Swine and influenza: a challenge to one health research.

The challenge of increasing swine production and a rising number of novel and known swine influenza viruses has prompted a considerable boost in research into how and why pigs have become such significant hosts for influenza viruses. The ecology of influenza A viruses is rather complicated, involving multiple host species and a segmented genome. Wild aquatic birds are the reservoir for the majority of influenza A viruses, but novel influenza viruses were recently identified in bats. Occasionally, influenza A viruses can be transmitted to mammals from avian species and this event could lead to the generation of human pandemic strains. Swine are thought to be "mixing vessels" because they are susceptible to infection with both avian and mammalian influenza viruses; and novel influenza viruses can be generated in pigs by reassortment. At present, it is difficult to predict which viruses might cause a human pandemic. Therefore, both human and veterinary research needs to give more attention to the potential cross-species transmission capacity of influenza A viruses.

The pandemic H1N1 influenza experience.

The pandemic H1N1 influenza that began in Mexico in the spring of 2009 spread rapidly to southern California within days and around the world within a few months. Because the genetic make-up of the new virus was novel, several months of lead-in time were required before a suitable vaccine for human use could be produced and distributed. The effort to confront the virus on the part of the World Health Organization which included almost every nation on earth and a vast array of scientists and public health officials was extensive and timely. However, it was the moderate severity of the virus itself that saved global public health from catastrophe. Because of the extensive publicity and research that occurred during the H1N1 pandemic, many lessons were learned that will be useful in confronting future influenza pandemics. A "One Health" approach to prevent, detect, and combat future pandemics is essential.

The pig as a mixing vessel for influenza viruses: Human and veterinary implications.

Influenza A viruses are highly infectious respiratory pathogens that can infect many species. Birds are the reservoir for all known influenza A subtypes; and novel influenza viruses can emerge from birds and infect mammalian species including humans. Because swine are susceptible to infection with both avian and human influenza viruses, novel reassortant influenza viruses can be generated in this mammalian species by reassortment of influenza viral segments leading to the "mixing vessel" theory. There is no direct evidence that the reassortment events culminating in the 1918, 1957 or 1968 pandemic influenza viruses originated from pigs. Genetic reassortment among avian, human and/or swine influenza virus gene segments has occurred in pigs and some novel reassortant swine viruses have been transmitted to humans. Notably, novel reassortant H2N3 influenza viruses isolated from the US pigs, most likely infected with avian influenza viruses through surface water collected in ponds for cleaning barns and watering animals, had a similar genetic make-up to early isolates (1957) of the H2N2 human pandemic. These novel H2N3 swine viruses were able to cause disease in swine and mice and were infectious and highly transmissible in swine and ferrets without prior adaptation. The preceding example shows that pigs could transmit novel viruses from an avian reservoir to other mammalian species. Importantly, H2 viruses pose a substantial risk to humans because they have been absent from mammalian species since 1968 and people born after 1968 have little preexisting immunity to the H2 subtype. It is difficult to predict which virus will cause the next human pandemic and when that pandemic might begin. Importantly, the establishment and spread of a reassorted mammalian-adapted virus from pigs to humans could happen anywhere in the world. Therefore, both human and veterinary research needs to give more attention to potential cross-species transmission capacity of influenza A viruses.