Aug. 23, 2012, 10:03 a.m.
Health officials say the US is in the midst of one of its largest outbreaks of West Nile virus.
The Centers for Disease Control and Prevention (CDC) say there have been 1,118 cases so far in 2012.
Fewer than 300 cases are usually reported this early in the year. There have been 41 US deaths from the mosquito-borne virus in 2012.
Aerial spraying began last week in Dallas, as the death toll in the state of Texas rose to 21.
Insecticide has been sprayed over the city twice, and officials are considering a third flight. Similar spraying was set to begin in Houston on Wednesday.
Health officials think a mild winter and early spring fostered the breeding of mosquitoes that bite infected birds.
Never before have so many illnesses been reported this early in the year, said Dr Lyle Petersen, who oversees the CDC's mosquito-borne illness programmes, adding that most infections are reported in August and September.
"We're in the midst of one of the largest West Nile outbreaks ever seen in the United States," he said.
West Nile virus was first reported in the US in 1999 in New York, peaking in 2002 and 2003, where severe cases of the disease reached nearly 3,000.
Only about one in five people infected with West Nile gets sick, and one in 150 of those infected will develop severe symptoms, including neck stiffness, disorientation, coma and paralysis.
Previous years have seen hot spots for the virus in south-east Louisiana, central and southern California, and areas around Dallas, Houston, Chicago and Phoenix.
But in 2012, there have been reports in 47 states, although about 75% of the cases are from five states: Texas, Mississippi, Louisiana, South Dakota and Oklahoma.