Quantitative risk assessment of human risks of methicillin-resistant Staphylococcus aureus (MRSA) from swine operations

Abstract

Describing quantitatively how large a risk is provides crucial information for helping to set risk management priorities. This chapter applies descriptive analytics to assess the size of the human health risks from a particular source. It continues the theme begun in Chap. 5 of examining human health risks from antibiotic-resistant infectious bacteria, but focuses specifically on long-standing concerns expressed by the public health community, news media, and members of the general public that methicillin-resistant Staphylococcus aureus (MRSA) transmitted from pigs to humans may harm human health. For readers not interested in details of this application, a brief summary is that previous studies of the prevalence and dynamics of swine-associated (ST398) MRSA have sampled MRSA at discrete points in the presumed causal chain leading from swine to human patients, including sampling bacteria from live pigs, retail meats, farm workers, and hospital patients. This chapter integrates available data from several sources to construct a conservative (plausible upper-bound) probability estimate for the quantitative human health harm (MRSA infections and fatalities) arising from ST398-MRSA from pigs. The estimated plausible upper bounds are approximately one excess human infection per year among all U.S. pig farm workers, and one human infection per 31 years among the remaining total population of the U.S., assuming that bacteria transmission events not yet observed are possible. The true risks may be smaller (possibly zero for members of the general population). Putting rough numerical bounds on the size of the risk such as “less than one case per 300 million people per decade” can help to engage System 2 thinking (cognitive, slow, deliberative; see Chap. 12) about costs and benefits of risk-reducing interventions; whereas leaving the risk unquantified is more likely to engage System 1 thinking (intuitive, quick, emotional; see Chap. 12), which typically reacts very strongly and adversely to the qualitative idea of contamination of food with invisible, harmful microbes.