Many different species, including ducks, chickens, pigs, whales, horses, and seals, have been infected with flu viruses. Besides birds, which host every known strain of influenza A viruses, various subtypes are exclusive to certain animals. Influenza A strains H3N2 and H1N1 affect people. The H1N1 and H3N2 virus infections in pigs and the H7N7 and H3N8 virus infections in horses are instances of distinct influenza A virus subtypes that have infected animals and led to outbreaks.
Influenza viruses that normally infect and circulate among one animal species may sometimes transcend species boundaries and cause sickness in another. Only H1N1 viruses, for example, were widespread in the swine population in the United States until 1998. Human H3N2 viruses were introduced into the pig herd in 1998, causing extensive illness in the pigs. H3N8 viruses from horses have recently crossed over and caused epidemics in dogs. Avian influenza A viruses may be spread between animals and humans in two ways: direct infection from birds or environments with virus contamination or an intermediate host, like a pig.
The flu A virus has eight unique gene segments. When influenza A viruses from two distinct species infect the same person or animal, their genomes are segmented, enabling them to combine and produce a new virus. Suppose that both a human and an avian influenza A virus infect a pig simultaneously. In that case, the new replicating viruses could combine their current genetic information (reassortment) and create a new influenza A virus.
The new virus will have a larger percentage of the genes from the human virus and genes like the hemagglutinin gene or neuraminidase gene from the avian virus. This virus can cause infection in humans and spread quickly. However, its surface proteins (hemagglutinin or neuraminidase) would be distinct from those present in human-infecting influenza viruses.
The term "antigenic shift" refers to a significant alteration in the influenza A virus. An antigenic shift occurs when individuals are infected with a novel influenza A virus subtype against which most people have minimal or no immune resistance. An influenza pandemic might arise if this new influenza A virus causes sickness in humans and is highly contagious.
A person who is co-infected with an avian influenza A virus and a human influenza A virus may experience the process of genetic reassortment. The genetic material in these viruses might recombine to develop a new influenza A virus with an avian hemagglutinin gene and additional human virus genes. Influenza A viruses that have been reassorted with a human influenza virus and contain a hemagglutinin against which humans have little or no protection are more likely to cause sustained human-to-human transmission and constitute a severe public health concern: pandemic influenza. As a result, rigorous analysis of influenza A viruses recovered from people infected with avian influenza A viruses are critical in detecting reassortment.
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Although human infections and outbreaks caused by certain avian influenza A viruses and swine influenza A viruses are uncommon, there have been rare cases of human infections and outbreaks caused by these flu viruses.
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