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. 2022 Sep 24;21(1):272.
doi: 10.1186/s12936-022-04288-2.

Asymptomatic and submicroscopic Plasmodium infections in an area before and during integrated vector control in Homa Bay, western Kenya

Collince J Omondi  1   2 , Wilfred O Otambo  3   4 , David Odongo  5 , Kevin O Ochwedo  5   3 , Antony Otieno  5 , Shirley A Onyango  3   6 , Pauline Orondo  3 , Benyl M Ondeto  5   3 , Ming-Chieh Lee  3   7 , Daibin Zhong  7 , James W Kazura  8 , Andrew K Githeko  3   9 , Guiyun Yan  7
Affiliations

Asymptomatic and submicroscopic Plasmodium infections in an area before and during integrated vector control in Homa Bay, western Kenya

Collince J Omondi et al. Malar J. .

Abstract

Background: Long-lasting insecticidal nets (LLINs) have been the primary vector control strategy until indoor residual spraying (IRS) was added in Homa Bay and Migori Counties in western Kenya. The objective of this study was to evaluate the impact of LLINs integrated with IRS on the prevalence of asymptomatic and submicroscopic Plasmodium infections in Homa Bay County.

Methods: A two-stage cluster sampling procedure was employed to enroll study participants aged ≥ 6 months old. Four consecutive community cross-sectional surveys for Plasmodium infection were conducted in residents of Homa Bay county, Kenya. Prior to the start of the study, all study households received LLINs, which were distributed between June 2017 and March 2018. The first (February 2018) and second (June 2018) surveys were conducted before and after the first round of IRS (Feb-Mar 2018), while the third (February 2019) and fourth (June 2019) surveys were conducted before and after the second application of IRS (February-March 2019). Finger-prick blood samples were obtained to prepare thick and thin smears for microscopic determination and qPCR diagnosis of Plasmodium genus.

Results: Plasmodium spp. infection prevalence by microscopy was 18.5% (113/610) before IRS, 14.2% (105/737) and 3.3% (24/720) after the first round of IRS and 1.3% (11/849) after the second round of IRS (p < 0.0001). Submicroscopic (blood smear negative, qPCR positive) parasitaemia reduced from 18.9% (115/610) before IRS to 5.4% (46/849) after IRS (p < 0.0001). However, the proportion of PCR positive infections that were submicroscopic increased from 50.4% (115/228) to 80.7% (46/57) over the study period (p < 0.0001). Similarly, while the absolute number and proportions of microscopy positives which were asymptomatic decreased from 12% (73/610) to 1.2% (9/849) (p < 0.0001), the relative proportion increased. Geometric mean density of P. falciparum parasitaemia decreased over the 2-year study period (p < 0.0001).

Conclusions: These data suggest that two annual rounds of IRS integrated with LLINs significantly reduced the prevalence of Plasmodium parasitaemia, while the proportion of asymptomatic and submicroscopic infections increased. To reduce cryptic P. falciparum transmission and improve malaria control, strategies aimed at reducing the number of asymptomatic and submicroscopic infections should be considered.

Keywords: Asymptomatic malaria; Kenya; Plasmodium falciparum; Submicroscopic infection; Vector control.

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Conflict of interest statement

The authors do hereby declare that they have no competing interests.

Figures

Fig. 1
Fig. 1
Map of study area showing part of Rangwe and Rachuonyo South sub-counties in Homa Bay county shaded red in the map of Kenya
Fig. 2
Fig. 2
Trial profile diagram showing the study participants recruitment criteria
Fig. 3
Fig. 3
Graph 2A shows the prevalence of Plasmodium infection by microscopy and qPCR among age groups during the 4 surveys. Graph 2B indicates the parasite density among age groups by microscopy
Fig. 4
Fig. 4
The graphs indicates the proportions of submicroscopic, asymptomatic and symptomatic infections during the four cross-sectional surveys
Fig. 5
Fig. 5
The graphs show the detection of malaria parasites by microscopy and qPCR during the 4 cross-sectional surveys
See this image and copyright information in PMC

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