Background
Chikungunya fever is a self-remitting febrile viral illness caused by Chikungunya virus (CHIKV). The CHIKV is an arthropod-borne virus (arbovirus) of alphavirus genus in the family Togaviridae. The term “Chikungunya” was derived from the African dialect Swahili or Makonde and translates as “to be bent over and refers to the “stooped-over posture” exhibited by individuals with the disease [
1]. The roots of this viral illness date back to 1953, when it was first detected in a Makonde Village in the Newala District of Tanzania [
2,
3]. CHIKV infection is usually characterized by an acute onset of fever, rash, and arthralgias, and is often accompanied by headache, joint swelling and conjunctivitis [
4‐
8]. Chikungunya disease is rarely fatal but is associated with significant morbidity. Although frequent outbreaks have been reported in the tropical countries of Africa and Southeast Asia, there are recent concern in Western countries and temperate zones around the world [
9,
10]. In Africa, high prevalence of the CHIKV has been reported with first case being isolated in Tanzania in 1953 [
1,
2], Union of the Comoros in 2005 [
11], Congo (DRC) during 1998–2000 [
12,
13], Central African Republic in 1999–2000 [
14] and Mauritius and Madagascar in 2005 and 2006 respectively [
15]. The dynamics attest to overall varying outbreak trends being observed in East/South/Central Africa and western Africa countries [
11]. Kenya has experienced two outbreaks of chikungunya fever in 2004 [
16] with the latest outbreak occurring in May 2016 in Northern Kenya (see Additional file
1) due to close proximity of mosquito breeding sites to human habitation and heavy rainfall [
17]. Large variations in prevalence within these countries have also been reported such as the 59 % seroprevalence of the CHIKV infection in Busia District and 24 % in Malindi in Kenya [
18].
The vectors principally responsible for transmission of the virus are
Aedes mosquitoes [
19,
20] where the virus actively replicates but the viral transmission occurs through the mosquitoes involved if the virus overcomes a series of anatomical barriers, i.e. the midgut and the salivary glands. In the past, large epidemics were related to the presence of the primary vector
Ae. aegypti, which is also the main vector of the dengue virus [
6,
21,
22].
Ae. aegypti was established in southern parts of continental Europe until the mid-1900s but subsequently disappeared for reasons that are yet to be completely understood [
21]. In Africa, CHIKV apparently is maintained in a sylvatic transmission cycle involving primates and forest-dwelling
Aedes mosquitoes [
23]. Sylvatic vectors that have been implicated in transmission include
Ae. africanus and
Ae. aegypti in East Africa [
24,
25].
Ae. aegypti predominantly breeds in stored fresh water, such as desert coolers, flower vases, water-tanks, etc., and in peri-domestic areas (discarded household junk items like vehicular tyres, coconut shells, pots, cans, bins, etc.) in urban and semi urban environments [
26,
27]. Adult mosquitoes rest in cool and shady areas and bite humans during the daytime.
In mosquito infected by CHIKV, the extrinsic incubation period (EIP), the time from initial acquisition of pathogens until transmission is possible [
28,
29], ranges from 2 to 9 days, with an average of 3 days [
30,
31]. CHIKV is transmitted by
Aedes mosquitoes, mainly by
Ae. aegypti. The
Ae. aegypti, is well distributed and is highly anthropophilic [
32‐
34], thus increases the risk of CHIKV transmission. Mosquito vectors display different degrees of vector competence for different CHIKV isolates [
35]. However, the invasive species
Ae. albopictus has played a major role in most of recent epidemics since its last emergence in Kenya in 2004 [
34,
36,
37]. Furthermore, recent studies have shown that transmission and spread of CHIKV in Africa and Asia is related to the CHIKV phylogroup and mosquito species [
11,
14,
38,
39]. In the present study, CHIKV strain isolated from the 2004–2005 outbreak in Lamu Island was considered, the East/South/Central Africa and Indian Ocean genotype [
14,
38]. Although the establishment of an arbovirus infection in a mosquito following ingestion of a virus is dependent on the amount of viral particles ingested by the mosquito and the susceptibility of the mosquito to infection by the virus [
40], the vector competence is a complex trait involving an interplay between vectors, pathogens and environmental factors [
35,
41,
42]. Temperature is regarded as one of the most important abiotic environmental factors affecting biological processes of mosquitoes, including interactions with arboviruses. Seasonal and geographic differences in temperature and anticipated climate change undoubtedly influence mosquito population dynamics, individuals’ traits related to vector biology (lifespan and vector competence for arboviruses), and disease transmission patterns. Extrinsic incubation temperature (EIT) has been shown to influence the replication and dissemination of arboviruses in vectors [
43] thus altering the Extrinsic Incubation Period (EIP) [
28,
29]. In the tropics, areas of high prevalence of the mosquitoes, with reported occurrence of CHIKV have variable temperature ranging between 25 and 34 °C throughout the year as part of climate characteristics. Yet information on the vector transmission of different populations of this species for CHIKV at different EIT is limited. Therefore the aim of this study was to compare the vector competence of coastal and Western Kenya
Ae. aegypti populations for CHIKV under varying EIT. The coastal and western regions of Kenya have mean annual ambient temperature of 32 and 26 °C respectively. The information generated from this study provides data on competence factors that would influence epidemiological patterns of chikungunya fever.
