Reported West Nile virus infections rose 25 percent from last week, reaching a record, according to the latest figures from the Centers for Disease Control and Prevention.
West Nile Virus by AJC1
As of Wednesday, the CDC reported 1,993 cases of West Nile disease and 87 deaths throughout the United States, making this year's outbreak the worst since the disease emerged in 1999. Lyle Petersen, director of the Division of Vector-Borne Infectious Disease at the CDC, said in a teleconference earlier this week that health officials found the mosquito-borne virus in all 48 continental states.
Seventy percent of West Nile cases were concentrated in South Dakota, Mississippi, Oklahoma, Michigan, Louisiana and Texas. Texas is the leader by a long shot, accounting for the largest share of illnesses in the country, many in the Dallas metropolitan region.
So far, Texas medical officials have observed 1,013 cases and 40 deaths from the virus. "As of this week, 2012 is now officially our worst year in the state of Texas for West Nile disease," said David Lakey, commissioner for the Texas Department of State Health Services, in the CDC conference call. "Our previous worst year was 2003, when we had had 439 cases of neuroinvasive disease and 40 deaths."
Lakey said parts of the state made progress against the disease using ground and aerial insecticide spraying to reduce mosquito populations that transmit the infection. "In the areas that had aerial spraying, the number of the mosquitoes that carry West Nile virus is decreased by 93 percent, where the weather allowed us to do two consecutive treatments," he said. Infections also have likely peaked, particularly in the northern parts of the state, he added.
In Chicago, spraying halved the percentage of mosquito pools testing positive for the virus, down to 11 percent. "Even though we are seeing slight improvements, last year it was 1 percent," said Efrat Stein, director of public affairs for the Chicago Department of Public Health. The city's surveillance report from this week showed 11 human cases this year so far.
Some big unknowns remain
Though health officials are still wary about pointing to a cause, several are now acknowledging that this year's extreme weather may have played a role in the outbreak, while ruling out other possible culprits.
"We just believe there are environmental conditions that occurred for whatever reason this year that just promoted a large West Nile outbreak," said Petersen. "Right now, we have no evidence to suggest that the virus has changed somehow or that there's a higher proportion of seriously ill people."
Stein also suspects the environment. "I think the combination of hot and dry has allowed for an increase in the abundance of West Nile virus-carrying mosquitoes," she said.
But the disease pops up sporadically throughout the country, colonizing some regions while dodging others, despite similar climate conditions. Texas suffers more infections than comparably hot and dry Arizona and New Mexico, for example.
Other regions have conditions that are more inviting for mosquitoes. "Why doesn't Florida get tons of West Nile virus?" asked Marm Kilpatrick, an assistant professor in ecology and evolutionary biology at the University of California, Santa Cruz. "The short answer is, we really don't know yet."
Kilpatrick said researchers have traced how variables like heat and rainfall can alter disease transmission. "We think we understand a bunch of the individual processes," he said. "The challenge is, we don't really understand how they interact and work together." He pointed out that heat speeds up how quickly a mosquito becomes infectious, but it also shortens the mosquito's life span.
Some cities are better prepared
Precipitation can also favor one type of mosquito over another, according to Robert Tesh, a professor of pathology at the University of Texas Medical Branch in Galveston. Culex mosquitoes, for example, need to keep their eggs immersed in water, so their breeding is associated with persistent water bodies like ponds or drainage systems, while Aedes mosquito eggs can survive dry spells, so they are more prevalent after rainfall.
Tesh noted that deliberate countermeasures also influence where West Nile takes hold and can explain why some cities are more strongly afflicted than others. Part of the reason Houston hasn't suffered from the outbreak as badly as others is the city's aggressive mosquito surveillance and insecticide spraying program, developed in the 1960s to counter another mosquito-borne virus, St. Louis encephalitis.
Health officials in Chicago treated 91,000 catch basins with larvacide in May, a precaution established after the 2002 West Nile outbreak that infected 225 people, according to Stein.
Though the disease is already endemic in the United States, Tesh thinks a warming climate may push the disease farther north into Canada as more areas become hospitable to mosquitoes.
Extreme weather may also factor in West Nile transmission. "In general, experiences show that such disasters do not increase the transmission of West Nile virus and other arboviruses [viruses transmitted by arthropods]," said Petersen. "However, small increases in the numbers of West Nile virus cases can occur, as was reported in several areas of Louisiana after Hurricane Katrina in 2005."
He said the CDC is working with local officials in Louisiana and Mississippi to monitor the situation in the aftermath of Hurricane Isaac.
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