by Shannon O’Neill
Climate change in Latin America and the Caribbean (LAC) has impacted precipitation and temperatures, which have been associated with increases in seasonal outbreaks of dengue fever. However, such correlations are often speculative due to the complexity of interactions involved in vector-borne diseases. Researchers Chadee and Martinez (2015) focused on the adaptive behaviors of the Aedes aegypti mosquito in efforts to fill some of the research gaps typically associated with the research of these diseases. This mosquito is a successful vector for various vector-borne diseases, including dengue fever, Zika virus, and chikungunya, and has shown adaptive behaviors. This research will provide the information to create better vector control strategies that can be applied in order to limit climate change impacts on the resurgence of these diseases.
Dengue fever, one of the viruses transmitted by the Aedes aegypti, threatens forty percent of the world’s population and is considered to be the most common arboviral disease in the world. The surge in this virus has been attributed to the asaptive breeding patterns of A. aegypti. Climate change in LAC countries has created water scarcity problems due to unpredictable rainfall, largely increasing the need for water storage units. A. aegypti has adapted to breed in artificial containers, therefore this increase in water storage has created the ideal breeding grounds for this mosquito to repopulate at greater rates.
Chadee and Martinez specifically wanted to better understand the influence of light conditions on this mosquito’s breeding patterns. This is important, as this mosquito has been known to infiltrate underground water systems. The field component of this study first took a thoroughly entomological survey for breeding sites of A. aegypti both inside and outside of buildings. Only wet containers that contained larvae and pupae were recorded. The survey also included inspecting underground sewer systems and septic tanks. Data loggers recorded the temperature of the logged containers every ten to fifteen minutes throughout the study. The containers were examined twice a day to determined mortality and emergence rates.
The laboratory component involved an A. aegypti strain obtained from Spain. The mosquitoes were kept in a room with a consistent temperature of 26°C with humidity levels ranging between 75-80%. Females obtained controlled blood meals, and then laid eggs in small tubs lined with germination paper. The eggs were incubated for forty-eight hours until they hatched. The mosquitoes and larvae were held in three different light conditions, continuous light, continuous darkness, and diurnal conditions, in order to determine how these conditions impact its life cycle.
The results of the field study found that drums, buckets, tanks, and septic tanks were the largest producers of female mosquitoes. Larvae and pupae found in plastic and metal cups, brick-holes, and eaves-gutters exposed to direct sunlight had a hundred percent mortality rate. Furthermore, there was a significant positive correlation between direct sunlight and pupae/larvae mortality. This provides insight as to why mosquitoes may be seeking underground breeding habitats.
The laboratory studies showed that the mosquitoes were capable of surviving under each of the conditions. Those in the dark containers developed at a quicker rate, but they also died sooner. The larvae under the continuous light conditions developed a little more slowly but lived as long as those in the natural, diurnal conditions, the control in this experiment.
The results from both components of the study display the behavioral plasticity of the A. aegpyti which contributes to its success. Such adaptability allows the mosquito to change its breeding sites dependent on weather conditions. For example, a previous study had shown that there had been a significant shift in the breeding sites from drums to underground septic tanks. This can be attributed to the fact that drums were associated with higher temperatures and therefore a greater mortality rate, causing the mosquito to look for better, cooler habitats underground. However, there may be a cost in such transformation of breeding habitat, as the laboratory experiments displayed a shortened life span in continuous dark conditions. Finally, warmer temperatures can also drive this mosquito to seek out cooler temperatures inside. Such habitats are ideal for the mosquito due to the readily available blood meal from humans.
The transition of A. aegypti from above ground containers to underground septic tanks explains why this mosquito is so hard to control. The researchers recommend several strategies in order to effectively manage this problem. First, underground areas need to be added to the search area when monitoring mosquito infestation. Polystyrene beads should be used in septic tanks to cover the water surface and prevent the larvae from getting the oxygen they need to develop. Additionally, holes that give mosquitos’ access to these areas should be repaired in order to keep the mosquitoes out. Finally, generic mosquito control strategies such as introducing sterile male mosquitoes, or transgenic mosquitoes with dominant lethality can be released to prevent repopulating.
The findings of this study conclude that the A. aegypti are successfully adapting to climate change. The results create a better understanding of how these dynamics are occurring, in order to effectively manage future outbreaks. Most of the strategies presented involve controlling the repopulation of the mosquito in underground breeding sites. Finally, more information is needed to develop plausible scenarios of how to effectively manage this mosquito as it continues to adapt to its changing environment.
Chadee,D, Martinez, R. 2015. Aedes aegypti (L.) in Latin American and Caribbean region: With growing evidence for vector adaptation to climate change? Acta Tropica 156, 137–143