A roundworm is any worm belonging to the phylum Nematoda, a massive group of parasitic and free-living organisms found in virtually every environment on Earth. They’re called “roundworms” because their bodies are cylindrical in cross-section, unlike the flat ribbon shape of tapeworms. Some species live harmlessly in soil or water, but others infect humans and animals, making roundworms one of the most common parasitic infections worldwide. An estimated 1.5 billion people, roughly 24% of the global population, are currently infected with soil-transmitted roundworms and related parasites.
What Roundworms Look Like
The simplest way to picture a roundworm is as a tube within a tube. The outer tube is the body wall, covered in a thick protective coating called a cuticle. The inner tube is a one-way digestive tract with a mouth at one end and an anus at the other. They have no eyes, no legs, and no segments. Their bodies are bilaterally symmetrical, meaning the left and right sides mirror each other. A fully developed roundworm is built from roughly 1,000 body cells, which is remarkably few compared to most animals.
Size varies enormously depending on the species. Some soil-dwelling nematodes are microscopic, barely visible without magnification. The giant intestinal roundworm that infects humans can grow to 35 centimeters (about 14 inches) long. Regardless of size, they all share that same smooth, unsegmented, cylindrical body plan.
The Main Species That Infect Humans
Several roundworm species cause disease in people, but the most significant is the giant intestinal roundworm, the single most common parasitic worm infection in the world. It spreads through contaminated soil. When someone swallows the microscopic eggs (usually from unwashed produce or contaminated water), the larvae hatch in the intestine, burrow through the gut wall, and travel through the bloodstream to the lungs. They spend 10 to 14 days maturing in the lungs, then crawl up the airways to the throat, get swallowed again, and finally settle in the small intestine as adult worms.
Most people with a light infection have no symptoms at all. Heavy infections, especially in children, can cause stunted growth and malnutrition. Very high worm burdens sometimes lead to abdominal pain, intestinal blockage, or even perforation. Occasionally a single adult worm migrates into the bile duct or appendix and causes an acute blockage there.
Roundworms From Pets and Wildlife
Dogs and cats carry their own roundworm species, and these can accidentally infect humans. The dog roundworm is the most well-known example. People, usually young children, become infected by swallowing eggs from contaminated soil or, less commonly, from undercooked meat. The larvae hatch and penetrate the intestinal wall, then travel through the bloodstream to the liver, lungs, heart, brain, muscles, or eyes.
Unlike in dogs, these larvae can’t complete their life cycle in a human body. Instead, they wander through tissues causing damage wherever they end up. When larvae invade internal organs like the liver and lungs (mostly in preschool-aged children), they trigger fever, coughing, rashes, weight loss, and an enlarged liver. When a larva reaches the eye, it can cause inflammation and permanent vision loss, typically in just one eye. In rare cases, larvae migrate to the brain and cause serious neurological complications.
Raccoons carry yet another roundworm species. Their droppings, often left in communal “latrines” on decks, attics, or sandboxes, contain eggs that become infectious after sitting in the environment for two to four weeks.
How Roundworm Eggs Survive in the Environment
Roundworm eggs are extraordinarily tough. They’re coated in a thick protective shell that allows them to survive in soil for months or even years under the right conditions. Most common disinfectants and chemicals don’t kill them. Heat, however, destroys the eggs instantly, which is why boiling water or flame treatment is recommended for contaminated surfaces.
Freshly deposited eggs aren’t immediately dangerous. They need two to four weeks in warm, moist soil to develop into their infectious stage. This window is important because prompt cleanup of animal feces significantly reduces the risk of infection.
How Roundworm Infections Are Diagnosed
The standard test is a stool sample examined under a microscope for eggs or worm fragments. This is sometimes called an ova and parasite test. Because eggs aren’t shed consistently, three or more stool samples collected on separate days give the most reliable results.
When stool tests come back negative but symptoms persist, doctors may use other approaches. Blood tests can detect antibodies the immune system produces in response to a roundworm infection, which is particularly useful for diagnosing tissue-migrating larvae that wouldn’t show up in stool. Imaging scans like X-rays, MRIs, or CT scans can reveal damage in organs where larvae have traveled. In some cases, a scope inserted through the mouth or rectum allows a direct look at the intestinal lining.
Treatment
Roundworm infections are highly treatable. The most commonly used medications work by disrupting the worm’s internal structural system, essentially preventing its cells from functioning. The worms die and are passed out of the body naturally. Other medications work by paralyzing the worms so they can no longer hold their position in the intestine and get expelled. Treatment is typically short, sometimes a single dose, sometimes a few days depending on the medication and the severity of the infection.
For tissue-migrating larvae (like those from pet roundworms that travel to the eyes or organs), treatment is more complex. The goal shifts to reducing inflammation and preventing further tissue damage while addressing the larvae themselves.
Preventing Infection
Most roundworm infections come down to one thing: accidentally swallowing eggs from contaminated soil, food, or surfaces. Prevention is largely about breaking that cycle.
- Wash hands after working or playing outdoors, especially before eating.
- Wash produce thoroughly, particularly if it was grown in or near soil.
- De-worm pets regularly under veterinary supervision, and clean up their feces promptly.
- Keep sandboxes covered when not in use, since animals may use them as a latrine.
- Discourage wildlife around your home by securing trash cans, closing off attic and basement access, and removing outdoor food and water sources.
If you find animal droppings in an enclosed space, take precautions during cleanup. Wear disposable gloves, avoid stirring up dust (lightly misting the area with water helps), and use hot water for laundering any contaminated clothing. Contaminated material should be bagged and sent to a landfill, burned, or buried. Since chemicals won’t kill the eggs, heat is your most effective tool for decontaminating hard surfaces.