We report the results of clinical and microbiological investigations on Listeria monocytogenes infections in a flock of 55 sheep and describe the implications for the safety of the raw milk and raw-milk cheeses produced in the on-farm dairy. The outbreak was caused by feeding grass silage, which was contaminated with 5 log10 CFU L. monocytogenes/g. Clinically, although having been fed from the same batch of silage, abortive (nine ewes), encephalitic (one ewe) and septicaemic (four ewes) forms of listeriosis were observed during the outbreak phase. As the starting point of feeding the contaminated silage was known we could calculate an incubation period of 18+/-2 and 26 days for the abortive and the encephalitic form of listeriosis, respectively. Pathologically, the septicaemic cases suffered from Listeria accumulation at comparable numbers in visceral organs but not in the brain. Only a single ewe developed central nervous symptoms and a rhomb-encephalitis was immunohistologically confirmed. In this case the infection proceeded from the nasal mucosa into the brain, with no infections of the liver, spleen and other visceral organs. Sampling of the cheese production chain, the farm environment and the persons living at the farm revealed the exposure of a farm-worker to an isolate genetically indistinguishable from the outbreak clone, obviously through the consumption of faecally contaminated bovine raw milk. The cheese under processing was free of Listeria because, as a result of intensive consultations, the farmer ensured a proper acidification of the cheese. The epidemiological findings suggest that food safety matters should be assessed in any case where infection of food-producing animals with potential human pathogens is observed.

The aim of this study was to determine the transmission routs of Listeria spp. in dairy farms manufacturing fresh cheese made from ovine and caprine raw milk and to evaluate the impact of Listeria monocytogenes mastitis on raw milk contamination. Overall, 5,799 samples, including 835 environmental samples, 230 milk and milk product samples, and 4,734 aseptic half-udder foremilk samples were collected from 53 dairy farms in the dairy intensive area of Lower Austria. Farms were selected for the study because raw milk was processed to cheese that was sold directly to consumers. A total of 153 samples were positive for Listeria spp., yielding an overall prevalence of 2.6%; L. monocytogenes was found in 0.9% of the samples. Bulk tank milk, cheese, and half-udder samples were negative for Listeria spp. Because none of the sheep and goats tested positive from udder samples, L. monocytogenes mastitis was excluded as a significant source of raw milk contamination. L. monocytogenes was detected at 30.2% of all inspected farms. Swab samples from working boots and fecal samples had a significantly higher overall prevalence (P < 0.001) of L. monocytogenes (15.7 and 13.0%, respectively) than did swab samples from the milk processing environment (7.9%). A significant correlation was found between the prevalence of L. monocytogenes in the animal and in the milk processing environment and the silage feeding practices. Isolation of L. monocytogenes was three to seven times more likely from farms where silage was fed to animals throughout the year than from farms where silage was not fed to the animals.