According to figures released today, last year's West Nile season was similar to the three seasons spanning 2004-2006, but not as bad as 2002 and 2003.
The most reliable way to track West Nile virus is by looking at the cases of neuroinvasive disease -- brain infections (encephalitis) and infections of the membranes surrounding the brain and spinal cord (meningitis). Because these infections are so serious, nearly all of them are recognized and reported to the CDC.
Neuroinvasive West Nile virus infections often result in lasting disability or death. One in 20 patients develops a polio-like paralysis that can be permanent.
In 2007, there were 1,227 cases of West Nile encephalitis and meningitis reported to the CDC. Sixty-three of these people were paralyzed. There were 117 deaths.
These numbers are similar to those seen in the previous three seasons, but not as bad as the record year of 2002, when there were nearly 3,000 cases of neuroinvasive West Nile virus infection and 284 deaths.
Because about seven out of 100 people who get West Nile infections get neuroinvasive disease, the CDC estimates that about 175,000 Americans got a West Nile virus infection in 2007.
There were 2,350 reported cases of West Nile fever, but because not all cases are reported, the CDC estimates that about 35,000 Americans came down with West Nile fever last summer.
The 2007 West Nile virus season started in June and peaked the first week of August. Cases began dropping sharply in September, and the season was all but over by November.
The virus expanded its reach into 19 U.S. counties that had not previously reported the virus, and reappeared in 1,148 U.S. counties. The states where residents had the greatest chance of getting a West Nile virus infection were North Dakota, South Dakota, Wyoming, Montana, and Colorado.
This year's West Nile season is right on track. As of June 24, 13 cases had been reported from seven states. But there's no way to tell how bad a year it will be.
For example, even though the west-central states had the highest incidence of West Nile infections, Idaho had only 10 cases of neuroinvasive disease -- 93% fewer than the 139 cases that state reported in 2006.
Here's a hint on how to avoid West Nile virus infection: The mosquitoes that spread the infection tend to bite at dawn and dusk. Although you should protect yourself against mosquito bites whenever you're outside, it's smart not to take any chances during the early morning and early evening hours.
The CDC's recap of the 2007 West Nile virus season appears in the July 4 issue of Morbidity and Mortality Weekly Report.