Antischistosomal Drugs

Antischistosomal drugs treat infection with Schistosoma species. Praziquantel is the only drug used widely: it increases calcium influx into the worm tegument, causing spastic paralysis and exposing the parasite to immune attack. It kills adult worms but is less active against immature schistosomula, so early infection can persist after treatment.

Pharmacokinetics. Rapidly absorbed with a large first-pass effect, absorption rises with food, half-life 1 to 3 hours, and metabolised by CYP enzymes. Oxamniquine is withdrawn from routine use.

• Praziquantel: 600 mg tablets, scored; bitter taste, so swallow with water and food.
• Paediatric praziquantel dispersible formulations (arpraziquantel) are available in some countries for children under 4 years.
• Oxamniquine: Schistosoma mansoni only, now little used.

Praziquantel dosing.

• Schistosoma haematobium and mansoni: 40 mg/kg single dose.
• Schistosoma japonicum and mekongi: 60 mg/kg in two or three doses on the same day.
• Repeat dose at 2 to 4 weeks if eggs persist, since early worms survive the first dose.
• Acute schistosomiasis (Katayama fever): corticosteroids first; give praziquantel 4 to 6 weeks later, and repeat after another 4 to 6 weeks.
• Neuroschistosomiasis: praziquantel 60 mg/kg with corticosteroids and antiseizure medicine.
• Hepatosplenic disease: praziquantel plus management of portal hypertension.

Mass treatment. Yearly school-age praziquantel treatment is used in endemic communities in East Africa, with extension to preschool children and adults.

Assessment of cure. Egg counts in urine or stool, or a point-of-care antigen test, 4 to 6 weeks after treatment.

Dose adjustment.

• Pregnancy: praziquantel is used in endemic areas when benefit outweighs risk, including in the first trimester.
• Breastfeeding: compatible.
• Liver disease: reduce dose in severe impairment.
• Children: use the same weight-based dose from age 4 years; younger children need specialist formulation.

Region. In East Africa, Schistosoma mansoni and haematobium are both widespread near lakes and slow water, and repeated exposure causes reinfection even after cure.

Contraindications. Hypersensitivity; ocular cysticercosis (treatment can damage the eye).

Adverse effects. Dizziness, drowsiness, headache, abdominal pain, nausea, urticaria, and fever; severe reactions in heavy infection are from dying worms. Driving is not advised for 24 hours after treatment.

Interactions. Rifampicin, carbamazepine, phenytoin and dexamethasone lower praziquantel levels. Cimetidine and grapefruit juice raise levels.

Monitoring. Clinical review at 4 to 6 weeks; check haematuria (haematobium), stool eggs (mansoni), and anaemia; ultrasound in chronic disease for fibrosis and obstructive uropathy.

Common prescribing errors. Treating acute schistosomiasis with praziquantel alone; failing to repeat treatment; treating without excluding neurocysticercosis in endemic areas with seizures.

Special groups.

• Older adults: no change.
• Kidney impairment: no change.

Referral and escalation. Refer neuroschistosomiasis, hepatosplenic disease with portal hypertension, bladder disease with obstruction or suspected cancer, and treatment failure after two doses.