Parasitic infections look different depending on where in the body the parasite lives. Skin parasites leave visible tracks, rashes, or bumps. Intestinal parasites may show up as worms in stool. Some parasitic infections produce no visible signs at all for weeks, showing up only as fatigue, digestive problems, or unexplained weight loss before anything appears on the skin or in the stool. Here’s what to look for across the most common types.
Skin Parasites: Tracks, Burrows, and Rashes
Scabies is one of the most recognizable skin parasites. The mites burrow just under the surface of the skin, leaving tiny raised serpentine lines that are grayish or skin-colored and can stretch a centimeter or more in length. These tracks most often appear in the webbing between the fingers, the folds of the wrist, elbow, or knee, and around the shoulder blades. The surrounding skin becomes intensely itchy, especially at night, and a bumpy red rash develops around the burrow sites.
Hookworm larvae that penetrate the skin from contaminated soil create a different pattern. Instead of short burrows, they produce irregular, raised, reddish tracks that snake across the skin. These tracks are typically about 3 mm wide and move several millimeters per day as the larva migrates beneath the surface. The result is a winding, trail-like rash that’s sometimes called “creeping eruption,” most commonly on the feet, legs, or buttocks.
Swimmer’s itch, caused by parasitic larvae found in freshwater, follows a faster timeline. Redness and itching begin while you’re still in the water or immediately after getting out. Within about 12 hours, the affected skin develops itchy raised bumps that can blister. The rash appears only on skin that was exposed to the water.
Head Lice and Nits
Lice are visible to the naked eye. Adult head lice are about the size of a sesame seed and move quickly through hair, making them hard to spot. Their eggs, called nits, are easier to find. They look like tiny white or yellowish-brown ovals cemented to individual hair strands, usually close to the scalp. The key way to tell nits from dandruff or hair product residue: dandruff flicks off easily, but nits do not. They’re glued to the hair shaft and require deliberate pulling or a fine-toothed comb to remove.
Worms in Stool
Some intestinal parasites are large enough to see when they pass in stool. Pinworms are the most common in the United States, especially in children. They’re small, thin, white worms, roughly the length of a staple. They’re most active at night, when females migrate to the skin around the anus to lay eggs, causing intense itching. You’re most likely to spot them by checking the area with a flashlight a few hours after bedtime.
Roundworms are far more dramatic in appearance. Adult females can measure 20 to 35 cm long (roughly the length of a ruler), while males are slightly smaller at 15 to 31 cm. These are pinkish-white, pencil-thick worms that are unmistakable when passed in stool. Roundworm infections are more common in tropical and subtropical regions, but travelers can bring them home.
Tapeworm segments, by contrast, look like small flat white or yellowish rectangles, roughly the size of a grain of rice. You might notice them in stool, on underwear, or on bedding. They can sometimes be seen moving.
Symptoms You Can’t See
Many parasitic infections produce symptoms that are felt rather than seen, at least initially. Giardia, one of the most common waterborne parasites, causes watery diarrhea, gas, bloating, and greasy stools that float. There’s nothing visually distinctive about the stool itself. The same is true for Cryptosporidium, which causes profuse watery diarrhea that can last for weeks.
Intestinal parasites frequently cause a combination of abdominal pain, nausea, fatigue, and unexplained weight loss. Some people develop increased appetite despite losing weight, because the parasite is competing for nutrients. Others notice mucus in their stool or develop iron-deficiency anemia over time, leading to pale skin and exhaustion.
One systemic sign that points specifically toward parasitic infection is a type of immune response that shows up on blood work. The body produces elevated levels of a specific white blood cell (eosinophils) when fighting certain parasites, particularly worms. Counts above 500 cells per microliter are considered elevated, with moderate elevations above 1,500 and severe above 5,000. This blood marker is often the first concrete clue when symptoms are vague.
Less Common but Distinctive Signs
Some parasitic infections produce highly specific visual markers. Acute Chagas disease, transmitted by “kissing bugs” found in parts of the Americas, can cause dramatic swelling of one eyelid when the parasite enters through the eye area. The swelling is distinctly one-sided and can last for weeks.
Certain parasitic infections cause lumps or nodules under the skin where larvae or adult worms have settled. These feel like firm, painless or mildly tender bumps and can appear on the scalp, trunk, or limbs depending on the species.
How Long Before Signs Appear
The gap between exposure and the first noticeable symptom varies widely. Cryptosporidium symptoms typically appear around 7 days after exposure, with a range of 2 to 28 days. Giardia takes longer, usually 1 to 4 weeks. Toxoplasma symptoms emerge in 6 to 10 days. Trichinella, acquired from undercooked meat, can cause gut symptoms within 1 to 2 days, but the muscle pain and facial swelling that make it recognizable may not develop for 2 to 8 weeks.
Skin parasites tend to show up faster. Scabies rash usually develops within 2 to 6 weeks on a first infection, but can appear within days if you’ve been infected before. Hookworm skin tracks begin forming within hours to days of larval penetration.
When It Feels Like Parasites but Isn’t
Persistent crawling, itching, or biting sensations on the skin sometimes have no parasitic cause. A condition called delusional parasitosis produces very real sensations of insects or worms crawling under the skin, even when no parasite is present. People with this condition often collect skin flakes, lint, or scabs in containers to show their doctor as proof. In an effort to relieve the sensations, they may scratch or pick at their skin enough to create sores, or apply chemicals and disinfectants that cause further irritation.
This is genuinely difficult to sort out, because many real skin conditions, from allergies to dermatitis to actual parasites, cause identical itching and crawling sensations. The skin damage from scratching and chemical use can also mimic other disorders. A thorough exam and sometimes skin scrapings or biopsies are needed to rule parasites in or out before attributing symptoms to another cause.