White Blood Cell Counts and Malaria

White blood cell (WBC) counts during malaria are generally characterized as being low to normal, a phenomenon that is widely thought to reflect localization of leukocytes away from the peripheral circulation and to the spleen and other marginal pools, rather than actual depletion or stasis. Leukocytosis is typically reported in a fraction of cases and may be associated with concurrent infections and/or poor prognosis. Remarkably few published studies have compared WBC counts in malarial parasite–infected and –uninfected residents of regions in which malaria is endemic, however. Human malaria can be caused by any of several species of Plasmodium parasites that occur together in various combinations in regions of endemicity. P. falciparum is responsible for almost all mortality attributed directly to malaria and is the focus of almost all research and intervention efforts. Compared with P. falciparum, however, P. vivax is the source of as much or more morbidity worldwide, despite its extremely low prevalence in sub-Saharan Africa. The tacit assumption that WBC counts are identical during infections with different Plasmodium species has been examined only minimally and tangentially. Although several methods for estimation of densities of blood-stage parasites by microscopy are in use, the most common is to count the number of asexual parasites seen relative to a given count of WBCs (usually 200 or 500 cells) and then to multiply the parasite: WBC ratio by 8000, the assumed number of WBCs per microliter of blood. These estimates are used in clinical and epidemiological studies and in evaluation of the effects of interventions on individuals and communities. The consequences of errors are strongly dependent on context but could be profound, as would be the case in studies that relate malarial symptoms or transmission to parasite densities.