2012 second-worst year ever for West Nile virus

• 'Reporting lag' is the reason for the spike
• 183 people have died
• Texas and California have had the most cases

West Nile virus cases in the U.S. hit 4,531 as of Tuesday, including 183 deaths, making 2012 the second-worst year ever for the mosquito-borne illness, the Centers for Disease Control and Prevention reported Wednesday.

Cases were up 8% from last week, to 282, including 15 more deaths reported, said Erin Staples, a West Nile expert for the CDC. The numbers don't represent a new wave of mosquito activity, but rather cases slowly working their way through to the CDC, she said. "It's a reporting lag. We're not hearing from our state partners that they're getting a deluge of cases."

It can take several weeks from when a person feels ill, goes to the doctor and then is tested for West Nile virus. Next, the report must go to the local health department and then to the state health department, which reports it to the CDC. The CDC then updates its numbers weekly, on Wednesdays. "So what we're seeing is probably illnesses that occurred in September," Staples said.

The peak of the disease appears to have hit at the end of August, when cases were going up as much as 35% a week. "As the cold weather sets in, particularly in the North and then moving south, that will stop the mosquito activity and then decrease the number of cases," Staples said.

The state that has been most affected is Texas with 1,580 cases, of whom 55 died. California is next with 285 cases and 11 deaths.

So far, 2012 has surpassed all years but 2003 for the number of cases. In the past week, it surpassed 2006 and it beat out 2002 the week before, Staples said.

Most people infected with West Nile virus will not have any signs of illness, but 20% will experience mild symptoms such as fever, headache, body aches and, in some cases, a skin rash on the trunk of the body and swollen lymph glands.

People older than 50 and those with compromised immune systems are most at risk. About one in 150 people will get more severe symptoms: headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness and paralysis.