YELLOW FEVER AND DENGUE VIRUSES CIRCULATION AMONG HUMAN AND MOSQUITO POPULATIONS IN NGURUMAN AND KERIO VALLEY, KENYA

Abstract

Dengue virus (DENV) and Yellow fever virus (YFV) are RNA viruses belonging to the genus Flavivirus and family Flaviviridae. Their emergence and re-emergence have become crucial public health problems, especially in Sub-Saharan Africa. Due to inadequate surveillance and diagnosis, prevalence and burden may be significantly underestimated. The main objective was to determine the circulation of DENV and YFV in mosquito and human populations in Kerio Valley and Nguruman, Kenya. 480 serum samples were collected from patients aged 5 to 85 years in a cross-sectional survey from July 2020 - May 2023 and tested for neutralizing antibodies against yellow fever virus and dengue viruses employing the Plaque Reduction Neutralization Test. In addition, 1822 pools of Aedes mosquitoes were collected and tested for DENV and YFV using cell culture and Reverse Transcriptase Polymerase Chain Reaction. A structural questionnaire was used to collect key demographic data. Using age, gender, and occupation as covariates, a multinomial logistic regression model was used to forecast risk for each of the most common viruses. Results indicated that, overall, neutralizing antibodies against at least one of the Flaviviruses analyzed were detected in 33.13%; 159/480 (95% CI, 50.1–59.0%) of the total samples, with larger proportions found in Kerio Valley (30.2%, 145/480) than in Nguruman (2.92%, 14/480) (P<0.0001). Generally, YFV had the highest seropositivity (32.5%) compared to DENV (0.63%). Dengue-neutralizing antibodies were detected only in Nguruman, where the seropositivity was low at 2%. This may be attributed to the dryland ecology in Nguruman, combined with climate change, which creates an ideal environment for Aedes mosquitoes, promoting DENV transmission. In Kerio Valley, women showed a significantly higher seroprevalence of neutralizing antibodies (60.82%) compared to men (p-value < 0.001), whereas in Nguruman, seropositivity rates were higher in men than in women at 47.47 per cent (P = 0.049). The study further shows that a diverse range of Aedes stegomyia mosquitoes exist in Nguruman and Kerio Valley with Aedes aegypti being the most predominant species (49.85%) followed by Aedes metallicus (16.5%), Aedes simpsoni (14.78%), Aedes Chausseri (2.37%) and Aedes africanus (1.7%). Out of the 1822 pools of mosquitoes tested, 2.8% (51/1822) showed cytopathic effects (CPE) on Vero cells (CCL-81 and E6). Nguruman had 3.35% of pools with CPE, while Kerio Valley had 1.94%. No significant difference was noted in viral activity between the two sites (p-value = 0.081, 95% CI). All 51 pools tested negative for flavivirus using the flavivirus universal primers (FU1 and CFD2). Further, upon sequencing, YFV and DENV viruses were not isolated from CPE-positive pools from both sites. The absence does not imply that these viruses are non-existent in these regions; rather, it may indicate low infection rates or viruses were not in circulation during the sampling period. The study concludes that yellow fever and dengue viruses are circulating among the human population in Nguruman and Kerio Valley, presenting a significant public health concern. Despite the presence of mosquito vectors, the viruses were not isolated from Aedes mosquitoes. The findings recommend strengthening surveillance of YFV and DENV in human and mosquito populations and using advanced methods like metagenomic testing for improved virus detection in mosquito from Nguruman and Kerio Valley.