Subclinical Plasmodium falciparum infections act as year-round reservoir for malaria in the hypoendemic Chittagong Hill districts of Bangladesh

• Evidence before this study: Most active screens for subclinical malaria in hypoendemic regions reveal a pattern of 20% Plasmodium prevalence by PCR only in asymptomatic individuals, with 1–4% RDT or microscopy positive prevalence. Few studies have looked at the risk factors and seasonal and spatial distribution of the subclinical individuals.
• Added value of this study: Subclinical malaria is not seasonal, nor confined to the same geographic hotspots as clinical malaria, but does have similar risk factors as clinical malaria. Subclinical malaria individuals were not asymptomatic.
• Implications of all the available evidence: Detection of the hidden subclinical reservoir is important to eliminate ongoing malaria transmission and will require novel approaches distinct from strategies for the control of clinical malaria.

SummaryObjectivesAn analysis of the risk factors and seasonal and spatial distribution of individuals with subclinical malaria in hypoendemic Bangladesh was performed.MethodsFrom 2009 to 2012, active malaria surveillance without regard to symptoms was conducted on a random sample (n = 3971) and pregnant women (n = 589) during a cohort malaria study in a population of 24 000.ResultsThe overall subclinical Plasmodium falciparum malaria point prevalence was 1.0% (n = 35), but was 3.2% (n = 18) for pregnant women. The estimated incidence was 39.9 per 1000 person-years for the overall population. Unlike symptomatic malaria, with a marked seasonal pattern, subclinical infections did not show a seasonal increase during the rainy season. Sixty-nine percent of those with subclinical P. falciparum infections reported symptoms commonly associated with malaria compared to 18% without infection. Males, pregnant women, jhum cultivators, and those living closer to forests and at higher elevations had a higher prevalence of subclinical infection.ConclusionsHypoendemic subclinical malaria infections were associated with a number of household and demographic factors, similar to symptomatic cases. Unlike clinical symptomatic malaria, which is highly seasonal, these actively detected infections were present year-round, made up the vast majority of infections at any given time, and likely acted as reservoirs for continued transmission.

Figure optionsDownload high-quality image (277 K)Download as PowerPoint slide