Roundworms are typically pink or white, smooth-bodied worms with tapered ends. The most common species to infect humans, Ascaris lumbricoides, can grow surprisingly large: females reach over 15 inches (40 cm) long and about a quarter inch (6 mm) in diameter, roughly the size and shape of a pencil. Males are somewhat smaller. But “roundworm” is actually a broad category covering several species, and they range from barely visible threads to these large intestinal worms.
Ascaris: The Large Intestinal Roundworm
Ascaris lumbricoides is the species most people mean when they say “roundworm.” These are the largest of the common human intestinal worms, and they’re hard to miss if they show up in stool. Live worms are pink or white with smooth, unsegmented bodies that taper to a point at both ends. They look somewhat like thick spaghetti noodles or rubber bands. Dead worms may appear more pale or yellowish and lose their firmness.
Female Ascaris worms are the larger sex, sometimes exceeding 15 inches long. Males tend to be shorter and thinner, with a curved or hooked tail end. If you see a worm this size in stool, there’s little question about what you’re looking at. Occasionally, a worm is passed during vomiting rather than in stool, which is alarming but not uncommon with heavy infections.
Pinworms: Tiny White Threads
Pinworms (Enterobius vermicularis) are the most common roundworm infection in temperate climates, especially in children. They look completely different from Ascaris. Adult females are only 8 to 13 mm long, roughly the length of a staple, and about half a millimeter wide. Males are even smaller at 2 to 5 mm. They’re white, thin, and thread-like.
The easiest way to spot pinworms is to check around the anal area at night, when females migrate out to lay eggs. They look like tiny white threads wiggling on the skin. The female has a distinctive long, pointed tail (which gives the species its common name), while the male’s tail is blunt. In stool, they can be easy to miss because of their small size, though you might notice one or two white filaments on the surface.
Hookworms: Small With Distinctive Mouths
Hookworms are another common human roundworm, though they’re rarely seen by patients because they attach to the intestinal wall rather than passing freely in stool. Adults are small: the species Ancylostoma duodenale reaches 8 to 15 mm, while Necator americanus tops out around 9 to 11 mm. Both are pinkish-white and slender.
What sets hookworms apart is their mouth. They have a specialized buccal capsule equipped with either sharp teeth or cutting plates, depending on the species, that they use to grip the intestinal lining and feed on blood. You wouldn’t see this detail without magnification, but it explains why hookworm infections cause anemia while other roundworms generally don’t. If hookworms do appear in stool (usually after treatment), they look like small, pale, slightly curved worms.
How To Tell Roundworms From Earthworms
People sometimes confuse roundworms with earthworms or other worm-like creatures. The key difference is segmentation. Earthworms belong to a completely different group (segmented worms) and have visible ring-like segments running the length of their body, along with a darker reddish-brown color. Roundworms have smooth, unsegmented bodies. Their name comes from the Greek root “nema,” meaning thread, and they do look thread-like in cross section: round rather than flat.
Roundworms also lack the muscular ridges and slimy coating that earthworms have. Their surface is smooth and slightly shiny. If you see something in stool that looks segmented, it’s more likely a tapeworm segment (which appears as small, flat, rice-grain-shaped pieces) than a roundworm.
What Roundworm Eggs Look Like
Roundworm eggs are invisible to the naked eye. They’re microscopic, measured in micrometers, and can only be identified under a microscope during a stool sample analysis. Ascaris eggs, for example, are 45 to 75 micrometers long (roughly the width of a human hair) and have a thick, bumpy outer shell that stains brown from bile in the intestine. Unfertilized eggs are slightly larger and more elongated, with a thinner shell.
Hookworm eggs are thin-shelled, colorless, and oval, measuring 60 to 75 micrometers. Pinworm eggs are similarly tiny and are usually collected using a tape test pressed against the skin around the anus in the morning, since the females deposit eggs there overnight rather than in stool.
What To Look For in Stool
If you’re checking stool because you suspect a roundworm infection, here’s what to watch for. Large Ascaris worms are obvious: long, pale, spaghetti-like strands, sometimes still moving. Pinworms are much harder to spot in stool and are more reliably found by checking the perianal area at night with a flashlight. Hookworms almost never appear in stool without treatment prompting their release.
Color can vary. Living worms tend to be pinkish-white, while worms that have been dead for some time may turn pale yellow or even slightly translucent. Movement is the clearest sign that what you’re seeing is a live worm rather than undigested food fiber, which is a common false alarm. Mucus strands, vegetable fibers, and bean sprouts are frequently mistaken for worms. If you’re unsure, placing the specimen in a clear container with water can help you see whether it has a defined worm-like structure with tapered ends.