Population structure and chimpanzee malaria

A new paper examines the parasite load of a group of wild chimpanzees for Plasmodium, the parasite that causes malaria DeNys:2013. Several strains of Plasmodium are endemic in wild chimpanzees, but their ecology is not well understood. For example:

As in humans [5], the development of acquired immunity probably plays an important role in explaining the observed patterns. Throughout this process, malaria parasites might also contribute to directly decimating young chimpanzees. Histopathological findings and molecular analyses performed on more than 30 dead adult chimpanzees from the same area and community do not support a marked pathogenicity of malaria parasites in adults, because malaria was thus far never identified as a possible cause of death [1]. For young chimpanzees, however, the question remains completely open. While it is known that mortality in young chimpanzees is high [17], their bodies are rarely accessible, either because they have less chances to be found opportunistically or because their carcasses are carried for several days by their mothers (data not shown). This makes the determination of young chimpanzees' cause of death a tricky issue and the involvement of malaria parasites currently cannot be ruled out.

The part that I think is the most useful addition to knowledge is this:

The general detection rate was 35 per cent overall, which is well in line with previous estimates in wild chimpanzees [1,2]. However, almost every individual chimpanzee was found positive at least once in the course of this short study. Taking into account sampling intensity, our data also suggest that at every point in time at least one individual of this group of 25 wild chimpanzees is infected. Given the low chimpanzee density in the region [7], it appears likely that circulation of malaria parasites is mainly supported by transmission within groups. Because these groups are, in general, quite small, it can be hypothesized that malaria parasites are locally (within groups) exposed to intense genetic drift. This could end up with group-specific infection with particular parasite species and/or shift of locally dominant parasite species through time, particularly when chimpanzee group size decreases.

If the ecology of malaria parasites is really so strongly shaped by the demography of small chimpanzee groups, that provides a mechanism for the maintenance and development of pathogenicity within the chimpanzee population. We know that human falciparum malaria seems to have come into humans from gorillas, which may well have similar issues with parasite transmission within and between groups. People have focused intensively on the role of cross-species infections as a mechanism for zoonotic disease in humans, but maybe we need to look equally seriously at the role of within-species population structure, particularly in the strongly differentiated groups characteristic of many wild primates.