Disease-Carrying Mosquitoes Reach "Critical Limit"; Cities Ponder Aerial Spraying

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With Zika at the forefront of the news, everyone's thinking about mosquitoes. The good news: There have been no local mosquito-borne transmissions of Zika virus in Texas to date, according to the Centers for Disease Control and Prevention. The CDC has identified 16 travel-associated cases of Zika in Dallas.  

The bad news is that West Nile Virus, another mosquito-borne disease that reached epidemic levels in Dallas in 2012, is back. So far this year, there have been six confirmed human cases in the county.

After Dallas County Commissioners authorized the use of aerial spraying to combat mosquitoes carrying West Nile Virus last week, cities across the county will have to decide whether or not they want to use the option. Over the next two weeks, the Department of Health and Human Services will oblige cities that decide they want the aerial spraying, said Zachary Thompson, DCHHS Director.

Though aerial spraying is usually referred to as a “last resort” after ground-spraying and larvicide are proved to be ineffective, Thompson said there is no emergency at this time. The option for cities to use aerial spraying is a “proactive measure,” he said.

However, at the beginning of this month, a disease index used by the county and the CDC had reached a “critical limit,” the same level during the 2012 outbreak. This figure is why Dr. Michael Merchant, Professor and Extension Urban Entomologist at Texas A&M AgriLife Extension Service, suspects Dallas County Health Department requested permission for aerial spraying. 

The index, called the Vector Index, is a combination of estimates of mosquito populations and percentage of possible mosquito vectors being trapped and found infective. In the first report of this month, the index had reached a level of 0.5 (and 0.6 in Tarrant County), a critical level, said Merchant.

“Most of the illnesses and deaths in Dallas County in 2012 occurred after this threshold had been passed,” said Merchant.

Each city will analyze their trap data over the next couple of weeks to decide whether or not they will do the spraying. Thompson emphasized that the vectors are often fluctuating and stressed that this was simply a preventative measure.

However, the efficacy of aerial spraying in killing WNV-carrying mosquitoes is often debated. Lack of understanding of long-term impacts of spraying on human health and the environment are also issues that citizens will need to take into account when going to their elected officials with their opinions on whether or not to spray.

The CDC and the U.S. Environmental Protection Agency say aerial spraying is the “the one method that can rapidly reduce the number of mosquitoes spreading disease over a large area,” wrote Robert Daguillard, an EPA agency spokesperson, in an email.

However, the final report from the CDC about the efficacy of aerial spraying in the 2012 epidemic is inconclusive. Though the report states that aerial adulticide spraying for WNV control had “a measurable impact” in preventing WNV neuroinvasive disease, the report also acknowledged that there was little or no meaningful change in mosquito abundance when the treatment zones were compared to the untreated areas.

Additionally,  in some treated areas reported “small but statistically significant increases in mosquito abundance.” 

The report concluded that the “reduction of human disease incidence in the aerially-treated areas supports the contention that the aerial spray applications reduced the abundance of the older, infectious mosquito population, resulting in a reduction of human infection risk, even though decreases in abundance were not documented.” This was concluded despite the fact the spraying occurred at a time when WNV was on the verge of natural decline. 

Yet many experts still say spraying is the most effective way to exterminate adult  mosquitoes. 

“Spraying from a plane is a more effective way to reach these mosquitoes compared to truck [ground] based spraying because it lands on the tree canopy first,” said Merchant. “It is less effective than ground spraying for the human-feeding Aedes mosquitoes [which carry the Zika virus], because they tend to lurk in lower sites.”

All in all, the analysis conducted by the CDC makes it hard to come to concrete conclusions about whether or not aerial spraying really makes a difference, as Jim Schutze writes in more detail in his 2013 article about the report.

Without the research to fully back it up, the cost of aerial spraying can seem daunting. In 2012, Dallas county spent $1.6 million on aerial spraying and the direct and indirect costs of WNV infections that summer were over $8 million, not to mention the long-term impacts on health of the virus victims.

In the past two weeks, however, the vector index has dropped below the critical point.

Thus, if aerial spraying is a “last resort,” as Thompson had said it was, then cities need to consider carefully whether or not to go through with the spraying immediately. 

Despite the fact that CDC, EPA and DCHHS studies found little to no short-term human health impacts in aerially sprayed communities, some say that the lack of studies analyzing the long-term effects of aerial spraying. This is particularly true of more vulnerable populations like children and the elderly.

“The amount of insecticide used in aerial spraying is so low (about 0.5 to 2 fl oz/acre) that the risk to most insects other than night flying mosquitoes is negligible for birds, fish, honey bees, butterflies and most insects that shelter under plants,” said Merchant. “Mosquitoes are exquisitely sensitive to these insecticides due to their small size and the fact that they are flying at the same time as the insecticides are floating in the air.”

The main target of WNV spraying is mosquito species Culex quinquefasciatus, which is principally a bird feeder. It feeds on birds in nests or roosting in trees, said Merchant. The spraying occurs after 9 p.m., when planes fly at low altitudes dropping the pesticide over designated areas.

Still, there remain no studies about the long-term impact of aerial spraying, though it has been shown pesticides can affect brain development in children. Symptoms not showing up for decades. 

As of now, the only option provided by DCHHS for cities to up the fight against West Nile Virus is choosing aerial spraying.

“I think (aerial spraying) is the dumbest thing you can possibly do,” said Howard Garrett, also known as The Dirt Doctor and an advocate for organic methods of fighting mosquito-transmitted diseases.

“There are much better techniques that work to actually control the mosquitoes,” Garrett said. “Most mosquito larvae are in protected containers, things like old tires and drainage lines and sewage lines and gutters on houses, a lot of which are covered up. Mosquitoes breed in wet mulch and that's why we recommend spraying liquid botanical, it's called BTi (Bacillus thuringiensis ‘Israelensis’ ).”

For aerial spraying to work, it needs to touch the adult mosquitoes directly. 

“(Bti) gets into those various kinds of breeding places a whole lot more effectively and when you get the homeowners to do it as well as the city, county, all using Bti, we can really make an impact on mosquitoes,” Garrett continued. “If anything, spraying makes the mosquito population worse because it ends up killing the beneficial pollinators.”

Garrett himself puts out barrels of water in his yard, which he allows to stagnate in order to attract mosquitoes and their larvae. He then sprays BTi and botanical larvicide to kill the larvae before they reach adulthood.

The aerial spray that DCHHS uses, called Duet, a pyrethroid insecticide, targets only adult mosquitoes, not larvae and is toxic to aquatic organisms and bees. In large doses, it can affect the nervous system, according to EPA studies.  The label states it is harmful to both humans and domestic animals if swallowed.

“Cities and county health departments run year-round programs to locate and eliminate or treat mosquito breeding sites, but despite our best efforts the problem is with us this year,” Merchant said. “The fact is that we can’t control today’s adult, disease-carrying mosquitoes (the ones health departments trap weekly) by targeting the larvae. That would be an ineffective action.”

Garrett would disagree, but says if spraying is the only option, use non-toxic organics.

“If you wanna spray something that kills them, you can spray non-toxic organic spray that has just as much killing power as the chemicals that the county and city sprayed last time,” Garrett said. “They're basically essential oil products. They also will kill beneficial insects, but the point is if you've got to spray something, you want to make the public feel you're doing the right thing, you could at least do something that's not toxic to people, wildlife, pets, livestock and all that.”

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