Data From ‘70s St. Louis Virus May Aid in West Nile Outbreak
In 1975, an outbreak of the mosquito-borne St. Louis encephalitis spread through 29 states, killing 95 people and infecting about 3,000 others. The following year, infection rates were down, and they have stayed down.
Health officials expect a similar trend in the country’s second major outbreak of West Nile virus, a mosquito-borne disease that appeared in 1999 and has killed seven people and infected more than 130 this year.
The first notable outbreak of St. Louis encephalitis occurred in 1974. The disease is still around, but annual cases have fallen to about 130. Last year, four people died in the Monroe, La., area from the illness.
In 1999, seven people died and 55 others were hospitalized in New York with West Nile. No cases have been reported this summer.
The successful control of the West Nile outbreak in New York and the St. Louis outbreak in the 1970s makes it unlikely this year’s outbreak will increase exponentially, said Sally Slavinski, an epidemiologist for the Mississippi Department of Health.
“Next year, we probably will see some [West Nile] activity, but hopefully not to the same degree, if we can use St. Louis as a model,” Slavinski said.
But Bernadette Burden, a spokeswoman for the Centers for Disease Control and Prevention, said Saturday that researchers have only four years of West Nile data compared with decades of data for St. Louis, making it more difficult to predict the future.
“We do believe, based on research, that it will continue its trek west. But to say how it will impact, we honestly don’t know,” she said.
The transmission rates of St. Louis encephalitis and West Nile are dependent on ecological and environmental factors such as rainfall and temperature, Slavinski said.
In addition, she said, studies have shown that humans build an immunity to St. Louis encephalitis and West Nile virus.
The death rate for St. Louis encephalitis ranges from 3% to 30%, according to the CDC. West Nile’s death rate nationally has been about 10%.
In addition to West Nile and St. Louis encephalitis, the mosquito-borne LaCrosse-California and Eastern equine encephalitis can also cause brain swelling. Western equine encephalitis has been reported north and west of Texas, and in several Midwestern states.
The seven deaths linked to West Nile this year have all been in Louisiana, although Mississippi officials are investigating a death they say appears to be linked to the virus.
The governors of both states have declared states of emergency, opening the door to federal aid for mosquito control.
Dr. Raoul Ratard, Louisiana’s epidemiologist, said the treatments for West Nile and St. Louis are similar, and the same health precautions should be taken, including use of mosquito repellents, elimination of standing water, and public spraying for mosquitoes.
