Substantial Antigenic Drift in the Hemagglutinin Protein of Swine Influenza A Viruses

Pia Ryt-Hansen*, Anders Gorm Pedersen, Inge Larsen, Charlotte Sonne Kristensen, Jesper Schak Krog, Silke Wacheck, Lars Erik Larsen

*Corresponding author for this work

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The degree of antigenic drift in swine influenza A viruses (swIAV) has historically been regarded as minimal compared to that of human influenza A virus strains. However, as surveillance activities on swIAV have increased, more isolates have been characterized, revealing a high level of genetic and antigenic differences even within the same swIAV lineage. The objective of this study was to investigate the level of genetic drift in one enzootically infected swine herd over one year. Nasal swabs were collected monthly from sows (n = 4) and piglets (n = 40) in the farrowing unit, and from weaners (n = 20) in the nursery. Virus from 1-4 animals were sequenced per month. Analyses of the sequences revealed that the hemagglutinin (HA) gene was the main target for genetic drift with a substitution rate of 7.6 × 10-3 substitutions/site/year and evidence of positive selection. The majority of the mutations occurred in the globular head of the HA protein and in antigenic sites. The phylogenetic tree of the HA sequences displayed a pectinate typology, where only a single lineage persists and forms the ancestor for subsequent lineages. This was most likely caused by repeated selection of a single immune-escape variant, which subsequently became the founder of the next wave of infections.
Original languageEnglish
Article number248
Issue number2
Number of pages17
Publication statusPublished - 2020


  • swineinfluenza A virus
  • genetic drift
  • antigenic drift
  • molecular clock
  • immune escape variants
  • selection
  • enzootic infections
  • substitution rate
  • hemagglutinin
  • sows

Cite this

Ryt-Hansen, P., Pedersen, A. G., Larsen, I., Kristensen, C. S., Krog, J. S., Wacheck, S., & Larsen, L. E. (2020). Substantial Antigenic Drift in the Hemagglutinin Protein of Swine Influenza A Viruses. Viruses, 12(2), [248].