Kenya is among the most affected tropical countries with pathogen transmitting Culicidae vectors. For decades, insect vectors have contributed to the emergence and distribution of viral and parasitic pathogens. Outbreaks and diseases have a great impact on a country’s economy, as resources that would otherwise be used for developmental projects are redirected to curb hospitalization cases and manage outbreaks. Infected invasive mosquito species have been shown to increasingly cross both local and global boarders due to the presence of increased environmental changes, trade, and tourism. In Kenya, there have been several mosquito-borne disease outbreaks such as the recent outbreaks along the coast of Kenya, involving chikungunya and dengue. This certainly calls for the implementation of strategies aimed at strengthening integrated vector management programs. Here we look at mosquitoes of public health concern in Kenya, while highlighting the pathogens they have been linked with over the years and across various regions.
The term “vector-borne” has become a commonly used term, especially in tropical and subtropical countries where emerging and re-emerging vector-related diseases frequently occur. One-sixth of human diseases is associated with vector-borne pathogens, with approximately more than half of the global population currently estimated to be in danger of contracting these diseases [
]. Hematophagous mosquitoes are the leading vectors among arthropods because of the significant role they play in disseminating microfilariae, arboviruses, and
parasites that seem endemic to sub-Saharan Africa [
,
]. The majority of these pathogens are maintained in zoonotic cycles and humans are typically coincidental dead-end hosts with a none-to-minimal role in the cycle of the pathogen [
].
Mosquito vectoral ability is greatly influenced by the availability of conducive breeding grounds, which is in turn, influenced by the spatial heterogeneity as well as the temporal variability of the environment [
]. Mosquitoes, pathogens, and hosts each endure and reproduce within certain ideal climatic conditions and changes in these conditions can greatly alter these pathogen transmission/competences. In this scope, temperature and level of precipitation are the most influential climatic components, but other factors such as sunshine length, sea level elevation, and wind have been shown to have considerable effects [
,
]. These vectors often adjust to changes in temperature by changing topographical distribution. For instance, the advent of malaria cases in the cooler regions of East African highlands may be attributed to climate change, which has led to an increase in mosquitoes in the highlands as they warm up [
]. Variability in precipitation may also have a direct influence on distribution of mosquito-borne diseases. When precipitation increases, the presence of disease vectors is also expected to rise due to the expansion of the existent larval habitat and emergence of new breeding zones [
]. Each mosquito species has unique environmental resilience limits dependent upon the availability of favorable aquatic larval habitats and the closeness of vertebrate hosts that serve as their source of blood meals. This reliance of mosquito species on aquatic environments is a constant part of their lifecycle and the availability of a suitable aquatic domain, which is a requirement for the development of eggs, larvae, and pupae, and basically determines the abundance of mosquito species.
Kenya represents a topographically diverse tropical/subtropical country which harbors a large diversity of mosquito species of public health importance. Many factors contribute to the extensive proliferation of mosquitoes ranging from global warming, sporadic floods, improper waste disposal, irrigation canals, presence of several lakes/rivers, and low altitudes around coastal regions. Consequently, an upsurge in emerging and re-emerging mosquito-borne pathogens over the years has been recorded with increased research efforts geared towards mosquitoes and their pathogens [
]. Increased urbanization, tourism, and international trade have led some of these species and pathogens to cross local and international borders to new territories. This dispersal poses both local and global health threats if proper mitigation measures are not put in place. Nevertheless, not all mosquito species are associated with human diseases, thus, this review highlights species of the main mosquito genera to which pathogens have been associated/detected and their countrywide distribution based on published data and reported cases. Additionally, this information provides a guide to proper mosquito control strategies by comparing the methods currently applied in the country and proposed alternative methods applied in other countries affected by mosquito disease burden.
Kenya, as a tropical country, is among the most affected sub-Saharan regions with mosquito-related ailments. The countrywide distribution of mosquito species is, however, not well documented with most studies focusing on disease endemic regions. Some of these regions include Garissa, Mandera, and Turkana situated in the north and north-eastern parts of the country and characterized as arid and semi-arid regions (
A). During rainy seasons, flood water acts as breeding areas for mosquito species. Rift Valley fever outbreaks were recorded in this area in 1997/98 and 2006/07 [
,
]. Other viruses reported from the regions include West Nile virus (WNV), Ndumu virus (NDUV), Babanki virus (BBKV), and orthobunyaviruses isolated from the flood water
and
species [
]. For decades, the coastal region of Kenya (Kwale, Kilifi, Mombasa, Lamu) has also reported multiple outbreaks resulting from mosquito-borne pathogens making it one of the most endemic regions. The contributing factors are majorly the low altitude which provides a conductive environment for mosquito breeding and high human population composed of both locals and global tourists. Mosquitoes of interest in this coastal region in terms of pathogen transmission include:
that transmits dengue fever and chikungunya [
,
];
species (
,
,
) which are associated with malaria and bancroftian filariasis sporozoites [
,
], and
[
] among others as demonstrated in
A–C.
