Pathogen–host–environment interplay and disease emergence

Engering, Anneke; Hogerwerf, Lenny; Slingenbergh, Jan

The second major component of the disease emergence framework concerns the emergence of a pathogen with novel characteristics bringing mostly sudden disease flare-up within the same host. This category comprises pathogens becoming hypervirulent ('virulence jumpers'), certain pathogens acquiring antibiotic or antiviral resistance, and some pathogens circumventing the effects of vaccination. Common to these situations is a pathogen overcoming an obstacle that was precluding access to major host resources, creating a 'winner takes all' scenario, often accompanied by severe clinical disease. Examples of pathogens with novel traits that inflict major host damage include recent H5 and H7 HPAI viruses in poultry, spilling back also to wild birds, 48, 49 and Escherichia (E.) coli O104:H4 with multiple antibiotic resistances that caused a major food safety and veterinary public health challenge in Germany in 2011. 50 Dynamics of emergence of a pathogen with novel traits A pathogen with novel traits can emerge as a result of mutation or because a latent trait with superior fitness becomes selected for, unlocking host resources that would otherwise remain out of reach. For virulence jumpers, the virulence increase may eventually turn less or even counter-productive, hindering transmission. 51, 52 Virulence-transmission tradeoff may take a variety of turns, depending also on additional factors such as the timing of transmission and associated virulence peak during the infection period, 53 and the optimal virulence level may differ for each pathogen-host-environment configuration. 54 For pathogens circumventing vaccines or antimicrobials, the superior fitness plays out as long as it lasts. It has to be noted that not all pathogens that gain resistance or escape vaccination count as EID; only the ones that bring sudden disease flare-up and/or that become dominant in a host community are truly emerging.