HIGH PROPORTION OF SUBCLINICAL PLASMODIUM-FALCIPARUM INFECTIONS IN ANAREA OF SEASONAL AND UNSTABLE MALARIA IN SUDAN

In the present longitudinal study, a cohort (n = 98) of children and a dults 5-30 years of age living in an area of highly seasonal and unsta ble malaria transmission were followed for malaria morbidity during se veral successive transmission seasons. Based on morbidity surveillance during 1993 and measurements of antibody titers to the Plasmodium fal ciparum ring-infected erythrocyte surface antigen (Pf155/RESA), the co hort was divided into three groups: those who had at least one episode of clinical malaria (Group 1, n = 31), those who did not suffer from clinical malaria but had (Group 2, n = 63) or had not (Group 3, n = 4) a significant increase in antibody titers against the Pf155/RESA anti gen. This increase was defined as equal to or greater than a four-fold increase in antibody titer in samples from same individuals taken at the beginning and the end of the malaria transmission season. Such inc reases in specific antibody levels suggested that the donors had been exposed to a P. falciparum blood-stage infection. Measurements of anti body titers to a peptide derived from the glutamate-rich protein exoan tigen gave data parallel to those for Pf155/RESA. A surprisingly high fraction of individuals in the study cohort (approximately 66%) showed evidence of infection without ensuing clinical disease (Group 2).