That was a major issue with mad cow.
Canada was using sick, processed animals in the feed.
To reduce risk of listeria:
Reheat until steaming hot the following types of ready-to-eat foods: hot dogs, luncheon meats, cold cuts, fermented and dry sausage, and other deli-style meat and poultry products. (Also, salmon and crabmeats)...Thoroughly reheating food can help kill any bacteria that might be present. If you cannot reheat these foods, do not eat them.
Wash hands with hot, soapy water after handling these types of ready-to-eat foods. (Wash for at least 20 seconds.) Also wash cutting boards, dishes, and utensils. Thorough washing helps eliminate any bacteria that might get on your hands or other surfaces from food before it is reheated.
Do not eat soft cheeses such as feta, Brie, Camembert, blue-veined or Mexican-style cheese. You can eat hard cheeses, processed cheeses, cream cheese, cottage cheese, and yogurt.
Do not drink raw, unpasteurized milk or eat foods made from it, such as unpasteurized cheese.
Observe all expiration dates for perishable items that are precooked or ready-to-eat.