CHAPTER TWO: LITERATURE REVIEW
2.0 Literature Review
The understanding of climate change and its influences on the developmental and activity levels for various vectors is a critical factor for concern in control and management of transmissible diseases. Presently, the concomitant changes in the global temperatures have resulted to elevation of the global temperatures by 1.50C to bout 4.50C over the last 1200 years (Williams et al. 2014). Although consistent changes are evident across the globe, the latitudinal differences present an additional variation across the entire world. Even within the latitudinal differences, the changes are not homogenous. For instance, the degree of climate warming at the equatorial zones is slower compared to that at the higher latitudes Williams et al. (2014) such as the Nearctic as well as at the Palearctic regions. Moreover, research shows that summer temperatures also warm at a slower rate compared to the winter temperatures (Williams et al. 2014). The climate warming (changes) experienced over the earth’s surface also affects precipitation distribution across the earth (Walton & Reisen, 2014). For instance, the warming processes and the changes in precipitation patterns over the last decade signals that northern high altitude are increasingly becoming wetter as African and Asian continents continue to be drier than before (Walton & Reisen, 2014).
Locally, the climate change perspectives have been investigated through the development of the CLIMPACTS models using local climate data, GHGs emission scenarios, a variety of other climate sensitivities and the downscaled GCM patterns. These models have been instrumental in generating country and regional change scenarios (Wratt et al. 2016) to help predict the near and far future climate changes. A review of all of the models about New Zealand suggests that the climate of the region may become warmer through time Office of the Prime Minister’s Science Advisory Committee (2013) with the projected warming rates varying between models. For instance, an average warming of about 0.90C across New Zealand in 1986 – 2005 is reported in (McGlone &Walker, 2011). Another study also denotes that New Zealand’s temperatures are expected to rise by about 3.50C above the remarkable 1986-2005 rise throughout the present century. However, the average variation indicates a lesser variation than the global averages and will be associated with increased number of warm days and decreased frost days. The models do not agree regarding the changes in precipitation conditions across New Zealand’s regional variations correspondent to the changes in temperatures. Different GCMs have instead produced varying results regarding the direction, the magnitude as well as the spatial patterns of precipitation changes compared to temperature changes (McGlone &Walker, 2011). As a result of these disagreements, the uncertainties in regional estimates of climate changes across New Zealand can be estimated based on a range of possible scenarios.
Climate changes linked to the continued warming of the world’s climate systems also affects the vector-borne diseases. The change factors linked to climate change and warming which affect disease-vector proliferation are described by Bader & Williams (2012) as under. First is the climate warming leads to expansion of biogeographic ranges of the arboviruses and vectors. Secondly, such changes have also been associated with the early springs and late summers arrivals. These anomalies often lead to extension of the annual activity periods of the mosquitoes and viruses. These expansions are common, especially in regions where the activity of these vectors and viruses were limited by low temperature conditions (Bader & Williams, 2012). Climate warming patterns also affect the changes in plants and animals distribution correspondingly influencing the distribution of mosquitoes and arboviruses. Increased precipitation also leads to flooding in some areas and extended droughts in other areas. Lengthening flooding duration, and drought phenomena creates grounds for mosquitoes spawning and increases the length of time for their activity respectively (Morin, Comrie & Ernst, 2013).
2.1 Development of mosquitoes under changing climatic conditions
Mosquitoes are known to be poikilothermic organisms whose lives are linked to the aquatic environments, especially during the first three stages of its development (Morin, Comrie & Ernst, 2013). Owing to the fact that mosquitoes’ temperatures fluctuates with that of the external environments, their metabolic rates, incubation periods of the associated viral pathogens, the periods between their hatching and emergence, etc. are tied to the ambient environments (Padmanabha et al. 2011). Consequently, the mosquitoes’ growths and developmental rates are affected entirely by temperature conditions, nutritional components, and larval density (Padmanabha et al. 2011). Of these, the climatic components that affects mosquitoes’ development are temperature and moisture conditions (moisture conditions provide favourable nutritional conditions for the developing mosquitoes), especially in the sub-adult stage.
Under these conditions, the mosquitoes can thermo-regulate behaviourally by selecting suitable conditions for resting and activity. For instance, the Culex tarsalis often find good hiding and development habitats from thick refugia such as vegetation, rodent burrows, dark swampy areas, etc. and egresses at dusk to feed on blood and plant saps. To the contrary, the immature stages are confined to the aquatic habitats created conveniently by the flooded swampy habitats created after the heavy rains (Padmanabha et al. 2011). According to Mohammed & Chadee (2011), the genetic structure of vector populations underlying their phenotypic variations under thermal tolerance, behavioural, developmental and bioenergetics mechanisms responsible for facilitating reproduction and survival in various environmental conditions will define the extent to which climate change will affect life histories of mosquitoes. For instance, during the development periods, favourable conditions e.g. increase in temperature, rainfall and humidity conditions often balance the developmental, physiological and survival rates and nutritional tradeoffs for mosquitoes (Mohammed & Chadee, 2011). These balances facilitate faster incubation, eggs-hatching, growths and survival rates for newborn mosquitoes thereby improving their population in occupant habitats.
