By Marice Richter
DALLAS (Reuters) - The number of U.S. West Nile virus infections has jumped more than 60 percent in the past week, putting this year's outbreak on track to be the worst on record, and federal health officials are stumped by its severity.
The U.S. Centers for Disease Control on Wednesday said 1,118 cases and 41 deaths have been reported so far this year, up from fewer than 700 cases and 26 deaths just one week ago.
That is the highest number of West Nile virus infections reported through the third week of August since the virus was first detected in the United States in 1999, the CDC said. The worst U.S. outbreak occurred in 2003, with 9,862 cases and 264 deaths that year.
Cases usually flare up in the summer because the illness is most often transmitted from infected birds to people by mosquitoes.
Victims may suffer fever and aches that can become severe or even cause death, especially in the elderly, children and other at-risk groups. There is no specific treatment for the infection.
Symptoms are often mild and many people stricken do not see a doctor, meaning cases are likely underreported.
More than half of this year's cases are in Texas, but the disease now has been detected in 47 states, with only Alaska, Hawaii and Vermont reporting no cases.
About 75 percent of the cases have been in Texas, Mississippi, Louisiana, Oklahoma and South Dakota, officials said.
Nationwide, 56 percent of cases are of the more serious type that can cause paralysis, meningitis or encephalitis, the CDC said. The remaining 44 percent are the milder form of West Nile Fever.
Officials said they are uncertain why this year's outbreak has been so severe. A mild winter, a hot summer and other factors such as fluctuations in the bird population are contributors, officials said.
Dallas, where a health state of emergency was declared this month, is experiencing an unprecedented epidemic, said Dr. Lyle Petersen, director of the division of vector-borne infectious diseases for the CDC.
The latest figures show 270 of the 586 Texas cases have been reported in Dallas County. Four additional deaths have occurred in the Dallas-Fort Worth area since state health officials reported 21 deaths statewide on Monday.
"Dallas has been hardest hit," said Dr. David Lakey, commissioner of the Texas Department of State Health Services.
So far 11 deaths have been reported in Dallas County this year compared to 10 in the period between 2003 -- when the disease was first detected in Dallas -- and 2011.
Aerial pesticide spraying in the city of Dallas and surrounding cities has been underway since last week.
Officials are bracing for more cases because West Nile peaks in mid-August and lasts through September. There is often a lag between the time of infection and the appearance of symptoms.
(Editing by Dan Burns and Xavier Briand)