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 following year, infection rates were down, and they have stayed down over the decades that followed.

Health officials expect a similar trend in the country’s second major outbreak of West Nile virus, a mosquito-borne disease that first 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 illness is still around today, but the number of cases has fallen to around 128 annually. 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 virus. The state hasn’t reported any cases 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 had only four years of West Nile data to work with compared to 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 both St. Louis encephalitis and West Nile are dependent on ecological and environmental factors, such as rainfall and temperature, Slavinski said.

“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.”

In addition, she said, studies have shown that humans build an immunity to both 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 West Nile virus has made its way to Kansas. Last week a horse that died in Cowley County was confirmed to have had the virus.