Experts Agree: Vector Control Still Best for Controlling Zika Virus

Last month, US Centers for Disease Control and Prevention (CDC) director Thomas Frieden, MD, MPH, and other health officials credited pesticide spraying—or “vector control”—with ending the outbreak of Zika virus in Miami’s Wynwood neighborhood.

But while the coordinated spraying efforts did significantly reduce the numbers of Zika-carrying Aedes aegypti mosquitoes in the area, activists and local residents have raised concerns about the impacts of the chemicals that were used, Naled and Bti, on the environment as well as potential effects on human health. The issue is front and center again now that a new case cluster of Zika has emerged in Miami’s “Little Haiti,” and new cases are still being identified in nearby Miami Beach. To date, there have been more than 150 cases of Zika linked to “local” mosquitoes reported in South Florida.

“At a human population level, there appears to be considerable evidence that the rewards of vector control [with Zika] are greater for human health than the risks, as is the case for most vector-borne diseases,” public health researcher Paul Parham, PhD, told Contagion. Dr. Parham, who lectures at the University of Liverpool (UK), has published research on the environmental impact of vector control. “But there are clearly other factors to take into account such as the effects on other local ecosystems.”

Indeed, bees have already been “collateral damage” in the fight against Zika virus. A report in Time magazine in August indicated that some 2 million bees in Dorchester County, South Carolina alone were killed as a result of anti-Zika pesticide spraying. And bees are far from the only concerns. Hundreds of angry Miami residents turned out for a “town hall” meeting in September saying that they’d rather risk being exposed to Zika-carrying mosquitoes than droplets from aerial sprays. They believe the flu-like symptoms associated with the mosquito-borne virus pose less of a threat than exposure to high concentrations of Naled. At-home use of Naled is currently banned in the United States, with high levels of exposure associated with side effects such as nausea, dizziness, and confusion. Officials have assured local residents that Naled is only being used at “safe levels,” and there is no evidence exposure can lead to serious illness. Still, Puerto Rico’s governor has refused to allow the pesticide to be sprayed there due to the potential risks, despite the fact the island has had more than 20,000 confirmed cases of Zika to date.

Concerns aside, both Miami-Dade and Broward County, Florida are still treating mosquito-heavy areas such as Little Haiti with larvacide, using truck-mounted turbines to spray the chemicals. Officials there told ABC Miami recently that these turbines are more efficient than handheld sprayers.

Unfortunately, experts such as Dr. Parham note that vector control is likely the safest and most effective option in the fight against the Zika virus, particularly in the absence of an effective vaccine and/or drug treatment. However, other approaches are being used. According to a Science magazine report on October 17, officials in Piracicaba, Brazil, have been experimenting with OX513A, a transgenic Aedes aegypti mosquito strain manufactured by biotech firm Oxitec that controls populations by passing its lethal gene, tTAV (tetracycline-repressible transcriptional activator variant), to offspring. Meanwhile, Niterói and Rio de Janeiro are using another specially bred strain of mosquitos, infected with a bacterium called Wolbachia pipientis, manufactured by the nonprofit Eliminate Dengue. Research suggests both genetically modified mosquito strains are safe; however, Florida residents have blocked the use of Oxitec’s product there so far. Hence, spraying remains the method of choice—for now.

“Overall, I would say that the aggressive use of vector control in [Miami] was justified by the epidemiological evidence and the desire to curb onward transmission before the attack rate become more significant—and the outbreak harder to control,” Dr. Parham said. “In the absence of vector control, we currently have very few tools to mitigate the current Zika epidemic and the alternative of not responding is not really an option.”

Brian P. Dunleavy is a medical writer and editor based in New York. His work has appeared in numerous healthcare-related publications. He is the former editor of Infectious Disease Special Edition.