For several decades, we have observed the redistribution of mosquito populations which are responsible of diverse infectious diseases such as Chikungunya, dengue, malaria, West Nile Virus or yellow fever. They have undergone epidemiological changes, emerging in new world areas, causing important economic losses as well as public health problems.

Among the responsible factors of this redistribution, the environmental and climate changes can modify some parameters like the vectorial capacity of mosquito populations, which is a key indicator of diseases transmission. Indeed, this concept measures the potential of infectious agent transmission by a mosquito population and it depends on different factors related to :

  • – the vector (longevity, fertility, vector competence and biting behaviour…)
  • the infectious agent (parasite, bacteria, virus) transmitting by the vector
  • the host
  • the environment (temperature, resource availability…)

Moreover, the vectorial capacity is strongly connected to the concept of the density of the vector population. For example, the increase of temperatures and rainfalls during the El Niño phenomenon between 1997 and 1998 caused malaria and Rift Valley fever outbreaks in Kenya [1], [2]. These later were might have been caused by an acceleration of parasite development but also by a modification of the size of the vector population. However, these similar climate modifications led to a reduction of the malaria transmission in Tanzania  [3].

Therefore, understanding the variation in vector-borne disease transmission intensity across time and space relies on a thorough understanding of the effect of environmental factors on vectorial capacity traits of mosquito populations. Indeed, larval development and growth of mosquitoes, as well as life history traits such as the body size of adults and longevity, depend on several environmental factors such as temperature and the availability of nutritional resources [4]–[8]. Thus, variations of these environmental parameters can lead to fluctuations in the size of mosquito populations, and these fluctuations are driven primarily by variations of larval development and growth.

Moreover, the notion of correlation between longevity and body size is controversial. It is often expected a positive correlation between these factors.While it is true that longevity often increases with body size [9], [10], this correlation can be unclear or even negative [4], [8], [11], [12]. The fact that these life-history traits respond differently to environmental factors might explain these opposite correlations. For example, undernourished juveniles generally become small adults [13] with increased longevity [8], [14], giving a negative correlation between the two traits among environments. In contrast, the colder temperature generally leads to larger adults [6] that live longer, in cold-blooded animals [15], giving a positive correlation.

The consequences of such interactions between extrinsic factors (temperature and resource availability) experienced during the larval stage on adult traits (longevity and body size) were sparsely investigated, whereas it would be useful for epidemiological models to consider the effect of more than one environmental variable through a shared metric.

That is the reason why researchers from the Pennsylvania State University (USA) in collaboration with the University of Neuchâtel (Switzerland) decided to explore the relationship between the longevity of mosquitoes and their body size according to the larval environment factors (temperature and resource). Their results are published in Parasite & vectors [16]. More precisely, they supposed a change of the correlation between the adult body size and longevity depending on the larval environment and they investigated how the interaction of these factors may influence this correlation of the 2 life-history traits responding differently to these environmental factors in Anopheles. gambiae, the main vector of malaria in Africa.

To explore this question, they reared mosquito larvae at 3 different larval temperatures (21, 25 and 29 °C) and 2 food levels. Larvae were fed with a standard diet (water + fish food) or with 50% of this standard diet until the emergence of female adults which were allowed then, to take a blood meal before they define their longevity and their body size measuring their wing length.

Via different statistic models, they analysed:

The effect of the larval environment on the time of pupation of female mosquitoes regarding the larval diet and temperature and the interactions between these factors.
The relationship between the body size and longevity according to the larval environment  in 2 steps in order to measure:
a) the direct effects of the larval environment on body size and on longevity including temperature, food and their interaction as factors for both analyses.
As wing length was not affected by an interaction of food and temperature, they directly analysed wing length regarding the larval diet and the larval temperature without interaction between these factors.
b) the direct and indirect effects of the different environmental factors, via the body size, on longevity. They analysed this trait regarding the interactions between larval diet, larval temperature and wing length

For the first analysis, results showed that the mean time to pupation decreased with the increase in temperature, while it increased with a lower larval diet.

Concerning the body size, researchers showed that mosquitoes reared with lower food level had shorter wings than those with a standard diet. Moreover, the wing length decreased with increasing temperature. However, no significant interaction between temperature and food was noticed due to the similar effect of temperature at the two food levels.

In addition, longevity was not significantly influenced by the temperature, larval nutrition, or their interaction, despite this trait increased from 26,4 days for mosquitoes reared at 21°C and standard food to 30,6 days for those reared at 29°C and low food.

These data confirmed there were direct effects of food and temperature during larval development on body size. Indeed, wing length decreased with increasing temperature and lower food levels. While no proof of direct effects of food and temperature on longevity was shown,  researchers observed indirect effects within individual larval environments observing a significant three-way interaction between wing length, food and temperature.
The correlation between the two life-history traits ranged from positive to negative among different treatments confirming that the relationship between size and longevity is not limited to a positive correlation and that the interaction between environmental factors can affect this relationship. Indeed, the correlation was positive with standard food but negative with low food at 21°C whereas this correlation was negative with standard food but positive with low food at 29 °C.

Summary of the different correlations between the extrinsic and intrinsic factors tested in this experiment:

TemperatureResource availabilityBody sizeBody size x Resource availability x Temperature
Body size+NANA

The complexity of this relationship between these 2 life-history traits could be explained by 2 phenomena :

1. The different ways the temperature influences on the teneral reserves and wing length during larval development.

As shown in Aedes. albopictus,  the relationship between teneral lipids and body size is linear with warm temperatures during larval development and exponential at lower temperatures  [17]. Likewise, the relation between weight and wing length also varies with the temperature of the larval environment in An. gambiae [18].

2. The influence of the larval environment (temperature and resources) on the feeding regimes of adults (blood and sugar consumption), which could have a large effect on mosquito longevity.

For instance, it exists a positive correlation between the larval food levels and the blood meal volume in An. gambiae [19]. Moreover, the larval environment influences the mosquito body size which can affect the first meal choice of mosquitoes with smaller mosquitoes being more likely to take a sugar meal [20]. In addition, larger female mosquitoes accumulate reserves from blood meals more efficiently and need fewer blood meals to develop mature eggs [9].

Recently, Hatala et al. [21] showed that age and body size, as well as larval density and diet, affected the spermatozoid quantity produced by Ae. albopictus males.

Therefore, besides to study the influence of the interaction between larval environment and the mosquito body size on the vectorial capacity and on the female fecundity, it might be interesting to study the relationship between the body size and the sperm quantity produced by male mosquitoes in a context of interaction between different larval environment parameters, adding the one of the temperature, in order to improve rearing protocols for male releases used in control method such as sterile insect technique.

Moreover, it would be interesting to test these extrinsic and intrinsic parameters as well as their interactions in the context of insecticide resistance.

Anyway, further experiments are necessary to understand this complex relationship and explore the physiological basis of mechanisms underlying the changes of the direction of the correlation between the environment in order to model the evolution of the life history traits of mosquitoes, the transmission of mosquito-borne diseases, and the influence on vector control.

Thanks for reading.

And don’t forget: Fight Malaria!

Bibliography :

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