These are the mosquitoes that cause West Nile virus. Here’s how they find them
Recent weather patterns in Sacramento may lead to more mosquito activity than usual to start the spring, the region’s mosquito control district said.
Sacramento-Yolo Mosquito and Vector Control District warned in a news release this month that recent weather conditions have brought about an increase of mosquitoes, which hibernate in the winter.
“Dry sunny days coupled with stagnant water left behind from significant rain this winter make the perfect combination for mosquitoes to breed,” the district explained.
After one of the region’s rainiest Februaries ever measured, followed by temperatures almost 10 degrees above average last weekend, what can the region expect as far as mosquito activity in coming weeks?
District Assistant Manager Samer Elkashef said rain in late winter and early spring will likely cause an “overabundance” of mosquito breeding sources.
“We’re expecting to see quite a bit of breeding at a more widespread scale,” Elkashef said. “We’re going to have to stay on top of that.”
Control efforts by the two-county district include trapping, drainage of stagnant water and in the most extreme cases, aerial spraying.
District Manager Gary Goodman said in a statement that it’s too early to predict how severe this year’s mosquito season as a whole might be, or to speculate on West Nile virus activity. But the conditions are definitely here, he said, for mosquitoes to “grow and multiply.”
The district notes that “aggressive day biting mosquitoes” have already been reported. But these are a seasonal species that do not pose West Nile threat.
Elkashef said Sacramento’s mosquito season is most active in the warmer months – from May to October – when the species carrying West Nile virus begin to populate the area.
“Our program is definitely built around West Nile detection and control,” he said. “Potentially, this water is giving a jump start to the populations. Our role is to kind of limit that start, so that when West Nile season comes in more toward the middle of summer when temperatures are warmer, those populations are reduced.”
Two of the most common mosquito species that pop up in Sacramento in early spring are inland floodwater mosquitoes and wetlands mosquitoes.
Mosquitoes and flooding
Parts of Northern California, especially the north Sacramento Valley, recorded exceptional amounts of flooding during February’s atmospheric river systems, which dumped record rainfall in many parts of the state and prompted Gov. Gavin Newsom to declare a state of emergency in more than two dozen counties.
The U.S. Centers for Disease Control and Prevention, in a resource page focused on mosquitoes’ prevalence after hurricanes, explains that mosquitoes can appear in very large populations in the immediate aftermath of flood events. But disease-carrying mosquitoes tend to pop up weeks or months after the flooding itself.
“Mosquito eggs laid in the soil by floodwater mosquitoes during previous floods hatch. This results in very large populations of floodwater mosquitoes. Most of these mosquitoes are considered nuisance mosquitoes. In general, nuisance mosquitoes do not spread viruses that make people sick. The types of mosquitoes that can spread viruses may increase two weeks to two months after a hurricane, especially in areas that did not flood but received more rainfall than usual.”
Despite the obvious lack of a hurricane hitting California, most of Sacramento fits the bill as an area with no flooding but higher-than-average rainfall. The heaviest rains in Northern California came during mid-to-late February.
The effect of Northern California flooding on mosquito populations is indirect, Elkashef said.
“Mosquitoes don’t breed in any water that’s moving. One of the concerns we have with the amount of moisture we saw this season in tandem with the rain ... With these warmer rain showers that come through, that’s gonna melt the snowpack and create runoff.”
The runoff from Sierra snowmelt can affect river levels, which in turn leads to seepage, Elkashef added. That seepage is one more breeding source to worry about.
Floodwater mosquitoes don’t typically pose virus risks to humans, as CDC said, but Elkashef said they are a vector for canine heartworm.
Mosquitoes can develop in many different habitats, as long as there’s standing water. But Sacramento and Yolo counties have a united mosquito control district in no small part due to the Yolo Bypass, which is a large rice-growing habitat.
“Rice fields are just large bodies of pretty much stagnant water,” Elkasher said.
Mosquitoes that pop up in the wetlands and the pastures of Yolo County this time of year don’t carry West Nile, but can be “very aggressive biters,” Elkasher said.
The Sacramento-Yolo Mosquito and Vector Control District, the Yolo County Water Resources Association and more than a dozen other state and local organizations help manage the Yolo Bypass.
The state Department Fish and Wildlife’s says key factors in controlling wetlands mosquito population are infrastructure maintenance, management of water and vegetation. These reduce stagnant water resources, and also promote natural mosquito predators.
Neither floodwater nor wetlands mosquitoes in Sacramento and Yolo counties are primary carriers of West Nile virus. West Nile is mostly found in house mosquitoes (culex pipiens) and encephalitis mosquitoes (culex tarsalis), both of which are active in summer and fall.
What would West Nile risk mean?
As Goodman said, it’s still too early to tell what West Nile risk might look like in the region this year. The disease and the mosquito species that carry it require warmer weather.
However, the mosquito-borne disease saw a local uptick last summer, with the county leading California in activity by mid-July. By the end of 2018, 241 dead birds in Sacramento County tested positive for West Nile, according to the Sacramento-Yolo Mosquito and Vector Control District.
In late July, the district determined that efforts by the city of Sacramento to add algaecide to the Pocket Canal disrupted water flow, attracting greater mosquito activity.
The city utilities department said last year that the disrupted flow led water to back up into storm drain inlets. These basins are supposed to remain dry, officials said at the time; if they fill with stagnant water, they create an ideal breeding habitat for mosquitoes.
The result was aerial spraying of pesticide, which Elkasher says is a “last resort” that was also employed in 2016, but not in 2017. Last year’s spraying affected mostly Land Park, the Pocket, Elk Grove and nearby areas.
In 80 percent of infected humans, West Nile virus comes without symptoms, according to CDC. But about 1 in 5 people can develop symptoms including fever, fatigue, headaches, skin rashes and eye pain; about 1 in 150 develop a more serious illness that affect brain tissue and the spinal cord.
There is no vaccine or cure to treat West Nile virus in humans.
The silver lining, Elkasher explained, is that West Nile species of mosquitoes prefer birds, not humans, for dinner. One of the reasons the 2015-16 Zika outbreak that spread to Florida was so significant, he said, was that humans “were the preferred food source for those mosquitoes.”
The control district routinely sets traps twice a week during West Nile season as a means of surveillance.
“We deploy traps to catch host-seeking females,” Elkasher said. “Those are females that are looking for a blood meal.”
Elkasher says trapped mosquitoes are tested for three viruses: West Nile, Western Equine Encephalitis and another strain of encephalitis.
Neither strain of encephalitis has been detected in the Sacramento area; however, they are tested as a precaution, and the non-equine variety has been observed in parts of Southern California.
Tips to avoid bites
The Sacramento-Yolo Mosquito and Vector Control District provides a list of seven “D’s” for averting mosquito activity or avoiding bites. They are:
Drain standing water.
Avoid being outdoors during dawn or dusk.
Dress in long sleeves and pants when outdoors.
Defend yourself with repellent, especially those containing DEET.
Keep screen doors and windows in good shape to keep mosquitoes out.
Contact mosquito district personnel, in the event of additional mosquito issues, at 1-800-429-1022.
Common household mosquito sources include swimming pools, bird baths, roof gutters, irrigated lawns and fish ponds, the district says.
Elkasher said anyone experiencing an exceptional amount of bug bites at their residence can call the Sacramento-Yolo control district and ask for a free walk-through inspection.
CDC additionally warns against using non-Environmental Protection Agency-registered insect repellents. These have not been shown to be effective against bugs.