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Listeria Food Poisoning

Dairy Pipeline: June 2000

Gerald M. (Jerry) Jones
Professor and Extension Dairy Scientist
Milk Quality & Milking Management
Virginia Tech
(540) 231-4764

Recent recalls of certain meat products resulted from contamination by Listeria monocytogenes, a bacteria which has been found in raw and pasteurized milk, ice cream, certain soft cheeses, poultry, fish, meat, and vegetables. It is the second major bacterial cause of foodborne death after Salmonella but higher than Campylobacter and E. coli. The severity of Listeria food poisoning can cause hospitalization for 90% of cases. As many as 10% of humans may be intestinal carriers of the bacteria. The majority of cases have been attributed to improper food handling, cooking, or storage in the home or food service operation, BUT it can be found as an infection of dairy cows. It has been reported that 2% of raw milk world-wide contains Listeria and that 16% of dairy cows are infected. Although usually inactivated by pasteurization, large microbial loads can make pasteurization ineffective. Listeria bacteria can be isolated from cows with mastitis, manure, soil, and in poorly fermented, moldy silage. Besides mastitis, it can cause abortions, circling disease, and death in animals. Mastitis usually occurs as chronic subclinical cases with occasional clinical flare-ups. New mastitis infections can be spread through dirty udders and teats, milkers' hands, and milking equipment. Attention should be paid to cleanliness of cows and loafing/exercise areas and use of clean and dry towels for cleaning teats before milking. These recommendations are important in prevention of any kind of mastitis. Silage should be properly preserved so that it will undergo desired fermentation. This includes dry matter content, rapid filling of tower silos and packing of horizontal silos to minimize air entrapment, and length of chop. Be sure to feed enough off the top or face to prevent spoilage. Pathogenic (disease-causing) microorganisms are ranked by scientific experts as the number 1 potential risk to public health, with 3-14% of the U.S. population becoming ill every year due to bacterial contamination of food somewhere between farm and table.

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