Outbreak of swine influenza in Argentina reveals a non-contemporary human H3N2 virus highly transmissible among pigs

Abstract

Sporadic outbreaks of human H3N2 influenza A virus (IAV) infections in swine populations have been reported in Asia, Europe and North America since 1970. In South America, serological surveys in pigs indicate that IAVs of the H3 and H1 subtypes are currently in circulation; however, neither virus isolation nor characterization has been reported. In November 2008, an outbreak of respiratory disease in pigs consistent with swine influenza virus (SIV) infection was detected in Argentina. The current study describes the clinical epidemiology, pathology, and molecular and biological characteristics of the virus. Phylogenetic analysis revealed that the virus isolate shared nucleotide identities of 96-98 % with H3N2 IAVs that circulated in humans from 2000 to 2003. Antigenically, sera from experimentally inoculated animals cross-reacted mainly with noncontemporary human-origin H3N2 influenza viruses. In an experimental infection in a commercial swine breed, the virus was of low virulence but was transmitted efficiently to contact pigs and caused severe disease when an infected animal acquired a secondary bacterial infection. This is the first report of a wholly human H3N2 IAV associated with clinical disease in pigs in South America. These studies highlight the importance of two-way transmission of IAVs and SIVs between pigs and humans, and call for enhanced influenza surveillance in the pig population worldwide.

Two years of surveillance of influenza a virus infection in a swine herd. Results of virological, serological and pathological studies

“…In 2008, the farm was positive for IAV where a wholly human-origin H3N2 virus was isolated from 40–50 days old pigs [8]. In October 2009, a novel IAV was identified, a reassortant with HA and NA gene segments from an H1N1 of the δ2 cluster and internal gene segments from an H1N1p virus [17].…”

“…In these farms, IAV infections are considered a nursery problem mostly affecting successive batches [3]. Previous serological and virological characterization studies showed that influenza virus strains that circulate in pigs in Argentina are not related to those reported in pigs elsewhere in the world with the exception of the H1N1p virus [3,8,17]. Upon independent multiple introductions of the H1N1p virus in pig populations around the world, multiple reassortment events have been and continue to take place with previously circulating swine IAV strains [17–20].…”

“…The pandemic H1N1 virus in 2009 (H1N1p) likely originated in swine and contained a constellation of gene segments derived from multiple reassortment events involving swine-, human-, and avian-origin IAV strains. The subsequent spillover of such virus back into pigs, had led to more intensive swine influenza surveillance efforts worldwide, particularly in commercial swine operations [1,8]. Several studies have suggested that influenza infection is far more common than suggested by confirmed clinical outbreaks [3,6,7,9].…”

Introductions and Evolution of Human-Origin Seasonal Influenza A Viruses in Multinational Swine Populations

“…Finally, our study did not consider pH1N1 viruses because prior studies provide global estimates of the frequency of human-to-swine transmission of these viruses since 2009 (16). Taken together, these data make it clear that our estimate of human-to-swine transmission represents a conservative, lowerbound estimate, and increased swIAV sequencing on a global scale is required to capture the full scale of human-to-swine transmission (43)(44)(45)(46).…”

Reverse zoonosis of influenza to swine: new perspectives on the human–animal interface

“…Central to the global evolution of genetic diversity of swIAVs is the repeated introduction of IAVs of human seasonal virus origin in swine [21,36–41]. In contrast to major pandemic influenza events that last 1–3 years and occur sporadically, seasonal influenza viruses are associated with annual epidemics in humans that occur in winter in temperate regions.…”