West Nile Death Toll Mounts, but Worst May Be Over, CDC Says

September 13, 2012

September 13, 2012 — The death toll in this year's outbreak of the mosquito-borne West Nile virus reached 118 on September 11, a 35% increase since the week before, and the number of reported cases involving the deadly neuroinvasive disease has continued to grow at a record-setting pace, the Centers for Disease Control and Prevention (CDC) announced yesterday.

For all those grim numbers, however, Lyle Petersen, MD, MPH, director of the CDC's Division of Vector-Borne Infectious Diseases, expressed optimism at a press briefing yesterday that the "worst of the outbreak is behind us."

"Based on historical data, we've turned the corner on the epidemic," said Dr. Petersen. "Experience has shown us that West Nile virus outbreaks in the United States tend to peak in mid- to late August." However, he stressed, Americans should continue to take precautions such as using mosquito repellent and ridding their property of standing water.

One sign of the agency's more sanguine outlook is the decision, announced by Dr. Petersen, to stop holding weekly telebriefings on the West Nile virus outbreak of 2012 with the media, barring a significant change in the number of cases. The CDC will continue, however, to post weekly updates on its Web site.

The number of West Nile virus infections reported as of Tuesday, September 11, stood at 2636, 32% more than on September 4. This is the second-highest year-to-date tally through September 11 since 2003, said Dr. Petersen. However, the total number of reported cases of infection is not a reliable measurement of an outbreak, he explained. Of the 20% of infected individuals who develop symptoms, the vast majority of them experience a short-lived fever, and only 2% or 3% of such cases are reported. Many individuals with West Nile fever never seek medical attention, and if they do, physicians often do not test for the disease, as the CDC recommends.

In contrast, 1 in from 150 to 250 infected individuals develops neuroinvasive disease such as meningitis or encephalitis, with a mortality rate of 8% to 10%. Because they are the most consistently reported to public health authorities, neuroinvasive cases are "the best indicator of the scope of an epidemic," said Dr. Petersen.

The number of neuroinvasive cases on September 11 stood at 1405, the highest through that date for any year since the virus was first detected in the United States in 1999. "We believe this year's outbreak is the largest to date, and the most serious," said Dr. Petersen.

Shortage of West Nile Test Kits Resolved

The CDC's belief that the West Nile virus outbreak has peaked seems to conflict with the rising number of reported infections and deaths, but Dr. Pedersen attributed the discrepancy to the time interval between a mosquito bite and a CDC case report.

"It takes a while for somebody to get infected, to get sick, to go to the doctor, to get tested, and the test results [to get] reported to the state health departments and then to us," he said. "So we're still largely monitoring cases that occurred several weeks ago, and that was the peak of the epidemic. We still expect large numbers of cases to come in for the next several weeks even though the number of infections may be going down."

Likewise, Dr. Pedersen called the number of deaths stemming from West Nile neuroinvasive disease "a lagging indicator."

"Someone who gets a severe disease, they may die a week, a month, or even 6 months later after the initial illness," he said. "Even if transmissions [of the virus] stopped tomorrow, we would expect the number of deaths to rise across the country."

The testing of patients for West Nile virus hit a speed bump late this summer when the number of cases temporarily overwhelmed the supply of test kits for the infection at some laboratories, according to a recent news report from the Center for Infectious Disease Research & Policy (CIDRAP) at the University of Minnesota. However, the primary manufacturer of the test kits, Focus Diagnostics, is now filling back orders, thanks in part to an approval from the US Food and Drug Administration for a new lot of test kits.

As a consequence, state health departments report that the "shortage is resolved," Roger Nasci, PhD, chief of the arboviral diseases branch of the CDC's Division of Vector-Borne Diseases, said at yesterday's press briefing. Laboratories and test manufacturers, Dr. Nasci said, are reassuring the public that they will "meet the need" for the remainder of the West Nile virus season.