Influenza A Virus Infections in Swine

Abstract

Influenza has been recognized as a respiratory disease in swine since its first appearance concurrent with the 1918 ''Spanish flu'' human pandemic. All influenza viruses of significance in swine are type A, subtype H1N1, H1N2, or H3N2 viruses. Influenza viruses infect epithelial cells lining the surface of the respiratory tract, inducing prominent necrotizing bronchitis and bronchiolitis and variable interstitial pneumonia. Cell death is due to direct virus infection and to insult directed by leukocytes and cytokines of the innate immune system. The most virulent viruses consistently express the following characteristics of infection: (1) higher or more prolonged virus replication, (2) excessive cytokine induction, and (3) replication in the lower respiratory tract. Nearly all the viral proteins contribute to virulence. Pigs are susceptible to infection with both human and avian viruses, which often results in gene reassortment between these viruses and endemic swine viruses. The receptors on the epithelial cells lining the respiratory tract are major determinants of infection by influenza viruses from other hosts. The polymerases, especially PB2, also influence cross-species infection. Methods of diagnosis and characterization of influenza viruses that infect swine have improved over the years, driven both by the availability of new technologies and by the necessity of keeping up with changes in the virus. Testing of oral fluids from pigs for virus and antibody is a recent development that allows efficient sampling of large numbers of animals.Keywords influenza virus, swine, pathogenesis, cytokines, apoptosis, virulence, cross-species infection, hemagglutinin, receptors, polymerases, diagnosisThe evolution and epidemiology of influenza A virus (IAV) infections in swine have been integrally tied to the segmented nature of the genome of the virus, which facilitates gene reassortment, and to the adaptability of these genes through mutation, which sometimes facilitates replication in a new host. 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. 129 In 1933, a similar virus was recovered from humans with this technique. 131 The viruses were essentially identical and served as the prototype for type A subtype H1N1 influenza viruses.All influenza viruses of significance that infect swine are type A viruses, defined by the highly conserved internal nucleoproteins and matrix proteins. Influenza A virus subtypes are based on the external hemagglutinin (HA) and neuraminidase (NA) proteins. The predominant viruses in swine populations in different parts of the world may have genes of different origin, even though the subtypes may be similar. 10,73,171 The original classic H1N1 swine influenza virus, which remained the predominant influenza virus in North American swine populations for nearl...

The differentiated airway epithelium infected by influenza viruses maintains the barrier function despite a dramatic loss of ciliated cells

“…Our results are consistent with data from IAV infections in swine. Airways infected by swIAV have been reported to be lined by flattened epithelial layers24.…”

Detection of African swine fever, classical swine fever, and foot-and-mouth disease viruses in swine oral fluids by multiplex reverse transcription real-time polymerase chain reaction

“…Available from: http://nationalhogfarmer.com/health-diseases/disease-prevention/oral-fluid-collection-enhances-testing-0915). 2,9,[14][15][16][17]19 In 2013, studies reported the experimental detection of FMDV genome or ASF virus (ASFV) antibody in swine OF samples. 13,24 Specifically, OF is defined as the fluid in the oral cavity collected by use of an absorptive device.…”

Bioaerosol and surface sampling for the surveillance of influenza A virus in swine

“…The previously described trH3N2 influenza virus appears to have an increased ability to acquire genes from viruses of human origin. Infection of swine with viruses originating from other species is an important factor in the evolution of endemic influenza strains within swine populations (Janke, ). A recent example of the importance of influenza virus of swine origin and its evolution is demonstrated by the influenza pandemic (H1N1) 2009 virus (H1N1pdm09), which first emerged in Mexico (Vijaykrishna et al., ).…”