Malaria (MOH, 2014). High malaria transmission usually occurs in

Malaria is a debilitating infection caused by
protozoan parasite in genus Plasmodium.
Five known Plasmodium species
infecting human include: Plasmodium
falciparum, P. vivax, P. ovale, P. malariae and P. knowlesii
with 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 distributed
while P. ovale, P. malariae and P. knowlesi
have 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 accounts
for 96% of all malaria infections (MOPHS, 2009;
MOH, 2014). In western
Kenya, Plasmodium falciparum remains
the most abundant (92%) with sporadic reports of P. malariae (6%) and P. ovale
at 2% (MOH, 2016a).

 

Malaria is transmitted by female Anopheles mosquitoes.
There are over 300 species  of female Anopheles mosquitoes which can transmit Plasmodium
species to human host (Lefevre et al., 2013). The three
most efficient malaria vectors in Kenya are Anopheles
gambiae, An. arabiensis and An. funestus (Githeko et al., 2000; Githeko et al., 2012). Anopheles gambiae was the most abundant
malaria vector along the lake region of western Kenya, however, recent studies
show resurgence of An. funestus (MOH, 2014). This could
be due to expanded ownership of treated bed nets which may have an impact on
exclusively indoor feeders (MOH, 2014). High
malaria transmission usually occurs in places where both An. gambiae and An. funestus
are present due to their ability to inhabit different breeding habitats and
increase in population in different seasons (Kelly-Hope et al., 2009). Anopheles funestus prefer to breed in
large permanent pools of water covered with aquatic vegetation while both   An. gambiae and An. arabiensis breed in small temporary
water bodies (Gimnig et al., 2001; Minakawa et al.,
2005; MOH, 2014). With introduction of
mixed-crop irrigation, variable aquatic habitats are likely to be created
thereby shifting vector composition and abundance.

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Malaria
remains a major public health burden worldwide due to associated high morbidity
and mortality (WHO, 2013). An
estimate of 429,000 deaths from malaria was reported in the year 2015 Most of
these deaths (92%) occurred in sub-Saharan Africa where children under the age
of five years were the most affected (WHO, 2016). In Kenya,
malaria remains a major public health challenge leading to almost half of all
outpatient attendance and about 20% of all inpatient in health centers (MOH, 2014). Decline in
malaria prevalence (from 38% to 27%) among children aged 6 months to 14 years
has been reported in western Kenya along the lake region (MOH, 2016b). Despite
the decline, malaria prevalence remains unacceptably high and the percentage
could be higher considering the nature of asymptomatic and submicroscopic
infections in endemic region which are unlikely to be detected by microscopy.
Therefore, detection tools with higher sensitivity and specificity will be of
great importance.

 

Worldwide decrease in malaria
incidence and mortality has been set at about 90% by the year 2030 (WHO, 2015a, 2015c). However, due to expanding human
population and increasing modification of the environment to sustain the daily
requisites, interaction between human and their environment is changing and may
lead to new epidemiological patterns of the major vector-bone diseases (Ozer, 2005). Environmental modification and
disruption of natural ecology have wielded and continue to wield profound
influence on emergence and increment of parasitic diseases (Patz et al.,
2000; Brunner and
Eizaguirre, 2016). Construction of dams and establishment of
irrigation schemes may impact malaria transmission patterns by creation of more
aquatic habitats necessary for malaria vectors’ breeding thereby placing huge
health burden to local communities (Keiser et al., 2005).

 

To accurately approximate malaria
transmission potential, more robust parasite detection methods are of utmost
importance. Molecular detection strategies have been proposed to better
accuracy of actual parasite prevalence (Mwingira et al., 2014). These molecular tools were recently introduced in
malaria endemic regions to help in close monitoring of control strategies and
epidemiological field studies (Andrade et al., 2010; Kamau et al.,
2011; Wampfler et al., 2013). Unlike light microscopy or rapid diagnostic
testing, molecular diagnosis is capable of detecting lower parasitemia
especially 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 and
treated may render designed intervention strategies less effective leading to
continuous malaria transmission. Moreover, gametocytes have been reported to
persist in low parasite densities which often go undetected by light microscopy
hence ensures efficient mosquito infection (Bousema and Drakeley, 2011a;
Mwingira et al., 2014; Nguitragool et al.,
2017). The current study is likely to reveal the actual
disease burden and severity by detecting submicroscopic and asymptomatic
individuals for appropriate action.

The purpose of this study will
therefore determine the impacts of mixed-crop irrigation on malaria
burden and scope of disease severity in
Homa bay, western Kenya. This will help to elucidate more information on gametocyte carriage and submicroscopic Plasmodium species infection in Homa Bay
County.

 

1.2. Statement of the problem

Most countries in sub-Saharan Africa
depend on irrigation and construction of dams to enhance crop productivity.
They have been acknowledged as a central path towards food security and
reduction of poverty in drought stricken regions (Keiser et al., 2005). In spite of their valuable contribution
towards food production, newly constructed dams and irrigation systems may trigger
variable 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 diseases
due to creation of ideal breeding habitats (Keiser et al., 2005; Kibret et al.,
2015; Kibret et al., 2017). But in other regions, it
lead to reduction on malaria transmission (Sharma et al., 2008).   

 

While many dam
and irrigation projects have been reported to increase malaria transmission,
there are also examples where there has been no apparent impact or even a
reduction in malaria prevalence (Yewhalaw et al., 2013). Therefore, understanding the impacts associated with establishment of
irrigation schemes on malaria transmission is very important towards designing
efficient control interventions. Homa Bay County is located in a malaria
endemic lake region in Kenya and modification of the environment that may
disturb the ecological balance by creating conducive conditions for vector
productivity may complicate fight against malaria. Its therefore imperative to
have full understanding of malaria transmission and infection status in the
region prior/during the implementation of irrigation system to fully understand
its likely effect for planning and implementation of interventions hence the
current study.

 

1.3. Justification of the study

Monitoring and detailed characterization
of malaria infection among populations residing in malaria endemic regions is
useful in mounting focused control strategies. Current studies suggest
increasing 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). Asymptomatic
malaria infection and submicroscopic gametocyte reservoirs have been further
considered as a threat to current control strategies (Lindblade et al., 2013). This is
because more studies from African countries have displayed their crucial role  towards maintenance of uninterrupted malaria
transmission 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, is
becoming a challenge especially in low-density parasite infections (Cheng et al., 2015).

 

Therefore, the need for most effective
parasite detection strategy to accurately characterize sub-microscopic malaria
infection and reveal true gametocyte prevalence is of utmost importance towards
understanding malaria transmission dynamics. Molecular techniques such as the
use of polymerase chain reaction (PCR) have progressively improved detection of
malaria parasites. For instance, the application of nested PCR in both
laboratory studies and clinical diagnosis has tremendously gained significance (Roper et al., 1996; Gal et al.,
2001; Mens et al., 2007). Current study
will be conducted in Homa Bay County where establishment of mixed-crop
irrigation is advancing. The application of these molecular techniques, are
likely to help determine the impacts of mixed-crop irrigation on malaria
transmission dynamics.