School-Age Children Are a Reservoir of Malaria Infection in Malawi

Abstract

Malaria surveillance and interventions in endemic countries often target young children at highest risk of malaria morbidity and mortality. We aimed to determine whether school-age children and adults not captured in surveillance serve as a reservoir for malaria infection and may contribute to malaria transmission. Cross-sectional surveys were conducted in one rainy and one dry season in southern Malawi. Demographic and health information was collected for all household members. Blood samples were obtained for microscopic and PCR identification of Plasmodium falciparum. Among 5796 individuals aged greater than six months, PCR prevalence of malaria infection was 5%, 10%, and 20% in dry, and 9%, 15%, and 32% in rainy seasons in Blantyre, Thyolo, and Chikhwawa, respectively. Over 88% of those infected were asymptomatic. Participants aged 6-15 years were at higher risk of infection (OR=4.8; 95%CI, 4.0-5.8) and asymptomatic infection (OR=4.2; 95%CI, 2.7-6.6) than younger children in all settings. School-age children used bednets less frequently than other age groups. Compared to young children, school-age children were brought less often for treatment and more often to unreliable treatment sources.

Conclusion: School-age children represent an underappreciated reservoir of malaria infection and have less exposure to antimalarial interventions. Malaria control and elimination strategies may need to expand to include this age group.

Fig 1. Malaria infection prevalence diagnosed PCR by season and district.

Prevalence was heterogeneous across districts and seasons (A) and across sites within district (B). Pearson chi-square P values were <0.001 for comparisons of prevalence of parasitemia across districts in both seasons and were <0.005 for comparisons of prevalence between seasons. Likelihood ratio P values for homogeneity of prevalence within districts were < 0.001.

Fig 2. Asymptomatic infections among malaria-infected individuals by season and district (A) and by age group (B).

Asymptomatic malaria was defined as PCR positive, with temperature <37.5°C and without reported fever within 48 hours. Prevalence differences were comparable (and Pearson chi-square P values >0.05) across districts in dry and rainy seasons and between seasons. In 2B, the odds of asymptomatic infection by age group were adjusted for season and sex and were statistically significant in Chikhwawa and Thyolo but not in Blantyre (P_{interaction age and district} = 0.001).

Fig 3. Proportion with submicroscopic infection among all malaria infections by age group.

Association of submicroscopic parasitemia with age and season was unchanged after adjustment for district, gender, net use, net ownership, and house materials.