Featured Malaria Publication - The occurrence of submicroscopic malaria infections and relevance to control meaures

19 June 2012

Malaria parasite prevalence in endemic populations is an essential indicator for monitoring the progress of malaria control, and has traditionally been assessed by microscopy. However, surveys increasingly use sensitive molecular methods that detect higher numbers of infected individuals, questioning our understanding of the true infection burden and resources required to reduce it.

Factors determining the occurrence of submicroscopic malaria infections and their relevance for control

Lucy C. Okell, Teun Bousema, Jamie T. Griffin, André Lin Ouédraogo, Azra C. Ghani and Chris J. Drakeley

In this article the team analyse a series of data sets to characterize the distribution and epidemiological factors associated with low-density, submicroscopic infections. They show that submicroscopic parasite carriage is common in adults, in low-endemic settings and in chronic infections. They find a strong, non-linear relationship between microscopy and PCR prevalence in population surveys (n=106), and provide a tool to relate these measures. When transmission reaches very low levels, submicroscopic carriers are estimated to be the source of 20–50% of all human-to-mosquito transmissions. These findings challenge the idea that individuals with little previous malaria exposure have insufficient immunity to control parasitaemia and suggest a role for molecular screening.

Measuring the prevalence of malaria infection in population surveys underpins surveillance and control of the parasite. During more than a century of malaria research, parasite infection has been assessed by light microscopy of blood films. This wealth of data is widely used to understand malaria epidemiology, to monitor and inform control strategy, to map the geographical distribution of malaria over time and to aid development of mathematical models. Rapid diagnostic tests (RDTs) based on antigen detection are now also used for prevalence surveys. However, both techniques have limited sensitivity. Molecular detection techniques for malaria have a much higher sensitivity and are increasingly revealing the widespread presence of infections with parasite densities below the detection threshold of either microscopy or RDTs. These results fundamentally challenge our current view of malaria epidemiology and burden of infection.

Read the full article in Nature Communications