A review analyzes the causes and consequences of asymptomatic malaria infections

Asymptomatic carriers of the malaria parasite represent a silent reservoir that sustains disease transmission. Therefore, such population needs to be targeted in malaria elimination settings. In a review published in Trends in Parasitology, researchers at ISGlobal and Manhiça Health Research Center (CISM) review the current knowledge on asymptomatic infections and their possible contribution to disease transmission in various phases of elimination.

The malaria parasite can cause infection without clinical symptoms (it is estimated that, in endemic regions, for every clinical infection there are four to five asymptomatic infections). These cases can contribute to seed the malaria outbreaks during the wet season when mosquitoes reappear, and to sustain disease transmission even in low-transmission settings. However, the frequency and contribution of asymptomatic infections during the different phases of disease elimination remain controversial. In order to better predict when and where the risk of asymptomatic transmission is most likely to occur, the authors do an in-depth review of the determinants that lead to this type of infection. Such factors involve both the parasite (its capacity to induce fever, its multiplication rate, its resistance to drugs) and the host (antibody and T cell-mediated immune responses, parasite-induced tolerance, genetic factors, pregnancy and health status). In addition, the authors analyze the association between malaria burden and antimalarial immunity and suggest that the asymptomatic reservoir will be larger in populations where the transmission rate is more rapid than the loss of antimalarial immunity.

"The relative contribution of asymptomatic infections will likely change during the course of elimination activities" says Dr. Alfredo Mayor, senior author of the study, "and the strategies must adapt to the different epidemiological scenarios". For this, the authors conclude, a better understanding of the complex interaction of factors influencing the clinical outcome of the infection is necessary.

Reference:

Galatas B, Bassat Q, Mayor A.Malaria Parasites in the Asymptomatic: Looking for the Hay in the Haystack. Trends Parasitol. 2015 Dec 19.