Mosquito-borne disease endemic regions based on distribution of pathogens and associated mosquito species (maps were constructed using the free and open-source Quantum GIS software (https://qgis.org/en/site/) using data compiled from
and
. (
) Abundance of mosquito-borne viruses detected/isolated in various counties. (
) Distribution of the major malaria vectors in numbers in different counties. (
) Counties in which
has been detected from mosquitoes.
Summary of the main mosquito species in which viruses have been detected/isolated in Kenya.
Genera |
Genera |
Species | Parasite |
Parasite Species |
Virus Isolated/Detected | Associated Human Ailment |
Associated Human Ailment | 1 |
Virus Isolated/Detected 1 |
Dominant Mosquito Species |
Dominant Mosquito Species | County of Virus Detection |
County of Virus Detection |
Counties of Vector Distribution |
Counties of Vector Distribution | Reference |
Reference |
||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Reference | Reference | ||||||||||||||||||
Aedes |
Aedes |
A. aegypti | |||||||||||||||||
Plasmodium falciparum |
Plasmodium falciparum A. aegypti |
DENV, CHKV | Malaria |
Malaria DENV, CHKV |
Mombasa, Mandera, Kilifi, Lamu, Busia | An. gambiae s.s., An. arabiensis, An. funestus, An. merus |
An. gambiae s.s., An. arabiensis, An. funestus, An. merus Mombasa, Mandera, Kilifi, Lamu, Busia |
[ | Kwale, Kilifi |
Kwale, Kilifi | 12,13,33,55,57,58] | ||||||||
[ | 71 | , | ] | 75, | 81,93, | 94 | A. africanus |
A. africanus |
YFV |
YFV |
Baringo, |
Baringo, |
[59] |
[59] |
|||||
A. albicosta |
A. albicosta |
DENV, CHKV |
DENV, CHKV |
Mombasa, Kilifi, Lamu, Kwale |
Mombasa, Kilifi, Lamu, Kwale |
[33] |
[33] |
||||||||||||
An. gambiae ss, An. arabiensis, An. funestus |
An. gambiae ss, An. arabiensis, An. funestus |
Taita-Taveta, Lamu, Kajiado, Embu, Nakuru, Baringo, Bungoma, Kirinyaga, Kiambu, Busia, Siaya, Kakamega, Vihiga, Homabay, Migori, Kisii, Kisumu, Nandi |
Taita-Taveta, Lamu, Kajiado, Embu, Nakuru, Baringo, Bungoma, Kirinyaga, Kiambu, Busia, Siaya, Kakamega, Vihiga, Homabay, Migori, Kisii, Kisumu, Nandi |
||||||||||||||||
An. gambiae ss, An. arabiensis |
An. gambiae ss, An. arabiensis |
Narok |
Narok |
A. circumluteolus | |||||||||||||||
A. circumluteolus |
RVFV, BBKV, NDUV, SMFV |
RVFV, BBKV, NDUV, SMFV |
Garissa |
Garissa |
An. arabiensis, An. funestus |
An. arabiensis, An. funestus |
Tana-River, Makueni, Machakos, Trans-Nzoia |
Tana-River, Makueni, Machakos, Trans-Nzoia | [4,36] | ||||||||||
A. fryeri |
A. fryeri |
DENV |
DENV | ||||||||||||||||
Mombasa, Kilifi, Lamu, Kwale | An. funestus |
An. funestus Mombasa, Kilifi, Lamu, Kwale |
[33] |
[33] |
|||||||||||||||
Samburu, Isiolo, Garissa, Mombasa, Uasin-Gishu, Nyamira | Samburu, Isiolo, Garissa, Mombasa, Uasin-Gishu, Nyamira | A. fulgens |
A. fulgens |
DENV, CHKV |
DENV, CHKV |
Mombasa, Kilifi, Lamu, Kwale |
Mombasa, Kilifi, Lamu, Kwale |
[33] |
[33] |
||||||||||
A. keniensis |
A. keniensis |
YFV |
YFV |
An. gambiae ss |
An. gambiae ss | Baringo |
Baringo |
[59] | Tharaka-Nithi |
Tharaka-Nithi[59] |
|||||||||
A. Luridus |
A. Luridus |
NDUV |
NDUV |
Tana River |
Tana River |
[4] |
[4] |
||||||||||||
A. mcintoshi |
A. mcintoshi |
RVFV, NDUV, PGAV, BUNV, BBKV, PGAV, SMFV, NRIV |
RVFV, NDUV, PGAV, BUNV, BBKV, PGAV, SMFV, NRIV |
Garissa |
Garissa |
[4, | 11, | ,40] | 36,40] |
[4 |
|||||||||
|
DENV, CHKV |
DENV, CHKV |
Mombasa, Kilifi, Lamu, Kwale |
Mombasa, Kilifi, Lamu, Kwale |
[33] |
[33] |
|||||||||||||
A. ochraceus |
A. ochraceus |
