Infections likely peaked between mid- and late-August, Peterson said, though reports to the CDC tend to lag by a week or two. “Historically, the data suggest we’ve turned the corner of the epidemic,” Peterson said. “We’re hopeful that the worst of the outbreak is behind us.”
This year, there have been at least 2,640 recorded human cases and 118 deaths. Cases reported this week increased 35 percent from the previous week, Peterson said. Texas continues to lead the nation, with 1,057 cases and 46 deaths.
In Ohio, 74 human infections were reported as of Wednesday, and two deaths have been reported, one each in Hamilton and Cuyahoga counties. Counties reporting human infections include Cuyahoga, 23; Montgomery, 8; Clark and Hamilton, 6 each; Allen, Franklin and Lucas, 4 each; Clermont, Lorain, Mercer, Putnam and Van Wert, 2 each; and Butler, Defiance, Miami, Richland, Ross, Sandusky, Stark, Warren and Wood, 1 each.
Of note this year has been the high percentage of neuroinvasive cases, when the infection results in meningitis or encephalitis. “Neuroinvasive cases are the best indicator of the size of a West Nile virus epidemic,” he said.
Nationally, at least 1,069 of the 2,640 cases reported were neuroinvasive, the highest reported for this time of year since the virus was identified in the country. Because they most often require hospitalization, neuroinvasive cases are most likely to be lab-confirmed infections and so most likely to be reported to state and federal health officials, Peterson said. Only about 2 to 3 percent of non-neuroinvasive West Nile cases, often called West Nile fever, are reported, usually because few people with milder cases see a doctor and even fewer undergo lab tests to confirm the infection, he said.
It is difficult to estimate the true scope of the outbreak, since about 80 percent of people infected with West Nile virus never get sick, and milder cases are never reported. Peterson said that for every neuroinvasive case, there are anywhere from 150 to 250 milder or asymptomatic cases.
As the outbreak winds down in the coming months, experts will begin focusing on why 2012 was such a bad year for West Nile virus, Peterson said.
“The 2012 epidemic has raised a number of challenging questions,” he said.
Officials want to know why Texas, particularly the Dallas area, was so hard-hit.
And Ohio health officials are wondering why, with a record-setting number of infected mosquitoes, human infections in the state have been relatively low compared to 2002, when 441 human cases and 31 deaths were reported.
“Thankfully, we haven’t seen human cases anywhere near what we saw then,” said Richard Gary, public health epidemiologist with the Ohio Department of Health.
But West Nile virus outbreaks have a complex life cycle involving weather patterns and shifting populations of birds, mosquitoes and people, Gary said. That is why, every year, health officials trap and test mosquitoes from around the state to know where the virus is active and how early it’s showing up.
“Its’ because of all these moving parts that you conduct the surveillance the way you do,” he said. “The one thing you can be sure of is that if you see the infection rates go up early in the year for mosquitoes, then the risk for human infections goes up.”
For local health departments, the focus is on taking steps to prevent infection by reminding people to use mosquito repellent, clean up standing water where mosquitoes breed and install or repair window screens.
“We don’t really know why, from year-to-year, it’s either good or bad,” said Bill Wharton, spokesman for Public Health - Dayton & Montgomery County. “It’s a little bit like flu. The honest answer is, you never know what kind of flu season you’re going to have until you’re in it. It’s the same for West Nile. The most we can try to do is try to make people aware of what the relative danger is this year and tell them what they need to do to protect themselves.”
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