CHAPTER ONE
1. Introduction
1.1 Background.
Dengue fever (DF) is an arboviral infection that occurs in tropical and sub-tropical regions of the
world. The infection, caused by either one of the four closely related virus serotypes (DENV1-4),
is usually self limiting but the severe forms, Dengue hemorrhagic fever (DHF) and Dengue
shock syndrome (DSS) can be lethal in some patients (Rigau-Perez et al. 1998). In recent
decades, the transmission of dengue virus (DENV) has increased predominantly in urban and
semi-urban areas, and with continued expansion of the geographic range of the disease, DENV is
a growing international public health problem (Gubler 1998; WHO 2012; Messina et al. 2014;
WHO 2015). DENV cases are under-reported in many parts of the world leading to an
underestimation of the dengue burden especially in Africa (Nathan and Dayal-Drager 2006; Sang
2006; Messina et al. 2014). However, it is estimated up to 300 – 400 million new cases occur
annually (Brady et al. 2012; Bhatt et al. 2013) and approximately 2.5- 4 billion people are at risk
of DENV infection globally (WHO 2012; Bhatt et al. 2013). Over half the world's population
living in about 128 countries is potentially at risk for DENV transmission; currently the disease
is ranked the most important and rapidly spreading human arboviral disease (Gubler 1998;
Mairuhu et al. 2004; WHO 2009; Brady et al. 2012).
In Africa, although many dengue cases are underreported due to inadequate surveillance and
misdiagnoses (Gubler et al. 1986; Nathan and Dayal-Drager 2006; Sang 2006), epidemic dengue
has been documented in several African countries in the last 50 years, with a much higher
1
,frequency in the East African region. During these epidemics, all the four dengue serotypes
(DENV -1, DENV-2, DENV- 3 and DENV-4) were involved, but DENV- 2 has been the most
frequently reported virus strain (Carey et al. 1971; Metselaar et al. 1980; Johnson et al. 1982;
Gonzalez et al. 1985; Gubler et al. 1986; Sang 2006). In Kenya since the first dengue epidemic
was reported (Johnson et al. 1982), cases of dengue fever have been on the increase, with more
recent outbreaks in the coastal city of Mombasa(Ellis et al. 2015; Lutomiah et al. 2016) and
Mandera in the North eastern region (Akhwale 2013). These and the periodic dengue outbreaks
primarily along the Kenya coast in recent years (Akhwale 2013; Ellis et al. 2015; Ochieng et al.
2015; Lutomiah et al. 2016) is a worrying trend.
Transmission of DENV is effected by mosquito vectors in the genus Aedes. Aedes aegypti is the
primary vector in most dengue epidemics worldwide , although other mosquitoes in the subgenus
Stegomyia such as Aedes albopictus and Aedes polynesiensis are competent as well (Gubler
1998; Mairuhu et al. 2004). The worldwide resurgence of dengue has been associated with
several factors, key among these being demographic and societal changes experienced in
different parts of the world in recent decades (Gubler 1989; Newton and Reiter 1992; Gubler
1996; Gubler 1998; Rigau-Perez et al. 1998). These coupled with inability to suppress vector
densities by many control programmes, have led to increased transmission of dengue especially
in urban areas of the tropical world (Gubler 1998). Ae. aegypti, the principal vector for dengue,
chikungunya (CHIKV), Zika and other emerging arboviruses, has adapted to domestic and
peridomestic environment where it exploits the ubiquitous artificial containers and other water
receptacles of various characteristics for its breeding (Service 1992; Focks and Chadee 1997;
Gubler 1998; Troyo et al. 2008; Armistead et al. 2012; Dom et al. 2013). Aedes mosquitoes are
2
,typically daytime feeders with peak biting times early morning and late in the evening before
dusk, this renders bed nets un-protective to humans. Thus control strategies are now being
directed towards source reduction through environmental management, and killing of larvae
using chemical and biological larvicides, and biological control agents (Focks et al. 2000;
Martinez- Ibarra et al. 2002; Kroeger et al. 2006; Tun-Lin et al. 2009; WHO 2009).
In Kenya, the two sub specific taxa, Aedes aegypti aegypti and Aedes aegypti formosus are the
primary dengue vectors (Trpis and Hausermann 1986) with the latter being more prevalent in
western Kenya (Yalwala et al. 2015). Of the two subspecies, reported to be sympatric in coastal
Kenya (Trpis and Hausermann 1986), Ae.ae. aegypti is more domestic/peri-domestic, than
Ae.ae.formosus which breeds mainly in forest tree holes. In the coastal town of Malindi in north
coast of Kenya Ae.aegypti immatures were found both indoors and outdoors at high numbers
(Midega et al. 2006). In a recent study (Saifur et al. 2012), Ae.aegypti immatures were found
mostly in outdoor habitats in what is thought to be an adaptive strategy by this vector. Human
activities within households play a crucial role in determining Ae. aegypti breeding, thus
productivity and abundance of these mosquitoes tends to fluctuate greatly in individual houses.
