More than 400 new U.S. cases of West Nile virus [infection] emerged in the last week in an outbreak that remains the 2nd worst on record but has begun to show signs of slowing. So far this year , 3545 cases have been reported to federal health officials as of 25 September 2012, up from 3142 reported the week before, the CDC said in its weekly update of outbreak data. About 38 per cent of all cases have been reported in Texas. Other states with large numbers of cases include Mississippi, Michigan, South Dakota, Louisiana, Oklahoma, and California.
A total of 147 people have died from the disease, compared with 134 reported one week ago. Just over half of the cases reported to the CDC this year  have been 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.
Experts believe the disease originated in Africa and was 1st detected in New York City in 1999. Outbreaks tend to be unpredictable. Hot temperatures, rainfall amounts and ecological factors such as bird and mosquito populations have to align just right to trigger an outbreak such as the one this year .
The CDC said the number of cases this year  is the highest reported to federal health officials through the last week in September since 2003, the year with the most cases.
People residing in the areas where cases have occurred are well advised to take measures to avoid mosquito bites, and owners of equine animals should have their animals vaccinated. One hopes that the incidence of WNV is indeed beginning to decline.