Listeria testing only takes place once every four years unless there's a reason to check more frequently.
Knowing this, some producers of foods like hot dogs and lunch meats change their sanitation efforts when they know they will be inspected.
The Food and Safety Inspection Services (FSIS) is wise to some food producer's techniques and issued a statement -- stop inconsistent sanitation and work habits.
Some practices include not producing high-risk meats, "drastically" reducing production, and changing equipment to ensure it hadn't been associated with listeria in the past.
The risks are great. If the producers aren't carrying on as usual, they risk skewing the results, and potentially harming consumers.
Listeria is virulent. It can live on manufacturing material for years.