Unstable malaria in Sudan: the influence of the dry season. Clone multiplicity of Plasmodium falciparum infections in individuals exposed to variable levels of disease transmission.

Studies of infection and immunity to malaria often take little account of the fact that the amount of infectious challenge individuals receive is very variable. Classic studies in areas of holoendemic transmission showed that clinical immunity develops quite rapidly during childhood, although the processes through which increasing levels of resistance to infection are acquired are still not understood. However, holoendemic transmission is one end of the spectrum of malaria epidemiology and the development of clinical immunity is also affected by factors such as the infection rate and the local parasite species composition. An exceptionally simple type of malaria transmission occurs during the short, autumnal malaria outbreaks of the Sudanese sahel-savannah belt, where a sparse 200-500 mm of rain falls in 2-3 summer months, Plasmodium falciparum causes > 95% of malaria cases in most areas, and the entomological inoculation rate (EIR) is very low by African standards; thus the population dynamics of malaria parasites are less affected by super-infection. A comparison of certain features of parasite genetic diversity, particularly the average number of parasite clones present in infections in the Sudanese sahel and in malaria study sites with different levels of transmission, is presented. It is proposed that increasing EIRs are associated with progressively smaller increases in the average number of malaria parasite clones per host and the implications of this relationship for studies on malaria infection and immunity are discussed.