Pathogen–host–environment interplay and disease emergence

Engering, Anneke; Hogerwerf, Lenny; Slingenbergh, Jan

Pathogens prone to display novel traits in the same host Pathogens capable of displaying novel traits while in the same host tend to feature a high mutation rate, and/or a capability of acquiring novel genetic material through re-assortment or recombination (viruses) or transfer of plasmids (bacteria). Enhanced virulence in RNA viruses is correlated with diversity of quasispecies, a collection of viruses with related sequences generated by mutation. 55 Below, examples will be discussed, starting with viruses, followed by bacteria, fungi and parasites. An important example of a virus capable to develop novel traits is influenza A virus, with a high mutation rate and formation of quasispecies, supporting the formation of a highly diverse gene pool across host reservoirs. 40 Human influenza viruses resistant to antiviral drugs have emerged globally, while H5 and H7 avian influenza subtypes are seen to acquire increased virulence through mutation. 40, 56 One more example of a virulence jump is given by the emergence of a highly pathogenic form of porcine reproductive and respiratory syndrome virus in China in 2006, based on an accumulation of point mutations and deletions. 57 Virulence jumps are also seen among plant viruses; the recombinant cassava mosaic virus UgV caused a very severe disease in cassava in the late 1980s in Sub-Saharan Africa. 58 Bacteria capable of acquiring novel genetic material through horizontal gene transfer can obtain virulence factors, toxins and/or acquire antimicrobial resistance. 59, 60 For example, E. coli O157:H7 obtained the large virulence plasmid pO157 and a bacteriophage expressing Shiga toxin leading to an emerging disease challenge in the food chain. 61 The plasmid containing New Delhi metallo-beta-lactamase forms another example; this plasmid is easily transferable by horizontal gene transfer and has conferred carbapenem resistance to many different Enterobacteriaceae species. 62 In fact, genes conferring antimicrobial resistance are ancient and known to circulate also in places out of reach of human and veterinary medicine. 63 Thus, the presence of antimicrobial resistance genes in microbial communities is not new, but in response to the presence of antimicrobials, the frequencies of these genes may increase through horizontal gene transfer and natural selection.