Malaria is a debilitating infection caused byprotozoan parasite in genus Plasmodium.Five known Plasmodium speciesinfecting human include: Plasmodiumfalciparum, P. vivax, P.
ovale, P. malariae and P. knowlesiiwith the latter being recently identified to infect human (Singh et al., 2004; Antinori et al.,2013).
Globally, Plasmodium falciparum and P. vivax are the most widely distributedwhile P. ovale, P. malariae and P. knowlesihave low prevalence (Cheesbrough,2009). In Kenya, four Plasmodium species have been detected with exception of P.knowlesi, but P.falciparum which causes the most life-threatening disease accountsfor 96% of all malaria infections (MOPHS, 2009;MOH, 2014).
In westernKenya, Plasmodium falciparum remainsthe most abundant (92%) with sporadic reports of P. malariae (6%) and P. ovaleat 2% (MOH, 2016a). Malaria is transmitted by female Anopheles mosquitoes.There are over 300 species of female Anopheles mosquitoes which can transmit Plasmodiumspecies to human host (Lefevre et al., 2013).
The threemost efficient malaria vectors in Kenya are Anophelesgambiae, An. arabiensis and An. funestus (Githeko et al., 2000; Githeko et al., 2012).
Anopheles gambiae was the most abundantmalaria vector along the lake region of western Kenya, however, recent studiesshow resurgence of An. funestus (MOH, 2014). This couldbe due to expanded ownership of treated bed nets which may have an impact onexclusively indoor feeders (MOH, 2014). Highmalaria transmission usually occurs in places where both An. gambiae and An. funestusare present due to their ability to inhabit different breeding habitats andincrease in population in different seasons (Kelly-Hope et al., 2009). Anopheles funestus prefer to breed inlarge permanent pools of water covered with aquatic vegetation while both An.
gambiae and An. arabiensis breed in small temporarywater bodies (Gimnig et al., 2001; Minakawa et al.,2005; MOH, 2014). With introduction ofmixed-crop irrigation, variable aquatic habitats are likely to be createdthereby shifting vector composition and abundance.
Malariaremains a major public health burden worldwide due to associated high morbidityand mortality (WHO, 2013). Anestimate of 429,000 deaths from malaria was reported in the year 2015 Most ofthese deaths (92%) occurred in sub-Saharan Africa where children under the ageof five years were the most affected (WHO, 2016). In Kenya,malaria remains a major public health challenge leading to almost half of alloutpatient attendance and about 20% of all inpatient in health centers (MOH, 2014). Decline inmalaria prevalence (from 38% to 27%) among children aged 6 months to 14 yearshas been reported in western Kenya along the lake region (MOH, 2016b).
Despitethe decline, malaria prevalence remains unacceptably high and the percentagecould be higher considering the nature of asymptomatic and submicroscopicinfections in endemic region which are unlikely to be detected by microscopy.Therefore, detection tools with higher sensitivity and specificity will be ofgreat importance. Worldwide decrease in malariaincidence and mortality has been set at about 90% by the year 2030 (WHO, 2015a, 2015c). However, due to expanding humanpopulation and increasing modification of the environment to sustain the dailyrequisites, interaction between human and their environment is changing and maylead to new epidemiological patterns of the major vector-bone diseases (Ozer, 2005). Environmental modification anddisruption of natural ecology have wielded and continue to wield profoundinfluence on emergence and increment of parasitic diseases (Patz et al.,2000; Brunner andEizaguirre, 2016). Construction of dams and establishment ofirrigation schemes may impact malaria transmission patterns by creation of moreaquatic habitats necessary for malaria vectors’ breeding thereby placing hugehealth burden to local communities (Keiser et al., 2005).
To accurately approximate malariatransmission potential, more robust parasite detection methods are of utmostimportance. Molecular detection strategies have been proposed to betteraccuracy of actual parasite prevalence (Mwingira et al., 2014). These molecular tools were recently introduced inmalaria endemic regions to help in close monitoring of control strategies andepidemiological field studies (Andrade et al., 2010; Kamau et al.,2011; Wampfler et al., 2013). Unlike light microscopy or rapid diagnostictesting, molecular diagnosis is capable of detecting lower parasitemiaespecially in asymptomatic infections which is a characteristic of populations in the malaria endemic zones (Kahama-Maro et al.
