Influenza viruses were first isolated from pigs in the United States in the early 1930s. These were H1N1-subtype viruses, and their isolation from pigs followed closely upon the 1918 human H1N1 influenza pandemic that claimed over 40 million lives around the world. In fact, influenza-like disease was recognized clinically among pigs in the United States during the late summer and fall of 1918, and recent evidence clearly indicates that the 1918 human virus and the 1930s swine viruses are closely related. However, it remains unclear whether these viruses appeared first in people and then spread to pigs, or vice versa. H1N1 influenza viruses have continued to infect pigs in the United States ever since the 1930s. In addition, H3 subtype viruses have infected pigs in Europe, Asia and, as of the fall of 1998, in the U.S.

Clinical disease in pigs:

Influenza remains one of the most commonly encountered swine respiratory diseases. It can present as either an enzootic disease problem in a herd or, more commonly, as explosive outbreaks of acute respiratory disease characterized by:

• fever and lethargy
• inappetence and weight loss
• nasal (and to a lesser degree ocular) discharge
• coughing and dyspnea (reflecting the underlying bronchopneumonia)
  • The severity of clinical illness is dependent upon:
    • the strain of influenza virus involved
    • the age of the pig
    • the immune status of the pig
    • environmental conditions
    • the presence or absence of concomitant stress factors
    • the presence of absence of additional respiratory tract bacterial and/or viral pathogens.

The course of disease in generally only 5-7 days, after which the pigs rapidly recover. Morbidity can often approach 100%, but mortality is uncommon unless the infection is complicated by secondary bacterial infections. In addition, the clinical signs may be milder if animals have been previously exposed or vaccinated or in neonates with residual maternal immunity. Despite the lack of mortality, swine influenza may have an economic impact because of delayed weight gain.

What roles do pigs play in influenza virus infection of humans?

1. Due to the segmented nature of the influenza virus genome (8 individual segments of RNA), influenza viruses can undergo genetic reassortment to produce new variant strains of virus. Pigs are hypothesized to be serve as the "mixing vessels" in which two influenza viruses co-infect and undergo reassortment.

• Genetic reassortment is the basis for one form of influenza virus variation called "antigenic shift". An antigenic shift occurs when an influenza virus of a new subtype enters a population and, because the animals/humans lack any pre-existing immunity to the new virus, it spreads rapidly throughout the population. "Pandemics" of human influenza tend to occur ~every 10-40 years.
  • In the human influenza pandemics of 1957 and 1968, it is clear that an avian virus reassorted with a pre-existing human virus to create the new variant virus. But human beings are not generally susceptible to infection with influenza viruses of avian origin, and vice-versa. (The H5N1 outbreak in Hong Kong in 1997 and the H9N2 cases in China and Hong Kong in 1999 being important exceptions). Pigs, however, are uniquely susceptible to infection with both avian and human viruses, as well as their own swine viruses, and are thus thought to serve as the intermediary for human/avian virus reassortment.
    • This susceptibility is because their tracheal epithelium contains virus receptor sialyloligosaccharides with both 2,3- (preferred by avian influenza viruses) and 2,6 (preferred by mammalian influenza viruses) N-acetylneuraminic acid-galactose linkages.
  • In direct support of this theory, human/avian virus reassortants isolated from pigs in Europe were subsequently isolated from children in the Netherlands. In addition, reassortant H3N2, H1N2, and H1N1 viruses containing genes from human, avian and swine influenza viruses have been isolated from pigs and in the U.S. since 1998. Furthermore, the a reassortant swine H1N1 virus was isolated from a person in Wisconsin in 1998, and reassortant swine H1N2 and H3N2 viruses have been isolated from turkeys in the U.S.

2. Classical swine influenza viruses can also be directly transmitted to humans as zoonotic infections, sometimes with fatal consequences. Human infections with swine influenza viruses have been documented in the U.S. at least 9 times since 1974, including fatal infections, as well as in Europe and in New Zealand. A recent study of swine farm families in Wisconsin documented statistically significant levels of exposure to swine influenza viruses, compared to an urban control population from Milwaukee.

3. Older strains of human influenza virus can be maintained among pigs, thereby allowing for re-introduction of antigenic variants back into the human population.

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