RELATIONSHIPS BETWEEN PLASMODIUM-FALCIPARUM INFECTION AND MORBIDITY IN A HIGHLY ENDEMIC AREA

A total of 736 outpatients diagnosed as having malaria using clinical criteria at a health centre in a highly endemic area of Papua New Guin ea were investigated parasitologically. Plasmodium falciparum-attribut able fractions were determined using a logistic regression model to co mpare parasite densities in cases with those of healthy individuals in community surveys. Thirty-seven percent of presumptive cases were fou nd to have raised P. falciparum parasitaemia. This corresponds to an a verage reporting rate for the population of 0.53 attributable episodes per annum. Whilst the maximum prevalence of parasitaemia in the commu nity was in children aged 5-9 years, the maximum age-specific incidenc e of attributable cases at the outpatient clinic was 2 cases per annum in the 2- to 4-year-old age group. The procedure for estimating attri butable fractions makes it possible to compare morbidity rates between age groups, and to examine how the relationship between morbidity ris k and parasite density changes with age, without diagnosing individual episodes. The average tolerance of parasites in an age group was meas ured by considering the level of parasitaemia associated with a given risk of malaria-attributable morbidity. In contrast to anti-parasite i mmunity, tolerance of parasites declines with age since at parasite is odensity the probability of being symptomatic increases with age.