With temperatures conditions increasing in New Zealand, these conditions are likely to be met satisfactorily and consequently, increase the population of mosquitoes in the region. Chaves et al. (2010) summarizes the effects of increased temperature, moisture and humidity conditions into three points. First, these conditions are likely to favour development rates for mosquito species that develops comparatively faster under warm conditions. Secondly, the conditions would increase maturity rates for the respective species. Thirdly mosquitoes will achieve adaptability to high temperature conditions. These adaptation characteristics would not only modify the suitability of the mosquitoes to inhabit expansive areas in New Zealand, but will also increase their population and life expectancy in the region (Chaves et al. 2010).
2.2 Blood-feeding patterns
The blood-feeding characteristic of arthropods such as mosquitoes is majorly geared towards nutrients acquisition to aid eggs production. The feeding activities also the primary means through which these vectors acquire and distribute disease causing pathogens (Yeap, 2014). Some of these include the dengue-causing virus, Zika virus, malaria-causing bacteria, etc. Consequently, the behavior and physiological processes that are associated with the mosquitoes blood-feeding processes also influences the frequency of the hosts-vector-pathogens interactions and this the frequency of diseases transmission between the host and the vector (Yeap, 2014). Owing to the fact that mosquitoes are poikilothermic, their frequency for blood feeding would typically increase as temperatures warm.
Similarly, warm temperature conditions also lower the period required for completion of the gonotrophic cycles of blood digestions, development of eggs feeding intervals and oviposition. The increase in mosquitoes feeding rates consequently increases their population in occupied regions (Chaves et al. 2010). As Williams et al. (2014) observes, the intrinsic incubation periods for pathogenic infections, dissemination, transmission and growths decreases significantly with increasing temperatures. This infers that vector-borne pathogenic transmissions through mosquitos’ bites typically become most efficient and effective under the warm temperature conditions compared to during the cold conditions. Bader & Williams (2012) research indicated that mosquito eggs take 1.5 to 2.5 days to hatch depending on the temperature conditions. Larval development and survival on the other hand was found to be maximal at 260C-300C. Development and survival rates however, reduced drastically at low temperatures. Based on these observations, it is evident that as global warming continues to occur globally and locally in New Zealand, favourable conditions are increasingly being created for mosquitos’ development and blood feeding capacities. These eventually leads to increases in the transmission rates, growths and survival rates for disease-causing pathogens transmitted through mosquitoes bites.
Bader, C.A. & Williams, C.R. (2012). Mating, ovariole number and sperm production of the dengue vector mosquito Aedes aegypti (L.) in Australia: broad thermal optima provide the capacity for survival in a changing climate. Physiol Entomol, 37, 136–144
Chaves, L.F., Harrington, L.C., Keogh, C.L., Nguyen, A.M., Kitron, U.D. (2010). Blood feeding patterns of mosquitoes: random or structured? Frontiers in Zoology, 7(3): 1-11. http://www.frontiersinzoology.com/content/7/1/3.
Morin, C.W., Comrie, A.C., & Ernst, K.C. (2013). Climate and dengue transmission: Evidence and Implications. Environ Health Perspect, 121, 1264–1272. http://dx.doi.org/10.1289/ehp.1306556
McGlone, M., &Walker, S., (2011). Potential effects of climate change on New Zealand’s terrestrial biodiversity and policy recommendations for mitigation, adaptation and research. Wellington, New Zealand: New Zealand Department of Conservation.
Mohammed, A. & Chadee, D.D. (2011). Effects of different temperature regimens on the development of Aedes aegypti (L.) (Diptera: Culicidae) mosquitoes. Acta Tropica, 119(2011): 38–43
Office of the Prime Minister’s Science Advisory Committee. (2013). New Zealand’s changing climate and oceans: The impact of human activity and implications for the future. An assessment of the current state of scientific knowledge by the Office of the Chief Science Advisor. Auckland: Office of the Prime Minister’s Science Advisory Committee.
Padmanabha, H. et al. (2011). Food availability alters the effects of larval temperature on Aedes aegypti growth. Journal of Medical Entomology, 48, 974-984.
Walton, W.E. & Reisen, W.K. (2014). Influence of climate change on mosquito development and blood-feeding patterns. In Viral Infections and Global Change (1st Ed.). S.K. Sigh (Ed). New Jersey, USA: John Wiley & Sons, Inc. Pp. 35-59.
Williams, C.R., Mincham, G., Ritchie, S.A., Viennet, E., & Harley, D. (2014). Bionomic response of Aedes aegypti to two future climate change scenarios in far north Queensland, Australia: implications for dengue outbreaks. Parasites & Vectors, 7(447): 1-7. http://www.parasitesandvectors.com/content/7/1/447
Wratt, D., et al. (2016). Climate Change: IPCC Fifth Assessment Report New Zealand findings. New Zealand: New Zealand Climate Change Centre.
Yeap, H.L. (2014). Assessing quality of life-shortening Wolbachia-infected Aedes aegypti mosquitoes in the field based on capture rates and morphometric assessments. Parasit Vectors, 7, 58.