JACKSON, Miss. - 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 next year, infection rates were down, and they have stayed down.
Health officials expect a similar trend with West Nile virus, a related mosquito-borne virus that first appeared in 1999 and has killed seven people and infected more than 130 this year in its worst U.S. outbreak.
The first notable outbreak of St. Louis encephalitis occurred in 1974. The illness is still around, but the number of cases has fallen to about 130 annually. Last year, four people died in the Monroe, La., area from the illness.
Transmission rates of St. Louis encephalitis and West Nile are dependent on ecological and environmental factors, such as rainfall and temperature, said Sally Slavinski, an epidemiologist for the Mississippi Department of Health.
"With these viruses, you need a lot of ecologic factors that are working in combination," Slavinski said. "You have to have a large mosquito pool that year of the right species."
Given the history of West Nile and related illnesses, it's not likely the outbreak will increase exponentially next year, Slavinski said. In 1999, seven people died and 55 others were hospitalized in New York with West Nile virus. The state hasn't reported any cases this summer.
"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.
In addition, she said, studies have shown that humans build an immunity to St. Louis encephalitis and West Nile virus.
During the 1975 St. Louis encephalitis outbreak, Illinois, Indiana and Ohio recorded the greatest number of cases, but Mississippi had the highest case rate - one out of every 1,000 people there got the illness. In Greenville, Miss., one in every 450 residents was affected.
The death rate for St. Louis encephalitis ranges from 3 percent to 30 percent, according to the Centers for Disease Control and Prevention. West Nile's death rate nationally has been about 10 percent.
Bernadette Burden, a CDC spokeswoman, said yesterday that researchers have only four years of West Nile data to work with, compared with decades of data for St. Louis, so they don't predict where West Nile might spread.
"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.
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 from Texas north and west and in several Midwestern states.
Health officials expect a similar trend with West Nile virus, a related mosquito-borne virus that first appeared in 1999 and has killed seven people and infected more than 130 this year in its worst U.S. outbreak.
The first notable outbreak of St. Louis encephalitis occurred in 1974. The illness is still around, but the number of cases has fallen to about 130 annually. Last year, four people died in the Monroe, La., area from the illness.
Transmission rates of St. Louis encephalitis and West Nile are dependent on ecological and environmental factors, such as rainfall and temperature, said Sally Slavinski, an epidemiologist for the Mississippi Department of Health.
"With these viruses, you need a lot of ecologic factors that are working in combination," Slavinski said. "You have to have a large mosquito pool that year of the right species."
Given the history of West Nile and related illnesses, it's not likely the outbreak will increase exponentially next year, Slavinski said. In 1999, seven people died and 55 others were hospitalized in New York with West Nile virus. The state hasn't reported any cases this summer.
"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.
In addition, she said, studies have shown that humans build an immunity to St. Louis encephalitis and West Nile virus.
During the 1975 St. Louis encephalitis outbreak, Illinois, Indiana and Ohio recorded the greatest number of cases, but Mississippi had the highest case rate - one out of every 1,000 people there got the illness. In Greenville, Miss., one in every 450 residents was affected.
The death rate for St. Louis encephalitis ranges from 3 percent to 30 percent, according to the Centers for Disease Control and Prevention. West Nile's death rate nationally has been about 10 percent.
Bernadette Burden, a CDC spokeswoman, said yesterday that researchers have only four years of West Nile data to work with, compared with decades of data for St. Louis, so they don't predict where West Nile might spread.
"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.
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 from Texas north and west and in several Midwestern states.