Up to new tricks – A review of cross-species transmission of influenza A viruses

Abstract

Influenza is a highly contagious disease that has burdened both humans and animals since ancient times. In humans, the most dramatic consequences of influenza are associated with periodically occurring pandemics. Pandemics require the emergence of an antigenically novel virus to which the majority of the population lacks protective immunity. Historically, influenza A viruses from animals have contributed to the generation of human pandemic viruses and they may do so again in the future. It is, therefore, critical to understand the epidemiological and molecular mechanisms that allow influenza A viruses to cross species barriers. This review summarizes the current knowledge of influenza ecology, and the viral factors that are thought to determine influenza A virus species specificity.

Influenza A Virus Infections in Swine

“…Infection of pigs with low-pathogenicity avian influenza (LPAI) viruses or seasonal human influenza viruses usually results in limited viral replication with mild clinical disease and lesions, unless the virus adapts through genetic mutation. 77 Even inoculation with viruses such as the 1918 human pandemic H1N1 virus and HPAI H5N1 that induce severe disease in other mammals (humans, primates, ferrets, and mice) only results in disease of similar or less severity than that induced by endemic swine viruses. 82,164 With viruses associated with human disease, challenge studies seeking to identify or characterize virulent strains have of necessity been conducted in animal models, primarily mice, ferrets, and macaques, and in the in vitro studies in influenzasusceptible cell lines.…”

“…Recent receptor studies on the respiratory tracts of humans and swine have revealed very similar distribution patterns, suggesting that avian viruses could just as readily infect the human respiratory tract as the swine respiratory tract, negating the need of the pig as an intermediate host. 142,147 Avian and human viruses do not readily infect swine, 77 but infection of pigs with intact avian and human viruses has been reported. 77,85 The presence of avian-and human-origin genes in the triple reassortants that comprise most of the influenza viruses currently circulating in swine also provides evidence for such infections.…”

“…142,147 Avian and human viruses do not readily infect swine, 77 but infection of pigs with intact avian and human viruses has been reported. 77,85 The presence of avian-and human-origin genes in the triple reassortants that comprise most of the influenza viruses currently circulating in swine also provides evidence for such infections. 73,157,171 These triple reassortant viruses with the cassette of internal protein genes from human-, avian-, and swine-origin viruses appear to have an increased capability of adding genes from viruses currently circulating in human populations.…”

“…In other studies, most of the viral proteins have been shown to have some influence on host range. 16,58,77,97,104 …”

“…8,19,77,85,89 Influenza was first recognized as a respiratory disease entity in swine during the 1918 human Spanish flu pandemic. 67 The virus was not isolated until 1931, when nasal discharge from pigs was inoculated into ferrets and subsequently into embryonated chicken eggs.…”