RVFV, NDUV, BUNV, BBKV, SNBV, SMFV |
RVFV, NDUV, BUNV, BBKV, SNBV, SMFV |
Garissa |
Garissa |
[4,11,36,37,38] | |||||||||||||
|
DENV, CHKV |
DENV, CHKV |
Mombasa, Kilifi, Lamu, Kwale |
Mombasa, Kilifi, Lamu, Kwale |
[33] |
[33] |
|||||||||||||
A. pembaensis |
A. pembaensis |
RVFV |
RVFV |
Kilifi |
Kilifi |
[4] |
[4] |
||||||||||||
|
DENV, CHKV |
DENV, CHKV |
Mombasa, Kilifi, Lamu, Kwale |
Mombasa, Kilifi, Lamu, Kwale |
[33] |
[33] |
|||||||||||||
A. sudanensis |
A. sudanensis |
BBKV, SNBV, WNV |
BBKV, SNBV, WNV |
Garissa |
Garissa |
[36] |
[36] |
||||||||||||
|
|
NDUV |
NDUV |
Tana River |
Tana River |
[39] |
[39] |
||||||||||||
Anopheles |
Anopheles |
An. funestus |
An. funestus |
ONNV |
ONNV |
Kisumu |
Kisumu |
[59] |
[59] |
||||||||||
|
BUNV |
BUNV |
Kajiado |
Kajiado |
[4] |
[4] |
|||||||||||||
|
NRIV |
NRIV |
Tana River |
Tana River |
[4] |
[4] |
|||||||||||||
An. gambiae |
An. gambiae |
BUNV |
BUNV |
Homabay |
Homabay |
[20] |
[20] |
||||||||||||
An. squamosus |
An. squamosus |
RVFV |
RVFV |
Garissa |
Garissa |
[11] |
[11] |
||||||||||||
Culex |
Culex |
Cx. bitaeniorhynchus |
Cx. bitaeniorhynchus |
RVFV |
RVFV |
Kilifi |
Kilifi |
[11] |
[11] |
||||||||||
|
NDUV |
NDUV |
Tana River |
Tana River |
[39] |
[39] |
|||||||||||||
Cx. cinereus |
Cx. cinereus |
NDUV |
NDUV |
Busia |
Busia |
[4] |
[4] |
||||||||||||
Cx. pipiens |
Cx. pipiens |
USUV |
USUV |
Kisumu |
Kisumu |
[4] |
[4] |
||||||||||||
|
NDUV |
NDUV |
Garissa, Tana River |
Garissa, Tana River |
[38,39] | ||||||||||||||
Cx. poicilipes |
Cx. poicilipes |
RVFV |
RVFV |
Kilifi |
Kilifi |
[11] |
[11] |
||||||||||||
Cx. quinquefasciatus |
Cx. quinquefasciatus |
RVFV |
RVFV |
Baringo, Garissa |
Baringo, Garissa |
[11,60] | |||||||||||||
|
WNV, SNBV |
WNV, SNBV |
Garissa |
Garissa |
[36,60] | ||||||||||||||
Cx. rubinotus |
Cx. rubinotus |
NDUV |
NDUV |
Baringo |
Baringo |
[4] |
[4] |
||||||||||||
Cx. univittatus |
Cx. univittatus |
RVFV |
RVFV |
Baringo |
Baringo |
[11] |
[11] |
||||||||||||
|
BUNV |
BUNV |
Homa Bay |
Homa Bay |
[20] |
[20] |
|||||||||||||
|
SNBV |
SNBV |
West Pokot, Nakuru, Busia |
West Pokot, Nakuru, Busia |
[4,37,61] | ||||||||||||||
|
WNV |
WNV |
Garissa, Turkana, West Pokot |
Garissa, Turkana, West Pokot |
[4,61] | ||||||||||||||
Cx. vansomereni |
Cx. vansomereni |
NDUV |
NDUV |
Tana River |
Tana River |
[39] |
[39] |
||||||||||||
| Kiambu |
Kiambu |
[4] |
[4] |
|||||||||||||||
Mansonia |
Mansonia |
||||||||||||||||||
An. arabiensis |
An. arabiensis |
Turkana |
Turkana |
||||||||||||||||
Wuchereria bancrofti |
Wuchereria bancrofti |
Bancroftian filariasis |
Bancroftian filariasis |
An. gambiae sl, An. funestus, Cx. quinquefasciatus |
An. gambiae sl, An. funestus, Cx. quinquefasciatus |
Kwale, Kilifi, Lamu |
Kwale, Kilifi, Lamu |
[50,91, |
[ | 95] | BBKV, SNBV |
BBKV, SNBV |
Nakuru |
Nakuru |
[4,37] | ||||
Cx. zombaensis |
Cx. zombaensis |
RVFV |
RVFV |
Nakuru |
Nakuru |
[62] |
[62] |
||||||||||||
|
BBKV |
BBKV |
Mn. africana |
Mn. africana |
RVFV |
RVFV |
Nakuru, Baringo, Garissa |
Nakuru, Baringo, Garissa |
[11,60,62] | ||||||||||
|
NDUV |
NDUV |
Baringo |
Baringo |
[4] |
[4] |
|||||||||||||
Mn. uniformis |
Mn. uniformis |
RVFV |
RVFV |
Baringo, Garissa |
Baringo, Garissa |
[40,60] | |||||||||||||
|
NDUV |
NDUV |
Baringo |
Baringo |
[36] |
[36] |
Summary of the dominant malaria and Bancroftian filariasis mosquito vectors distributed in Kenya.
The main mosquito species associated with human etiologies in the country belong to four genera (
