Conventional Vector Control: Evidence it Controls Arboviruses
Scott Ritchie and Gregor Devine
from: Arboviruses: Molecular Biology, Evolution and Control (Edited by: Nikos Vasilakis and Duane J. Gubler). Caister Academic Press, U.K. (2016) Pages: 281-290.
Arboviruses cause significant human morbidity and mortality. Most arboviral infections for which a vaccine is not registered are controlled by managing the mosquito vector. Unfortunately, 'evidence based' studies that provide proof that this impacts infection or disease are almost nonexistent. Many arboviral diseases, especially encephalitides such as St. Louis encephalitis and Murray Valley encephalitis, are so rare in time and space that sentinel animals and mosquito infection rates must be used as proxies for disease activity. Thus, the impact of vector control interventions on mosquito populations is often the only measure of success we have. That effect is assessed using a continuum of investigations that range from simple bioassays and small plot/semi-field trials (does the intervention kill the target vector?) to far larger and rarer field trials (does the intervention reduce populations of the target vector?). The logistics and cost of these larger trials restricts their uptake but they are a basic requirement for demonstrating operationally useful impacts. These evaluations (including those that measure impacts on the aquatic habitat) must demonstrate population level effects on adult vectors over significant scales. They must compare treated and untreated control areas under statistically robust designs. Of all the arboviral diseases, dengue is by far the most prevalent and widespread. Its transmission patterns are complex and driven by climate, herd immunity, a superbly adapted mosquito, and modern patterns of human movement. Despite its importance, just as for rarer arboviruses, myriad studies show the impact of dengue control strategies on mosquitoes but few demonstrate an effect on the transmission of the virus. This chapter highlights the difficulties of dengue management in modern urban environments and considers why it is so hard to emulate the modern day successes of many malaria control programs. It discusses whether an emphasis on managing the aquatic habitat is a historical hangover from the well-funded, vertically-managed eradication programs of the 1900s and notes the paucity of trials that support investment in any existing vector control tool. The limited control options and operational funds available suggest that the old paradigms of dengue prevention and eradication are no longer practicable and need to be augmented by more targeted but less ambitious outbreak responses that focus on the few tools that might justify the expense of deployment read more ...