Meeting at Manson House, London, 11 December 1997: Unstable malaria in theSudan: The influence of the dry season - Plasmodium falciparum population in the unstable malaria area of eastern Sudan is stable and genetically complex.

This paper reviews surveys carried out, over a period of 6 years between 19 89 and 1995, to examine Plasmodium falciparum population structure in Asar village in eastern Sudan, an area of unstable malaria, the incidence of whi ch is confined to a few weeks following the short rainy season (June-Octobe r). The first phase of the study involved regular cross sectional surveys, between 1989 and 1993 during the seasons of malaria incidence, while the se cond involved surveys during the malaria-free months of the dry seasons. Th e parasites were examined for 20 polymorphic loci, including enzyme electro phoretic variants, proteins detected by 2 dimensional polyacrylamide gel el ectrophoresis, antigens detected by monoclonal antibodies, and in vitro res ponses to antimalarial drugs. In addition, alleles of the polymorphic genes for merozoite surface proteins 1 and 2 (MSP-1, MSP-2) were examined using the polymerase chain reaction and oligonucleotide probes. Great genetic com plexity was observed among the parasites which appeared during the short tr ansmission seasons. A large proportion of the patients who were infected du ring the transmission season maintained asymptomatic, subpatent parasitaemi as throughout the subsequent dry season, often as genetically complex infec tions.