Tissue Nematodes

Wuchereria bancrofti is the most common cause of lymphatic filariasis worldwide. It is transmitted by several mosquito genera (Culex, Anopheles, and Aedes, depending on region).

Vector. Mosquitoes ingest microfilariae during a blood meal from an infected human. The larvae develop through several stages within the mosquito's flight muscles over one to two weeks. They then migrate to the proboscis as infective L3 larvae, which are transmitted at a subsequent blood meal.

Tissue location. Adult worms reside within the lymphatic vessels and lymph nodes, principally in the lower limbs, genital region, and, less commonly, the upper limbs and breast. They cause progressive lymphatic vessel damage and dysfunction.

Microfilariae. Adult female worms release microfilariae into the bloodstream. In most endemic regions they show nocturnal periodicity. They circulate in peripheral blood predominantly at night (typically peaking around midnight) and sequester in the pulmonary vasculature during the day.

This pattern is believed to have evolved in synchrony with the nocturnal biting behaviour of the principal mosquito vectors. It has direct diagnostic implications. In most settings, blood samples for microfilarial detection must be timed to nocturnal collection, or daytime provocation testing used as an alternative.

Major clinical associations. Chronic lymphatic damage from repeated adult worm infection and the associated host inflammatory response produces the classic clinical picture of lymphatic filariasis.

• Lymphoedema, most dramatically presenting as elephantiasis: gross, disfiguring chronic swelling, most often of the lower limbs, with skin thickening and secondary bacterial infection over time.
• Hydrocele in men: accumulation of lymphatic fluid around the testis. It is one of the most common presentations of bancroftian filariasis.
• Acute episodes of adenolymphangitis (fever, painful lymphadenitis, and lymphangitis), often precipitated by secondary bacterial infection of already damaged lymphatics. They occur intermittently and contribute cumulatively to progressive chronic lymphatic damage.
• Tropical pulmonary eosinophilia: a distinct hyperresponsive immunological syndrome with paroxysmal cough, wheeze, and marked eosinophilia. It occurs in a subset of infected individuals as an aberrant immune response to microfilariae trapped in the pulmonary circulation. Notably, microfilariae are not demonstrable in peripheral blood in this syndrome.

Brugia malayi and Brugia timori cause a form of lymphatic filariasis clinically similar to, but generally somewhat less extensive than, Wuchereria bancrofti disease. It is geographically restricted to parts of South and Southeast Asia.

Vector. Transmitted principally by Mansonia mosquitoes (and, for some regional strains, Anopheles). The intramosquito developmental process is broadly similar to Wuchereria.

Tissue location. Adult worms reside in the lymphatics, with a more restricted anatomical distribution than Wuchereria. They tend to affect the lymphatics of the lower limbs (below the knee) more than the genital lymphatics. This correlates with a lower incidence of hydrocele and genital involvement relative to bancroftian filariasis.

Microfilariae. Also typically exhibit nocturnal periodicity in most endemic areas. They circulate in peripheral blood with the same diagnostic timing implications as Wuchereria.

Major clinical associations. Chronic lymphoedema and elephantiasis, generally more often confined to the lower leg and foot than the more extensive limb and genital involvement sometimes seen with Wuchereria. Acute adenolymphangitis episodes occur similarly to bancroftian disease.

Onchocerca volvulus is the cause of onchocerciasis (river blindness). It is transmitted by blackflies breeding in fast-flowing rivers and streams, which accounts for the strong geographic association of disease with riverine communities in endemic areas. These are principally in sub-Saharan Africa, with additional foci historically in Latin America and Yemen.

Vector. Simulium blackflies acquire microfilariae during a blood meal from an infected human. Larvae develop within the fly over one to several weeks before becoming infective L3 larvae transmitted at a subsequent bite.

Tissue location. Adult worms reside within fibrous subcutaneous nodules (onchocercomata), typically found over bony prominences, and do not circulate in blood. The nodules can persist for years, harbouring adult worms that continuously release microfilariae into the surrounding skin and adjacent tissue.

