SEASONAL-VARIATION IN AGGLUTINATION OF PLASMODIUM-FALCIPARUM-INFECTEDERYTHROCYTES

Agglutination and rosette formation are in vitro characteristics of Pl asmodium falciparum-infected erythrocytes, which have been associated with host protective immune responses and also with parasite virulence . The present study was carried out in an area of seasonal and unstabl e malaria transmission in eastern Sudan. Plasma samples were obtained before, during, and after the transmission season from a volunteer coh ort of 64 individuals seven years of age and older. These plasmas were assayed for their ability to agglutinate cultured parasitized erythro cytes originally obtained from acute malaria infection samples taken f rom five of the cohort members. Our data show that the capacity of don or plasma samples to agglutinate parasitized cells depended largely on the time of sampling relative to the transmission season, at least wi thin this epidemiologic setting. Thus, although less than half of the pretransmission season samples could agglutinate any of the five lines of cultured parasites, all post-transmission season samples could agg lutinate at least one of the parasite lines, with 74% agglutinating tw o or more lines. This increase in the agglutination capacity of indivi dual plasma samples after the transmission season occurred essentially regardless of whether and individual had experienced a clinical malar ia attack during the transmission season. The study thus confirms the acquisition of agglutinating antibodies following episodes of clinical malaria, but also demonstrates that such acquisition can take place i n the absence of disease, presumably as a consequence of subclinical i nfection. This is the first demonstration of marked seasonal fluctuati ons in the capacity of individuals' sera to agglutinate parasitized re d blood cells. Possible explanations for this effect include a decreas e in the levels of agglutinating antibodies between seasons, or shifts in the antigens being recognized by such antibodies from one transmis sion season to the next. Finally, we showed the existence of marked se asonal fluctuation in the levels of agglutinating antibodies, either b ecause levels of such antibodies are not sustained between seasons or because the antigens recognized change from one season to the next.