Several parasitic worms in the order Spirurida can invade human skin, producing symptoms that range from migrating red streaks and itchy swellings to painful blisters with a worm visibly poking through. These are not a single disease but a collection of infections caused by different species, each with its own route into the body, its own preferred tissue, and its own geography. Some, like guinea worm, are on the verge of global eradication; others, like certain Dirofilaria and Onchocerca species, are expanding into regions where clinicians rarely think to look for them.
What Spirurida Worms Actually Are
Spirurida is a large order of roundworms (nematodes) whose life cycles almost always involve an arthropod intermediate host, whether a mosquito, a blackfly, a flea, or a tiny freshwater crustacean. In nature, they mainly parasitize animals. Humans usually enter the picture as accidental or dead-end hosts, picking up larvae through insect bites, contaminated water, or undercooked meat. Once inside a person, these worms tend to settle in connective tissue, subcutaneous fat, or along tissue planes just beneath the skin, which is why dermatological symptoms dominate the clinical picture even when the parasites can also reach the eyes, the central nervous system, or the chest cavity.
Gnathostomiasis and the Creeping Skin Lesion
Gnathostoma species are among the most common Spirurida culprits behind skin disease in humans. People typically become infected by eating raw or undercooked freshwater fish, eels, frogs, or poultry that harbor the larvae. The infection is most frequently reported in Southeast Asia, Mexico, and parts of Central and South America, but increasing global travel and the popularity of dishes like sashimi and ceviche have carried cases well beyond those regions.
Once swallowed, the larvae cannot complete their full life cycle in a human host, so they wander. That wandering is the problem. In a systematic review of the published literature, skin involvement accounted for roughly 84% of gnathostomiasis cases, making it far more common than the neurological, ocular, or visceral forms of the disease.1PubMed Central. Human gnathostomiasis – A systematic review and analysis of the literature The classic presentation is a migratory swelling: a painful, itchy lump that moves under the skin over days or weeks as the larva tunnels through tissue. Some patients instead develop creeping eruption, a serpiginous red track on the skin surface that marks the larva’s path.
Treatment typically involves antiparasitic drugs. In a comparative study, albendazole given daily for 21 days cured about 92% of cutaneous gnathostomiasis patients, while a single dose of ivermectin cured about 76%, though the difference between the two was not statistically significant.2The American Journal of Tropical Medicine and Hygiene. Treatment of Cutaneous Gnathostomiasis with Ivermectin When a larva can be located, surgical removal is sometimes preferred, especially if it is near the eye or another sensitive area.
Onchocerciasis and Skin That Mimics Eczema
Onchocerca volvulus, the cause of river blindness, is probably the best-known Spirurida parasite affecting human skin. It is transmitted by blackflies that breed near fast-flowing rivers in sub-Saharan Africa and, to a lesser extent, parts of Latin America and Yemen. Adult worms live coiled inside fibrous nodules (onchocercomas) under the skin, where they can survive for over a decade, steadily releasing microfilariae that spread through the surrounding tissue and trigger intense immune reactions.
The skin findings can be deceptive. Onchodermatitis often looks like ordinary eczema, with papular, lichenoid, atrophic, or pigmentary changes that would not raise suspicion if a clinician is unaware of the patient’s travel history. The encysted worms themselves tend to present as common skin cysts or benign-looking lumps.3International Journal of Dermatology. Onchocerciasis (river blindness): larva-induced eczema (onchodermatitis) from an important oculocutaneous tropical disease spilling over into North America and Europe This is relevant because migration and travel mean onchocerciasis cases now show up in North American and European clinics, where the diagnosis can easily be missed.
A related but distinct species, Onchocerca takaokai, was recently documented infecting a human for the first time. A young man in Japan developed a linear migratory red streak on his forearm. Tissue examination and DNA analysis confirmed the worm was O. takaokai, a parasite normally found in wild boars.4PubMed Central. First human infection with Onchocerca takaokai (Spirurida: Onchocercidae) presenting as creeping eruption in Japan The case is a reminder that animal Onchocerca species can spill over into people, producing skin signs that look very different from classic river blindness.
