Trematodes

The genus Schistosoma (blood flukes) is biologically distinct from every other trematode. Three species cause the great majority of human disease.

Schistosomes differ from other trematodes in three ways:

• They have separate sexes (male and female worms), rather than the hermaphroditic pattern typical of trematodes generally.
• They live as paired adults within blood vessels rather than in the intestinal or biliary lumen.
• They are acquired by direct skin penetration by free-swimming cercariae rather than by ingestion of an encysted metacercarial stage.

Schistosoma haematobium, endemic across Africa and parts of the Middle East, is unique among the major schistosome species in that adult worm pairs reside within the venous plexus draining the urinary bladder and pelvic organs (the vesical and pelvic venous plexus). It produces predominantly urinary tract disease.

Schistosoma mansoni, endemic across Africa, parts of the Middle East, and South America and the Caribbean, resides as adult pairs within the mesenteric venules draining the large intestine, particularly the inferior mesenteric vessels. It produces predominantly intestinal and hepatic disease.

Schistosoma japonicum, endemic in parts of East and Southeast Asia, resides within the superior mesenteric venules draining the small intestine. It also produces intestinal and hepatic disease.

It is generally associated with higher egg output per worm pair and a broader zoonotic reservoir (including cattle, pigs, and other domestic and wild mammals) than Schistosoma mansoni. This contributes to a more difficult overall control picture in endemic areas.

Infection begins when free-swimming cercariae, released from an infected snail into freshwater, make direct contact with human skin. They actively penetrate it within minutes, aided by proteolytic enzyme secretion that breaks down skin tissue. No wound, ingestion, or vector bite is required.

Penetration can produce an itchy papular skin reaction at the site of entry, termed cercarial dermatitis or "swimmer's itch" when it occurs. The reaction is often mild or unnoticed with the human-adapted schistosome species.

It is generally more pronounced when caused by non-human (avian or other animal) schistosome species. These cannot complete their life cycle in humans and provoke a more vigorous localised immune reaction, as the cercariae are destroyed in the skin rather than surviving to migrate further.

Following successful penetration, the cercaria loses its forked tail and transforms into a schistosomulum, the migratory larval stage. It enters the bloodstream and lymphatics, travels through the lungs, and eventually reaches the liver. There it matures and pairs with a worm of the opposite sex.

The paired adults then migrate together against venous blood flow to their species-specific final venous location (vesical plexus for haematobium, mesenteric venules for mansoni and japonicum).

Schistosomes require a specific freshwater snail species as an obligate intermediate host. Each major schistosome species is associated with a particular snail genus:

• Bulinus for Schistosoma haematobium
• Biomphalaria for Schistosoma mansoni
• Oncomelania for Schistosoma japonicum

This specificity directly shapes the geographic distribution of each disease. Transmission can only occur where both the human reservoir and the appropriate snail species coexist in shared water sources.

Eggs passed in human urine (haematobium) or stool (mansoni, japonicum) that reach freshwater hatch into free-swimming miracidia. These actively seek out and penetrate the appropriate snail species.

Within the snail, the parasite undergoes extensive asexual multiplication through sporocyst generations. It produces and releases large numbers of free-swimming cercariae back into the water.

This biological amplification step parallels the general trematode pattern. A single successful miracidial infection of one snail can generate many thousands of cercariae. Relatively limited human faecal or urinary contamination of a water body can therefore sustain substantial ongoing transmission risk.

Schistosome eggs are a key diagnostic and pathological feature. They differ characteristically by species in both shape and the position of a distinctive spine.

• Schistosoma haematobium eggs are elongated with a prominent terminal spine at one pole.
• Schistosoma mansoni eggs are similarly elongated but bear a prominent lateral spine projecting from the side of the shell rather than the end.
• Schistosoma japonicum eggs are more rounded/oval with only a small, easily overlooked lateral knob or rudimentary spine rather than a prominent spine of either type. This can make japonicum eggs somewhat less immediately recognisable on microscopy than the other two species.

Unlike the eggs of most other helminths, schistosome eggs are already embryonated (containing a fully developed miracidium) at the time they are laid. They must reach freshwater relatively promptly to hatch and continue the cycle, rather than requiring a period of environmental maturation.

The species-specific final location of adult worm pairs within particular venous plexuses directly determines both the route of egg excretion and the principal pattern of resulting disease.

The great majority of the pathology in schistosomiasis does not result from the adult worms themselves. They are relatively well tolerated immunologically once established, in part through host antigen mimicry that reduces immune attack on the adult worm surface. The pathology results from the host granulomatous inflammatory response to eggs that become trapped in tissue rather than successfully passing out of the body in urine or stool.

Schistosoma haematobium egg deposition around the bladder and pelvic venous plexus results in eggs being shed primarily into the urine. Trapped eggs provoke granulomatous inflammation within the bladder wall and ureters.

Schistosoma mansoni and Schistosoma japonicum egg deposition within the mesenteric venous system results in eggs being shed primarily into the stool. Trapped eggs provoke granulomatous inflammation within the intestinal wall.

A substantial proportion of eggs are also swept by portal venous blood flow back into the liver. There they lodge within portal tracts and provoke a further, and more clinically significant, hepatic granulomatous and fibrotic response.

Schistosoma haematobium produces predominantly urinary tract disease. Chronic granulomatous inflammation of the bladder wall leads over years to:

• bladder wall thickening
• calcification (visible on imaging as a calcified bladder outline in longstanding disease)
• ureteric strictures with resulting hydronephrosis and risk of chronic kidney disease

Haematuria, classically terminal haematuria (blood appearing at the end of micturition), is a clinically distinctive feature. It is often the first noticed symptom, sometimes from childhood in endemic areas, and can be mistakenly normalised as unremarkable in some heavily endemic communities.