Microfilariae. Unlike the lymphatic filariae, Onchocerca microfilariae reside in the skin and connective tissue (not the bloodstream). They lack the nocturnal periodicity of Wuchereria and Brugia, being found in skin at any time. They can also migrate into the eye, which underlies the organism's capacity to cause blindness. This skin location is the basis for the standard diagnostic skin snip technique.

Major clinical associations.

• Chronic dermatitis, with pruritus (often severe and a major cause of disability and sleep disturbance in endemic communities).
• Progressive skin changes including thickening, depigmentation (leopard skin appearance), and atrophy over years of chronic infection.
• Ocular disease results from microfilarial migration into the eye and the associated host inflammatory response to dying microfilariae. It produces progressive keratitis, chorioretinitis, and optic nerve damage, culminating in irreversible blindness in chronic, heavy, longstanding infection. Historically it was a major cause of blindness in affected communities.
• Subcutaneous nodules are usually painless. They are more a diagnostic sign (palpable adult worm nests) than a major source of morbidity in themselves.

Loa loa, the African eye worm, is transmitted by day-biting Chrysops (deer/mango) flies in the rainforest and swamp-forest regions of West and Central Africa.

Vector. Chrysops flies ingest microfilariae during a daytime blood meal. Larval development occurs within the fly over one to several weeks before transmission of infective L3 larvae.

Tissue location. Uniquely among the filarial worms, adult Loa loa worms migrate actively through subcutaneous connective tissue throughout the body over months to years. They do not remain fixed in lymphatics or form stationary nodules.

They can transiently migrate across the subconjunctival space of the eye. This behaviour is responsible for the organism's common name and its most clinically distinctive presentation.

Microfilariae. Circulate in peripheral blood with diurnal periodicity (the reverse pattern from Wuchereria and Brugia), peaking during daytime hours. This matches the daytime biting behaviour of the Chrysops vector. Again there are direct implications for the timing of diagnostic blood sampling.

Major clinical associations.

• Calabar swellings: transient, localised, non-tender subcutaneous areas of angioedema, typically on the limbs. They result from a local host hypersensitivity response as an adult worm migrates through the tissue. They are the most characteristic clinical sign and often prompt clinical suspicion.
• Subconjunctival migration of an adult worm across the surface of the eye. It produces conjunctival irritation, oedema, and the disconcerting but usually self-limited direct visibility of the moving worm beneath the conjunctiva. It is a distinctive and diagnostically striking presentation.

A specific and clinically important treatment consideration is that individuals with very high Loa loa microfilarial density are at risk of severe, potentially fatal encephalopathy if treated with certain antifilarial drugs (notably ivermectin) that cause rapid microfilarial killing.

This is why Loa loa microfilarial density assessment is sometimes required before mass drug administration in areas where Loa loa is co-endemic with Onchocerca volvulus or Wuchereria bancrofti.

Dracunculus medinensis, the Guinea worm, causes dracunculiasis. The disease is targeted for global eradication and is now reduced to an extremely small number of cases annually.

It is transmitted not by a biting arthropod vector in the conventional sense, but via an intermediate host copepod (a small aquatic crustacean, colloquially a "water flea") ingested in drinking water.

Vector/intermediate host. Humans become infected by drinking water contaminated with copepods harbouring infective L3 larvae. The copepod is digested in the stomach, releasing larvae that penetrate the intestinal wall.

Tissue location. Following intestinal penetration, larvae migrate through the retroperitoneum and mature over approximately a year. The mature adult female eventually migrates to the subcutaneous tissue, most classically of the lower limb, where it induces a painful blister. The male is much smaller, dies after mating and is not typically seen clinically.

Life cycle completion (no microfilarial bloodstream stage). Unlike the filarial worms, Dracunculus does not produce circulating microfilariae.

Instead, the blister ruptures, often precipitated by the affected individual immersing the limb in water to relieve the intense burning pain. The adult female worm's uterus prolapses through the skin ulcer and releases large numbers of first-stage larvae directly into the water upon contact. Copepods ingest them to continue the cycle.

Eradication programmes directly exploit this transmission mechanism, by preventing contact between the emerging worm and drinking water sources.

