Vector control officials are reminding the public to take precautions against mosquito bites after a recent upswing in the number of mosquito samples testing positive for West Nile virus locally and statewide.
Turlock Mosquito Abatement District and San Joaquin County Mosquito Vector Control District are both reporting steep increases in mosquito samples that have tested positive for West Nile Virus. Plus, Merced County officials said they recently found that area’s first positive West Nile virus mosquito samples of the year, in addition to a sentinel chicken that tested positive for the virus in Los Banos.
West Nile virus can be transmitted to humans through mosquito bites, and is some cases the disease can be fatal. About 20 percent of people infected with West Nile have a fever, headache and body aches lasting for weeks. A more rare, life-threatening neurological disease can result in tremors, convulsions, paralysis and long-term disabilities.
Monica Patterson, vector biologist with Turlock Mosquito Abatement District, which covers Stanislaus County south of the Tuolumne River, said her agency this year thus far has reported 48 mosquito samples, pulled from different sites, that have tested positive for West Nile. That compares to five positive samples pulled last year for the period Jan. 1 through July 20. Patterson’s agency pulled a total of 76 positive mosquito samples in 2015. “That’s a significant increase,” Patterson said.
Statewide, the number of positive West Nile mosquito samples has also gone up significantly, according to the state’s West Nile virus website. With 30 counties reporting, a total of 896 mosquito samples through July 15 this year have tested positive, compared to 497 mosquito samples statewide that tested positive during the same period last year, according to the website.
Patterson said the recent higher level of rains and warmer spring temperatures – followed by temperature spikes – encouraged mosquito development. The virus itself also fares better in warmer weather.
Similarly, San Joaquin Mosquito Vector Control District reported 23 mosquito samples that tested positive for West Nile from July 10 through July 16, compared to only two positive samples during the same period last year. The week prior, the district reported 21 mosquito samples that tested positive for West Nile, compared to eight during the same period last year. Aaron Devencenzi, public information officer for San Joaquin County Mosquito and Vector Control District, said this year’s numbers for San Joaquin County have been the highest concentration of West Nile since 2004. Patterson said the same is probably true for her district.
“Because of the high virus (levels) we are finding throughout the county, it’s important for people to protect themselves from being bitten,” Devencenzi said.
Lloyd Douglass, manager of East Side Mosquito Abatement District, which covers Stanislaus County north of Tuolumne River, said his agency saw more mosquitoes testing positive for West Nile Virus toward the end of June, although those numbers have decreased to normal levels since. Douglass said he meets regularly with vector control officials in neighboring districts to exchange information. "The virus is a community problem and everyone has to help take care of themselves, eliminate sources and call us if they notice an uptick in the mosquito population,” Douglass said.
Merced County officials have also noticed recent West Nile activity. Allan Inman, Manager-Entomologist at Merced County Mosquito Abatement District, said a sentinel chicken in the Los Banos area tested positive for the virus Thursday. Mosquito samples pulled Friday from two pools in rice-growing areas also tested positive for the virus. Officials also trapped a few positive mosquitoes in North Merced.
Thus far, no human cases of West Nile have been found this year in Merced County. One human West Nile case was reported in Merced County last year.
Inman said early intervention and using planes to spray has helped cut down on the mosquitoes, although he warned “things are really ripe to explode” because of the wetter spring and more water on the ground (mosquitoes tend to multiply in standing water). Inman said students from UC Merced recently received approval for labs to assist with testing for West Nile virus mosquitoes. Although the UC Merced students will do their testing independently, they will share data and information with the county. “That should double our capacity,” Inman said.
An additional concern for local vector control officials is the Zika virus, also carried by mosquitoes. However, the Aedes aegypti and Aedes albopictus mosquitoes known to carry the virus haven’t been found in Merced, Stanislaus or San Joaquin counties. Aedes aegypti mosquitoes were found in Madera and near Fresno in 2013.
Earlier this year a 19-year-old Stanislaus County man contracted Zika while traveling outside the U.S., although no human cases in Stanislaus County have been reported.
The Zika virus results in a minor illness for some of those infected, but infected pregnant women may give birth to babies with microcephaly. The birth defect is characterized by small skulls and underdeveloped brains.
Patterson said her agency has been actively setting traps for the mosquitoes known to carry Zika for the past four years. Other neighboring agencies have led similar efforts to prevent Zika. “We are still looking, we are aware of the potential threat and have been aware for quite awhile,” Patterson said.
Inman noted UC Merced students will also look for Aedes aegypti, the mosquito known to carry the Zika virus.
Tips for reducing West Nile Virus risks:
- Eliminate all sources of standing water on your property. Standing water typically supports mosquito breeding.
- Apply insect repellent containing the ingredient DEET or Picaridin when outdoors, according to label instructions.
- Avoid spending time outside when mosquitoes are most active, at dawn and duck, and especially for the first two hours after sunset.
- When outdoors, wear long pants, long-sleeved shirts and other protective clothes.
- Keep mosquitoes out of your home by using tight-fitting screens on doors and windows.
- Contact your veterinarian for information on vaccinating horses against West Nile Virus.
- Report large mosquito infestations to East Side Mosquito Abatement District 209-522-4098, Turlock Mosquito Abatement District 209-634-1234, San Joaquin Mosquito and Vector Contol District 209-982-4675, and Merced County Mosquito Abatement District at 209-722-1527.
* source: San Joaquin Mosquito and Vector Control District