Red cell selectivity in malaria: a study of multiple-infected erythrocytes

To characterize red cell susceptibility to invasion in malaria, a selectivity index (SI) was calculated as the ratio of observed number of multiple-infected red cells to that expected from a random process (Poisson distribution). In patients with falciparum malaria (n = 100) SI decreased with increasing parasitaemia (P < 0·001), and correlated inversely with plasma lactate concentrations, chosen prospectively as a measure of disease severity (r = −0·36, P < 0·001). For parasitaemias <5%, the SI was lower in patients with severe malaria (geometric mean 1·35; 95% confidence interval 1· 01– 1·80) than in uncomplicated malaria (2·31; 1·89-2-81; P=0·003), despite similar parasite counts. The geometric mean (range) SI in vivax malaria (n = 20), 7·69 (1·67, 29·75), was significantly greater than that in falciparum malaria at comparable parasitaemias (≤2%), 2·44 (0·45, 14·05), P < 0·001, suggesting that about 13% of circulating erythrocytes were susceptible to invasion by Plasmodium vivax. This translates into susceptibility for about 2 weeks after emergence from the bone marrow, if age is the sole determinant of this process. In falciparum malaria selectivity was inversely proportional to severity; lack of selectivity could reflect either a 'favourable' host red cell phenotype, or an indiscriminate parasite population. Both are dangerous for the host.