To the Editor: Resistance of Plasmodium falciparum to antimalarial drugs is one of the most worrying problems in tropical medicine. For P. falciparum malaria acquired in French Guiana, the combination of quinine and doxycycline is one of the first-line recommended treatments (1). Since 1996, only 2 treatment failures with quinine have been reported from that country (2). An elevated 50% inhibitory concentration (IC₅₀), classified as in vitro quinine resistance, was reported for 17% of 32 P. falciparum isolates obtained during 1983–1987 in French Guiana (3). Throughout 1994–2005, isolates were susceptible to quinine, with a mean IC₅₀ <200 nmol/L (4).

We report quinine treatment failure in a 35-year-old man who was infected during a 3-month stay in Saül, a rural area of French Guiana. The patient did not use antivectorial or antimalarial prophylaxis. The patient sought treatment with fever 4 days after returning to France on June 22, 2010 (day 0), and a diagnosis of P. falciparum malaria was made on the basis of results of a rapid diagnostic test performed by a private medical laboratory. The man, who weighed 58 kg, was treated as an outpatient with 500 mg of quinine to be taken orally 3×/d for 7 days; he did not receive doxycycline. He was admitted to the Laveran Military Teaching Hospital in Marseille on July 15 (day 24 and first day of recrudescence) for uncomplicated malaria with a P. falciparum parasitemia level of 4%. He was given artemether, 80 mg/d, by intramuscular injection for 3 days. Blood samples taken on day 27 (third day of recrudescence) and day 52 (4 weeks of recrudescence) were negative for P. falciparum.

In vitro testing of drug susceptibility was performed by the standard 42-hour ³H-hypoxanthine uptake inhibition method (5). We assessed susceptibility to 11 antimalarial drugs on the fresh isolate and after culture adaptation (Table). The laboratory-adapted strain 3D7, tested 3× on the same batch of plates, was used as reference. The strain isolated from the blood sample on day 24 (first day of the recrudescence) showed reduced susceptibility to quinine (1,019 nmol/L), chloroquine (427 nmol/L), and monodesethylamodiaquine (157 nmol/L). The isolate was susceptible to all other antimalarial drugs tested. We assessed gene polymorphisms of pfcrt (P. falciparum chloroquine resistance transporter), pfmdr1 (P. falciparum multidrug resistance 1 protein), and pfnhe-1 (P. falciparum Na⁺/H⁺ exporter 1); the copy number of pfmdr1 involved in quinoline resistance; and gene polymorphisms of dhfr (dihydrofolate reductase, involved in proguanil or pyrimethamine resistance), dhps (dihydropteroate synthetase, involved in sulfadoxine resistance), and cytB (cytochrome B, involved in atovaquone resistance) (6).

The pfnhe ms4760 microsatellite showed a profile 3, with 1 repeat of DNNND and 2 repeats of DDDNHNDNHNN. Studies of the pfnhe-1 polymorphism of worldwide culture-adapted isolates showed that increased numbers of DNNND were associated with decreased quinine susceptibility (7). Association of 2 repeats of DNNND and a high quinine IC₅₀ value was found in a case of clinical failure of quinine in a traveler returning from Senegal (8).

Reinfection was excluded because the patient had stayed in mainland France since his return. The patient reported that he took the quinine as instructed. We report here a clinical and parasitologic failure of quinine treatment associated with high IC₅₀ but not linked with the ms4760 pfnhe-1 profile involved with quinine in vitro reduced susceptibility. A hypothesis that may explain our data is that unlike previous studies (7), in which the less susceptible strains originated from Asia, this isolate came from South America. The profile associated with quinine resistance for chloroquine resistance and pfcrt genotypes could be different in the 3 malaria-endemic continents. There are no data on ms4760 pfnhe-1 of P. falciparum isolates from South America. In another recent study (9), a multivariate analysis performed on 83 clinical isolates from Madagascar and 13 African countries did not confirm this association between quinine susceptibility and pfnhe-1 microsatellite polymorphisms.

The pfcrt gene had a point mutation on codon 76 (76T) and pfmdr1 on codons 184F, 1034C, 1042D, and 1246Y. These data are in agreement with those from previous studies that showed the mutation 76T in the pfcrt gene led to decreased susceptibility to chloroquine, amodiaquine, and quinine. The isolate had only 1 copy of pfmdr1. The data on mutations and copy number of pfmdr1 are consistent with data in Brazil (10). Nevertheless, the lack of gene amplification and specific point mutations in pfmdr1 were not associated with decreased in vitro susceptibility of quinine. dhfr and dhps genes had a 5-mutation haplotype, 51I C59 108N I164-S436 437G 540E 581G A613, which suggested in vitro resistance to proguanil, pyrimethamine, and sulfadoxine. This case confirms the need to always add doxycycline to quinine for treatment of P. falciparum malaria acquired in French Guiana as well as other parts of South America.