Metrifonate is a safe, low-cost alternative drug for the treatment of Schistosoma haematobium infections. It is not active against S mansoni or S japonicum. It is not available in the USA.
Metrifonate, an organophosphate compound, is rapidly absorbed after oral administration. After the standard oral dose, peak blood levels are reached in 1–2 hours; the half-life is about 1.5 hours. Clearance appears to be through nonenzymatic transformation to dichlorvos, its active metabolite. Metrifonate and dichlorvos are well distributed to the tissues and are completely eliminated in 24–48 hours.
The mode of action is thought to be related to cholinesterase inhibition. This inhibition temporarily paralyzes the adult worms, resulting in their shift from the bladder venous plexus to small arterioles of the lungs, where they are trapped, encased by the immune system, and die. The drug is not effective against S haematobium eggs; live eggs continue to pass in the urine forseveral months after all adult worms have been killed.
In the treatment of S haematobium, an oral dose of 7.5–10 mg/kg is given three times at 14-day intervals. Cure rates on this schedule are 44–93%, with marked reductions in egg counts in those not cured. Metrifonate was also effective as a prophylactic agent when given monthly to children in a highly endemic area, and it has been used in mass treatment programs. In mixed infections with S haematobium and S mansoni, metrifonate has been successfullycombined with oxamniquine.
Some studies note mild and transient cholinergic symptoms, including nausea and vomiting, diarrhea, abdominal pain, bron-chospasm, headache, sweating, fatigue, weakness, dizziness, and vertigo. These symptoms may begin within 30 minutes and persist up to 12 hours.
Metrifonate should not be used after recent exposure to insec-ticides or drugs that might potentiate cholinesterase inhibition. Metrifonate is contraindicated in pregnancy.