For the second time this summer the Allegheny County Health Department will release pesticides in pockets of the East End tonight to combat a burgeoning number of mosquitoes contaminated with West Nile virus.
Sixty-nine mosquito samples have tested positive for the virus in the county this year, which is one shy of the total number of mosquito samples found with West Nile in all of 2011.
"A small percentage of all of the mosquitoes that are tested have been positive, but that's higher than usual by a fair amount," said Ron Voorhees, acting director of the department. The percentage infected was not available Monday.
Zenivex, a pesticide that kills adult mosquitoes, will be released through a truck-mounted sprayer at a rate of 1 to 3 ounces per acre in the lower neighborhoods of Homewood South, Homewood Cemetery, Aberdeen Street in Edgewood and the Hunter Park section of Wilkinsburg from 8 to 11 p.m.
The pesticide is said to be harmless to people and pets when released in low doses.
The first mosquito in the area with West Nile virus was caught in mid-May, more than a month earlier than the first contaminated mosquitoes in 2011, health officials said.
In an interview last month, Bill Todaro, county entomologist, cited the reason for West Nile cases popping up in May as the early arrival of spring, which allowed adult mosquitoes to get out sooner than usual.
The pesticide was first released in the East End two weeks ago and the health department decided to order a second spraying when samples determined that there was still a viable adult population of infected mosquitoes.
Storm-water basins were also treated earlier this summer to prevent the mosquitoes from breeding in standing water, a prime environment for reproduction.
"Any place where water is collecting in or near a yard is a site where mosquitoes can reproduce, and we really recommend that people get out and try to limit the places where water can collect," Dr. Voorhees said.
Mosquitoes contract West Nile through infected birds and pass it to humans by bite. The virus is not passed to the insect's larvae.
No human cases have been reported this year, and only one man was infected last year.
"For the vast majority of people it is an extremely mild illness," said Dr. Voorhees. "Most people won't even know they have the illness."
Symptoms associated with mild cases of West Nile virus are similar to those of having a flu, such as a fever and aches lasting about a week as the immune system fights off the virus.
About one in every 150 people infected with West Nile develops encephalitis, a more severe form of the virus that can cause headache, fever, chills, lethargy, changes in behavior, sensations and cognition, and in rare cases lead to death.
Health officials ask residents to rid their yards of standing water, use insect repellent and avoid spending time outdoors at dusk and dawn, when mosquitoes are most active.
Concerns about potential mosquito breeding sites can be reported to the Allegheny County Health Department at 412-687-ACHD.
Staff writer Rob Wennemer contributed.