Loiasis and Calabar Swellings
Loa loa, the African eye worm, is transmitted by deer flies (Chrysops species) in the rainforest belt of West and Central Africa. Adult worms migrate freely through subcutaneous tissue, and a hallmark of the infection is the Calabar swelling: a localized, non-pitting area of edema that tends to appear around the hands and wrists, lasting a few days before resolving and then recurring in the same or a different spot. The swellings are thought to be an allergic reaction to worm antigens, though the exact mechanism and the predilection for the hands and wrists have never been fully explained.5PLoS Neglected Tropical Diseases. Understanding Calabar swellings: Assessing subcutaneous loiasis using ultrasound
The most dramatic moment in loiasis is when the adult worm crosses the conjunctiva of the eye, visibly slithering across the white of the eye over the course of minutes to hours. This is alarming but usually more distressing than dangerous. Removing a visible subconjunctival Loa loa can be done under local anesthesia by incising the tissue and extracting the worm with forceps. If the worm is actively mobile and keeps retreating from the surgeon’s approach, one described technique involves placing a pharmacological barrier of lidocaine with epinephrine in the surrounding tissue to block the worm’s escape routes and immobilize it for retrieval.6PLoS Neglected Tropical Diseases. Removal of adult subconjunctival Loa loa amongst urban dwellers in Nigeria The integrity of the worm should be preserved during extraction, because a broken worm can provoke severe local inflammation.
Subcutaneous Dirofilariasis
Dirofilaria repens is a filarial worm of dogs and other carnivores, transmitted by mosquitoes. Humans are accidental hosts, and the parasite typically cannot mature fully or produce offspring in people. Instead, the immature worm often lodges in the subcutaneous tissue and provokes a nodule. These nodules can appear almost anywhere on the body, including the eyelids, and are frequently mistaken for cysts, lipomas, or even tumors until surgical removal reveals the worm inside.7PubMed. A human case of subcutaneous dirofilariasis caused by Dirofilaria repens in Vietnam: histologic and molecular confirmation
Some patients develop migratory skin changes instead of a fixed nodule. In a severe case documented in Slovenia, a woman experienced recurring angioedematous skin swellings measuring roughly 15 cm across on her upper trunk and armpit areas. The swellings would appear, persist for a few days, then vanish, only to reappear somewhere else. When tissue was eventually excised, it contained necrotizing granulomas around structures with the thick, ridged cuticle characteristic of Dirofilaria.8Emerging Infectious Diseases. Severe and Rare Case of Human Dirofilaria repens Infection with Pleural and Subcutaneous Manifestations, Slovenia
Because Dirofilaria species do not produce microfilariae (the tiny larval stage detectable in blood) in humans, standard blood tests for filarial infections will miss them. Diagnosis usually comes down to removing the nodule and examining the worm histologically or with DNA analysis.9PubMed. Thirty cases of human subcutaneous dirofilariasis reported in Rostov-on-Don (Southwestern Russian Federation) This creates a diagnostic gap: many cases are discovered only incidentally during surgery for what was assumed to be a benign lump.
Guinea Worm Disease
Dracunculus medinensis is in many ways the most viscerally alarming member of this group. People become infected by drinking water containing tiny crustaceans (copepods) that carry the larvae. Over about a year, the larvae mature inside the body, and the female worm, which can reach 70 to 80 cm in length, migrates to the subcutaneous tissue, usually in the feet or legs.10PubMed Central. Guinea worm disease: epidemiology, control, and treatment When she is ready to release larvae, she creates a painful, intensely itchy blister on the skin. The pain drives many people to soak their feet in water, which is exactly what the worm needs: the blister ruptures on contact with water, the worm’s head emerges, and she releases a cloud of larvae into the water source, completing the cycle.11PubMed Central. Guinea Worm Disease: A Neglected Diseases on the Verge of Eradication
There is no vaccine and no drug that kills the worm. The traditional treatment is to wind the emerging worm slowly around a stick over days or weeks, taking care not to break it, because a ruptured worm can cause severe secondary infection. Prevention depends entirely on filtering or treating drinking water and keeping infected individuals away from water sources during the emergence period.
Guinea worm disease has been the target of one of the most successful public health campaigns in history. From an estimated 3.5 million cases in 1986, global cases dropped to just 14 in 2023 and 15 in 2024.12Morbidity and Mortality Weekly Report. Progress Toward Eradication of Dracunculiasis (Guinea Worm Disease) — Worldwide, January 2024–June 2025 As of mid-2025, indigenous transmission was still occurring in six countries: Angola, Cameroon, Chad, Ethiopia, Mali, and South Sudan. However, a complication has emerged in recent years: hundreds of animal infections, mostly in domestic dogs, have been detected, particularly in Chad and Cameroon. Animal infections increased by about 30% between 2022 and 2023 before declining substantially in the first half of 2024.13MMWR. Morbidity and Mortality Weekly Report. Progress Toward Global Dracunculiasis (Guinea Worm Disease) Eradication, January 2023–June 2024 Civil unrest in Mali, South Sudan, and Sudan further complicates surveillance. Still, the program appears close to reaching zero human cases.
