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The amount of chloroquine (CQ) resistance is usually estimated by carrying out World Health Organization (WHO) in vivo tests. However, this is a time-consuming task and results are difficult to interpret where re-infection is frequent.1 The identification of the relationship between the Plasmodium falciparum CQ resistance transporter (Pfcrt) T76 mutation and CQ resistance resulted in a series of field studies that investigated the usefulness of this molecular marker in estimating CQ resistance.2–5 Data from Mali suggest that its prevalence is 2–3-fold higher than in vivo CQ resistance.1 Two indexes, the genotype resistance index (GRI) and the genotype failure index (GFI), which correspond to the ratio between the age-adjusted frequency of the Pfcrt T76 mutation and the prevalence of parasitologic or clinical failure, respectively, were defined. They were proposed as useful tools to estimate, over large areas and without the need of carrying out expensive and time-consuming in vivo tests, CQ resistance, at least when it is less than 30–50%. However, the GRI and the GFI have not been applied or validated extensively. We therefore computed, based on the prevalence of the Pfcrt T76 mutation, the GFI in four sites in Bobo Dioulasso, Burkina Faso in two different years (1998 and 2001) and related it to the prevalence of CQ resistance estimated by in vivo 14 days test in 1998–1999 and 2000–2001.
This is part of a larger study that investigated CQ and sulfadoxine-pyrimethamine efficacy in children between 6 months and 15 years of age. This study was reviewed and approved by institutional Ethical Board at Centre Muraz. Informed consent for participation in the study was obtained from the children’s parents or guardians. In the following analysis, only CQ efficacy has been considered. Details of the study methodology have been reported elsewhere.6 Outcomes were defined according to the WHO modified classification for monitoring antimalarial drug resistance.7 Total treatment failure (TTF) was defined as the sum of early treatment failure (ETF), late clinical failure (LCF), and late parasitologic failure (LPF).
Blood samples for the molecular analysis were collected on no. 3 filter paper (Whatman, Maidstone, United Kingdom) at day 0 before treatment. The DNA was extracted using the Chelex-100 method.8 Detection of the Pfcrt T76 mutation was done by using a polymerase chain reaction, followed by-sequence-specific restriction enzyme digestion (http://www.medschool.umaryland.edu/CVD/plowe.html). The GFI was computed by dividing the frequency of the Pfcrt T76 mutation by the prevalence of TTF.
Between 1998 and 2001, a total of 1,068 patients were enrolled in the four sites. The mean age of the children included in the study was similar in the four study sites and varied between 51.7 months in Lena in 1998–1999 to 52 months in Toussiana in 1998 and 2001. Outcome was known for 884 (83%) patients who completed the follow-up. The TTF increased from 20.4% (69 of 337) in 1998–1999 to 28.7% (157 of 547) in 2000–2001 (odds ratio [OR] = 0.6; 95% confidence interval [CI] = 0.4–0.9, P = 0.0006), and was composed of mostly LCF or LPF. Such an increase occurred in all sites, although the difference between the two periods was statistically significant (OR = 0.5, 95% CI = 0.2–0.9, P = 0.03) at only one site (Lena).
Seven hundred sixteen pre-treatment blood samples (340 in 1998 and 376 in 2001) were randomly selected and analyzed for polymorphism in the Pfcrt 76 locus. In all sites, the prevalence of Pfcrt T76 mutation increased between 1998 and 2001 (Figure 1
), but the difference between the two years was statistically significant only in Accartville (OR = 0.5, 95% CI = 0.3–1, P = 0.03). In Bama, the prevalence of the mutation was low (22% in 1998 and 33% in 2001) and significantly different from the other sites (P < 0.00001 for both years). In three sites (Accartville, Lena, and Toussiana) the prevalence of the Pfcrt T76 mutation was always higher than the TTF (Figure 1) and the GFI ranged between 1.7 and 3. However, in both years in Bama, the prevalence of the Pfcrt T76 mutation was almost identical to that of the TTF with a GFI of 1.1.
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Another specific and selective factor in Bama is the much higher frequency of Anopheles gambiae M (Mopti) compared with the other sites where both forms M and S (Savanna) occur.16 High transmission has been associated with a faster spread of CQ resistance, once this has emerged.15 In Uganda, CQ resistance seems to spread faster, regardless of drug pressure in low and high transmission areas.17 In Bama, where malaria transmission is high, this does not seem to happen. The high frequency of An. gambiae M could be involved in this phenomenon, although it is not clear how.
A major limitation of this study is that we observed a GFI close to 1 only in one site, although the study was conducted in two different years (1998 and 2001). In the other three sites, estimating the amount of CQ resistance by dividing the prevalence of the Pfcrt T76 mutation by 2 or 3 seems to confirm the usefulness of the GFI.1 It would be important to determine whether this is still the case in areas of extremely high transmission, such as Bama.
Received July 12, 2004. Accepted for publication December 20, 2004.
Acknowledgments: We thank the parents of the children included in this study for their participation. We also thank the health staff of the health centers where the study was conducted for their collaboration.
Financial support: We thank the French Co-operation for its support of the field study and the Belgium Co-operation for support of the laboratory work through a training grant to Halidou Tinto. This work was also partially support by Société de Pathologie Exotique through a prize awarded to Halidou Tinto.
* Address correspondence to Umberto D’Alessandro, Institute of Tropical Medicine, Nationalestraat 155, Antwerp B-2000, Belgium. E-mail: udalessandro{at}itg.be 
Authors’ addresses: Halidou Tinto, Boroma Sanou, Jean Bosco Ouédraogo, and Tinga Robert Guiguemdé, Centre Muraz/Institut de Recherche en Sciences de la Santé, BP 153 Bobo Dioulasso, Burkina Faso, Telephone: 226-20-97-01-02; Fax: 226-20-97-28-24, E-mail: tintoh{at}hotmail.com. Jean-Claude Dujardin, Chantal van Overmeir, Annette Erhart, and Umberto D’Alessandro, Institute of Tropical Medicine, Nationalestraat 155, Antwerp B-2000, Belgium, Telephone: 32-3-247-6354, Fax: 32-3-247-6362, E-mail: udalessandro{at}itg.be. Eric van Marck, Faculty of Medicine, University of Antwerp, Universiteitsplein 1 B-2610, Antwerp, Belgium, Telephone: 32-3-820-2540, Fax: 32-3-820-2501, E-mail: Eric.Van.Marck{at}uza.be.
Reprint requests: Umberto D’Alessandro, Institute of Tropical Medicine, Nationalestraat 155, Antwerp B-2000, Belgium.
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