Incidence and seasonal variation of Listeria species in bulk tank goat's milk

Four hundred and fifty raw goat's milk samples obtained from the bulk tanks of 39 goat farms were analyzed for Listeria spp. over a 1-year period. Modified versions of the U.S. Department of Agriculture Food Safety and Inspection Service (USDA-FSIS) and Food and Drug Administration (FDA) protocols were used for recovery of Listeria. Overall, 35 (7.8%) samples yielded Listeria spp. with Listeria monocytogenes identified in 17 of the 35 (3.8%) Listeria-positive samples. Listeria innocua was detected in 26 (5.8%) samples. Eight milk samples contained both L. monocytogenes and L. innocua. Milk samples from 18 of the 39 (46.2%) farms were positive for Listeria at least once during this 1-year study. The modified USDA-FSIS method, which used Listeria repair broth rather than University of Vermont (UVM) broth for primary enrichment followed by a 4-h nonselective incubation period, yielded more Listeria-positive samples (77.1%) than the FDA method (51.4%). All L. monocytogenes isolates belonged to serotypes 1 (62.6%) or 4 (37.4%). Moreover, five different Listeria ribotypes were identified from 34 selected L. monocytogenes isolates, 2 of which were deemed to be of clinical importance. Listeria isolation rates were markedly higher during winter (14.3%) and spring (10.4%) as compared to autumn (5.3%) and summer (0.9%) with these trends similar to those previously reported for cow's milk.