Zika-Spreading Mosquitoes Are Becoming More Resistant to Common Pesticides

— -- Health officials working to contain Zika outbreak in northern Miami are now investigating whether the Zika-spreading mosquitoes have become resistant to common pesticides used to combat the insect -- as studies suggest has been the case in other parts of the world.

Currently, 14 people have been found to be infected with the Zika virus from mosquito bites in a 1-square-mile area in northern Miami, according to the U.S. Centers for Disease Control and Prevention. Mosquito control measures were implemented after four people were found infected with Zika in July, but health officials said on Monday those initial mosquito control measures were not enough.

The Aedes aegyti mosquito has been called a "cockroach" mosquito for its ability to live indoors and reproduce even in tiny pools of water. The insect is the primary way the Zika virus is spread, although the disease can also be transmitted through sexual contact.

"Aggressive mosquito control measures don't seem to be working as well as we would have liked," CDC Director Tom Frieden told reporters on Monday after 10 additional Zika cases were announced. He pointed out that it was unclear if the insects themselves are biologically resistant to the chemicals used in common insecticides used or if there were other environmental factors like standing water that was not visible that lead to the mosquito population bouncing back.

Frieden said an expert was investigating the mosquitoes to test if they are genetically resistant to pesticides but that it could take weeks to get the findings.

The case has highlighted a problem health departments and mosquito control districts have dealt with for years: some mosquito species are becoming resistant to pesticide. In the Florida Keys, the local mosquito control district has been looking at new ways to diminish mosquito populations as some of the 46 species present have become more resistant to pesticide. They have even considered a test run of genetically modified mosquitoes near Key West.

Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center, said the Aedes aegypti mosquito is especially hard to combat for multiple reasons.

"There's a history of Aedes being relatively resistant to conventional pesticide," Schaffner said. "When we say they're resistant that means the mosquito inherently can shrug off the pesticide."

Research on why the mosquitoes are so resilient have found the insects may have some genetic mutations that help them survive. Scientists in Thailand found that the Aedes aegypti mosquito had genetic mutations that made the pesticide less likely to bind to them, according to a 2016 study published in Parasites & Vectors. A bacteria called Bacillus thuringiensis israelensis used as larvicide has been found to be less effective on multiple mosquito species, including the Aedes aegypti mosquito, according to a 2014 study published in BMC Genomics.

Schaffner explained that when the initial pesticide stops working, mosquito control is forced to turn to other chemicals that are not as ideal for use in a public area.

"They are wonderfully biodegradable, they can be dispersed in fine mist ... and they're very, very safe," he explained. "When we have to start using others, there are sometimes issues."

Some secondary pesticides may be more likely to cause irritation, he noted.