Hookworm at the Beach

My Facebook feed has been full of stories about the Canadian couple who acquired hookworm, or as the story stresses, larva migrans, while vacationing in the Dominican Republic. Of course, many of the stories tell you that the solution for this is that you should always wears shoes on the beach.

Oh, really?

Here, for instance, is what came from George Takei’s feed, highlighting the story from the New York Post.

Prudent to wear protective footwear at all times?

I’m pretty sure that headline is wrong (though very vaguely on the right track).

And if one should really wear shoes on beaches because of low-probability events (we know it’s low-probability because it became a news story!), then it also makes perfectly good sense to wear HazMat suits all the time to protect against mosquitoes: Zika, Malaria, Dengue, West Nile, Yellow Fever, etc..

Since many of the news stories just got it wrong what happened here (even the Washington Post was rather incomplete), I thought I’d give my perspective on what I think happened.

I’ve written about hookworm before, in Hookworm and Hookworm Revisited. Here’s a quickie refresher on the hookworm life-cycle.

Hookworm eggs end up in feces. There they develop into larvae, which when stepped on, chew their way into the foot (often, the feces have degraded by then sufficiently that it’s not clear that feces were there), and head for the bloodstream. Once in the bloodstream, they go to the lungs, are coughed up, swallowed, and then make it to the intestines. They then attach to the intestine wall, mate, and lay the eggs that end up in a new batch of feces.

The best way for a country to get rid of hookworm is to have decent sanitation, and you’d think the Dominican Republic would know about such things (particularly for tourists). And they do . . . for human hookworm.

But notice that in the stories they talk about “cutaneous larva migrans”. That basically means “migrating larvae in the skin”, so what’s that all about? The life-cycle of the hookworm doesn’t say anything about digging their way all over one’s feet.

Well, there are various species of hookworm, specializing in different mammals to attack. And they work by keying on special proteins in their (potential) hosts’ bodies to find their way into the bloodstream. And that means that if they get into the wrong species of host, they get lost. Thus the hookworm tracks.

So, what this Canadian couple got was almost assuredly canine hookworm, not human hookworm. (If they’d gotten human hookworm, they probably would not have even noticed!) So the problem in the Dominican Republic was a large population of stray dogs (which pass the hookworm amongst themselves). That’s what you need to look for (and clean up) at beaches if you are concerned about this problem. You’d think the Dominican Republic would want to do something about reducing the stray dog population, or consider maybe regularly making sure the dogs get effective hookworm medications.

There are things you can look out for, and a strategy to use, if you are at a tropical beach that seems to have a fair number of stray dogs. This strategy relies upon a weakness of the hookworm.

They need moisture.

The eggs need moisture to keep from drying out and dying in the 3-8 days it takes (depending on temperature) for them to develop into larvae. Yeah, while still encased in feces, there is sufficient moisture inside, but as that breaks down and dries out, so do the eggs. Once the eggs become larvae, they still need moisture for the week or two they can live outside a body (depending on conditions, again). They can access moisture by climbing up and down in the soil as need be, but that also uses up their energy reserves. The larvae also need some moisture to start digging into the skin.

This need for moisture really shows up in maps of where hookworm used to (and in some cases, still does) thrive.

Occurence of Hookworm in North America (early 1900s)

You can see, in the US, it’s just in the southeast. The north is too cold, and the west is too dry.

So now let’s use our knowledge to think about a tropical beach. The sand by the water’s edge is too wet—it washes away the eggs. The main part of the beach has sand that is baked all day long and dries out pretty thoroughly, and to a decent depth. So the headline that it was a “romantic walk on the beach” doesn’t seem real likely.

However, I bet you there are trees on the beach. Those trees will shade the sand and keep it from drying out. Also, those trees will drop leaves that turn into real soil that does an even better job of holding onto moisture. And, we all know dogs have a particular affinity for relieving themselves near trees.

So, I’m guessing the couple spent some time in the shade while barefoot and that that is where they picked up the canine hookworm. (And of course, that means that your strategy if you see a lot of stray dogs around is to stay out of the shade of trees on the beach if you are barefoot.)

There was also this article from the CBC in which a doctor says that sandals offer no protection. I don’t think this is fully accurate. They offer some protection, in that the covered part of your foot is protected, thus reducing the probability of getting infected. But sand can still come over the sides (but if it is only sand that’s been sitting in the sun for a while and is fully dried out, again, that offers even more protection). And what about full-time barefooters with really thick soles? They probably have a bit more protection, too, but the hookworm would still have no problem getting through thinner skin, such as on the arch or between the toes.

Don’t confuse hookworm with ringworm. Generally, ringworm is a fungus in the same family as athlete’s foot (not a worm/helminth). You get a ring because it spreads from the center of where it starts, kind of like Fairy Rings.

Finally, there are a few other aspects of this I found interesting. That Canadian couple had to see at least 3 doctors before they found one capable of correctly diagnosing their problem. Whoa. Also, they were unable to get the treatment, Ivermectin, because the company that makes it just doesn’t sell in Canada. If you look at that map of hookworm occurrences, you might be able to figure out why. They did manage to get some, though, but crossing into Detroit and picking some up there.

In the end, yes, you can wear shoes and HazMat suits all the time. I myself prefer to know what I am dealing with and then try to generate effective strategies to counter or minimize them.

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