by Joseph DeSisto
Rat lungworm disease — even the name sounds awful. But to understand the disease, we first have to understand the life cycle of the worm that causes it which, incidentally, is as fascinating as it is terrifying.
The rat lungworm (Angiostrongylus cantonensis) is a kind of parasitic roundworm or nematode which, unsurprisingly, is mainly a parasite of rats. It’s favorite host is the brown “Norway” rat, now found throughout the world where it has been spread by human travels. The worms enter a rat as larvae less than a millimeter long, first entering the bloodstream and then migrating, like salmon up a stream, to the host’s brain. Here they gorge themselves with brain tissue until they become sub-adults. The brain-filled, sub-adult lungworms are almost half an inch in length — and still not done growing.
Before they reach adulthood, the worms migrate again via the circulatory system, this time stopping when they reach the heart’s right ventricle, or the pulmonary arteries. In the heart tissue, the worms finally mature, mate, and lay their eggs — but the eggs, too, must migrate. The eggs are laid directly into the bloodstream, and because the pulmonary arteries lead directly to the lungs, that’s where the eggs end up. Hence, the name “lungworm,” even though they might as well be called “heartworms” or even “brainworms.”
Even now, the lungworms have yet to finish their ricochet across your — I mean, the rat’s — body systems. When the eggs hatch, baby worms travel up the respiratory tract, leaving the lungs and entering the esophagous, where they enter the digestive system.
Despite being microscopic, the baby lungworms are extremely tough. They have to be, because they are going all the way, from throat to stomach to intestines and beyond. Finally, when the rat defecates, its feces are loaded with baby lungworms, all ready to start infectious lives of their own.
If they are lucky, the scent of rat dung will catch the nose of a passing snail or slug. Land snails are usually scavengers that will eat almost any non-living biological material, from dead leaves to carrion to, yes, dung. Should a snail care to take a bite, it will quickly become infected with hordes of developing lungworms.
As any city-dweller will attest, rats will eat almost anything, including, it just so happens, snails. The life cycle of the rat lungworm continues with a rat eating an infected snail or slug, and with the worms travelling up to the rat’s brain to eat, grow, and make wormy babies of their own. For a more technical description of the rat lungworm and its strange life cycle, I recommend Cowie’s 2013 review paper.
Rat feces aren’t just eaten by snails, and snails aren’t just eaten by rats. As a result, rat lungworms accidentally infect animals they aren’t supposed to, such as flatworms, shrimp, frogs, birds and, yes, humans. People all over the world eat snails, both on purpose and by accident. Rat lungworm disease in China is usually attributed to eating market-bought raw snails (Lv et al. 2008). During a 2002 outbreak in Jamaica, where snails aren’t as popular, infections were the result of contaminated vegetables (Lindo et al. 2002).
Eating cooked snails is fine, since the cooking process kills the lungworm larvae — it’s raw escargot that can cause problems. Snails and slugs in gardens can also leave a trail of worm larvae in their slime, so washing vegetables in lungworm-inhabited areas can be important.
What happens when a person accidentally eats a rat lungworm? In a human, the worm follows the same cycle as it does when in a rat, going from circulatory to nervous to circulatory to respiratory to digestive systems, and back out to be eaten again by snails.
Medical problems come from the sub-adult worms, as they eat away at the host’s brain tissue. Worms are pretty big things to have squirming around in your head, and as they burrow through nervous tissue, they can cause enough damage that the brain becomes inflamed. The result is eosinophilic meningitis, a series of symptoms of which lungworms are just one possible cause. In some cases the damage can cause behavioral changes in the host — one victim developed severe photophobia, and was terrified of light (Ramirez-Avila et al. 2009).
Rat lungworm disease is not common but can be serious, and potentially fatal. Most cases occur in the tropics, especially in Southeast Asia and the Pacific, where the worm is native. Recently, however, lungworms have become more common across the world, as rats and certain snails have been introduced by humans (Kliks and Palumbo 1992).
A small outbreak in Hawaii occurred only a decade ago (Hochberg et al. 2007) and made the news. Global trade in food has also been a factor — the contaminated vegetables that caused the outbreak in Jamaica may very well have been grown halfway around the world (Lindo et al. 2002). As the world becomes economically smaller, strange local diseases can become worldwide problems.
And yet, for all this gloom and doom, the reason I wrote this article in the first place is that the rat lungworm is actually a pretty cool animal. It’s easy to view wormy parasites like nematodes as simple and unsophisticated creatures. But if the rat lungworm can teach us anything, it’s that even “simple” animals can have incredibly complex and, yes, amazing life cycles. And maybe, just maybe, even the most nightmarish of animals can be, in its own twisted way, sort of, well … beautiful.
Have a lovely and parasite-free day.
(Disclaimer: My interest is in science education. I am not a doctor, and nothing in this article should be interpreted as medical advice. If you are here because you’re worried you might actually have rat lungworm disease, please stop browsing the Internet and talk to a real doctor. Thank you.)
Cowie R.H. 2013. Biology, systematics, life cycle, and distribution of Angiostrongylus cantonensis, the cause of rat lungworm disease. Hawai’i Journal of Medicine & Public Health 72(6): 6-9.
Hochberg N.S., S.Y. Park, B.G. Blackburn, J.J. Sejvar, K. Gaynor, H. Chung, K. Leniek, B.L. Herwaldt, and P.V. Effler. 2007. Distribution of eosinophilic meningitis cases attributable to Angiostrongylus cantonensis, Hawaii. Emerging Infectious Diseases 13(11): 1675-1680.
Kliks M.M. and N.E. Palumbo. 1992. Eosinophilic meningitis beyond the Pacific Basin: the global dispersal of a peridomestic zoonosis caused by Angiostrongylus cantonensis, the nematode lungworm of rats. Social Science and Medicine 34(2): 199-212.
Lincoln M. 15 April 2015. Rat lungworm disease spreads fear across Hawaii Island. Hawaii News Now. Retrieved from http://www.hawaiinewsnow.com/
Lindo J.F., C. Waugh, J. Hall, C. Cunningham-Myrie, D. Ashley, M.L. Eberhard, J.J. Sullivan, H.S. Bishop, D.G. Robinson, T. Holtz, and R.D. Robinson. 2002. Enzootic Angiostrongylus cantonensis in rats and snails after an outbreak of human eosinophilic meningitis, Jamaica. Emerging Infectious Diseases 8(3): 324-326.
Lv S., Y. Zhang, P. Steinmann, and X. Zhou. 2008. Emerging angiostrongyliasis in mainland China. Emerging Infectious Diseases 14(1): 161-164.
Ramirez-Avila L., S. Slome, F.L. Schuster, S. Gavali, P.M. Schantz, J. Sejvar, and C.A. Glaser. 2009. Eosinophilic meningitis due to Angiostrongylus and Gnathostoma species. Clinical Infectious Diseases 48(3): 322-327.