THEORETICAL-STUDIES OF THE EFFECTS OF HETEROGENEITY IN THE PARASITE POPULATION ON THE TRANSMISSION DYNAMICS OF MALARIA

Periodicity in malaria transmission has generally been ascribed to sea sonal fluctuations in mosquito population density or spatial heterogen eity with periodic migration. In this paper we demonstrate that simple models of strain heterogeneity can generate periodic behaviour as a c onsequence of the interaction between parasite strains and host immuno logical defences. As the degree of cross-immunity between strains incr eases, the system moves from a regime of independent strain transmissi on and coexistence, through increasingly coupled behaviour, to the dis placement of the strain of lower transmissibility by the strain with a higher basic reproductive rate (R(0)). Cross-immunity thus serves bot h to bring the strains into competition, and also to couple the dynami cs. We find analytical and numerical results on strain coexistence to show how the range of possible outcomes may be read as an effect of th e tension between these two effects of cross-protection.