Arboviral Disease Season: It's On!

Diagnostic testing of CSF offers two advantages over serologic testing. One is that if testing is done early – say, 4-5 days after illness onset – IgM antibodies may not be present yet in serum, but they will likely be in the CSF.

"When you get out to 9 or 10 days, though, there’s not really a difference. Everybody should have IgM antibodies in their CSF and serum," Dr. Fischer explained.

The other advantage of testing the CSF is that IgM antibodies to West Nile virus can be maintained in the serum for a long time. Their presence could be due to an asymptomatic infection that occurred the previous year. In contrast, finding IgM antibodies in the CSF of a patient with neurologic symptoms boosts the likelihood that the patient has a focal viral infection.

In response to an audience question as to why West Nile virus neuroinvasive disease has been on the wane in the United States for most of the past decade, Dr. Fischer was quick to reply that no one knows. The most popular hypothesis is that it’s due to human herd immunity, but he considers that unlikely. More plausible in his view is that the vertebrate hosts of the virus – birds – have become immune in a given area.

Or there may be an entirely different and as yet unrecognized explanation, he continued. After all, western equine encephalitis virus was a not uncommon cause of neuroinvasive disease in the western and central United States between the 1960s and the early 1990s, yet for unknown reasons no cases have been reported since 1999.

"If you look back historically, St. Louis encephalitis virus infection, when it did occur more in the United States, occurred in very large outbreaks with 20- to 30-year intervals in between. The first U.S. case of West Nile disease in humans was identified in New York in 1999, so we’re really only 12 years into our West Nile outbreak. Maybe it’ll be like St. Louis encephalitis virus: quiet for 20-30 years before coming back with a big year," he said.

Dr. Fischer stressed that the views he expressed are his own and not necessarily those of the CDC. He reported having no financial conflicts.