread, however, around the world, probably with slugs, snails, and perhaps rats. In Hawaii, a recently introduced slug species is thought to be contributing to an increase in the parasite there.
Another culprit is the Giant African Land Snail, Achatina fulica. The snail originated in East Africa, but has been spread widely, either deliberately — for food or as pets — or accidentally with plants, soil, or other shipped goods. One snail, transported with sperm already stored in its body, can start an entire population in a new home, and the snails are a known crop pest, attracted to cucumbers and beans among other things. If the snail also carries the parasite, it will easily spread it to a new location, and to people, on fresh produce.
Globally, hundreds of human cases of angiostrongylosis occur each year.
Related Content
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Sources
“Achatina fulica.” Global Invasive Species Database.
Clinical Parasitology 9th Ed. Beaver, Paul Chester, Rodney Clifton Jung, and Eddie Wayne Cupp. Philadelphia: Lea and Febiger, 1984.
Foundations of Parasitology 6th Ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2000.
Rosemary Drisdelle - Rosemary Drisdelle has been published many times as a nonfiction writer and several times as a poet. Her first book, Parasites: Tales of ...
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