Influenza has been recognised as a respiratory disease in swine since its first appearance concurrent with the 1918 'Spanish flu' human pandemic.

According to Dr B.H. Janke of Iowa State University in a paper in Veterinary Pathology, 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 necrotising 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 characterisation 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.

Dr Janke concluded that testing of oral fluids from pigs for virus and antibody is a recent development that allows efficient sampling of large numbers of animals.

Reference

Janke B.H. 2014. Influenza A virus infections in swine: pathogenesis and diagnosis. Veterinary Pathology. 51:410-426

April 2014