Various factors may influence productivity of Ae. aegypti larval habitats in different container
types, these include, the frequency of water replenishment, the availability of food for the larvae
(Subra 1983) the degree of sunlight exposure (Maciel-de-Freitas et al. 2007; Paul et al. 2018) and
container covering.
Few studies on the ecology of larval Ae. aegypti have been conducted in Kenya (Trpis and
Hausermann 1986; Midega et al. 2006; Tun-Lin et al. 2009; Yalwala et al. 2015), despite the
evidence of dengue virus transmission in the country in recent years (Akhwale 2013; Ellis et al.
3
, 2015; Ochieng et al. 2015; Vu et al. 2017). Moreover, data on routine surveillance of Ae. aegypti
or other potential vectors is lacking. The presence of DENV and other arboviruses in Kenya
(LaBeaud et al. 2015), coupled with conditions suitable for the proliferation of the Aedes vector,
increases the risk for dengue outbreaks in this region. In the absence of a viable vaccine
(Halstead 2012; WHO 2018; CDC 2019) , epidemiological surveillance and vector control
remain the best practices for preventing dengue outbreaks (Hiscox et al. 2013; Ghosh and Dar
2015; Rather et al. 2017). Effective vector control and development of meaningful surveillance
methods depends on a good understanding of larval and adult vector ecology of which little is
known in Kenya.
The purpose of this study was to characterize breeding habitats of Ae. aegypti, determine
seasonal distribution and abundance of Aedes aegypti larvae and pupae in rural and urban sites in
coastal Kenya, identify households that are consistently productive for Ae. aegypti pupae and
determined the ecological and socio-demographic factors associated with the persistence and
abundance of pupae in households. The study also sought to determine susceptibility of Aedes
aegypti larvae to Bacillus thuringiensis var israelensis (Bti). This study makes a meaningful
contribution to the dengue vector surveillance in the region, in addition to providing information
that is vital guiding vector control efforts.
4
1. Introduction
1.1 Background.
Dengue fever (DF) is an arboviral infection that occurs in tropical and sub-tropical regions of the
world. The infection, caused by either one of the four closely related virus serotypes (DENV1-4),
is usually self limiting but the severe forms, Dengue hemorrhagic fever (DHF) and Dengue
shock syndrome (DSS) can be lethal in some patients (Rigau-Perez et al. 1998). In recent
decades, the transmission of dengue virus (DENV) has increased predominantly in urban and
semi-urban areas, and with continued expansion of the geographic range of the disease, DENV is
a growing international public health problem (Gubler 1998; WHO 2012; Messina et al. 2014;
WHO 2015). DENV cases are under-reported in many parts of the world leading to an
underestimation of the dengue burden especially in Africa (Nathan and Dayal-Drager 2006; Sang
2006; Messina et al. 2014). However, it is estimated up to 300 – 400 million new cases occur
annually (Brady et al. 2012; Bhatt et al. 2013) and approximately 2.5- 4 billion people are at risk
of DENV infection globally (WHO 2012; Bhatt et al. 2013). Over half the world's population
living in about 128 countries is potentially at risk for DENV transmission; currently the disease
is ranked the most important and rapidly spreading human arboviral disease (Gubler 1998;
Mairuhu et al. 2004; WHO 2009; Brady et al. 2012).
In Africa, although many dengue cases are underreported due to inadequate surveillance and
misdiagnoses (Gubler et al. 1986; Nathan and Dayal-Drager 2006; Sang 2006), epidemic dengue
has been documented in several African countries in the last 50 years, with a much higher
1
,frequency in the East African region. During these epidemics, all the four dengue serotypes
(DENV -1, DENV-2, DENV- 3 and DENV-4) were involved, but DENV- 2 has been the most
frequently reported virus strain (Carey et al. 1971; Metselaar et al. 1980; Johnson et al. 1982;
Gonzalez et al. 1985; Gubler et al. 1986; Sang 2006). In Kenya since the first dengue epidemic
was reported (Johnson et al. 1982), cases of dengue fever have been on the increase, with more
recent outbreaks in the coastal city of Mombasa(Ellis et al. 2015; Lutomiah et al. 2016) and
Mandera in the North eastern region (Akhwale 2013). These and the periodic dengue outbreaks
primarily along the Kenya coast in recent years (Akhwale 2013; Ellis et al. 2015; Ochieng et al.
