Category Archives: Nematodes

Rat Lungworm Disease: How it Works

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.

The brown rat, common in cities, is the primary host of the rat lungworm. Photo by  Ian Kirk, licensed under CC BY 2.0.

The brown rat (Rattus norvegicus), common in cities, is the primary host of the rat lungworm. Photo by Ian Kirk, licensed under CC BY 2.0.

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.

An adult female rat lungworm. Photo from Lindo et al. (2002), in public domain.

An adult female rat lungworm. Photo from Lindo et al. (2002), in public domain.

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.

The gray garden slug, which has been recorded as an intermediate host for the rat lungworm. In other words, if you eat them, cook them first. Photo by Bruce Marlin, licensed under CC BY-SA 3.0.

The gray garden slug (Deroceras reticulatum), which has been recorded as an intermediate host for the rat lungworm. In other words, if you eat them, cook them first. Photo by Bruce Marlin, licensed under CC BY-SA 3.0.

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.

A summary of the rat lungworm life cycle (click to enlarge). Figure from the Centers for Disease Control and Prevention, in public domain.

A summary of the rat lungworm life cycle (click to enlarge). Figure from the Centers for Disease Control and Prevention, in public domain.

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.)

Cited:

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.

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Worms Have Mothers Too: Meet the Heteroderids

by Joseph DeSisto

Today’s story comes from another group of agricultural pests — not insects this time, but cyst-forming nematodes in the family Heteroderidae. What are nematodes, you ask? Nematodes are one of many animal phyla referred to as “worms.” These worms in particular are unsegmented, surrounded by a hard cuticle or exoskeleton, and often microscopic.

There are around 25,000 known species of nematodes, but as many as a million are likely undiscovered. Some are free-living and eat bacteria or other tiny animals, but most are parasites either on animals (including humans) or plants. The heteroderids belong to the last category — here are some:

A whole bunch of nematode cysts, among potato roots. Photo by Xiaohong Wang, in public domain.

A whole bunch of potato cyst nematodes, Globodera pallida. Photo by Xiaohong Wang, in public domain.

The wormy-looking things are not worms at all, but the roots of a potato plant. Instead, the worms are packed away in cysts, the tiny yellow spheres nestled among the roots.

Within each of these cysts is a cluster of eggs which, when conditions are right, will emerge as worms. If conditions do not support hatching, the eggs can survive in these cysts for up to 20 years! Below is a recently hatched cyst nematode, beside an unhatched egg. This image was taken with an electron microscope — the colors do not reflect the colors of the actual animals.

The soybean cyst nematode (Heterodera glycines). Photo from Agricultural Research Service, in public domain.

The soybean cyst nematode (Heterodera glycines). Photo from Agricultural Research Service, in public domain.

Although called “cyst nematodes,” the heteroderids are not unique in forming cysts. Lots of nematodes do it — the reason it’s so important to cook pork thoroughly is that parasitic nematode cysts can survive in meat long after the pig has been slaughtered. What’s unusual about many heteroderids is how and when they form their cysts, but to appreciate that we need to start at the beginning.

For now, let’s consider the soybean cyst nematode, the species in the image above (see Davis and Mitchum 2005). When the nematode first hatches, it is microscopic and barely small enough to burrow into the root. Once inside, it secretes chemicals that trick the plant into forming special tissues where the worm can feed.

After reaching a certain size, a male soybean cyst nematode leaves the inside of the root and migrates over the surface, looking for mates. The female is lazier — not leaving her special feeding site, she eats and grows until her body expands so much that her rear end bursts out of the root, exposed to the soil. At this point she has eaten so much that if you were to dig up the soybean plant, you could see her exposed body without a microscope.

The soybean (Glycine max) is one of the most economically important crop plants in the world -- and the most important food plant for the soybean cyst nematode. Photo by Harry Rose, licensed under CC BY 2.0.

Originally from East Asia, the soybean (Glycine max) is now one of the most economically important crop plants in the world — and the most important food plant for the soybean cyst nematode. Photo by Harry Rose, licensed under CC BY 2.0.

She’s not done yet, though. With her rear end now outside the root and able to swell freely, the nematode “lays” her eggs into a jelly-filled cavity, still inside her body. This hardens to form an egg sac until finally, the nematode herself dies and her dried cuticle becomes the cyst’s protective skin. For the female cyst nematode, laziness becomes the biggest sacrifice a mother can make. Her eggs, numbering up to 400, can now survive for years until it’s time to hatch.

Soybean cyst nematodes can be pretty damaging to crops, but pale in comparison to the potato cyst nematode (Globodera pallida — see what I did there?). Nicol et al. (2011) estimate that just in the United Kingdom, yearly losses to pallida total roughly £50 million. Fields infested by the potato cyst nematode have reported crop losses of up to 50%. That’s a lot of potatoes. Thanks to strict quarantine protocols, the worm has not successfully invaded the United States, but pallida is so scary that a small outbreak in Idaho caused Japan to boycott U.S. potatoes for decades until imports were resumed in 2006.

Controlling nematodes is not easy, and this is especially true for cyst-formers. Not only are the cysts extremely resilient, the adult worms are hard to monitor and control as they spend their lives sheltered inside the roots of their host plants. Conventional pesticides, even those designed to kill nematodes, are seldom effective. Including all species, nematode damage to crops worldwide is estimated to cost $80 billion every year (Nicol et al. 2011).

A newly hatched cyst nematode, Meloidogyne incognita, entering the root of a tomato plant. Photo by William Wergin and Richard Sayre, licensed under CC BY 2.0.

A newly hatched cyst nematode, Meloidogyne incognita, entering the root of a tomato plant. The colors aren’t real — this was taken with an electron microscope. Photo by William Wergin and Richard Sayre, licensed under CC BY 2.0.

Modern science, however, may provide solutions. Last year the complete genome of the potato cyst nematode was sequenced, giving scientists a first look at the role genes play in the worm’s complex life cycle (Cotton et al. 2014). Cotton et al. were able to isolate the genes responsible for several key proteins, each vital to the ability of cyst nematodes to invade, manipulate, and feed on their hosts. Hopefully these discoveries will pave the way for newer, more effective methods of controlling pallida and its relatives.

For a more technical description of the heteroderid life cycle, I recommend Davis and Mitchum’s 2005 paper in Plant Physiology. If you enjoy reading about life histories and especially chemical ecology, you will find I have barely scratched the surface of what there is to know about these strange and amazing creatures.

Cited:

Cotton J.A., C.J. Lilley, L.M. Jones, T. Kikuchi, A.J. Reid, P. Thorpe, I.J. Tsai, H. Beasley, V. Blok, P.J.A. Cock, S.E. den Akker, N. Holroyd, M. Hunt, S. Mantelin, H. Naghra, A. Pain, J.E. Palomares-Ruis, M. Zarowiecki, M. Berriman, J.T. Jones, and P.E. Urwin. 2014. The genome and life-stage specific transcriptomes of Globodera pallida elucidate key aspects of plant parasitism by a cyst nematode. Genome Biology 15(3):

Davis E.L. and M.G. Mitchum. 2005. Nematodes: Sophisticated parasites of legumes. Plant Physiology 137(4): 1182-1188.

Nicol J.M., S.J. Turner, D.L. Coyne, L. den Nijs, S. Hockland, and Z. Tahna Maafi. 2011. Current nematode threats to world agriculture. In: J. Jones, G. Gheysen, and C. Fenoll (Eds.), Genomics and Molecular Genetics of Plant-Nematode Interactions (21-43). Netherlands: Springer.