By Amanda Gardner
FRIDAY, Aug. 17 (HealthDay News) -- Officials in Dallas County, Texas began aerial spraying of insecticides overnight Thursday to combat the worst outbreak of West Nile virus in the nation this year, and one of the worst outbreaks since the virus was first identified in the United States in 1999.
Dallas County has recorded 10 deaths and hundreds of cases of the mosquito-borne illness so far, while Texas overall has seen 465 cases and 17 deaths, according to state statistics. This is putting 2012 "on track to be the worst year ever for West Nile virus," said Christine Mann, a spokeswoman for the Texas Department of State Health Services.
Planes began spraying an area of Dallas Thursday night until early Friday, and more areas were expected to be sprayed later Friday, the NBC News affiliate in Dallas/Fort Worth reported Friday. Dallas mayor Mike Rawlings declared a state of emergency before approving the aerial spraying in his city.
Louisiana, Mississippi and Oklahoma have also been hit hard by West Nile virus this summer. According to the U.S. Centers for Disease Control and Prevention, Louisiana has reported 37 cases and six deaths, Mississippi has reported 33 cases and one death, and Oklahoma has reported 40 cases and one death.
The United States is experiencing the biggest spike in West Nile virus since 2004, health officials reported.
Dr. Marc Fischer, a medical epidemiologist with the CDC's Arboviral Diseases Branch in Fort Collins, Colo., said a "seasonal outbreak occurs every year but so far this year the activity seems to be greater and a little earlier than in recent years."
It's difficult to pinpoint why virus activity is higher this year and why it is higher in certain regions of the country.
"That's impacted by a number of factors, environmental factors like weather, heat, precipitation, the birds that are around to amplify the virus and maintain it, the mosquitoes that spread the virus, and human behavior," he said.
This summer's hot, dry weather across much of the middle of the country has provided ideal conditions for some species of mosquito. The heat speeds up the insects' life cycle, as well as the virus' replication process, the Associated Press reported.
Despite the outbreak in north Texas, not everyone is pleased with the insecticide spraying. Some residents worry that the chemicals could be harmful to children, pets and useful insects such as honeybees and ladybugs, the AP reported.
The chemical used in the insecticides -- synthetic pyrethroid -- is similar to a natural substance found in chrysanthemums. The U.S. Environmental Protection Agency says pyrethroids pose no significant risk to wildlife or the environment, although no pesticide is 100 percent safe, the news service said.
Eighty percent of people who are infected with West Nile virus develop no or few symptoms, while 20 percent develop mild symptoms such as headache, joint pain, fever, skin rash and swollen lymph glands, the CDC's Fischer said.
Less than 1 percent will develop neurological illnesses such as encephalitis or meningitis. People at greater risk for serious illness are those who are older than 50 and who have certain underlying medical conditions such as cancer, diabetes or high blood pressure, the CDC said.
Severe symptoms can include high fever, headache, neck stiffness, disorientation, coma, tremors, convulsions, muscle weakness, loss of vision, numbness and paralysis. These symptoms may last several weeks, according to the CDC.
In more extreme cases, the virus can lead to serious neurologic illness, such as encephalitis or meningitis (inflammation of the brain or surrounding tissues), or death. People older than 50 and those with certain medical conditions, such as cancer, diabetes, hypertension, kidney disease and organ transplants, are at greater risk for serious illness, according to the CDC.
Fischer said that "children can certainly be infected with West Nile virus but are less likely to get serious neurological illness."