Undetected malaria carriers identified as a likely source of infection

People who have low-level malaria infections that are not detected by standard tests may be a source of up to 20-50 per cent of onward transmissions, a new study has found.

These carriers have a low number of parasites in their blood and are usually unaware that they have malaria, but mosquitoes taking a bite on these people can still become infected and then go on to transmit the parasite to other people.

Experts from Imperial College London and the London School of Hygiene and Tropical Medicine, with colleagues from Radboud University Nijmegen, the Netherlands and Centre National de Recherche et de Formation sur le Paludisme in Burkina Faso, put together data from 106 surveys from endemic countries which tested for malaria using both sensitive molecular techniques and routine microscopy.

The sensitive methods reveal on average twice as many malaria infections, showing that low-level, sub-microscopic infection is common. The findings of the study are published today in the journal Nature Communications.

The authors show that submicroscopic malaria carriage is more likely in areas of low malaria transmission, in adults, and in people with long-term, chronic malaria infections.

Although these carriers are much less likely to transmit their infection via mosquitoes than people who have a greater number of parasites, there are so many of them in some areas that they are likely to be a significant source of transmission.

Dr Lucy Okell, from the Medical Research Council (MRC) Centre for Outbreak Analysis and Modelling at Imperial College London and lead author of the study, commented: ''The data show that low-density, submicroscopic malaria infections are most common in areas with low levels of malaria transmission, which is surprising since people are less likely to have immunity from previous malaria attacks. Control programmes are increasingly considering the use of screen and treat programmes, and our results suggest that in some areas it may be worth investing in more sensitive diagnostic methods.''

Chris Drakeley, Reader in Infection & immunity at LSHTM and a senior author on the paper, said: ''The results highlight the complexity of malaria transmission and the need to integrate new methods into standard survey procedures to better refine malaria control and elimination programmes.''