Monitoring for mefloquine-resistant Plasmodium falciparum in Africa: implications for travelers' health.

The effectiveness of mefloquine to prevent malaria caused by Plasmodium falciparum is influenced by the sensitivity of the malaria parasites to this drug. Concern has been raised that resistance to mefloquine may develop in sub-Saharan Africa as has been observed in Southeast Asia. Case reports, along with blood smears to confirm the diagnosis and blood samples to determine the mefloquine concentration, were provided on any Peace Corps volunteer serving in sub-Saharan Africa who was diagnosed with malaria. We defined prophylaxis failures probably due to mefloquine resistance as patients with P. falciparum malaria confirmed at the Centers for Disease Control and Prevention, reported compliance with prophylaxis, no ingestion of mefloquine between date of illness onset and date of blood drawing, and a mefloquine level > or = 620 ng/ml in blood drawn within five days of onset of illness. Between January 1, 1991 and September 6, 1996, 44 (31%) of 140 volunteers with confirmed P. falciparum had blood drawn within five days of onset of illness. Twenty-nine (66%) had not fully complied with prophylaxis. Five of 15 prophylaxis failures in four countries had mefloquine levels > or = 620 ng/ml. Failure of mefloquine prophylaxis is primarily due to noncompliance. Evidence of probable resistance to mefloquine among strains of P. falciparum was found in five Peace Corps volunteers in sub-Saharan Africa. Clusters of well-documented prophylaxis failures need to be followed-up by therapeutic in vivo studies to document parasite resistance to mefloquine. Reduced sensitivity to mefloquine does not (yet) appear to be a significant problem in sub-Saharan Africa.