Increased Incidence of Listeria Species in Dairy Processing Plants Having Adjacent Farm Facilities

The processing environments of 30 dairy facilities were surveyed for the presence of Listeria species. Two different primary enrichment media - University of Vermont Modified Listeria Enrichment Broth and Listeria Repair Broth - were employed to increase the probability of identifying positive samples. Samples were also tested using both an enzyme-linked immunosorbent-based (ELISA-based) assay and a gene probe assay. A total of 346 sponge samples were evaluated for the presence of Listeria . Listeria spp. were identified via one or more of the assays 122 (35.3%) times. Fifty-five of the positive samples (37.2%) contained Listeria monocytogenes and 93 (62.8%) contained Listeria innocua . Of the 30 plants tested, 9 had a dairy farm contiguous to the processing facilities. Our results show that these plants are more likely to be contaminated (9/9) than those plants without on site dairy farms (17/21). Analysis of the Listeria spp. results indicated that contamination was significantly higher (α = 0.1) at those plants with an on-site dairy farm (x = 50.1 %) than those plants without an on-site dairy farm (x = 33.5%). Plants producing dairy ingredients, frozen milk products or fluid milk were all shown to have significantly higher incidence rates than expected. Conversely, plants producing cultured dairy foods, or a combination of cultured dairy foods and fluid milk were found to have significantly lower incidence rates than expected. There was no statistically significant difference in contamination by area within the plants.