Prevalence, heterogeneity of asymptomatic malaria infections and associated factors in a high transmission region

Background: Although current reports have shown a reduction in malaria cases, the disease still remains a major public health problem in Kenya. In most endemic regions, the majority of infections are asymptomatic which means those infected may not even know and yet they remain infectious to the mosquitoes. Asymptomatic infections are a major threat to malaria control programs since they act as silent reservoirs for the malaria parasites.Objective: The study sought to determine the prevalence of asymptomatic malaria infections, whether they show heterogeneity spatially, across age groups and across time as well as their determinants in a high transmission region.Study Design: This was part of a larger prospective cohort study on malaria indices in the HDSS.Study Setting: The study was conducted in the Webuye Health and Demographic Surveillance Site in Bungoma East Sub-County.Study Subjects: Quarterly parasitological surveys were conducted for a cohort of 400 participants from randomly selected households located in known fever “hotspots” and “coldspots”. Follow-up of all the participants continued for a period of one year. Generalized estimating equations were used to model risk factors associated with asymptomatic parasitemia.Results: Of the total 321 malaria infections detected during the five cross-sectional surveys conducted over the period of one year, almost half (46.3%) of these were asymptomatic. Overall, most of the asymptomatic cases (67%) were in households within known fever “hotspots”. The proportion of infections that were asymptomatic in the coldspots were 73.1%, 31.8%, 13.3%, 55.6% and 48.2% during the first, second, third, fourth and fifth visits respectively. In the known fever “hotspots”, the proportion of infections without symptoms was 47.7%, 48.5%, 35%, 41.3% and 47.5% during the first, second, third, fourth and fifth visits respectively. Factors associated with asymptomatic malaria include; the village one lives: people living in village M were twice likely to be asymptomatic (A.O.R: 2.141, C.I: 0.03 - 1.488), age: children aged between 6 to 15 years were more than twice likely to be asymptomatic (A.O.R: 2.67, C.I. 0.434 - 1.533) and the season: infections during the dry season (January) were less likely to be asymptomatic (A.O.R: 0.26, C.I: -2.289 - 0.400).Conclusion: The prevalence of asymptomatic infections in this region is still very high. The highest proportion of asymptomatic infections was registered in a fever coldpspot village which may explain why the village is a fever coldspot in the first place. There is a need for active surveillance to detect the asymptomatic cases as well as treat them in-order to reduce the reservoir. Targeting interventions to the asymptomatic individuals will further reduce the transmission within this region.