DAIRY MANAGEMENT PRACTICES AND THE TRANSMISSION OF ZOONOTIC PATHOGENS IN MILK
Title: Quantitative risk assessment of Listeriosis due to consumption of raw milk
Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 24, 2011
Publication Date: August 8, 2011
Citation: Latorre, A., Pradhan, A., Van Kessel, J.S., Karns, J.S., Boor, K., Rice, D., Mangione, K., Grohn, Y., Schukken, Y. 2011. Quantitative risk assessment of Listeriosis due to consumption of raw milk. Journal of Food Protection. 74(8): 1268-1281.
Interpretive Summary: Listeriosis is an uncommon but severe human disease caused by the foodborne bacterial pathogen, Listeria monocytogenes. Immuno-compromised individuals such as pregnant women (and their unborn children) and the elderly are most susceptible to listeriosis. Milk and dairy products have been implicated in several foodborne listeriosis outbreaks in the United States and L. monocytogenes has been frequently isolated from dairy farm environments and from the feces of dairy cattle. Dairy cattle often harbor this pathogen without showing any signs of illness. Milk can become contaminated with L. monocytogenes during the milking process when small amounts of feces get into the milking system. In one national survey, raw, bulk tank milk on 6.5% of dairy farms was contaminated with L. monocytogenes. While pasteurization greatly reduces any risk to the public from L. monocytogenes, raw milk sales are legal in more than half of the United States. The objectives of this study were to estimate the risk of illnesses due to L. monocytogenes to consumers who choose to drink raw milk purchased from permitted raw milk dealers, and to people on farms who consume raw milk. Thre scenarios were modeled for purchased milk: direct purchase of raw milk from the farm storage tank, purchase from a farm store, and purchase from a reatail store. Results showed that the risk of listeriosis associated with consumption of raw milk is low. Results also demonstrated that screening the milk for L. monocytogenes as part of the milk permitting process reduces the risk of listeriosis associated with raw milk consumption. Overall, the temperature of the home refrigerator was the model parameter that had the greatest impact on the risk of listeriosis (l. monocytogenes can grow at refrigeration temperatures). Despite the low risk of acquiring listeriosis from a single raw milk serving, the serious consequences of the disease (miscarriages, stillbiths, meningitis, death) should not be disregarded by raw milk consumers.
The objectives of this study were to estimate the risk of illnesses for raw milk consumers due to L. monocytogenes contamination in raw milk sold by permitted raw milk dealers, and the risk of listeriosis for people on farms who consume raw milk. Three scenarios were evaluated for raw milk sold by dealers: raw milk purchased (i) directly from bulk tanks, (ii) from on-farm stores, and (iii) from retail stores. To assess the effect of mandatory testing of raw milk by regulatory agencies, the number of listeriosis cases per year were compared where (i) no raw milk testing was done, (ii) only a screening test to issue a permit was conducted, and (iii) routine testing was conducted and milk was recalled if it was found to be positive for L. monocytogenes. The median number of listeriosis cases associated with consumption of raw milk obtained from bulk tanks, farm stores, and retail stores for an intermediate-age population was estimated as 6.6' 10-7, 3.8' 10-5, and 5.1' 10-5 cases/year, respectively. In populations with high susceptibility to listeriosis the estimated median number of cases per year was 2.7' 10-7 (perinatal) 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 purchased at farm stores, and 2.1' 10-5 (perinatal) and 1.0' 10-4 (elderly) for milk obtained from retail stores. For raw milk consumed on farms, the median number of listeriosis cases was estimated to be 1.4 ' 10-7 cases/year. 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.