Research on listeriosis in milk cows with intramammary inoculation of Listeria monocytogenes.

The purpose of our work was to produce 600 litre quantities of milk containing (per ml) 5000 viable Listeria monocytogenes, for certification of standard pasteurization technique. Preliminary research indicates deep (10 cm) inoculation via teat canal with 3 injections, (10(3), 10(5), 10(7)) per 10-day interval, was better than intraocular or per os route for establishing chronic L. monocytogenes mastitis. Although milk yield diminished 25%, chronic L. monocytogenes shedding persisted for 8 months. Sheeding in individual cows varied from 10(3) to 10(5) listeriae/ml. Dexamethasone injections increased L. monocytogenes shedding in milk ca. 1000-fold. Three of 30 cows were pregnant during the experiment and produced normal calves, indicating that intramammary L. monocytogenes infection was localized and did not migrate to the pregnant uterus. Four of 30 cows developed CNS symptoms of listeriosis, and one died. From 11 cows, L. monocytogenes was isolated at necropsy, from mammary tissues (9/11), supramammary lymph nodes (9/11), liver (6/11), spleen (5/11), and brain (6/11) by cold enrichment culture. Serum antibody titres of infected cows increased from 1: 160 to 1: 10,280 in most cows. Large scale pasteurization trials led to the conclusion that the present pasteurization guidelines of the FDA are adequate to inactivate listeriae in whole milk.