Discussion
Arboviruses are ecologically complex, and interactions between larval mosquitoes and their aquatic environment can influence adult transmission dynamics. Moreover, due to the impact of climate on vector ecology, competence and their risk of transmitting viruses may be sensitive to projected changes in global temperatures. In this study, we evaluated the effect of ambient temperatures and changes of EIT on the risk of vector transmission and competence of the
Ae. aegypti for CHIKV. We provide evidence that the incubation temperatures of vector directly impact virus transmission by influencing the likelihood of infection and dissemination of CHIKV. We established that the MIR of
Ae. aegypti sampled from the coastal area with ambient temperature of 32 °C was higher than those sampled from the western Kenya that has ambient temperature 26 °C regardless of the EIT. Meanwhile for
Ae. aegypti emanating from lower ambient temperature of 26 °C, there was increased MIR when EIT was increased from 26 to 32 °C. This suggests that virus transmission is likely to be affected more by higher environmental temperature due to possible effects of the temperature on the biological processes moderating the vector competence [
61]. It has earlier been noted that temperature may limit virus transmission in areas where the vectors is present noting that an increase in environmental temperature for adult mosquitoes reduces the EIP most likely due to an increase in the metabolism of the adult mosquito and replication speed of the virus [
28,
61,
62]. Equally, temperature changes experienced in the immature stages of the mosquito development before infection may affect vector virus interactions by changing physical and physiological characteristics of mid-gut barriers which would impact virus infection and transmission [
63,
64]. This is in agreement with previous studies which have established that ambient temperature affect the biological processes of mosquitoes and plays a key role in modulating mosquito vector competence for pathogens [
65‐
67]. Previous studies have indicated that increases in adult-holding incubation temperatures have usually been associated with enhanced vector competence [
62,
68‐
74]. However, some studies have identified reduced vector competence and activity in nature associated with increases in incubation temperature [
64,
75‐
77]. It has long been recognized that increases in incubation temperature reduce the extrinsic incubation period (the time from initial acquisition of pathogens until transmission is possible) [
28], which render virus transmission more likely under such incubation period. Along the same lines, increases in temperature reduce the adult lifespan of mosquitoes and may impinge transmission [
69,
70,
78]. Temperature effects may drastically alter risk of disease transmission, especially under conditions where the extrinsic incubation period approaches the lifespan of the mosquito. This result differs with other systems where arboviral vector competence was reduced in female mosquitoes that were reared at higher compared to lower temperatures [
68‐
70,
78]. However, vector capacity of a mosquito population is a complex phenomenon that is influenced by a number of factors such as host seeking behavior and longevity of the infected mosquitoes apart from temperature and inherent factors [
79] and thus further studies are recommended on how these factors can combine to affect the MIR.
Mosquito susceptibility to arbovirus infection resides primarily in the midgut and can vary greatly between mosquito species and geographical strains of the same species and even within individuals of the same population [
80]. Vector competence, which is the capacity of an arthropod to acquire an infection and transmit it to a subsequent host, can greatly vary among individuals and between populations [
56] and has been previously linked to genetics [
81] as well as by climate variables such as temperature [
70,
82]. Disseminated infection is generally accepted as a measure of a mosquito’s ability to transmit a virus through biting [
56,
70]. The rate of dissemination, when expressed as a percentage of the number of mosquitoes infected, may provide information about the effect of a “midgut escape barrier” moderating whether gut infections are able to disseminate into the hemolymph. In the current study, the dissemination rates of infected
Ae. aegypti for CHIKV in the legs was high at higher ambient temperature regardless of the EIT. Notably, the disseminated infection rates for the CHKV in
Ae. aegypti in the legs was higher in mosquito emanating from ambient temperature 32 °C (Coastal Region) regardless of the EIT while those from ambient temperature of 26 °C (Western Kenya) dissemination rates was significantly higher at higher EIT of 32 °C. These results suggest that the midgut barriers preventing dissemination were strongly influenced by the ambient and rearing temperature. Thus, it can be speculated that there may be an increased midgut escape barrier in mosquitoes derived from the higher rearing temperatures. At temperature of 26 °C during the adult stage resulted in the lowest rates of viral dissemination. Rates of dissemination were higher at 32 °C relative to cooler holding temperatures of adults. These results corroborate observations found for laboratory studies examining susceptibility to dengue virus infection and length of the extrinsic incubation period in
Ae. albopictus and
Ae. aegypti [
83‐
85]. However, we found no association between vector dissemination in between the midgut and the head. The explanation for these observed effects of mosquitoes with disseminated infections is not entirely clear, but it does suggest complex effects of temperature on virus infection and dissemination and by extension, mosquito competence.
Acknowledgements
The authors wish to thank all the technical team in KEMRI- VHF, Nairobi for their technical assistance and unwavering encouragement while collecting data and doing the experiments in the laboratory that was used in this publication.