Last year’s outbreak of the West Nile virus was not only the largest Denton has seen in many years, but it also affected far more younger people than in the past, officials said Monday.
The city’s committee on the environment studied Denton’s mosquito response plan over the past several months, and the City Council informally agreed to its recommendations during a work session Monday. During the discussion, Ken Banks, director of environmental services, reported on the committee’s work, which tried to balance the competing interests of mosquito control and the impact of such control measures on the environment.
Perhaps the most significant change was striking the reference that people age 50 and older were more likely to become sick from a West Nile infection. That was not the case in 2012, Banks said.
“We had a larger number of younger people showing both the symptoms and the neuroinvasive form of the disease,” Banks said.
Of the 21 people who were diagnosed with West Nile fever in Denton last year, the average age was 54. Eight people, or 38.1 percent, were 50 or younger. Of the 15 more serious cases of West Nile neuroinvasive disease, five people were 25 or younger. Another seven were 25-50 years old.
In late 2012, the Denton County Health Department denied a public information request by the Denton Record-Chronicle for basic demographic data of West Nile victims, citing health privacy laws. The Texas Attorney General’s office upheld the decision after the Record-Chronicle appealed the denial.
Banks told the City Council there had been some discussion that the 2012 outbreak could have come from a new mutation of the virus. A November 2012 story in The Washington Post cited concerns from doctors experienced in treating West Nile who were seeing damage to different areas of the brain than they had seen before, including those that govern speech, language and cognition.
A Houston-area virologist told the Post that samples from birds and mosquitoes showed signs of genetic change in the virus.
Denton first adopted a plan to monitor and respond to mosquitoes in 2003.
This year’s changes reflect the first modifications to the plan since its implementation, Banks said.
Some of the recommendations reflect changes that already were essentially in effect, Banks said. For example, the city no longer looks to the number of horses or birds testing positive for West Nile to shape its response plan.
Most horse owners have their animals vaccinated against West Nile, and state labs no longer test birds.
When enough mosquito traps test positive for West Nile, Denton also will add another pesticide it will use to kill mosquito larvae. The original plan called for city crews to spread broad-spectrum, oil-based larvacides in infested areas, but the city will use Spinosad instead, Banks said.
Similar to the Bacillus thuringiensis israelensis, or BTI, briquettes, Spinosad is organically derived.
The pesticide is more toxic than BTI, Banks said in an interview after the meeting, but would only be applied aquatically to kill larvae, which limits its impact elsewhere in the environment.
The city also will stop capturing and redistributing mosquito fish, which eat the larvae, because crews are finding that area ponds and streams appear to have an indigenous population, Banks said.
While the city will be ready to respond more quickly if the risk reaches upper levels, the city won’t change the trigger for targeted, nighttime ground spraying for adult mosquitoes.
The committee used the 2012 data both from mosquito traps and reports of human cases to study whether ground spraying at different thresholds would have had different results. Although the data were limited, Banks said it showed that the current trigger — ground spraying after two human cases and/or three consecutive positive mosquito traps — appeared to best balance the need to control mosquitoes and minimizing the impact of the spraying.
Mayor Mark Burroughs encouraged Banks and the city staff to find ways to work with homeowners associations to help distribute the BTI briquettes.
Council members asked Banks whether the city’s response plan would ever include a trigger for aerial spraying. The Centers for Disease Control and Prevention offered aerial spraying last year when the Texas outbreak was at its peak, and while the county and many area cities accepted the offer, Denton opted out.
Banks said he could not envision a scenario in the city that would trigger the need for an aerial response.
While state labs begin accepting mosquitoes for testing in May, Banks told council members it was about time for residents to begin thinking about what they can do in their backyards.
Neighborhoods with older drainage infrastructure and newer neighborhoods with a propensity to irrigate during the drier periods seem to be the most prone to creating mosquito habitats, he said.
“That’s [irrigating] become a problem,” he said.
PEGGY HEINKEL-WOLFE can be reached at 940-566-6881 and via Twitter at @phwolfeDRC.