, 2011; Mwingira et al.,2014). These asymptomatic infections if not detected andtreated may render designed intervention strategies less effective leading tocontinuous malaria transmission. Moreover, gametocytes have been reported topersist in low parasite densities which often go undetected by light microscopyhence ensures efficient mosquito infection (Bousema and Drakeley, 2011a;Mwingira et al., 2014; Nguitragool et al.
,2017). The current study is likely to reveal the actualdisease burden and severity by detecting submicroscopic and asymptomaticindividuals for appropriate action. The purpose of this study willtherefore determine the impacts of mixed-crop irrigation on malariaburden and scope of disease severity inHoma bay, western Kenya. This will help to elucidate more information on gametocyte carriage and submicroscopic Plasmodium species infection in Homa BayCounty. 1.2. Statement of the problemMost countries in sub-Saharan Africadepend on irrigation and construction of dams to enhance crop productivity.They have been acknowledged as a central path towards food security andreduction of poverty in drought stricken regions (Keiser et al.
, 2005). In spite of their valuable contributiontowards food production, newly constructed dams and irrigation systems may triggervariable impacts on malaria transmission depending on vector management,epidemiological setting and health seeking behavior (WHO, 2005). In some regions, it lead to increased vector borne diseasesdue to creation of ideal breeding habitats (Keiser et al.
, 2005; Kibret et al.,2015; Kibret et al., 2017). But in other regions, itlead to reduction on malaria transmission (Sharma et al., 2008). While many damand irrigation projects have been reported to increase malaria transmission,there are also examples where there has been no apparent impact or even areduction in malaria prevalence (Yewhalaw et al., 2013). Therefore, understanding the impacts associated with establishment ofirrigation schemes on malaria transmission is very important towards designingefficient control interventions.
Homa Bay County is located in a malariaendemic lake region in Kenya and modification of the environment that maydisturb the ecological balance by creating conducive conditions for vectorproductivity may complicate fight against malaria. Its therefore imperative tohave full understanding of malaria transmission and infection status in theregion prior/during the implementation of irrigation system to fully understandits likely effect for planning and implementation of interventions hence thecurrent study. 1.3. Justification of the studyMonitoring and detailed characterizationof malaria infection among populations residing in malaria endemic regions isuseful in mounting focused control strategies.
Current studies suggestincreasing burden of asymptomatic infections and submicroscopic gametocyte reservoir in malaria endemic regions (Okell et al., 2009; Karl et al.,2011; Diallo et al.
, 2012; Okell et al.,2012; Lindblade et al., 2013). Asymptomaticmalaria infection and submicroscopic gametocyte reservoirs have been furtherconsidered as a threat to current control strategies (Lindblade et al.
, 2013). This isbecause more studies from African countries have displayed their crucial role towards maintenance of uninterrupted malariatransmission hence should be a priority for intervention (Laishram et al., 2012; Stresman et al., 2014; Nzobo et al.,2015; Waltmann et al., 2015). However,limitations of light microscopy which is the mainstay of malaria diagnosis, isbecoming a challenge especially in low-density parasite infections (Cheng et al.
, 2015). Therefore, the need for most effectiveparasite detection strategy to accurately characterize sub-microscopic malariainfection and reveal true gametocyte prevalence is of utmost importance towardsunderstanding malaria transmission dynamics. Molecular techniques such as theuse of polymerase chain reaction (PCR) have progressively improved detection ofmalaria parasites.
For instance, the application of nested PCR in bothlaboratory studies and clinical diagnosis has tremendously gained significance (Roper et al., 1996; Gal et al.,2001; Mens et al., 2007).
Current studywill be conducted in Homa Bay County where establishment of mixed-cropirrigation is advancing. The application of these molecular techniques, arelikely to help determine the impacts of mixed-crop irrigation on malariatransmission dynamics.