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Bulletin No. 16 - Fall 2003
Fourth International Symposium on Emerging and Re-emerging Pig diseasesGeneral Session
MENGELING WL
How viruses change.
Proceedings of the 4th International Symposium on Emerging and Re-emerging Pig Diseases, 2003, 3-8
The ways by which viruses change are reviewed: point mutation, recombination, reassortment, deletions and insertions, with particular attention to porcine reproductive and respiratory syndrome (PRRS) and influenza viruses.
- Point mutations ("antigenic shift" in influenza virus) result from RNA polymerase errors of base incorporation and are associated or not with effect on phenotype (virulence). Currently available tools provide a consensus sequence of a virus but cannot provide a perfect, comprehensive picture of the genetic diversity due to mutations. Moreover, the actual effect of mutations on virulence still requires a better understanding.
- Recombination: cells must be infected simultaneously, or almost so, with more than one viral strain. High levels of replication and long-lasting persistence of viruses increase the probability of recombination between them. Three categories of recombination are defined: between and among virulent strains (the more likely under field conditions), between and among virulent and attenuated (vaccine) strains, between and among attenuated strains (multi-strain vaccine). In all cases, the levels of virulence and replication of the recombinant strain should be the main matters of concern.
- Reassortment ("antigenic drift" in influenza virus) involves discrete segments of genome and is limited to viruses with segmented genome. The strains of influenza virus are defined thanks to the combination of two major surface proteins (hemagglutinins and neuraminidases respectively responsible for the attachment and the release of the virus to and from cells) which are as many expressions of reassortment. Interspecies reassortants such as the swine H3N2 subtype increase the complexity of this phenomenon.
- Deletions and insertions have a limited impact in comparison with the foregoing ways of genetic change, at least regarding PRRSV and SIV. However, they are not devoid of interest since porcine respiratory coronavirus (PRCV), for example, is assumed to be a respiratory variant of porcine transmissible gastroenteritis virus (TGEV) as a result of deletion.
Virus changes are thus to be considered more a result of successful mistakes than a designed tactical plan. One must also keep in mind that other factors may urge virus changes, namely high densities of animals in intensive management, the increase in long-distance transports and the use of artificial insemination. In future, the main difficulty will be to keep the lead over the evolution of pathogens.
