Tag Archives: West Nile

Septic Tanks: urban breeding grounds for virus-carrying mosquitoes

Contributed by Guest Blogger: L. Herrera-Torres ’14

Like in several other landforms in tropical regions, Puerto Rico is victim to seasonal increases in the Dengue fever and West Nile Virus, which are transmitted via the mosquitoes Aedes aegypti and Culex quiquefasciatus respectively. In order to test whether or not active septic tanks with raw sewage provide an adequate environment for the development of mosquitoes (particularly Aedes aegypti) and therefore aid in the spread of these diseases, a test was conducted in a southern municipality of Puerto Rico, called Salinas. In the community of Playa-Playita 89 septic tanks with varying structural integrity and water quality were sampled for the presence and abundance of mosquito larvae using floating funnel traps and 93 septic tanks were tested for the presence and abundance of adult mosquitoes using screened, plastic emergence traps.
Predictably, Culex quinquefasciatus, the vector of West Nile virus, which has been proven to thrive in polluted waters, was found in 74% of the septic tanks in larval form and in 97% in adult form. However, the results of vector for Dengue fever (the main focus of the experiment) were more surprising.
Previously Ae. Aegypti was known to be well adapted to urban areas and were often found in artificial containers, but it was still generally accepted that these larvae developed in clean water. However in 18% of the septic tanks sampled revealed that Ae. Aegypti was present in this water despite its contamination and had a positive association with the cracking of septic tank walls, uncapped tanks, and larger tank surface area. Similar results were found for Ae. Aegypti adults. 49% of the tanks showed both their presence and abundance as well as their positive correlation with cracking, uncapping, and septic water pH. The correlation between the amount of larvae collected from the septic tanks and the amount of adult mosquitoes recorded strongly insinuates that this environment is conducive to mosquito reproduction and development and is not just a resting place as others have suggested.
These findings led the researchers to believe that Ae. Aegypti can develop in sewage water and that septic tanks provide ideal conditions for mosquito productivity and can serve as potential to maintain dengue transmission during the dry season.