The Culex tarsalis mosquito is the main vector for West Nile virus in the western United States. |
(Courtesy of James Gathany/CDC)
Public health officials this past summer announced that this year’s West Nile outbreak was shaping up to be the worst in U.S. history.
By Sept. 4, nearly 2,000 cases of West Nile disease, 87 ending in death, had been reported to the Centers for Disease Control and Prevention. Of these cases, 54 percent were classified as neuroinvasive disease, such as meningitis or encephalitis. Forty-eight states had identified infections in people, birds, horses, or mosquitoes so far.
The 1,993 cases represent the most West Nile cases reported to the CDC through the first week in September in any year since the virus was detected in the U.S. in 1999. More than 70 percent of this year’s cases were in six states: Texas, South Dakota, Mississippi, Oklahoma, Louisiana, and Michigan.
“We’re in the midst of one of the largest West Nile virus outbreaks ever seen in the United States,” Lyle Petersen, MD, director of the CDC’s Division of Vector-Borne Infectious Diseases, announced at an Aug. 23 press conference.
Although the peak of West Nile virus epidemics typically occurs in mid-August, it takes a couple of weeks before people become ill and the disease is diagnosed, according to Dr. Petersen, meaning reports are delayed a week or more.
“Thus, we expect many more cases to occur, and the risk of West Nile virus infection will probably continue through the end of September,” he said.
Experts are at a loss to explain the reasons for the outbreak’s severity and why some parts of the country have been harder hit than others. For instance, Texas accounts for almost 45 percent of all West Nile cases and 50 deaths this season. Dallas County leads the state in West Nile infections with 296 cases including 13 that ended in death.
Texas Department of State Health Services Commissioner David Lakey, MD, was also at the August press conference. Prior to 2012, Texas experienced its worst West Nile outbreak in 2003 when the disease was first discovered there, he explained. That year, 439 cases of neuroinvasive disease including 40 deaths occurred statewide.
“If you look at Dallas County data and add up the total deaths from 2003 to 2011, they had 10 deaths. We now in this year, in Dallas County, have more deaths than their entire history in the past,” Dr. Lakey said.
Hot temperatures appear to promote the spread of West Nile virus, and many major outbreaks in Europe, Africa, and now the United States have occurred during abnormally hot weather, according to Dr. Petersen.
“We don’t really know why it’s worse this year than in previous years,” he acknowledged. “Hot weather, we know, from experiments done in the laboratory, can increase the transmissibility of the virus through mosquitoes, and that could be one contributing factor.”
Starting in the late 2000s, interest in West Nile virus generally began to wane with a decline in the number of human cases in many parts of the U.S., according to Dr. Erik Hofmeister, a veterinary medical officer with the U.S. Geological Survey’s National Wildlife Health Center. Located in Madison, Wis., the center has played a key role in government West Nile virus surveillance and research programs since the virus first arrived in the Western Hemisphere more than a decade ago.
Dr. Hofmeister, an expert in mosquito-borne diseases, says wild birds, which are the primary reservoirs for West Nile virus, might help explain the sudden spike in human infections this year. He refers to evidence collected by NWHC scientists, among others, that shows certain bird species, such as gray catbirds and northern cardinals, in particular regions can develop antibodies to the virus. It is theorized that the size of these immune avian populations has a direct correlation with the virus’s transmission among people. The greater the number of immune birds, the fewer the opportunities mosquitoes have to spread the virus; the opposite is also true.
Dr. Hofmeister cited a recent study published in PLOS ONE in which researchers monitored the prevalence of antibodies against West Nile virus in house finches and house sparrows in Los Angeles from 2003 through 2011. By measuring seroconversion rates in sentinel chickens and tracking the number of West Nile neuroinvasive disease cases reported to the L.A. County Department of Public Health, researchers concluded that as avian immunity to the virus diminishes, the number of human West Nile cases increases.
“What we might be seeing this summer to a large part is a decrease in herd immunity within the birds,” Dr. Hofmeister said, explaining that birds with West Nile virus antibodies eventually die and are replaced by immunologically naive individuals.
Although there’s no certainty that the West Nile outbreak of 2012 will dwarf those of previous years, the CDC’s Dr. Petersen says it’s certainly notable. “If things continue on their trajectory … this will be amongst the biggest or the biggest outbreak that we have experienced in the United States,” he said.