Less Common Culprits
A few other Spirurida worms occasionally turn up in human skin or mucosa, though they are rare enough that they tend to be reported as individual case studies rather than studied in large series.
Gongylonema pulchrum normally parasitizes the esophageal lining of cattle, sheep, and other ruminants. Humans can become infected by accidentally swallowing dung beetles or cockroaches that harbor the larvae. The worm then embeds in the mucosa of the mouth, lips, or throat, where it causes a sensation of something crawling or moving beneath the surface. In one reported case from Slovenia, a patient actually extracted the worm himself from beneath his tongue and correctly identified it using an internet search.14Journal of Helminthology. First case of human Gongylonema pulchrum infection in Slovenia While not strictly a skin infection, the mucosal location and the crawling sensation fall squarely in the territory of what people fear when they hear about worms in the body’s outer layers.
Thelazia callipaeda, the oriental eyeworm, parasitizes the conjunctival sac and tear ducts. It is transmitted by small fruit flies and normally infects dogs, cats, and various wild mammals, but human cases occur regularly in parts of Asia and southern Europe. A study in Japan highlighted the parasite’s remarkably wide host range and the significant role of wildlife as a reservoir, raising concerns that human cases could become more common as the parasite’s geographic range continues to expand.15Parasitology International. New host record of Thelazia callipaeda (Nematoda: Spirurida) with a notably wide host range and shared zoonotic lineage in Japan
Why These Infections Get Missed Outside the Tropics
A recurring theme across Spirurida skin infections is misdiagnosis. A nodule from Dirofilaria repens gets excised as a suspected cyst. Onchodermatitis is treated as eczema for months before someone thinks to ask about travel. Gnathostomiasis gets worked up as an allergic reaction or cellulitis. The problem is not that these infections are inherently difficult to diagnose; rather, they are simply not on the radar of most clinicians in temperate countries.
Travel history is the single most useful diagnostic clue. A migratory swelling or a creeping red track on the skin of someone who recently ate raw freshwater fish in Thailand, spent time near rivers in West Africa, or lives in an area where mosquito-borne dog heartworm is common should prompt consideration of a Spirurida parasite. Without that context, standard blood tests are often unhelpful. As noted earlier, Dirofilaria infections in humans do not produce the blood-borne microfilariae that serological tests for filariasis are designed to detect.16PubMed Central. Human gnathostomiasis – A systematic review and analysis of the literature In many cases the definitive diagnosis only comes after a worm is removed and examined under a microscope or identified by DNA analysis.
The Wildlife Reservoir Problem
Many Spirurida parasites that infect human skin are fundamentally animal parasites. Humans are bystanders in a cycle that typically runs between wild or domestic animals and their arthropod vectors. This matters for two reasons: it means that eliminating human cases does not eliminate the parasite, and it means that new spillover events are always possible as human activity increasingly overlaps with wildlife habitat.
Dirofilaria immitis, the dog heartworm, illustrates this. While it primarily causes pulmonary disease in humans rather than skin involvement, studies in Brazil’s Atlantic Forest have found the parasite circulating between domestic dogs and wild carnivores like ring-tailed coatis. Infected wild carnivores can act as reservoir hosts in the presence of competent mosquito vectors, sustaining transmission even if domestic dogs are treated.17PLoS Neglected Tropical Diseases. Filarids (Spirurida: Onchocercidae) in wild carnivores and domestic dogs from the Brazilian Atlantic forest A similar dynamic applies to D. repens, the species more commonly associated with subcutaneous nodules in people.
Guinea worm presents the starkest version of this problem. The eradication campaign has driven human cases to near zero, but hundreds of animal infections, mainly in dogs, threaten to maintain the parasite’s transmission cycle indefinitely.18PubMed Central. Dracunculiasis Eradication: End-Stage Challenges Dogs are thought to acquire infection by eating fish entrails or other aquatic animals containing infected copepods, creating a transmission pathway that the traditional strategy of filtering human drinking water cannot fully interrupt. These animal infections in limited geographic pockets are now the main obstacle standing between the current program and global eradication.
The Onchocerca takaokai case from Japan fits the same pattern. The worm’s natural host is the wild boar, and the human infection appears to have been an incidental spillover.19PubMed Central. First human infection with Onchocerca takaokai (Spirurida: Onchocercidae) presenting as creeping eruption in Japan As diagnostic tools improve and more unusual worms get identified to the species level by DNA, the catalog of Spirurida species capable of infecting human skin is likely to keep growing. Whether that reflects genuinely new spillovers or simply better detection of infections that have always occurred at low levels is an open question that parasitologists are still sorting out.