West Nile Virus Cases Up By Nearly A Third In The Last Week

* Dallas-Fort Worth area is epicenter of the outbreak

* Some 53 percent of cases are severe neuroinvasive form (Adds comments from federal and Texas officials)

By Marice Richter

DALLAS, Sept 12 (Reuters) - The number of U.S. cases of West Nile virus climbed by nearly a third in the latest week as the spread of the mosquito-borne disease accelerated and threatened to make the 2012 outbreak the deadliest on record, government figures showed on Wednesday.

So far this year, 2,636 cases have been reported to federal health officials, up from 1,993 the previous week, the Centers for Disease Control and Prevention said in its weekly update of outbreak data. A total of 118 people have died from the disease, compared with 87 reported one week ago.

"We are still seeing many, many new cases continue to come in," said Dr. Lyle Petersen, director of the Division of Vector-Borne Infectious Diseases for the CDC. The number of deaths is expected to continue rising, Petersen said.

The disease has been reported in people, birds or mosquitoes in 48 U.S. states, so far absent only in Alaska and Hawaii.

Two-thirds of the reported cases have been in just six states - Texas, Louisiana, South Dakota, Mississippi, Michigan and Oklahoma.

Texas accounted for about 40 percent of all human cases, and the Dallas-Fort Worth area remained the epicenter of the outbreak. Texas recorded 1,182 cases and 53 deaths in its latest report, which has more up-to-date numbers for the state than the CDC has reported.

Aerial spraying in the Dallas area in recent weeks has been effective in reducing the mosquito population that transmits West Nile from infected birds to people and other mammals.

"This is the worst year ever for West Nile illness in Texas," said Christine Mann, a spokeswoman for the Texas Department of State Health Services. "At this point, it is too early to determine if we've reached the peak of West Nile season, which is typically in August."

Peterson of the CDC said the outbreak may not have peaked in Texas and other states where summer-like weather is continuing.

Elsewhere in the country, the peak for new infection was likely in August, CDC officials believe, and the spike in new cases is likely due to the lag time between infection and states reporting cases.

Of the cases reported to the CDC this year, 1,405, or 53 percent, are of the severe neuroinvasive form of the disease, which can lead to meningitis and encephalitis.

The milder form of the disease causes flu-like symptoms and is rarely lethal.

West Nile outbreaks tend to be unpredictable. Hot temperatures, rainfall amounts and ecological factors such as the bird and mosquito populations have to align just right to trigger an outbreak like the one this year.

The CDC said the number of cases this year is the highest reported to federal health officials through the second week in September since 2003, the year with the most cases.

This year may not turn out to have the highest number of total cases but because of the high number of neuroinvasive cases, "this year's outbreak is the largest to date and the most serious," Petersen said.

This year's outbreak is already more than three times the size of last year's, when 712 cases were reported nationally, with 43 deaths.

The disease is thought to have originated in Africa and was first detected in New York City in 1999. (Additional reporting by Paul Thomasch; Editing by Corrie MacLaggan and Todd Eastham)