West Nile virus, the last infectious disease to emerge in the 20th century, is here to stay -- and within 14 years has already displaced another mosquito-borne infection that once was also a summertime affliction, scientists report Wednesday.
The remarkable speed with which West Nile has established itself in the United States -- and beyond -- has taken scientists by surprise.
What began as a mystery disease in Queens in the summer of 1999 has transformed into a seasonal menace in the 48 contiguous states, much of Canada and large swaths of South America.
Before its emergence here, West Nile was primarily found in the Middle East and Africa. No one knows how it arrived in this hemisphere.
"It's really kind of amazing how fast it spread," said Dr. Lyle Petersen, director of vector-borne diseases at the Centers for Disease Control and Prevention's division in Fort Collins, Colo.
The infection is caused by the bite of a female mosquito, which requires blood to produce fertile eggs. It waxes and wanes from one summer to the next -- sometimes occurring in outbreaks, in other years as only sporadic cases.
The hotter the weather, the faster West Nile viruses replicate inside mosquitoes. Record-breaking heat tends to be associated with an elevation in infections, Petersen said.
West Nile, which relies on mosquitoes feeding on infected birds, has virtually replaced St. Louis encephalitis virus in the United States. The encephalitis virus is also carried by birds, Petersen said. In both infections, mosquitoes bite infected birds, then transmit the infection when feeding on humans.
The encephalitis pathogen, though still around, rarely causes human disease.
"St. Louis encephalitis virus has been endemic in the United States ever since we started looking at it in the 1930s," Petersen said. "It is related to West Nile virus.
"They're both flaviviruses, 'flavi' meaning yellow because both of them are related to the yellow fever virus."
But West Nile reproduces at an astoundingly higher rate in birds. That means mosquitoes are more likely to become infected, Petersen said.
Dr. Jorge Benach, chairman of microbiology at Stony Brook University and director of the School of Medicine's Center for Infectious Diseases, said he's not surprised the encephalitis virus is on the decline.
"St. Louis encephalitis virus was transmitted only sporadically," Benach said. "But West Nile has come in with enormous strength. Even though West Nile came into the United States in 1999, it moved across the country very quickly."
"Migrating birds took it everywhere," Benach said.
West Nile is carried by 326 bird species and 65 mosquito species, according to a scientific paper by Petersen and a team of scientists in Wednesday's Journal of the American Medical Association.
Three key types of mosquitoes are the main drivers of the disease.
On Long Island, and elsewhere on the Eastern Seaboard, the species known as Culex pipiens, a classic dawn-to-dusk flier, largely spreads the infection.
In Western and Plains states, the closely related Culex tarsalis is the main biter, while in the South, it's Culex quinquefasciatus.
Biologist Thomas Daniels, of Fordham University, who studies at the Louis Calder Center's biological field station in upstate Armonk, said preliminary evidence suggests an invasive mosquito species, Ochlerotatus japonicus, which is relatively new in New York, may also be a carrier.
Petersen, however, says he is certain it will never replace the hardy Culex biters at the epicenter of the disease.