Asymptomatic and Sub-Microscopic Malaria: a Challenge to Elimination Efforts

WHO says that, “In settings where malaria is actively being eliminated or has been eliminated, a “case” is the occurrence of any confirmed malaria infection with or without symptoms.” Several recent studies describe the importance of paying attention to asymptomatic infections.

In the Bagamoyo District of Tanzania Sumari and colleagues collected blood samples and examined them for Plasmodium falciparum prevalence using rapid diagnostic test (RDT), light microscopy (LM) and reverse transcription quantitative PCR. While overall prevalence was higher in symptomatic children using all three methods, asymptomatic children had a higher prevalence of gametocytes using light microscopy and PCR.  They concluded that, “The higher gametocytemia observed in asymptomatic children indicates the reservoir infections and points to the need for detection and treatment of both asymptomatic and symptomatic malaria.”

The health effects of asymptomatic plasmodial infections (API) on children were documented in Rwanda. These included “Plasmodium infection was associated with anaemia, fever, underweight, clinically assessed malnutrition and histories of fever, tiredness, weakness, poor appetite, abdominal pain, and vomiting” and were generally more common with submicroscopic infection.

Besides children other groups are at risk from API.  Malaria during pregnancy is a life and health threat to both the pregnant woman and the unborn child. Thirty-seven percent of asymptomatic pregnant women who had just delivered in Colombia were found to have parasitemia. Using microscopy only 8% were identified, such that without PCR the true extent of the problem would not have been identified. Thus, there is also concern for submicroscopic malaria and well as API generally. Asymptomatic and submicroscopic infections in areas co-endemic for P. falciparum and P. vivax are major contributors to anemia, not only in children but also in adults.

Working along the China-Myanmar border area, Zhao et al. explained that, “Sensitive methods for detecting asymptomatic malaria infections are essential for identifying potential transmission reservoirs and obtaining an accurate assessment of malaria epidemiology in low-endemicity areas aiming to eliminate malaria.” Thus they tried three molecular detection methods side-by-side, namely nested PCR targeting the rRNA genes, nested RT-PCR to detect parasite rRNA, and CLIP-PCR to detect parasite rRNA.

Interestingly the presence of fever is no guarantee that malaria parasites will be found. A study in Gabon demonstrated that among febrile patients only 1% had parasites found through microscopy compared to 32% through molecular testing. These studies have demonstrated the need for a better understanding of malaria transmission across different zones and strata in a country in the light of asymptomatic and submicroscopic malaria, especially gametocytemia. This should lead to better targeting of case detection, improved treatment and better compliance with preventive measures.

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