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. Mod ified versions of the U.S. Department of Agriculture Food Safety and inspec tion Service (USDA-FSIS) and Food and Drug Administration (FDA) protocols w ere used for recovery of Listeria. Overall, 35 (7.8%) samples yielded Liste ria spp, with Listeria monocytogenes identified in 17 of the 35 (3.8%) List eria-positive samples. Listeria innocua was detected in 26 (5.8%) samples. Eight milk samples contained both L monocytogenes and L. innocua. Milk samp les from 18 of the 39 (46.2%) farms were positive for Listeria at least onc e during this 1-year study. The modified USDA-FSIS method, which used Liste ria repair broth rather than University of Vermont (UVM) broth for primary enrichment followed by a 4-h nonselective incubation period, yielded more L isteria-positive samples (77.1%) than the FDA method (51.4%). All L. monocy togenes isolates belonged to serotypes 1 (62.6%) or 4 (37.4%). Moreover, fi ve different Listeria ribotypes were identified from 34 selected L. monocyt ogenes isolates, 2 of which were deemed to be of clinical importance. Liste ria 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 sim ilar to those previously reported for cow's milk.