A hundred per cent of fields positive in a thick film: a useful indicator of relative changes in morbidity in areas with seasonal malaria

The classical method of estimating the density of Plasmodium falciparum in blood, by counting the number of trophozoites per leucocyte, is compared with a method in which the proportion of microscope fields in a thick film that include at least one asexual form is evaluated. Mean densities of 144 and 1920 trophozoites/microliters blood gave 9.5% and 99.5% positive fields, respectively, and < 5% of slides with 100% positive fields were of blood with < 2000 trophozoites/microliters. The proportion with high parasitaemia (PHP) in a population, defined as the proportion of individuals with 100% positive fields, is proposed as a simple and reliable indicator of relative changes in malaria morbidity due to seasonal fluctuations or control activities. However, the use of this index is limited to areas with intermediate malaria stability. Data from a longitudinal survey in Burundi, presented to illustrate the use of PHP, show that PHP undergoes important fluctuations related to transmission. In contrast to the parasite index, the amplitude of fluctuations in PHP decreases with age as a result of acquired protective immunity. Although two health centres, one in a vector control area and one in an area with no control, reported similar proportions of cases of clinical malaria among their patients, PHP was about three times lower in patients from the vector control area. The estimation of the efficacy of a malaria control programme from simple clinical information appears unreliable. Sentinel health centres, each equipped with a microscope to estimate PHP, may often be better indicators of changes in morbidity.