Schistosomiasis and Other Trematode Infections¶
Chapter 241 | Part 5: Infectious Diseases
KEY CLINICAL POINTS¶
- Schistosomiasis is caused by six Schistosoma species (S. mansoni, S. japonicum, S. intercalatum, S. haematobium, S. mekongi, S. guineensis) with distinct geographic distributions and transmission routes.
- Diagnosis relies on detection of schistosome eggs in stool/urine, serology, imaging, and PCR; treatment is praziquantel (40-60 mg/kg PO), with no effective drugs for dracunculiasis.
- Climate change and environmental factors significantly impact snail intermediate host distribution, altering schistosomiasis transmission dynamics globally.
1. DEFINITION & OVERVIEW¶
Trematodes (flukes) are helminths causing human infections through complex life cycles involving snail intermediate hosts. Key infections include schistosomiasis (blood flukes) and food-borne trematode infections (liver, intestinal, lung flukes).
Table 241-1 Major Human Trematode Infections¶
| TREMATODE | TRANSMISSION ROUTE | GEOGRAPHIC DISTRIBUTION |
|---|---|---|
| Schistosoma mansoni | Skin penetration by cercariae from Biomphalaria snails | Africa, Brazil, Venezuela, Surinam, Caribbean (low risk) |
| Schistosoma japonicum | Skin penetration by cercariae from Oncomelania snails | China, Indonesia, Philippines |
| Schistosoma guineensis/S. intercalatum | Skin penetration by cercariae from Bulinus snails | Central African rainforests |
| Schistosoma mekongi | Skin penetration by cercariae from Neotricula aperta snails | Cambodia, Lao PDR |
| Clonorchis sinensis | Ingestion of metacercariae in freshwater fish | Asia (Korea, China, Vietnam) |
| Opisthorchis viverrini/felineus | Ingestion of metacercariae in freshwater fish | Southeast Asia, Europe |
| Fasciola hepatica/gigantica | Ingestion of metacercariae on aquatic plants/water | Global (F. hepatica), Africa/Asia (F. gigantica) |
| Fasciolopsis buski | Ingestion of metacercariae on aquatic plants | Bangladesh, China, India, Indonesia |
| Paragonimus westermani/kellicotti | Ingestion of metacercariae in crayfish/crabs | Tropical/subtropical Asia, Americas, sub-Saharan Africa |
1.1 Schistosomiasis¶
Caused by Schistosoma species, characterized by adult worms in mesenteric/pelvic veins, egg deposition in tissues, and immune-mediated granulomas. S. haematobium causes urogenital disease; others cause intestinal disease.
1.2 Food-Borne Trematode Infections¶
Infections from liver (Clonorchis, Opisthorchis, Fasciola), intestinal (Fasciolopsis, Echinostoma), and lung flukes (Paragonimus). Transmission via ingestion of metacercariae in raw fish, plants, or crustaceans.
2. EPIDEMIOLOGY¶
Schistosomiasis affects 251.4 million people globally (78% in sub-Saharan Africa), with 75.3 million treated in 2021. Dracunculiasis (Guinea worm) has 13 human cases in 2022, with snail reservoirs in dogs/cats complicating eradication. Climate change impacts snail distribution and transmission.
2.1 Schistosomiasis¶
Children acquire infection at 3-4 years via contaminated water; prevalence peaks at 10-15 years. Adult worms persist 3-5 years, with 30-year lifespan in severe cases. 70% of cases in sub-Saharan Africa.
2.2 Dracunculiasis¶
13 human cases in 2022 (Chad, South Sudan). Transmission via Cyclops intermediate hosts; no effective drugs. Prevention relies on safe water access.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
Schistosomes have complex life cycles with snail intermediate hosts. Eggs trigger granulomatous inflammation and fibrosis. Immune evasion via surface protein binding and proteolytic enzymes. S. haematobium is IARC Group 1 carcinogen (bladder cancer).
