Ask the doctors: Hookworm larvae infect humans by burrowing in their skin

Dear Doctor: I read a terrible story about a couple who went to the Caribbean and got hookworms by going barefoot. Is that only possible in the tropics, or should I be worried here in the United States? I hate wearing shoes in the summer.

Dear Reader: Let’s start with a recap of the story. A couple visiting a Caribbean resort came home with more than a tan. After a barefoot walk on the beach, their feet began to itch. They chalked it up to bug bites and, amid much scratching, continued their vacation. Once home, however, they developed large, weeping blisters on their feet, followed by subcutaneous markings that looked as though something might be burrowing beneath their skin.

It was. During that barefoot stroll, the couple had unwittingly walked through an area contaminated by hookworm larvae. Larvae are present anywhere that an infected animal (or human – a bit more on that in a moment) has left its stool. In this case, the hookworms were a species that lives in the intestines of dogs, cats and wild animals. This was actually good news because humans are imperfect hosts for this particular parasite: In other words, the larvae can’t mature or reproduce. As a result, infections typically resolve without medication in about four to six weeks. (In the couple’s case, once the infection was identified by inspection of a stool sample, they were given a course of antiparasitic drugs.)

The key to understanding what happened lies in the life cycle of the hookworm. The parasite, a nematode, thrives in moist, warm areas and thus is common in the tropics and subtropics. Its eggs are deposited into the soil via the feces of an infected host. After the eggs hatch and the larvae mature, they are ready to burrow into the skin of whichever creature crosses their path.

When hookworm larvae enter their ideal host, they become a serious health threat. They complete a complex journey through the body that brings them to their final destination, the inside of the small intestine. Once there, they latch onto the intestinal walls with the hook-like mouths that give them their name, and begin to feed on the host’s blood.

Two different species of hookworm can infect humans – Necator americanus and Ancylostoma duodenale – with the former the more prevalent. At least 575 million and up to 740 million individuals are affected by hookworm, making it one of the most common parasitic infections in the world. Without treatment, infection by these species causes severe anemia. Over time, this results in weakness, extreme fatigue, shortness of breath and irregular heartbeat. Children with hookworm infection often have stunted growth and cognitive problems, and become susceptible to other illnesses. Adults become so exhausted that they cannot function fully.

Although infection by N. americanus was once a grave public health problem in the United States, particularly in the Southeast, it is now quite rare, thanks to education and eradication programs, as well as improvements in sanitation. However, hookworms that infect pets and wildlife remain common. If a barefoot walk were to take you across soil contaminated by the feces of an animal (or human) infected with hookworm, then you could indeed be at risk.