May Marks the Beginning of West Nile Virus Season

May is the start of West Nile Virus season in Tennessee, and the Department of Health will once again monitor test results from mosquito pools across the state.
This year, Tennesseans have a new tool in the arsenal of repellents approved by the Centers for Disease Control and Prevention to prevent mosquito borne illnesses.  IR3535 was recently approved for use in repelling mosquitoes and has performed comparably to more familiar repellents, such as DEET and picaridin products.

“The best way to prevent West Nile Virus infection is to prevent mosquito bites,” said State Medical Entomologist Abelardo Moncayo, PhD. “Our testing programs help identify areas in which mosquitoes have been found to carry the virus, so we can make people aware of the risk of infection and they can take proper steps to protect themselves and their families.”

There are currently four repellents that are recommended for use against mosquitoes and can be applied to the skin:  DEET, picaridin, oil of lemon eucalyptus and IR3535. Any insect repellent product should be applied following the label instructions. Apply repellents only to exposed skin and/or clothing, never under clothing. DEET should not be used on infants younger than two months old, and DEET at 30 percent concentration is the maximum level recommended for children and infants over two months old. Oil of lemon eucalyptus should not be used on children younger than three years of age. None of these products should be applied around the mouth or eyes at any age. Picaridin and IR3535 are as effective as low concentrations of DEET and there are no age restrictions recommended for these two products.

In addition, certain products containing permethrin are also recommended for use on clothing, shoes and camping gear, but not on skin. Permethrin is highly effective as an insecticide and a mosquito repellent.

Tennessee continues to be impacted by WNV with 11 human cases confirmed in 2007. While human cases in Tennessee last year dropped to half the number confirmed in 2006, health officials warn that citizens must still be vigilant in protecting themselves from the virus.

“The fact that our numbers for human West Nile cases were down last year may give people a false sense of security about their risk this year,” said Moncayo. “Last year’s drought may have contributed to the drop in human cases. While numbers of West Nile Virus cases are hard to predict, it is possible that the abundance of rain in Tennessee this spring could lead to larger mosquito populations this year and potentially more West Nile Virus.”

Mosquitoes become infected with WNV by feeding on infected birds and can transmit the virus to humans through their bites. Most human WNV infections are either asymptomatic or mild. About 20 percent of infections result in symptoms that may include fever, headache and body aches. However, these initially mild infections have occasionally led to more severe symptoms, including fatigue, memory problems, headaches, depression, tremors, muscle weakness and paralysis. Eighty percent of people who become infected with WNV have no noticeable symptoms. The virus cannot be spread from one person to another.

WNV can occasionally cause an infection of the central nervous system in humans. Severe infections, which occur in less than one percent of human cases, may cause meningitis or encephalitis and result in high fever, neck stiffness, stupor or disorientation. Severe cases may also cause muscle weakness or paralysis.

While anyone can become infected with WNV, certain populations are at higher risk of infection. High risk groups include the elderly, persons who abuse alcohol and those with chronic health conditions such as diabetes, hypertension, renal disease and cardiovascular disease.

Mosquito populations in Tennessee are at their peak May through October. Human cases of WNV usually occur beginning in July or August. There is no human vaccine for WNV; therefore, Tennesseans are urged to take preventive measures to avoid being bitten by infected mosquitoes.

• Most mosquitoes likely to transmit WNV bite at dusk and dawn, so avoid being outdoors at these times. If you must go outside during these times use repellent or wear long sleeves, long pants and socks to protect yourself.
• Eliminate standing water near your home, which can serve as a breeding ground for mosquitoes. Many containers, even those as small as a bottle cap, can hold enough water for mosquitoes to breed.
• Keep windows and doors closed or cover them with screens to prevent mosquitoes from entering your home.
• Use insect repellent containing either DEET, Picaridin, oil of lemon eucalyptus or IR3535. (Pure oil of lemon eucalyptus is not registered by the Environmental Protection Agency as an insect repellent, and is not covered by this recommendation.)

Officials with the TDOH State Laboratory will test mosquitoes and birds for WNV. Tennessee’s state lab tests only American robins, blue jays and American crows for WNV. These three species are chosen because they are common in Tennessee, are easily recognized by the public, and have proven to be the most efficient species for WNV detection. Birds and mosquitoes are submitted for testing by county surveillance officials. WNV cannot spread directly from birds to people; however, dead birds should not be handled with bare hands. Anyone who finds a robin, crow or blue jay that has died recently is urged to contact their local health department.

Horses can also be infected with WNV. Tennessee had four confirmed cases of WNV in horses in 2007. Horse owners should be sure that their animals are currently vaccinated for WNV and for eastern equine encephalitis, which is also carried by mosquitoes.

For more information on West Nile Virus, visit the TDOH Web site at http://health.state.tn.us/ceds/WNV/wnvhome.asp.