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VOL. 128 | NO. 96 | Thursday, May 16, 2013

West Nile Warning

County sees earliest ever occurrence of mosquitoes infected with virus

By Jennifer Johnson Backer

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As summer approaches, Memphians who enjoy being outdoors in the evening might want to consider taking protective measures.

Glenn Williamson of Vector Control tops off the tank on an Ultra Low Volume (ULV) sprayer. Nine spray trucks mist for mosquitoes throughout the city four days a week for three hours after sunset.  

(Photos: Lance Murphey)

The Shelby County Health Department has detected mosquitoes infected with West Nile virus in Memphis, Bartlett, Germantown, Collierville and parts of unincorporated Shelby County – the earliest occurrence of positive West Nile Virus pools on record.

“It is unusual,” said Ture Carlson, an entomologist with the Health Department. “2012 was also a bit unusual in that we had a record-setting April for high temperatures. This is about a month earlier than usual.”

Carlson said it’s not unusual for Shelby County to send up to 145 pools a week to Nashville for testing. Pools usually contain 10 to 50 mosquitoes, depending on how many are caught by the traps. Already, 10 pools have tested positive for West Nile Virus.

Last year, 15 people in Shelby County were infected with the virus, up from 12 in 2011 – the most cases out of any county in Tennessee. There were no Shelby County fatalities last year and two in 2011. Nationally, the Centers for Disease Control and Prevention reported a total of 5,674 cases in 2012, including 286 deaths. That was the highest number of cases on record since 2003.

Infected southern female house mosquitoes catch the virus from birds and can spread it to people and other animals when they bite. While about four out of five people who are infected with West Nile will not show any symptoms, it can be severe and even result in death.

In severe cases, patients can develop high fevers, neck stiffness, coma, convulsions, paralysis and muscle weakness. In rare cases, the virus can be spread through blood transfusions, organ transplants, breastfeeding and even during pregnancy from mother to baby.

Male southern house mosquitoes do not bite; rather it’s the females who need what Carlson called “a blood meal” to lay their eggs on damp soil or directly on the surface of water.

Entomologist Ture Carlson stands by one of nine Ultra Low Volume (ULV) spray trucks. The Vector Control trucks mist for mosquitoes throughout the city four days a week. 

Since April, the Health Department has applied microbial larvicides to standing bodies of water known to produce mosquitoes as part of a proactive effort to decrease the number of mosquitoes, Carlson explained.

Shelby County Vector Control also will continue truck-mounted spraying in the evening with Permethrin, a synthetic chemical widely used as an insecticide and repellent. The spray kills adult mosquitoes when they are actively flying in the air.

“We use it because it is has low mammalian toxicity – and it’s very safe,” he said. “Our biggest concern is the bees, which is why we spray at dusk. Most bees are most active at dawn and during the day. At night they are usually back in their hives and safe from the spray.”

Stan Cope, an expert on mosquito-transmitted diseases with Memphis-based Terminix, says the county is taking all the right preventative measures.

“When you have infected mosquitoes out and flying around, it’s 100 percent proper to be putting the spray out because you have to kill those infected mosquitoes,” he said.

Cope said it’s likely the Health Department is detecting the virus earlier in mosquitoes this year because of a milder winter – which allowed more female southern house mosquitoes to survive.

The Shelby County Health Department recommends that people who plan to spend time outside in the evening measures use a mosquito repellant containing one of the four active ingredients approved by the U.S. Environmental Protection Agency: DEET, Picaridin, Oil of Lemon Eucalyptus and IR3535.

Residents also should:

• Eliminate standing water where mosquitoes can lay eggs. Check properties for objects – including old tires, flower pots and drip plates, tin cans, buckets, and children’s toys – that collect rainwater and either drain or dispose of the water

• Install or repair windows and door screens

• Empty, clean and refill birdbaths and small wading pools weekly

• Empty and refill pets’ water bowls every few days

• Repair failed septic systems

• Repair leaky outside faucets

• Clean rain gutters and down spouts

• Secure swimming pool covers tightly and remove any standing water after rainfall

• Store wheelbarrows, canoes and boats upside down

• Stock ornamental lawn ponds with mosquito fish (Gambusia), which eat mosquito larvae. Gambusia fish are available free from the Vector Control Program – call 222-9715 for availability.

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