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Updated: Aug 13, 2026
06:56
Influenza Virus Propagation in Embryonated Chicken Eggs
Published on: March 19, 2015
Evolution of pig influenza viruses
U Schultz1, W M Fitch, S Ludwig
1Institut für Virologie, Justus-Liebig Universität Giessen, Federal Republic of Germany.
Virology
|July 1, 1991
Summary
The nucleoprotein (NP) gene of swine influenza virus shows avian-like sequences, suggesting reassortment. However, other genes cluster with human strains, indicating a common evolutionary path possibly linked to the 1918 Spanish Flu pandemic.
Area of Science:
- Virology
- Molecular Evolution
- Influenza Virus Research
Background:
- The nucleoprotein (NP) gene of classical swine influenza virus (A/Swine/1976/31) exhibits avian-like characteristics at the amino acid level, despite nucleotide sequence similarity to early human strains.
- This observation led to the hypothesis of recent strong selection pressure or reassortment with avian influenza genes.
Purpose of the Study:
- To investigate the evolutionary origins and genetic relationships of swine influenza virus genes.
- To test the hypothesis that the Sw/31 NP gene was under recent strong selection pressure.
Main Methods:
- Sequencing of internal and nonstructural protein genes from A/Swine/1976/31.
- Phylogenetic analysis of nucleotide and amino acid sequences.
- Comparison with human and avian influenza virus gene sequences.
Main Results:
- Sequencing data and phylogenetic trees contradicted the reassortment hypothesis.
- All examined genes of Sw/31 clustered with early human strains at the nucleotide level.
- Most genes showed closer amino acid sequence relationships to avian strains, similar to the NP gene, indicating concomitant evolution.
Conclusions:
- The genetic data support the hypothesis that a human influenza A (H1N1) virus entered the pig population around the time of the Spanish Flu (1918/19).
- The release from human-specific selection pressure in pigs may have allowed swine virus genes to revert to optimal avian sequences.
- Modern swine influenza viruses (e.g., A/Swine/Germany/2/81) differ significantly from classical swine viruses, with genes derived from an avian influenza virus pool.
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