The United States has seen at least 1,118 cases of West Nile virus this year, the most since the mosquito-borne disease was first detected in 1999, with 41 deaths, authorities said Wednesday.
The Center for Disease Control said Texas alone accounted for almost half of all cases, followed by Mississippi, Louisiana, South Dakota and Oklahoma, and advised Americans to take measures to prevent mosquito bites.
It said the number of cases is the highest recorded by the third week of August since the virus was first detected here more than a decade ago, and that it expects the toll to rise.
"Typically we see a peak in the West Nile virus season around the middle of August, and it takes some time for cases to get reported to us," CDC spokeswoman Candice Burns Hoffman told AFP.
"We are expecting that in the coming weeks we will see more cases."
The entire year of 2011 saw just 712 cases and 43 deaths, the CDC said.
The outbreak has been concentrated in and around Dallas, Texas, where authorities last week began using aircraft to spray pesticide.
Texas has had a total of 537 cases this year, causing 19 deaths, according to the CDC. Neighboring Louisiana has had six deaths out of 73 total cases.
The disease, first discovered in Uganda in 1937, is carried by birds and spread to humans by mosquitoes.
The CDC said the uptick could be related to this year's unusually mild winter, but that several factors influence the scope of an outbreak, including the birds that carry the disease, the mosquito population and human behavior.
Hoffman said the best way to prevent infection was to wear bug repellent and long sleeves, and to avoid going outside at dawn and dusk, when mosquitoes are most active.
Authorities are also urging Americans to drain standing pools of water where mosquitoes breed.
Severe symptoms of the virus include high fever, vision loss and paralysis, while milder symptoms range from headaches to skin rashes.
There are no specific treatments or cures for the disease, though the milder symptoms tend to go away on their own. Eighty percent of those infected will not show any symptoms at all, according to the CDC.