September 26, 2018
Parasitic worm.

A Parasite Primer

by Wellness Letter

If you live in a developed country such as the U.S., you may not give much thought to parasites and the sometimes horrifying infections they can cause. Rather, you may think they are a problem only in impoverished parts of the world—as exemplified by the case of the North Korean soldier who defected to South Korea last November and was found to have worms crawling in his gut, some almost a foot long. You may be surprised to know, however, that many parasitic infections also occur in the U.S.

For example, there were 32 outbreaks of Cryptosporidium linked to swimming pools or water playgrounds in the U.S. in 2016, up from 16 in 2014. You may also recall the massive “crypto” outbreak 25 years ago in Milwaukee, which sickened more than 400,000 people over nine days, caused by a glitch in one of the city’s water-treatment plants. This was the largest waterborne disease outbreak ever recorded in the U.S.

Here’s a primer on such parasites to help you understand the ins and outs of these creepy critters and how to protect yourself against them.

What is a parasite?

The term parasite is commonly used to refer to an organism that lives in (or on) another organism, satisfying its own nutrient needs at the expense of the host. The two main classes are microscopic protozoa (one-celled organisms, such as Giardia and Cryptosporidium) and helminths (multi-celled organisms, commonly called worms, which, in their adult phase, can often be seen with the naked eye).

How do people get infected?

There are many ways, depending on the specific parasite. Protozoa can get into the gastrointestinal tract, for example, when someone consumes contaminated food or water.

Helminths typically enter the body as eggs, from water or soil that was contaminated with animal or human feces: When people drink the water or eat raw, unpeeled or unwashed vegetables grown in that soil, they will ingest the eggs. Or they may become infected if they come into contact with soil directly—as when children play outside or people garden—and then touch their mouth with their dirty hands. It’s also possible to become infected with certain helminth eggs by eating meat from a contaminated animal, or by consuming contaminated aquatic plants or seafood.

In the case of hookworm and some other worms, people usually get infected simply by walking barefoot in contaminated soil (the eggs develop into a form that can penetrate the skin).

Certain parasites, like those that cause malaria, are spread by insects, referred to as vectors. (The main focus of this article, however, is on non-vector-borne parasites.)

Parasite Prevention: 5 Tips

If you're traveling to a developing country—or encounter local warnings at home—heed these tips to avoid parasitic infections. They also apply if you are hiking anywhere in the world.

How do parasites harm the body?

Signs and symptoms of infection vary but may include anything from seizures and heart failure to pregnancy complications, blindness, and even death. Protozoa like Giardia, Cryptosporidium, and Cyclospora can cause severe diarrhea, while intestinal helminths, notably, can impair the nutritional status of the host in several ways, such as by reducing a person’s appetite, causing blood loss in the intestine (resulting in anemia), feeding on the person’s tissues (leading to protein and iron losses), and reducing the absorption of nutrients in the intestine.

Some helminths can also cause muscle pain and skin lesions—and can even infect the brain, heart, liver, and other vital organs, leading to life-threatening illness.

What parasitic infections are most common in the U.S?

The most common intestinal parasite in the U.S. is the human pinworm, acquired by ingesting the eggs, which may be transferred via the unwashed hands of infected people after they touch their anus (female pinworms lay their eggs outside the anus after they pass through the intestines of an infected person). The eggs may also be spread through clothing, towels, or other contaminated surfaces. Pinworms can cause anal itching, though often there are no symptoms.

Giardia, which can be acquired by drinking untreated water, is responsible for the most cases of parasite-related diarrhea (giardiasis) in the U.S. Also common are infections from Cryptosporidium (and to a lesser extent Cyclospora), protozoa that can cause diarrhea and that are spread by water or food that’s contaminated with feces.

There are also several diseases that the CDC calls “neglected parasitic infections” (NPIs) because few resources have been allocated to them, yet they infect large numbers of people in the U.S. and can be severe—but can be treated and prevented.

These include cysticercosis (which can cause seizures), toxocariasis (which infects cats and dogs and, as a result of contact with their feces, humans), and toxoplasmosis (a leading cause of foodborne illness, which infects over 800,000 people in the U.S. each year and can be especially dangerous for pregnant women and immunocompromised individuals). Toxoplasmosis is also transmitted through exposure to cat feces (pregnant women who have cats should clean litter boxes every 12 to 24 hours with gloves on, or have someone else clean them).

Another common NPI is trichomoniasis, which is transmitted sexually. Nearly 4 million people in the U.S. have it, but most are not aware because there are usually no symptoms, so they may unknowingly pass the parasite on to others.

Why are parasitic infections less common in the U.S. than in developing countries?

For one thing, the U.S. has cleaner water, better hygiene and sanitation, and adequate shelter that keeps out parasite-carrying insects. Climatic conditions also play a role. For example, the high temperatures and humidity associated with long rainy seasons in many developing countries, such as India and Brazil, as well as parts of Africa, can increase parasite growth and reproduction. People in the U.S. may develop a parasitic disease when traveling to developing countries, or bring it with them if they have immigrated to the U.S. from one of those countries, or if they are in close contact with new immigrants who are infected.

But poverty in the U.S. can also lead to conditions just as bad as those in developing countries where certain parasites are ubiquitous, as was seen in a 2017 study in the American Journal of Tropical Medicine and Hygiene. It found that one in three people in a poor rural Alabama county had evidence of hookworm infection. This was not surprising considering that raw sewage was common there, including pools of feces-contaminated water near where children played.

How are parasitic infections diagnosed?

A variety of lab tests can be used, depending on your symptoms and signs and your travel history. Because diagnosis can be difficult, your doctor may order more than one test. The tests include stool analysis by microscopy and newer molecular analyses, as well as blood tests to identify antigens from the pathogen or antibodies produced by the body as a result of the infection.

This article first appeared in the UC Berkeley Wellness Letter.

Also see Vector-Borne Diseases on the Rise.