Mechanisms of viral emergence

Domingo, Esteban

Vet. Res. (2010) 41:38 as will the overlap of domains (in viral proteins or nucleic acids) in general. Structural and biochemical studies have indicated that residues located at the surface of viral particles tend to be subjected to less structural constraints than residues located in the interior of viral capsids or envelopes and, therefore, they may be prone to accept amino acid substitutions. Tolerance to change of surface amino acid residues may facilitate the emergence of new cell receptor specificities mediated by antigenic change with obvious implications for viral disease emergence and re-emergence [10, 34, 43, 90] . Partial immunity to FMDV, evoked by synthetic peptides that mimicked the major antigenic site of the virus, produced the selection of antigenic variants in cattle, and the variants displayed alterations of cell tropism [80, 81] . Escape mutants selected by ineffective vaccines have been reported in other viral systems [85] . In addition, tolerance to amino acid substitutions at surface sites of capsid or envelope proteins may result in antigenic variation independent of immune selection, as documented with several DNA and RNA viruses [10] . Long-term coexistence of viruses and their hosts has given coevolution a good chance to reach some relative equilibrium that has excluded ''super-virulent'' pathogens from our biosphere, because if they existed they would be eliminated with their hosts. However, transient disequilibria may often occur when viruses cross a species barrier. The great majority of viruses are not pathogenic, although most of the viruses that have been studied in detail are associated with disease, for obvious reasons. Many viruses do not cause disease in their natural reservoir hosts but can be highly pathogenic when transmitted to a new host species. This is amply documented by the zoonotic nature of most of the viruses that have emerged in the human population (influenza virus type A from aquatic birds, HIV-1 from Old World primates, SARS coronavirus from bats probably via palm civets, Nipah and Hendra viruses from fruit bats via pigs, among others) [1, 33, 40, 55, 59, 69, 77, 89] .