Host factors affecting the delay of reappearance of Plasmodium falciparum after radical treatment among a semi-immune population exposed to intense perennial transmission

To investigate host factors affecting the delay of reappearance of malaria parasites after radical treatment, a study was underdaken in Dielmo, Senega l, an area of intense perennial malaria transmission. A 7-day course of qui nine was administered to 173 asymptomatic persons from 1 to 85 years of age and reappearance of malaria parasites in the peripheral blood was monitore d weekly for 14 weeks. Additional thick blood films were made in case of fe ver as part of a daily clinical surveillance. The median times before reapp earance of Plasmodium falciparum were 22, 39, and 53 days among persons 1-6 , 7-14, and greater than or equal to 15 years of age, respectively (P < 0.0 001). Multivariate analysis indicated that the daily rate of reappearance o f P. falciparum was 2.2 (95% confidence interval [CI] = 1.2-4.5) times lowe r in sickle cell trait carriers than in AA individuals, and 1.5 (95% CI = 1 .1-2.1) times lower in bed nets users than in non-users. The risk ratio for the daily risk of reappearance was significantly related to the level of p arasitemia before treatment. No influence of glucose-6-phosphate dehydrogen ase deficiency, HLA-B53, and DR13 were observed. Findings show that monitor ing during a few weeks the reappearance of malaria parasites after treatmen t among a small cohort of individuals naturally exposed to malaria is relev ant for investigating host resistance factors. This suggest that small, low -cost, field trials may be very informative on the potential of new malaria vaccine candidates.