Last Saturday the Wall Street Journal reported that a growing number of people in the United States are showing up in doctors’ offices with parasitic infections. They’re not infected with relatively harmless parasites like pinworm -- they’re carrying potentially debilitating species that were once found mainly in tropical countries. Some people are bringing hitchhikers home from their Costa Rican vacation, but the vast majority of infections are in people who are poor and living in either the South or the mountains of Appalachia. And that means these parasites aren’t accidental visitors to our shores – they’re living among us now.
Now, I can’t solve the problems of poverty or health care access on a blog. But the article mentioned that medical schools often give short shrift to these parasites, so many doctors don’t even know they exist. That, I can do something about. For the next three weeks our Friday Parasite will focus on the parasites discussed in the WSJ article, describing their life cycles, how they manage to get inside people, and what they can do once they’re there.
First up: Toxocara canis – a parasitic roundworm that doesn’t even want to be in you. As the name suggests, the worms are normally found inside of dogs, where they live in a kind of arrested development because they can only grow to maturity inside very young puppies.
Here’s how the Toxocara life cycle is supposed to go: Mature adults mate inside the intestine of an infected puppy. The females lay fertilized eggs, which enter the outside world when the puppy defecates. The eggs spend a couple of weeks developing in the soil. When they’re swallowed by a puppy they hatch in the puppy’s intestine, then burrow through the intestinal wall, enter the bloodstream, and get swept into the lungs, where they’re coughed up and reswallowed. Back in the intestine, they quickly develop into adults. But most dogs aren’t young puppies, and if Toxoxara finds itself in a dog that is older than 5 weeks it doesn't bother with the trip through the lungs. Instead, the larval worms use the blood as a highway to major organs like the heart and liver, where they encyst themselves. And there, they wait. Most worms never leave this stage, but if they happen to be in a female dog and she gets pregnant, the larvae leave their cysts to infect her pups through their placentas and her milk.
Toxocara becomes a human health problem because the larval worms behave the same way in any mammal that swallows them – migrating through the body and encysting in major organs, even though there’s no chance of getting into a puppy from there. The good news is that most people don’t develop serious symptoms after they’re infected, and the worms die off on their own. But some people - particularly children - unfortunate enough to harbor large numbers of worms can wind up with tissue damage as the larvae migrate through their bodies.
Icky? Indeed. But there's an easy two-prong strategy to avoiding this parasite. First, attack it at the host end by deworming Fido and picking up after him -- the eggs take 2 weeks to mature, remember? And second, limit transmission to the human set with the boring but effective act of hand washing -- even if your kids howl about it.
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