Describe the trematodes with their general features. Discuss schistosomiasis and the food-borne flukes.

General Features of the Trematodes

• Flukes are leaf-shaped, unsegmented flatworms with an oral and a ventral SUCKER, and a blind-ending gut with no anus
• All are hermaphrodite except the schistosomes, which have separate sexes, the slender female lying permanently in the gynaecophoric canal of the stouter male
• All trematodes require a snail as the first intermediate host, which is why their distribution is determined entirely by the distribution of particular snails — and why snail control is a central control measure
• The larval sequence is egg → miracidium → sporocyst → redia → cercaria (released from the snail) → metacercaria (the encysted infective stage on vegetation or in a second intermediate host)
• The mode of human infection divides the group cleanly — the schistosomes penetrate the skin as free-swimming cercariae, whereas all the others are ingested as metacercariae on water plants, or in fish or crustaceans

Schistosomiasis

• The pathology is caused by the eggs, not the worms. The adult flukes are coated with host antigen and provoke little response; but eggs trapped in tissue induce a granulomatous, T-cell mediated reaction followed by fibrosis, which is what produces every chronic manifestation
• Clinical stages — swimmer's itch at cercarial penetration; katayama fever 4 to 8 weeks later, an acute serum-sickness reaction as egg-laying begins, with fever, urticaria, hepatosplenomegaly and marked eosinophilia; then chronic organ disease over years
• Diagnosis — eggs in urine (S. Haematobium, midday terminal specimen) or stool; rectal or bladder biopsy showing eggs; serology (useful in travellers but not in an endemic population); circulating antigen assays; and PCR
• Treatment is praziquantel, which is effective against all species; corticosteroids are added in Katayama fever and in cerebral or spinal disease
• Control — mass treatment with praziquantel, snail control, safe water supply and sanitation, and health education against urinating and defaecating in water

Diagnosis and Treatment of the Trematodes

• Most fluke eggs are operculated — they have a lid at one end — which distinguishes them at a glance from nematode and cestode eggs. The schistosomes are the exception, having a spine instead
• The specimen depends on the fluke — stool for the intestinal and liver flukes, urine for S. Haematobium, and sputum (and stool, from swallowed sputum) for Paragonimus; requesting the wrong one accounts for many missed diagnoses
• Eosinophilia is prominent, particularly during the migratory phase, and is often the finding that prompts the search
• Praziquantel treats almost all trematodes — with the important exception of fasciola, which requires triclabendazole

Larval Stages of the Trematodes

Egg passed in faeces, urine or sputum reaches fresh water → The miracidium hatches — a ciliated free-swimming larva that must find a snail within hours or die → It penetrates the snail and becomes a sporocyst → The sporocyst produces REDIAE (absent in the schistosomes, which produce daughter sporocysts instead) → These produce large numbers of cercariae — so one miracidium yields thousands of cercariae, which is the asexual multiplication that compensates for the improbability of finding a snail → Cercariae leave the snail. In the schistosomes they are fork-tailed and penetrate human skin directly → In all others they encyst as metacercariae on water plants or in a second intermediate host — fish, crab or crayfish — and are ingested

Applied Aspects

• Consider paragonimiasis in "sputum-negative tuberculosis" in the north-east, especially with eosinophilia and a history of eating raw or pickled crab; the treatment is short and effective
• Ask for eggs in the right specimen; a routine stool examination will not find S. Haematobium or Paragonimus
• Take a travel and residence history in unexplained haematuria or portal hypertension; schistosomiasis is not endemic in India but is common among Indians who have worked in Africa and the Gulf
• Praziquantel is the answer for almost every fluke, but not for Fasciola — an exception worth remembering, since the drug is otherwise so uniformly effective
• Fluke infections are strongly linked to cancer — Clonorchis and Opisthorchis with cholangiocarcinoma, S. Haematobium with squamous bladder carcinoma; treating the infection is therefore cancer prevention
• Food habits drive the epidemiology, so control is dietary and cultural as much as pharmacological — cooking fish, crab and water plants breaks every one of these cycles
• The snail is the obligatory first host of every trematode, so snail control and avoidance of snail-infested water interrupt the whole class
• Asexual multiplication in the snail is why a single egg matters — one miracidium yields thousands of cercariae, so even light faecal contamination of water sustains transmission
• Katayama fever is a hypersensitivity reaction to the onset of egg-laying, not to the worms themselves, and it responds to corticosteroids alongside praziquantel
• Terminal-spined eggs go with urine and lateral-spined with stool, which is the practical rule for requesting the right specimen in schistosomiasis
• Fascioliasis is the exception to praziquantel, and a patient with biliary flukes who fails to respond should be reconsidered for triclabendazole
• Schistosome eggs cause the disease, not the worms, which is why the pathology is granulomatous and fibrotic and why it continues long after the worms are killed
• Pipe-stem fibrosis gives portal hypertension with preserved liver function, so a patient may bleed from varices while their synthetic function remains normal — a presinusoidal block, unlike cirrhosis
• Liver flukes are Group 1 carcinogens, so treating Clonorchis and Opisthorchis infection is a cancer-prevention measure as much as a symptomatic one
• Fasciolopsiasis is endemic in Assam, Bihar and West Bengal, acquired from raw water chestnut, and is a genuine Indian problem rather than a textbook curiosity
• Look for operculated eggs to recognise a fluke at a glance; the schistosomes, with their spines instead, are the only exception
• Schistosomes are the only trematodes with separate sexes and the only ones that penetrate skin rather than being eaten — two exceptions that go together and are worth remembering as a pair
• Cercarial dermatitis ("swimmer’s itch") may be caused by avian schistosomes that cannot mature in man, producing an itchy rash without any subsequent infection
• Praziquantel is remarkably safe and single-dose, which is what makes mass treatment of schistosomiasis feasible in the way that mass deworming is
• Snail control alone rarely succeeds; it works in combination with mass treatment, sanitation and safe water, as with most parasitic control programmes
• Ask where a patient has worked; schistosomiasis in an Indian patient is almost always imported, and haematuria in a returning migrant worker should raise it
• Bladder cancer from S. Haematobium is squamous, not transitional, which is a useful pointer when the histology is unexpected
• Cooking is the single intervention that prevents most fluke infection, since all except the schistosomes are acquired by eating an infective stage