) as discussed below. Their distribution in Kenya has mainly been studied in disease endemic regions and as a result of outbreaks. These data are summarized in
and
.
3.4.
The
mosquito species belong to the sub-family Culicinae and are normally characterized by their large body size and asymmetrical wing scale structure with sparkling on their wing veins and legs. They show some resemblance with
and
mosquito but the simplicity of their tarsal claws in structure and a truncated abdomen in females sets it apart [
]. Furthermore,
preferably breed in ponds and permanent waters with aquatic plants where larvae can burrow into the dead plants at the bottom or cling to the roots of the live ones [
]. Its genera comprise 25 globally distributed species in two subgenera:
(10 species) and
(15 species).
The distribution of these species comes with increased vectorial role of human etiologies.
a common species in the Americas, is a known vector of Venezuelan equine encephalitis [
]. Recently, Argentina detected SLEV and BUNV for the first time from this species, proving its competent nature to transmit the viruses [
]. In south-east Asia,
and
have been linked to various cases of Brugian and lymphatic filariasis [
]. Africa, as one of the most affected regions of vector-borne diseases, harbors two main and probably the most common
spp. (
and
). These species have been reported to vector
’s lymphatic filariasis [
,
], RVF [
], Zika [
], among other diseases. Kenya is not an exception, as studies have shown that the two
spp. are distributed within the country and harbor pathogens of human health importance [
,
,
,
,
].
LaBeaud et al. [
] showed that
mosquitoes could have significantly contributed to the vectorial transmission of RVF in the 2006–2007 outbreak in the north-eastern region. Out of the 12,080 mosquitos collected during the study, 682 were identified to be
spp. Further investigation based on molecular screening identified three out of eight pools of
spp. to be positive for RVFV.
like many other female mosquitoes, feed on vertebrate blood for egg development. These vertebrates can lead to amplification of a disease in case of outbreaks. Virus screening from blood-fed
spp. by Lutomiah et al. [
] during this same RVF outbreak, showed that goats and sheep were the greatest amplifiers of the virus.
Owing to the fact that
and
preferably breed around permanent water bodies, flooded lagoons, and swamps, they have been shown to dominate lake regions in the country. A study by Lutomiah et al. [
] on the abundance of mosquito vectors in various ecological zones in the country revealed high abundance of
spp. in Baringo (71%) and Kisumu (23%), the home places of Lake Baringo and Lake Victoria. Interestingly, it is here (Baringo) that positive strains of RVFV and NDUV had earlier been detected in
and
[
,
]. This was further evidenced by Ajamma et al. [
], where these species dominated in abundance in the islands and mainlands of both Lake Victoria and Lake Baringo.
Even though Zika virus and
have been detected from
spp. in some African states [
,
], this has not been the case in Kenya. In a study at the Kenyan Tana delta on vectorial potential of
spp. to transmit
all 236 species collected tested negative on PCR (polymerase chain reaction) analysis [
]. However, the study recommended further assessment to be done by infecting
spp. experimentally with
References
microfilaria to identify whether or not they can support development of microfilaria up to the infective stages.