title: Assessment of the relative success of sporozoite inoculations in individuals exposed to moderate seasonal transmission.
abstract: BACKGROUND
  The time necessary for malaria parasite to re-appear in the blood following treatment (re-infection time) is an indirect method for evaluating the immune defences operating against pre-erythrocytic and early erythrocytic malaria stages. Few longitudinal data are available in populations in whom malaria transmission level had also been measured.
  
  
  METHODS
  One hundred and ten individuals from the village of Ndiop (Senegal), aged between one and 72 years, were cured of malaria by quinine (25 mg/day oral Quinimax in three equal daily doses, for seven days). Thereafter, thick blood films were examined to detect the reappearance of Plasmodium falciparum every week, for 11 weeks after treatment. Malaria transmission was simultaneously measured weekly by night collection of biting mosquitoes.
  
  
  RESULTS
  Malaria transmission was on average 15.3 infective bites per person during the 77 days follow up. The median reappearance time for the whole study population was 46.8 days, whereas individuals would have received an average one infective bite every 5 days. At the end of the follow-up, after 77 days, 103 of the 110 individuals (93.6%; CI 95% [89.0-98.2]) had been re-infected with P. falciparum. The median reappearance time ('re-positivation') was longer in subjects with patent parasitaemia at enrolment than in parasitologically-negative individuals (58 days vs. 45.9; p = 0.03) and in adults > 30 years than in younger subjects (58.6 days vs. 42.7; p = 0.0002). In a multivariate Cox PH model controlling for the sickle cell trait, G6PD deficiency and the type of habitat, the presence of parasitaemia at enrolment and age >/= 30 years were independently predictive of a reduced risk of re-infection (PH = 0.5 [95% CI: 0.3-0.9] and 0.4; [95% CI: 0.2-0.6] respectively).
  
  
  CONCLUSIONS
  Results indicate the existence of a substantial resistance to sporozoites inoculations, but which was ultimately overcome in almost every individual after 2 1/2 months of natural challenges. Such a study design and the results obtained suggest that, despite a small sample size, this approach can contribute to assess the impact of intervention methods, such as the efficacy vector-control measures or of malaria pre-erythrocytic stages vaccines.
authors: Unité d'Epidémiologie, Institut Pasteur de Dakar, B.P. 220 Dakar, Sénégal. tall@pasteur.sn
authors: Adama
authors: A
authors: Tall
authors: Cheikh
authors: C
authors: Sokhna
authors: Ronald
authors: R
authors: Perraut
authors: Didier
authors: D
authors: Fontenille
authors: Laurence
authors: L
authors: Marrama
authors: Alioune B
authors: AB
authors: Ly
authors: Fatoumata D
authors: FD
authors: Sarr
authors: Aïssatou
authors: A
authors: Toure
authors: Jean-François
authors: JF
authors: Trape
authors: André
authors: A
authors: Spiegel
authors: Christophe
authors: C
authors: Rogier
authors: Pierre
authors: P
authors: Druilhe
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000818:Animals; D000852:Anopheles / Q000469:parasitology*; D000962:Antimalarials / Q000627:therapeutic use; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D007223:Infant; D007299:Insect Bites and Stings / Q000453:epidemiology / Q000469:parasitology*; D016778:Malaria, Falciparum / Q000097:blood / Q000188:drug therapy / Q000453:epidemiology / Q000635:transmission*; D008297:Male; D008875:Middle Aged; D018512:Parasitemia / Q000188:drug therapy / Q000453:epidemiology / Q000469:parasitology / Q000635:transmission*; D010963:Plasmodium falciparum / Q000302:isolation & purification* / Q000469:parasitology; D011803:Quinine / Q000627:therapeutic use; D012008:Recurrence; D012424:Rural Population; D012621:Seasons; D012675:Senegal / Q000453:epidemiology; D034101:Sporozoites / Q000469:parasitology; D013997:Time Factors; D055815:Young Adult
chemical_list: D000962:Antimalarials; D011803:Quinine