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The numbers for West Nile virus cases continue to rise, up 35 percent in the last week. But the Centers for Disease Control and Prevention is confident the nation has turned the corner on its worst-ever epidemic of West Nile virus disease.
"Based on historical data, in most of the country the epidemic probably peaked around the end of August," the CDC's Lyle Petersen told reporters in a teleconference Wednesday. "We're hopeful that the worst of the outbreak is behind us."
The current tally is 2,636 cases and 118 deaths.
But those numbers don't tell the whole story. Petersen says only about 3 percent of West Nile cases get reported. A more reliable measure comes from the number of people who get the dangerous form of West Nile, so-called neuroinvasive disease. That's infection of the brain or surrounding tissue.
The number of neuroinvasive cases so far this year is 1,406 — about half the highest-ever recorded back in 2003. But because of reporting lags, Petersen is pretty sure this year's total West Nile neuroinvasive infections will match or surpass the 2,866 reported in 2003.
This gets us close to a real appreciation of West Nile's big impact on America.
The CDC figures that for every case of neuroinvasive West Nile infection, 150 to 250 other people get innocuous or mild infections.
So if 3,000 Americans get neuroinvasive West Nile infections this year — a rough guess of the eventual total — "that could translate into roughly 450,000 infections," Petersen says.
Three-quarters of people with infections don't have symptoms. The rest — about 100,000 Americans — will have suffered fever, headache, body aches, nausea and other miseries of symptomatic West Nile infection in 2012. Most likely, almost 300 will have died.
But why is this year's West Nile epidemic likely to be the worst since the virus landed on America's shores 13 years ago — probably inside a mosquito that stowed away on a plane or ship? And why was Dallas, with 40 percent of the cases, so susceptible?
The CDC is planning to investigate those questions in the months ahead. Researchers plan to look at who got infected and where they were, along with weather information, data on West Nile infection rates among the Culex mosquitoes that carry the virus, and data on populations of susceptible birds such as robins, blue jays and crows. They're also looking at the virus itself, but so far see no genetic signal that it's changed.
Clearly weather is important. Outbreaks don't occur in abnormally cold seasons, Petersen says, and they often occur during heat waves. But that's not always the case.
Between now and next spring, he and his colleagues will try to make mathematical models using all these data to see if they can explain why 2012 was such a bad year.
But Petersen warns not to expect too much. "The models we've made to date have not been very successful, simply because it's very, very complicated," he says. "It may take more years of observation to tease all this out."