Epidemiology of Plasmodium falciparum Infections in a Semi-Arid Rural African Setting: Evidence of Reactive Case Detection in Northwestern Kenya

Abstract

In northwestern Kenya, Turkana County has been historically considered unsuitable for stable malaria transmission because of its unfavorable climate and predominantly semi-nomadic population; consequently, it is overlooked during malaria control planning. However, the area is changing, with substantial development, an upsurge in travel associated with resource extraction, and more populated settlements forming. Recently, numerous malaria outbreaks have highlighted the need to characterize malaria transmission and its associated risk factors in the region to inform control strategies. Reactive case detection of confirmed malaria cases at six health facilities across central Turkana was conducted from 2018 to 2019. Infections in household members of index cases were detected by malaria rapid diagnostic tests (RDTs) and PCR tests, and they were grouped according household and individual characteristics. The relationships between putative risk factors and infection were quantified by multilevel logistic regression models. Of the 3,189 household members analyzed, 33.6% had positive RDT results and/or PCR test results. RDT-detected infections were more prevalent in children; however, PCR-detected infections were similarly prevalent across age groups. Recent travel was rarely reported and not significantly associated with infection. Bed net coverage was low and net crowding was associated with increased risks of household infections. Infections were present year-round, and fluctuations in prevalence were not associated with rainfall. These findings indicate year-round, endemic transmission with moderate population immunity. This is in stark contrast to recent estimates in this area. Therefore, further investigations to design effective intervention approaches to address malaria in this rapidly changing region and other similar settings across the Horn of Africa are warranted.

Plasmodium falciparum importation does not sustain malaria transmission in a semi-arid region of Kenya

“…However, this assumption is challenged by reports of malaria both from routine cases in local health facilities as well as several malaria outbreaks [16,17]. Consistent with these observations, we recently reported that nearly one-third of the household members of acute malaria patients in Central Turkana were also infected with Plasmodium falciparum, the major malaria parasite species in Kenya, suggesting that malaria is endemic in these communities [18]. Given the recent increase in local travel into and out of the region that is concentrated along a limited number of travel routes, it seems plausible that parasite importation, if implicated in sustaining local transmission, could be intervened upon to reduce the burden of malaria.…”

“…Community participants presenting with malaria were enrolled at three urban health facilities (Ngiitakito, St. Monica, and St. Patrick) located within the town of Lodwar and three rural health facilities (Kerio, Nadoto, Nakechichok) located along the Turkwel river that connects Lodwar to Lake Turkana over approximately 60 km [18]. We first compared haplotype distributions by travel reported (and whether a traveler was visiting Turkana or returning), case type, and urbanicity for individuals residing in the study For csp, we found a majority (43/72; 60%) of haplotypes were observed in both people who did and who did not report travel; only 4/72 (5.6%) csp haplotypes were unique to participants reporting travel, 3 of which were unique to visitors.…”

“…Such preventive measures typically include distributions of insecticide-treated bed nets and vector control with indoor residual spraying or other approaches. These are rare in Turkana County [18], though their effectiveness may be attenuated by the unique ecological and epidemiologic features of the region, which include little surface water with only evanescent vector breeding sites, a (semi-)nomadic population, and limited opportunities for bed net use. Among the potential measures for transmission reduction would be prevention of importation, but our finding that malaria is not only prevalent but also sustained by local transmission suggests that interrupting importation is not likely to be an effective malaria control strategy on its own.…”

“…The main routes into this remote county go through Lodwar by road and air. As previously reported [18], participants presenting with malaria at three urban (Ngiitakito, St. Monica, and St. Patrick) and three rural (Kerio, Nadoto, Nakechichok) health facilities in central Turkana from August 2018 to October 2019 were enrolled along with their household members. The enrollment clinics were selected to represent a broad range of transmission settings around Lodwar.…”

“…Molecular detection of P. falciparum on DBS from inbound passengers was performed as previously described for community samples [18] using a duplex qPCR assay targeting P. falciparum pfr364 and human beta-tubulin. Parasite densities were determined based on a standard curve of mock DBS constructed from dilutions of P. falciparum 3D7 in whole blood ranging from 0.1 parasites/μl to 2000 parasites/μl.…”