2015; Lutomiah et al. 2016) is a worrying trend.
Transmission of DENV is effected by mosquito vectors in the genus Aedes. Aedes aegypti is the
primary vector in most dengue epidemics worldwide , although other mosquitoes in the subgenus
Stegomyia such as Aedes albopictus and Aedes polynesiensis are competent as well (Gubler
1998; Mairuhu et al. 2004). The worldwide resurgence of dengue has been associated with
several factors, key among these being demographic and societal changes experienced in
different parts of the world in recent decades (Gubler 1989; Newton and Reiter 1992; Gubler
1996; Gubler 1998; Rigau-Perez et al. 1998). These coupled with inability to suppress vector
densities by many control programmes, have led to increased transmission of dengue especially
in urban areas of the tropical world (Gubler 1998). Ae. aegypti, the principal vector for dengue,
chikungunya (CHIKV), Zika and other emerging arboviruses, has adapted to domestic and
peridomestic environment where it exploits the ubiquitous artificial containers and other water
receptacles of various characteristics for its breeding (Service 1992; Focks and Chadee 1997;
Gubler 1998; Troyo et al. 2008; Armistead et al. 2012; Dom et al. 2013). Aedes mosquitoes are
2
,typically daytime feeders with peak biting times early morning and late in the evening before
dusk, this renders bed nets un-protective to humans. Thus control strategies are now being
directed towards source reduction through environmental management, and killing of larvae
using chemical and biological larvicides, and biological control agents (Focks et al. 2000;
Martinez- Ibarra et al. 2002; Kroeger et al. 2006; Tun-Lin et al. 2009; WHO 2009).
In Kenya, the two sub specific taxa, Aedes aegypti aegypti and Aedes aegypti formosus are the
primary dengue vectors (Trpis and Hausermann 1986) with the latter being more prevalent in
western Kenya (Yalwala et al. 2015). Of the two subspecies, reported to be sympatric in coastal
Kenya (Trpis and Hausermann 1986), Ae.ae. aegypti is more domestic/peri-domestic, than
Ae.ae.formosus which breeds mainly in forest tree holes. In the coastal town of Malindi in north
coast of Kenya Ae.aegypti immatures were found both indoors and outdoors at high numbers
(Midega et al. 2006). In a recent study (Saifur et al. 2012), Ae.aegypti immatures were found
mostly in outdoor habitats in what is thought to be an adaptive strategy by this vector. Human
activities within households play a crucial role in determining Ae. aegypti breeding, thus
productivity and abundance of these mosquitoes tends to fluctuate greatly in individual houses.
Various factors may influence productivity of Ae. aegypti larval habitats in different container
types, these include, the frequency of water replenishment, the availability of food for the larvae
(Subra 1983) the degree of sunlight exposure (Maciel-de-Freitas et al. 2007; Paul et al. 2018) and
container covering.
Few studies on the ecology of larval Ae. aegypti have been conducted in Kenya (Trpis and
Hausermann 1986; Midega et al. 2006; Tun-Lin et al. 2009; Yalwala et al. 2015), despite the
evidence of dengue virus transmission in the country in recent years (Akhwale 2013; Ellis et al.
3
, 2015; Ochieng et al. 2015; Vu et al. 2017). Moreover, data on routine surveillance of Ae. aegypti
or other potential vectors is lacking. The presence of DENV and other arboviruses in Kenya
(LaBeaud et al. 2015), coupled with conditions suitable for the proliferation of the Aedes vector,
increases the risk for dengue outbreaks in this region. In the absence of a viable vaccine
(Halstead 2012; WHO 2018; CDC 2019) , epidemiological surveillance and vector control
remain the best practices for preventing dengue outbreaks (Hiscox et al. 2013; Ghosh and Dar
2015; Rather et al. 2017). Effective vector control and development of meaningful surveillance
methods depends on a good understanding of larval and adult vector ecology of which little is
known in Kenya.
The purpose of this study was to characterize breeding habitats of Ae. aegypti, determine
seasonal distribution and abundance of Aedes aegypti larvae and pupae in rural and urban sites in
coastal Kenya, identify households that are consistently productive for Ae. aegypti pupae and
determined the ecological and socio-demographic factors associated with the persistence and
abundance of pupae in households. The study also sought to determine susceptibility of Aedes
aegypti larvae to Bacillus thuringiensis var israelensis (Bti). This study makes a meaningful
contribution to the dengue vector surveillance in the region, in addition to providing information
that is vital guiding vector control efforts.
4
