Quantitative Risk Assessment of Listeriosis Due to Consumption of Raw Milk

The objectives of this study were to estimate the risk of illness for raw milk consumers due to Listeria monocytogenes in raw milk sold by permitted dealers, and the risk for people on farms who consume raw milk. Three scenarios were evaluated for raw milk sold by dealers: raw milk purchased directly from bulk tanks, from on-farm stores, and from retail. To assess the effect of mandatory testing of raw milk by regulatory agencies, the number of listeriosis cases per year was compared where no raw milk testing was done, only a screening test to issue a permit was conducted, and routine testing was conducted and milk was recalled if it was L. monocytogenes positive. The median number of listeriosis cases associated with consumption of raw milk from bulk tanks, farm stores, and retail for an intermediate-age population was 6.6 × 10⁻⁷, 3.8 × 10⁻⁵, and 5.1 × 10⁻⁵ cases per year, respectively. In populations with high susceptibility, the estimated median number of cases per year was 2.7 × 10⁻⁷ (perinatal, i.e., pregnant women and their fetuses or newborns) and 1.4 × 10⁻⁶ (elderly) for milk purchased from bulk tanks, 1.5 × 10⁻⁵ (perinatal) and 7.8 × 10⁻⁵ (elderly) for milk from farm stores, and 2.1 × 10⁻⁵ (perinatal) and 1.0 × 10⁻⁴ (elderly) for milk from retail. For raw milk consumed on farms, the median number of listeriosis cases was 1.4 × 10⁻⁷ cases per year. A greater risk of listeriosis was associated with consumption of raw milk obtained from retail and farm stores as compared with milk obtained from bulk tanks. This was likely due to additional time-temperature combination steps in the retail and farm store models, which increased the chances for growth of L. monocytogenes in raw milk. A close relationship between prevalence of L. monocytogenes in raw milk and the values of disease incidence was observed. Hence, a reduction in the number of cases per year in all populations was observed when a raw milk-testing program was in place, especially when routine testing and recalling of milk was conducted.