Chronic urinary schistosomiasis is a well-established risk factor for squamous cell carcinoma of the bladder. This is a recognised long-term complication of longstanding, heavy, and inadequately treated infection.

Genital involvement is an increasingly recognised and clinically significant manifestation. It includes female genital schistosomiasis (with granulomatous lesions of the cervix, vagina, and vulva, associated with increased risk of HIV acquisition due to mucosal disruption) and male genital involvement.

Schistosoma mansoni and Schistosoma japonicum produce predominantly intestinal and hepatic disease. Intestinal granulomatous inflammation from trapped mucosal and submucosal eggs can cause abdominal pain and diarrhoea (sometimes bloody). Heavy chronic infection can cause colonic polyposis.

The more clinically significant chronic complication, however, is hepatic. Trapped eggs within the portal tracts provoke progressive periportal ("pipestem") fibrosis. Unlike cirrhosis from most other causes, this pattern tends to spare the hepatic parenchyma and liver synthetic function relatively well, even as it produces marked presinusoidal portal hypertension.

Splenomegaly, oesophageal varices, and variceal haemorrhage are major causes of morbidity and mortality in chronic heavy infection.

Additionally, an acute systemic hypersensitivity syndrome, Katayama fever, can occur several weeks after primary heavy exposure to any of the major schistosome species. It typically affects previously unexposed travellers to endemic areas. It corresponds to the onset of egg production and the resulting acute immune response to the sudden appearance of large numbers of eggs and circulating antigen.

Features are fever, urticaria, myalgia, abdominal pain, cough, and marked eosinophilia. It generally precedes the development of the chronic organ-specific disease. It is a distinct, self-limited (though occasionally severe) acute clinical entity in its own right.

Fasciola hepatica, the sheep liver fluke, infects humans incidentally following ingestion of aquatic plants, classically watercress, contaminated with encysted metacercariae. Sheep and cattle are the principal natural definitive hosts. A freshwater snail (Lymnaea species) is the required intermediate host.

The cycle is broadly analogous in outline to the general trematode life cycle pattern: egg to freshwater, miracidium, snail intermediate host with asexual multiplication, cercariae, encystment as metacercariae on aquatic vegetation.

Following ingestion, larvae excyst in the small intestine, penetrate the intestinal wall, and migrate directly through the peritoneal cavity to reach the liver capsule. They then burrow through hepatic parenchyma. This produces an acute, invasive hepatic phase with fever, right upper quadrant pain, hepatomegaly, and marked eosinophilia.

The larvae finally reach and mature within the larger bile ducts, where chronic infection can produce biliary obstruction and cholangitis.

Fasciola is unusual among the intestinal/biliary trematodes for causing substantial pathology during larval tissue migration itself (the acute hepatic phase). Pathology is driven predominantly by egg deposition with Schistosoma, and by chronic adult worm presence in the duct lumen with the other liver flukes.

Clonorchis sinensis and Opisthorchis species (Opisthorchis viverrini, Opisthorchis felineus) are the Asian and Southeast Asian/Eastern European liver flukes respectively. They are acquired by ingestion of raw or undercooked freshwater fish containing encysted metacercariae.

The life cycle has two intermediate hosts (snail, then fish). It is directly analogous to Diphyllobothrium's transmission pattern among the cestodes, though via an entirely unrelated organism.

Ingested metacercariae excyst in the duodenum and migrate up the biliary tree via the ampulla of Vater to reach the intrahepatic bile ducts. Adult worms mature there and can survive for many years. They produce chronic biliary tract inflammation and obstruction.

Of particular long-term importance is a well-established association between chronic Clonorchis sinensis and Opisthorchis viverrini infection and cholangiocarcinoma. This serious and clinically important oncological consequence distinguishes these organisms from most other helminths in terms of their carcinogenic potential.

Paragonimus westermani, the Oriental lung fluke, is acquired by ingestion of raw or undercooked freshwater crustaceans (crabs or crayfish) containing encysted metacercariae. The life cycle has two intermediate hosts (snail, then crustacean). It parallels the fish-borne liver flukes but substitutes a crustacean second intermediate host.

Ingested metacercariae excyst in the small intestine, penetrate the intestinal wall, and migrate through the peritoneal cavity and diaphragm. They characteristically pair within the lung parenchyma. There adult worms become encapsulated within the host's own fibrous tissue response, forming cyst-like structures.

Eggs are released from these structures into the bronchial tree. They are either coughed up in sputum or swallowed and passed in stool, giving rise to the two principal diagnostic specimen types for this infection.

Clinically, chronic pulmonary paragonimiasis can closely mimic pulmonary tuberculosis. It produces chronic cough, haemoptysis (often rust-coloured or blood-streaked, reflecting chronic low-grade bleeding from the worm's location and the surrounding tissue reaction), chest pain, and abnormal chest imaging.

Paragonimus infection is a well-recognised and clinically important differential diagnosis to actively consider in endemic areas. Suspect it when a patient presents with a tuberculosis-like clinical and radiological picture but fails to respond to standard antituberculous therapy or has negative sputum studies for Mycobacterium tuberculosis.

Aberrant larval migration to ectopic sites, most importantly the brain, is a recognised and more serious complication, producing cerebral paragonimiasis with seizures and focal neurological deficits.