In a leading medical journal this week US researchers describe the 90-year history of swine flu and warn about working with “freezer” viruses, which they suspect is how an H1N1 virus that closely resembles a temporarily “extinct” 1950s strain resurrected some 20 years later and continues to circulate as a seasonal flu strain today.
The report is by two disease experts at the University of Pittsburgh Graduate School of Public Health and appears in the 29 June 2009 online issue of the New England Journal of Medicine.
The current strain of H1N1 swine flu is descended from a virus that sickened pigs in 1918 at the Cedar Rapids Swine Show in Iowa, they write.
Senior author Dr Donald S. Burke who is dean of the University of Pittsburgh Graduate School of Public Health, told the press:
“At the same time the 1918 flu pandemic was rapidly spreading among humans, pigs were hit with a respiratory illness that closely resembled symptoms seen in people.”
“Early experiments confirmed that this 1918 swine virus and a human strain emerged about the same time,” he said.
Burke explained that since that time, the genes of this ancestral strain of swine flu have “re-assorted” with those of other flu strains at least four times, resulting in the new 2009 strain, which still bears similarities to the original virus.
In their report, Burke and lead author Dr Shanta M. Zimmer, assistant professor, University of Pittsburgh School of Medicine, explain that although originally traced to Mexico, the exact physical origins of the 2009 H1N1 pandemic virus are unknown.
But because today’s virus has the same ancestors as older flu strains, this could be how some people appear to have partial immunity to the new pandemic virus, they write.
Burke and Zimmer also describe how the H1N1 virus temporarily became “extinct” in humans in 1957 and then re-appeared 20 years later.
They write of a small contained outbreak of H1N1 among 230 soldiers at Fort Dix in New Jersey that did not spread outside the base, and yet 20 years later, in 1977, H1N1 influenza re-emerged in people living in the former Soviet Union, Hong Kong and northeastern China. However, genetic studies have since shown that this re-emerged strain was not the Fort Dix strain as first suspected, but a very close relative of an earlier strain from 1950.
This leads the authors, and others, to suspect that in 1976, during a “flurry” of investigation to trace the Fort Dix strain, someone accidentally released a “freezer” virus that had been preserved since 1950. There is no proof that this is what happened, it is merely a theory based on the genetic similarity of the 1950 strain to the 1977 strain, which still circulates as a seasonal flu virus today.
Burke and Zimmer explain that the danger of a virus is determined by two things: how lethal it is and how easily it passes from host to host. A virus that passes readily from human to human and from animals to humans and vice versa has a greater chance of surviving and adapting through mutation, which is how H1N1 has managed to survive this long.
“Studying the history of emergence and evolution of flu viruses doesn’t provide us with a blueprint for the future, but it does reveal general patterns, and this kind of information is critical if we are to be as prepared as possible,” said Burke.
Burke and Zimmer suggest that the emergence of yet another serious global health threat from an animal source shows how important it is to understand zoonotic viruses (viruses that jump from species to species), and call for more live studies of virus development in animals, as well as more “field epidemiologic studies, and surveillance in animal populations, along with the development of computational models”.
And, since the current swine flu strain is thought to have originated outside the US, it is also important to collaborate closely with other countries in “efforts to predict and control future pandemic threats,” they write.
“Historical Perspective — Emergence of Influenza A (H1N1) Viruses.”
Zimmer, Shanta M., Burke, Donald S.
DOI: 10.1056/NEJMra0904322
Source: University of Pittsburgh Schools of the Health Sciences.
Written by: Catharine Paddock, PhD