Accidental release blamed in swine flu outbreak

By Allison M. Heinrichs
Tuesday, June 30, 2009

Sick pigs at the 1918 Cedar Rapids Swine Show in Iowa and scientists' accidental release of an "extinct" flu virus in 1977 played key roles in creating a strain that has swept the globe and sparked fear of a more deadly flu season this winter.

University of Pittsburgh infectious disease experts reviewed nearly a century of epidemiology reports to trace the origins of swine flu, the H1N1 virus, that emerged in Mexico this spring. It has sickened at least 27,000 people and killed more than 100 in the United States.

"Our review is the perfect combination of history, public health, science and politics, really," said Dr. Shanta Zimmer, an assistant professor at Pitt's School of Medicine and lead author of the research paper, which will be published in the July 16 issue of New England Journal of Medicine.

"Hopefully during this pandemic, global scientists and public health communities will work together and everybody will communicate openly," she said. "Just like in politics, we learn from what works and what doesn't in medicine."

Her paper, co-authored by Dr. Don Burke, dean of Pitt's Graduate School of Public Health, gives clues as to why this strain of swine flu predominantly infects children and young adults, while older people seem to have immunity.

In 1918, while a flu virus from birds was infecting humans with deadly results, that same virus infected pigs. For nearly the next 40 years, the virus evolved, sporadically infecting humans. Then, in 1957, the virus vanished when another virus emerged, outcompeting it.

As abruptly as it disappeared, the original virus emerged again to infect people in the former Soviet Union, Hong Kong and northeastern China in 1977.

Since the virus had not evolved since 1950 -- which is unusual -- and because scientists were examining swine flu viruses in light of a small, confined outbreak in Fort Dix, N.J., the previous year, Zimmer believes the infection was caused by an accidental release of a frozen sample preserved for study.

"It was literally frozen in time," she said.

Since then, the virus has continued to circulate with other influenza viruses during flu seasons.

Flu viruses also were evolving in pigs, combining and mutating to become more genetically complex. Occasionally, those viruses would infect people who worked with pigs, but they lacked the ability to spread from human-to-human.

The swine flu virus infecting people has traits of those viruses from pigs, as well as from the 1918 virus that died out in 1957 and re-emerged in 1977.

Because people older than 52 likely would have been sickened with the human strain before it disappeared in 1957, they appear to have some immunity.

According to the Centers for Disease Control and Prevention, the average age of swine flu victims is 12; the average age of hospitalized patients is 20; and of those who died, 37.

Earlier this month, the Allegheny County Health Department stopped testing every likely case of swine flu, instead concentrating on select groups of concern. Before it stopped testing, nobody older than 55 tested positive for swine flu, and 71 percent were 18 or younger.

The National Institutes of Health, which paid for the Pitt research, will use it to learn more about how to write computer programs that predict the emergence of diseases and the best ways to head off their spread, said James Anderson, program director at the National Institute of General Medical Sciences, a division of NIH.

"This work on understanding ... the time sequence in which the virus emerged is important background information in creating models," Anderson said. "We are constantly learning, all the time. This is really unknown territory here, and all the data we can gather helps."