Spread of resistant bacteria and resistance genes from animals to humans--the public health consequences.

The paper reviews the lines of evidence which link the use of antimicrobial drugs for food animals with the emergence of antimicrobial drug resistance in bacteria pathogenic to humans, with a particular focus on the public health aspects. Deductions from the epidemiology of food-borne infections, ecological studies, outbreak investigations, typing studies and direct epidemiological observations show that resistant bacteria are transferred from food animals to man. In addition to transfer in the food chain, exchange of mobile genetic elements among commensal and pathogenic bacteria contributes to the emergence of drug resistance. There is growing evidence that this has measurable consequences for human public health. One consequence is increased transmission supported by unrelated use of anti-microbials in humans. Other consequences are related to reduced efficacy of early empirical treatment, limitations in the choices for treatment after confirmed microbiological diagnosis, and finally a possible coselection of virulence traits. Recent epidemiological studies have measured these consequences in terms of excess mortality associated with resistance, increased duration of illness, and increased risk of invasive illness or hospitalization following infections with resistant Salmonella.