Major clinical associations. The dominant clinical picture is the emergence of the adult female worm through a painful skin blister/ulcer, typically on the lower leg or foot. It is associated with intense local burning pain, which drives the water-immersion behaviour central to transmission.

Secondary bacterial infection of the ulcer is a common and significant complication. It occasionally produces cellulitis, abscess, sepsis, or, if a joint is involved, septic arthritis and lasting disability.

Management classically involves slow extraction of the worm over days to weeks by gradually winding it onto a stick as it emerges, a practice with ancient historical precedent.

Trichinella spiralis causes trichinellosis. It is transmitted via ingestion of undercooked meat (classically pork, though also game meat including bear and wild boar in some regions) containing encysted larvae. No arthropod vector is needed, and no intermediate host distinct from the meat-source animal itself.

Route and tissue location. Ingested encysted larvae are released by gastric digestion. They mature rapidly to adults within the small intestinal mucosa. This brief adult intestinal phase is the only stage in which Trichinella resembles the other intestinal nematodes.

Adult females release newborn larvae (rather than eggs) directly into the intestinal mucosa. These larvae enter the bloodstream and lymphatics and disseminate throughout the body.

They penetrate and encyst specifically within skeletal muscle fibres, with a preference for highly vascular, metabolically active muscle such as the diaphragm, extraocular muscles, tongue, and masseter. Each larva induces transformation of the host muscle cell into a specialised "nurse cell". This supports the larva's long-term survival, encysted and viable, for years.

Microfilariae. Trichinella does not produce a bloodstream microfilarial stage. Newborn larvae disseminate briefly via blood and lymphatics during the acute invasive phase before encysting in muscle. This is a transit phase rather than a sustained circulating reservoir stage.

Major clinical associations. An early gastrointestinal phase (corresponding to intestinal adult worm activity) produces nonspecific diarrhoea, abdominal pain, and nausea in the first one to two days after ingestion.

The subsequent muscle invasion phase, as larvae disseminate and encyst, produces the classic clinical picture:

• Fever, marked myalgia and muscle tenderness.
• Periorbital and facial oedema (a notably characteristic early sign), and splinter haemorrhages.
• Striking peripheral eosinophilia, reflecting the intense tissue-invasive phase.

Severe, heavy infection can involve cardiac muscle (myocarditis) and the central nervous system (encephalitis). These are the principal causes of mortality in severe trichinellosis, though this is uncommon with typical light to moderate infection.

Toxocara canis (dog) and Toxocara cati (cat) are intestinal nematodes of their respective definitive animal hosts. Humans are only an incidental, dead-end intermediate-type host. This results in aberrant tissue migration rather than the normal life cycle progression to an intestinal adult stage.

Vector/route. No arthropod vector is involved. Humans become infected by ingesting embryonated eggs from soil contaminated with dog or cat faeces. Classic exposure includes young children with pica or geophagia and soil contact in areas frequented by dogs. Less commonly, infection follows ingestion of undercooked meat from a paratenic host harbouring larvae.

Tissue location. Ingested eggs hatch in the human small intestine. Larvae penetrate the intestinal wall and disseminate via the bloodstream to various tissues.

Because the human is not the normal definitive host, the larvae cannot complete their expected migratory life cycle (which in the dog or cat would proceed to intestinal adult maturation). Instead they wander unpredictably through host tissue, most notably the liver, lungs, and eye, without ever maturing to an egg-producing adult in the human host. The larvae can also become dormant and persist for extended periods within tissue.

Microfilariae. Not applicable. Toxocara does not produce a bloodstream microfilarial stage, since the parasite never reaches sexual maturity in the human host.

Major clinical associations.

• Visceral larva migrans results from disseminated larval migration through internal organs, principally the liver and lungs. It produces fever, hepatomegaly, pulmonary symptoms (cough, wheeze), and marked eosinophilia. It occurs predominantly in young children with a history of pica or heavy soil exposure.
• Ocular larva migrans results from larval migration into the eye. It typically presents in an older child or young adult as unilateral visual disturbance.
• Ocular larva migrans can produce a granulomatous retinal mass that is a well-recognised mimic of retinoblastoma. This is an important differential diagnosis, given the very different management and prognostic implications of the two conditions.