Citywide Mosquito Management Effort in Puerto Rico During Zika Outbreak Offers Lessons for Future
The 2016 outbreak of Zika virus in the Americas pointed a spotlight at the state of the global public health community’s capacity to respond to vector-borne disease. Various government agencies, non-government organizations, and other stakeholders jumped into action, and lessons learned from their collective efforts are informing the development of new response plans for the next such event.
One of those efforts was an integrated vector management (IVM) program implemented across Caguas City, Puerto Rico, by the U.S. Centers for Disease Control and Prevention to reduce the local population of Aedes aegypti mosquitoes and transmission of Zika virus (along with dengue and chikungunya). In a city of more than 140,000 people, it was one of the largest coordinated IVM programs ever undertaken, according to a retrospective examination of the results published in February in the Journal of Medical Entomology.
In a previous, smaller-scale studies, researchers had found that placing mosquito traps known as autocidal gravid ovitraps (AGOs) at a density of three traps per home in the yards of most houses in a community could reduce the number of female adult Ae. aegypti mosquitoes caught per trap to two to three per week—a number that was then associated with lower incidence of Zika and chikungunya in field-collected mosquitoes.
In Caguas in 2016, the team led by Roberto Barrera, Ph.D., entomology and ecology activity chief at the CDC’s Division of Vector-Borne Diseases, sought to reach the same threshold and were indeed successful. Launching the program in October 2016, they reduced the number of adult female Ae. aegypti mosquitoes caught weekly per trap to roughly two by the following March, a level that was maintained through the end of the program in August 2017.
“The public health intervention described here to control an ongoing Zika epidemic showed entomological impact, by reducing the density of Ae. aegypti to levels that were protective against [chikungunya] infection in a previous, smaller scale study of mass AGO trapping,” Barrera and colleagues write in their report. “The results from this investigation showed that such a threshold can be attained in a middle-size city such as Caguas.”
Getting there, though, was no small task. Barrera and colleagues began with engaging 50 community leaders to build buy-in among residents for the vector-management efforts and to facilitate relations throughout Caguas between residents and vector-management personnel. They also conducted several town-hall meetings with a wider group of community leaders and provided information to Puerto Rico’s 311 call center for explain the effort to concerned residents.
Then, traps were deployed on more than 20,000 properties across the city, with an average of 3.3 traps per property (total 78,126 traps). Three-hundred sixty of these traps were surveillance traps that were monitored once per week. Additional activities included distributing educational handouts to residents, removing or dumping small containers of standing water, and applying larvicides in other water containers not used for pet or human consumption. Completing the work required as many as 100 hired personnel each month, and the city was broken into eight zones, which were treated one at a time in random order between January and June 2017. (This approach, known as a cluster randomized stepped wedge method, was useful both for practical purposes—to allow a limited number of personnel to implement the IVM program over time rather than across the entire city at once—as well as for ethical reasons: Leaving some zones as untreated controls in the presence of an ongoing Zika outbreak was not an acceptable option.)
Barerra’s team monitored the surveillance traps weekly through August 2017, and they also tested captured mosquitoes for presence of Zika, dengue, and chikungunya viruses. Analysis of the data showed that mosquito presence stabilized at about two adult females per trap per week when about 60 percent of properties in a zone were treated, and it fell below two per week when 80 percent were treated. “These results suggest that vector control coverage does not need to reach 80 percent to effectively reduce Ae. aegypti density to low and stable levels in this city, which can substantially reduce costs,” the researchers note in their report.
Because the IVM program in Caguas took place late in the outbreak of Zika, the researchers say they can’t make definitive conclusions about the program’s effect on virus transmission to humans, but they take the threshold of mosquito presence reached—linked to reduced transmission in previous studies—as a positive sign.
Barrera and colleagues emphasized in their report the importance of the integrated, multifaceted approach to vector management. “This investigation did not attempt to isolate individual effects of community outreach, education, source reduction, larviciding, or mass trapping. Rather, we pursued evaluating the impact of this integrated approach. … One can argue that the importance of using integrated vector control approaches is to compensate for deficiencies of individual vector control tools,” they write.
The program also sheds light on challenges likely to be faced in similar efforts elsewhere, such as the number of field personnel needed and the time required to train them. But, as mosquito-borne diseases continue to make their presence known, this effort in Puerto Rico in 2016 will nevertheless stand as an example to follow in future outbreaks.
Journal of Medical Entomology