The Centers for Disease Control and Prevention said 4,891 people have contracted the disease, up from 4,725 a week ago. The virus killed four more people in the past week, bringing this year's death toll to 223.

The outbreak appears to be slowing, with 166 new cases reported during the last week, down from 199 the week before, according to CDC data. The week's death toll dropped to four, down from 36 during the previous week.

Just over half the cases reported to the CDC this year have been of the severe nueroinvasive form of the disease, which can cause meningitis and encephalitis. West Nile Fever, the less severe form, causes flu-like symptoms and is rarely lethal.

More than 70 percent of cases have been concentrated in 10 states: Texas, California, Louisiana, Mississippi, Illinois, South Dakota, Michigan, Oklahoma, Nebraska and Colorado.

Texas has been the hardest hit, accounting for more than a third of all cases, with Dallas-Fort Worth at the center of the outbreak.

According to the latest tally from the Texas Department of State Health Services, 1,718 people in the state have contracted the virus this year, and 78 people have died.

"We're still seeing new cases, but we're seeing the pace slow down quite a bit," said Chris Van Deusen, a spokesman for the Texas Department of State Health Services.

CDC officials said warm weather is a factor in a West Nile outbreak and colder weather suppresses mosquito activity. The worst year for West Nile-related deaths was 2002, when 284 people died as a result of the virus, the CDC said.

(Editing by Cynthia Johnston and Stacey Joyce)

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