A new analysis of the current swine-origin H1N1 influenza A virus suggests that transmission to humans occurred several months before recognition of the existing outbreak.
‘Using computational methods, developed over the last ten years at Oxford, we were able to reconstruct the origins and timescale of this new pandemic,’ said Dr Oliver Pybus of Oxford University’s Department of Zoology. ‘Our results show that this strain has been circulating among pigs, possibly among multiple continents, for many years prior to its transmission to humans.’ Dr Pybus, along with Andrew Rambaut from the University of Edinburgh and colleagues, used evolutionary analysis to estimate the timescale of the origins and the early development of the epidemic. They believe that it was derived from several viruses circulating in swine, and that the initial transmission to humans occurred several months before recognition of the outbreak. The team conclude that ‘despite widespread influenza surveillance in humans, the lack of systematic swine surveillance allowed for the undetected persistence and evolution of this potentially pandemic strain for many years.’
Their is indeed a need for systematic surveillance of influenza in swine for it was being noted that new genetic elements in swine can result in the emergence of viruses with pandemic potential in humans. Transmission of swine influenza virus from pigs to humans is not common and does not always cause human influenza, often only resulting in the production of antibodies in the blood. The meat of the animal poses no risk of transmitting the virus when properly cooked. If transmission does cause human influenza, it is called zoonotic swine flu. People who work with pigs, especially people with intense exposures, are at increased risk of catching swine flu. In the mid-20th century, identification of influenza subtypes became possible, which allows accurate diagnosis of transmission to humans. Since then, fifty confirmed transmissions have been recorded, Rarely, these strains of swine flu can pass from human to human. In humans, the symptoms of swine flu are similar to those of influenza and of influenza-like illness in general, namely chills, fever, sore throat, muscle pains, severe headache, coughing, weakness and general discomfort. It was first proposed to be a disease related to human influenza during the 1918 flu pandemic, when pigs became sick at the same time as humans. This H1N1 form of swine flu is one of the descendants of the strain that caused the 1918 flu pandemic. As well as persisting in pigs, the descendants of the 1918 virus have also circulated in humans through the 20th century, contributing to the normal seasonal epidemics of influenza.