3.1 Life Cycle¶
Snail intermediate host → cercariae → skin penetration → adult worms in portal/venous system → egg deposition in tissues. Eggs induce Th2 immune response with eosinophilia.
3.2 Pathogenesis¶
Egg-induced granulomas cause periportal fibrosis, portal hypertension, and organ-specific damage. S. haematobium eggs cause bladder squamous cell carcinoma.
4. CLINICAL FEATURES¶
Three stages: acute (Katayama fever), active (organ-specific symptoms), chronic (fibrosis, portal hypertension). Urogenital schistosomiasis causes bladder inflammation, hematuria, and carcinoma. Pulmonary schistosomiasis from portacaval shunting.
4.1 Schistosomiasis Stages¶
Acute: fever, eosinophilia, pulmonary infiltrates. Active: intestinal pain, hepatosplenomegaly. Chronic: portal hypertension, fibrosis, cholangiocarcinoma.
4,2 Food-Borne Trematode Infections¶
Liver flukes: biliary colic, cholangitis, pancreatitis. Intestinal flukes: malabsorption, abdominal pain. Lung flukes: pulmonary cysts, CNS complications.
5. DIFFERENTIAL DIAGNOSIS¶
Acute fevers: malaria, salmonellosis, tropical eosinophilia. Urogenital symptoms: bladder cancer, tuberculosis. Pulmonary symptoms: tuberculosis, fungal infections. Eosinophilia: helminth infections, allergic reactions.
6. INVESTIGATIONS & DIAGNOSIS¶
Diagnosis via stool/urine microscopy for eggs, serology (FAST-ELISA, immunoblot), PCR, and imaging (ultrasound, CT). Cercarial dermatitis presents as maculopapular rash. Neuroschistosomiasis requires CSF analysis.
6.1 Diagnostic Methods¶
Egg detection in stool (50 mg) or urine (10 mL). Formol-ether concentration for low egg counts. PCR for sensitive detection. Serology for travelers.
6.2 Imaging¶
Ultrasound for hepatosplenomegaly, portal hypertension. CT/MRI for biliary duct thickening, pulmonary granulomas. Endoscopy for urogenital tract evaluation.
7. MANAGEMENT & TREATMENT¶
Praziquantel (40-60 mg/kg PO) is first-line for schistosomiasis. No drugs for dracunculiasis; excision required. Food-borne flukes treated with praziquantel or triclabendazole. Prevention via water sanitation and snail control.
7.1 Schistosomiasis¶
Praziquantel 40-60 mg/kg PO for S. mansoni/japonicum. Repeat dosing if symptoms persist. Glucocorticoids for acute Katayama fever.
7.2 Food-Borne Trematodes¶
Praziquantel 25-75 mg/kg for liver/intestinal flukes. Triclabendazole for Fasciola (10 mg/kg x2). Avoid raw freshwater fish in endemic areas.
8. PROGNOSIS & COMPLICATIONS¶
Chronic schistosomiasis leads to portal hypertension, fibrosis, and cholangiocarcinoma. Dracunculiasis causes severe ulcers and secondary infections. Climate change threatens future transmission patterns.
8.1 Long-Term Outcomes¶
Hepatosplenomegaly resolves with treatment, but fibrosis persists. S. haematobium increases bladder cancer risk. Severe cases may progress to uremia or liver failure.
8.2 Complications¶
Portal hypertension, esophageal varices, obstructive uropathy, and ectopic infections. Neuroschistosomiasis may cause transverse myelitis or encephalopathy.
9. SPECIAL CONSIDERATIONS¶
Schistosomiasis is a poverty-related disease; prevention requires water sanitation. Travelers should avoid freshwater contact in endemic areas. HIV co-infection may exacerbate genital schistosomiasis.
10. KEY POINTS & CLINICAL PEARLS¶
- Praziquantel is the only effective drug for schistosomiasis; no treatment exists for dracunculiasis. 2. Snail control and water sanitation are critical for prevention. 3. Egg detection in stool/urine is the gold standard for diagnosis. 4. Climate change impacts snail distribution and transmission dynamics.