Describing chronic malaria infections

I read with interest the recent thought-provoking article by Dennis Shanks [1]. This discusses the state of anti-disease resistance that may be eventually achieved by a person if s/he is repeatedly exposed to infection with Plasmodium falciparum, assuming that no infectious episode has a fatal outcome. This ‘steady state’ between parasite dispersal and host survival can be observed in those adult residents of P. falciparum-endemic areas who have not succumbed to exposure during childhood. The author makes a compelling case for using the term ‘malaria-tolerant’ in preference to ‘malaria-immune’ that is found far more frequently in the published literature. The reason for this is that such individuals often harbour blood stage parasites below a threshold of parasitemia required for them to show clinical manifestations of malarial infection, and thus they are apparently tolerant to the residual but persistent presence of parasites in their peripheral circulation. Hence, their naturally acquired protective immune response, a form of premunition, reduces infection for the most part to subclinical levels but does not eradicate all parasites. It is likely that these asymptomatic yet chronic carriers of malaria parasites provide a reservoir of infection for possible transmission within their local community if and when an Anopheles mosquito takes a blood meal from them, especially if this coincides with a time when they may be experiencing a recrudescence of patent infection.