DETECTION OF VERY-LOW LEVEL PLASMODIUM-FALCIPARUM INFECTIONS USING THE NESTED POLYMERASE CHAIN-REACTION AND A REASSESSMENT OF THE EPIDEMIOLOGY OF UNSTABLE MALARIA IN SUDAN

We have used the nested polymerase chain reaction (PCR) to assay for l ow level Plasmodium falciparum infections that were below the threshol d of detection of blood film examination. This revealed a substantial group of asymptomatic, submicroscopically patent infections within the population of a Sudanese village present throughout the year although clinical malaria episodes were almost entirely confined to the transm ission season. In our September, January, April, and June surveys, the PCR-detected prevalences were 13%, 19%, 24%, and 19%, respectively. T hese figures reveal a much higher prevalence of dry season infection t han previous microscopic surveys have indicated. Furthermore, 20% of a cohort of 79 individuals were healthy throughout the September to Nov ember transmission season but were PCR-positive for P. falciparum in a least one of a series of samples taken in the ensuing months. Levels of exposure to P. falciparum infection were therefore higher than was previously believed in this region, highlighting the fact that many in dividuals were infected but healthy for most of the year. The reservoi r parasite population was thus larger and more stable than previously thought, a finding that is consistent with the high levels of genetic variation at polymorphic loci reported from analysis of P. falciparum parasites in this area.