This recent and very serious Listeria outbreak in the US has been reported by Food Quality News as the worst in modern history with a death toll of 29.
Listeria can, once it takes hold within the food area, be very difficult indeed to eliminate. The organism is associated with areas of wet and standing water, such as floors and drains. Using very high pressure hoses can exacerbate the problem causing micro-aerosols which spread far and wide around the food room, contaminating other surfaces and food equipment.
A higher risk of Listeria within raw materials and ingredients needs to be highlighted within the assessment of hazard significance as part of the HACCP Plan. Then the risk to the food stream and factory can be appreciated and effective controls proposed and implemented. Those controls will centre around the key pre-requisites of selection and maintenance of materials used in construction of the food room, design and hygiene of equipment, and effective, validated cleaning and disinfection procedures using clean and well designed cleaning equipment and chemicals.