So far this year, 3,969 cases of the mosquito-borne illness have been reported to federal health officials as of Oct. 2, up from 3,545 reported the week before, the Centers for Disease Control and Prevention (CDC) said in its weekly update of outbreak data.
The pace of new cases has slowed since late summer, but the total cases reported by the first week in October is still the highest since a record-setting outbreak in 2003, when there were 9,862 cases for the full year.
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.
A total of 163 people have died from the disease, compared with 147 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.
Almost 70 percent of the cases have been reported in eight states: Texas, Mississippi, Michigan, South Dakota, Louisiana, Oklahoma, Illinois and California.