Mefloquine-resistant falciparum malaria on the Thai-Burmese border.

Mefloquine is the treatment of choice for uncomplicated multiresistant falciparum malaria, and in combination with sulphadoxine and pyrimethamine (MSP) at a single dose of 15/30/1.5 mg/kg, respectively, has been used in Thailand for the past 6 years. In 1985-86, MSP cured over 98% of 5192 patients with falciparum malaria on the Thai-Burmese border. 4 years later we studied the efficacy of MSP in 395 patients at the same location. The cure rate at 28 days was 70.8% (95% Cl 67-77.2%). The proportion of early treatment failures (in whom parasitaemia did not clear) had risen from 0.27 to 3.7% (p less than 0.0001). Failure rates were 50% in children under 6 years old, 29% in the 6-15 age group, and 19% in adults (p less than 0.001). Patients with early treatment failure were retreated with 25 mg/kg mefloquine, but 27% had a further recrudescence of infection within 28 days. The mean (95% Cl) serum mefloquine concentration at the time of first recrudescence was 638 (546-730) ng/ml, a value previously associated with successful treatment. Mefloquine concentrations were no lower in those with recrudescent infections than in age-matched successfully treated patients, suggesting that pharmacokinetic factors were not responsible for the high treatment-failure rate. Plasmodium falciparum has developed resistance to mefloquine rapidly, despite the addition of sulphadoxine and pyrimethamine and strict control of drug administration. The MSP combination should now be abandoned.