Excess Infections Due to Antimicrobial Resistance: The “Attributable Fraction”

Antimicrobial use causes a transient decrease in an individual's resistance to colonization by noncommensal bacteria ("competitive effect") and increases the likelihood of infection upon exposure to a foodborne pathogen. The additional "selective effect" of antimicrobial resistance results in a >3-fold increase in vulnerability to infection by an antimicrobial-resistant pathogen among individuals receiving antimicrobial therapy for unrelated reasons. Combining the increase in vulnerability to infection with the prevalence of taking an antimicrobial agent, it is possible to estimate the attributable fraction, or the number of excess infections that occurred as a result of the unrelated use of an antimicrobial agent to which the pathogen was resistant. Calculations based on estimates of the annual infection rates and attributable fractions of infections with nontyphoidal Salmonella and Campylobacter jejuni suggest that resistance to antimicrobial agents results annually in an additional 29,379 nontyphoidal Salmonella infections, leading to 342 hospitalizations and 12 deaths, and an additional 17,668 C. jejuni infections, leading to 95 hospitalizations.