title: The dynamics of natural Plasmodium falciparum infections.
abstract: BACKGROUND
  Natural immunity to Plasmodium falciparum has been widely studied, but its effects on parasite dynamics are poorly understood. Acquisition and clearance rates of untreated infections are key elements of the dynamics of malaria, but estimating these parameters is challenging because of frequent super-infection and imperfect detectability of parasites. Consequently, information on effects of host immune status or age on infection dynamics is fragmentary.
  
  
  METHODS
  An age-stratified cohort of 347 individuals from Northern Ghana was sampled six times at 2 month intervals. High-throughput capillary electrophoresis was used to genotype the msp-2 locus of all P. falciparum infections detected by PCR. Force of infection (FOI) and duration were estimated for each age group using an immigration-death model that allows for imperfect detection of circulating parasites.
  
  
  RESULTS
  Allowing for imperfect detection substantially increased estimates of FOI and duration. Effects of naturally acquired immunity on the FOI and duration would be reflected in age dependence in these indices, but in our cohort data FOI tended to increase with age in children. Persistence of individual parasite clones was characteristic of all age-groups. Duration peaked in 5-9 year old children (average duration 319 days, 95% confidence interval 318;320).
  
  
  CONCLUSIONS
  The main age-dependence is on parasite densities, with only small age-variations in the FOI and persistence of infections. This supports the hypothesis that acquired immunity controls transmission mainly by limiting blood-stage parasite densities rather than changing rates of acquisition or clearance of infections.
authors.affiliation: Swiss Tropical and Public Health Institute, Basel, Switzerland.
authors.forename: Ingrid
authors.initials: I
authors.lastname: Felger
authors.forename: Martin
authors.initials: M
authors.lastname: Maire
authors.forename: Michael T
authors.initials: MT
authors.lastname: Bretscher
authors.forename: Nicole
authors.initials: N
authors.lastname: Falk
authors.forename: André
authors.initials: A
authors.lastname: Tiaden
authors.forename: Wilson
authors.initials: W
authors.lastname: Sama
authors.forename: Hans-Peter
authors.initials: HP
authors.lastname: Beck
authors.forename: Seth
authors.initials: S
authors.lastname: Owusu-Agyei
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Smith
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D000953:Antigens, Protozoan / Q000235:genetics; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D005260:Female; D005838:Genotype; D005869:Ghana; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D016778:Malaria, Falciparum / Q000276:immunology / Q000469:parasitology*; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D010963:Plasmodium falciparum / Q000235:genetics* / Q000276:immunology; D015800:Protozoan Proteins / Q000235:genetics; D055815:Young Adult
chemical_list: D000953:Antigens, Protozoan; D015800:Protozoan Proteins; C065974:merozoite surface protein 2, Plasmodium