Invasive Giant African Land Snails

Tomas Vokaty/Creative Commons

Of all the exotic animals that have invaded Florida—and there have been a lot, from Gambian pouched rats to wild boars to Burmese pythons—my favorite is a nematode called the rat lungworm. It’s thin as a sewing thread and less than an inch long. Other invaders are crassly direct in their trouble-making. Recently, for example, a study published in PNAS (Proceedings of the National Academy of Sciences) blamed the invading giant snakes for the disappearance of mammals in the Everglades. It cited a 99% drop in sightings of opossums and raccoons, a 94% drop in sightings of whitetailed deer, and so on. The pythons eat the natives, says the report; it’s as simple as that. 


But the rat lungworm only hurts other animals by getting eaten. That happens because, like thousands of other nematode worms, it’s a parasite. You’ve probably heard of its cousin Trichinella spiralis, the reason we overcook pork. It’s famous, of course, for taking up residence in the human intestine. The rat lungworm is different. It takes up residence in the brain, causing headache, stiff neck, vomiting, visual disturbances, and malfunctions in the sense of touch. But let’s not rush to that. First, a little background, a little biography. 


The obvious sign of rat lungworm is the dramatic appearance of giant land snails. A giant land snail looks like other snails, moist wrinkles of yellow hide pulsing as its two antennae probe ahead of the muscular foot that constitutes its body. These creatures are native to Africa, but lately they have been crawling the lanais of North America. Like the giant snakes, they probably arrived as pets, though imported produce is another possibility. The giant land snail can grow a shell a foot long. Even one half that size is a rippling, muscular handful. They are not picky eaters; leafy greens will do, but so will 500 other kinds of plants. The snail laps at any handy food. Since its tongue fairly bristles with tooth-like spikes, the result is to gouge holes in the food, whether it’s a fruit, a leaf, or even a stucco wall. And they can reproduce like mad, each snail laying twelve hundred eggs per year. (They’re hermaphrodites, by the way, so any two can fertilize each other.)  Farmers in Florida are feeling apprehensive. So are home owners; that plaster-licking habit can render buildings unsafe for use.


But what does this have to do with worms? It is, as they say in Hollywood, complicated.


The rat lungworm begins life as an egg in the arteries that supply the lungs of a rat. From there, it follows the blood stream into the lungs proper, then crawls to the throat. The rat swallows it. Now it travels through the gut, finally making its exit in the rat’s droppings. 


Getting swallowed is, in fact, a big part of the lungworm’s life plan. Ideally, some animal with low standards—a giant land snail, say—will come along and swallow it with the rat’s droppings. It’s not built for crawling, but in a pinch, it can swim in search of a snail by thrashing about like a fire hose. 


Inside a snail, the lungworm matures further. It bides its time, waiting for its snail host to meet a gruesome fate. (Remember, its life depends on getting swallowed over and over.) Ah, but what would eat a softball-sized gobbet of snail? 


A rat would. They’re not picky. The worm survives the eating. Once inside the rat, it travels to the brain. There, it finally reaches adulthood. The rat doesn’t mind. Parasite and host have spent countless generations adapting to each other. They get along. The worm swims the bloodstream to the lungs, and there, having engaged in romance, lays its eggs. We’ve come full circle. 


What keeps the well-informed Floridian from treating the giant land snail as an economy-sized escargot is the lungworm. A single giant land snail may contain thousands of lungworm larvae. If we eat the snails, the worms, unschooled in our pretensions, treat us as rats. They proceed to the brain. However, they find us inhospitable hosts, and they die. It’s at this point that we fall sick, as our bodies react to the disintegration of the parasite. The result is a kind of meningitis. Grave as the symptoms sound, they generally pass in a week or two. Still, nobody seems to enjoy them. 


Of course we could choose not to eat giant land snails. The problem is that they may have left their lungworms lying around in produce. We may also eat something—a crab or a shrimp, say—which has itself eaten an infected snail.


The pythons look more dangerous, of course. I often hear people fretting that they’ll slither into houses and eat babies. But for direct impact on human lives, look to the snails and their cargo of tiny parasites. 

Related Post: Gordiid Worms

4 comments:

  1. My vote goes to contaminated produce. Some years back I decided to try and get some giant land snails...it proved just about impossible. Florida's usually been pretty anal about what comes in (with good reason) if it eats plants, so I doubt these guys were intentional introductions.

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  2. Makes sense. I think I got the claim about them being introduced as pets from the Florida wildlife people, as quoted in a news article. But produce has long been known as their main method of travel.

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  3. Have you ever checked out "Only a Gringo Would Die for an Anteater?" I forget the name of the author, but it's by a fellow who was a vet in the '60's-'70's NYC. He cites an instance where an old, decrepit dog who had trouble getting under motion was attacked by garden slugs--which were running unusually large in that locality, apparently--while lying down outside. The dog incurred several sores or wounds from the rasping tongues over a period of time, until the owner captured a slug in the act and the vet was able to put two and two together.

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  4. No, I never even heard of that. It sounds fascinating!

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