Immunocompromised humans such as pregnant women or the elderly are highly susceptible to virulent Listeria. It has become a leading cause of infection in kidney transplant patients. The bacillus can also cross the placenta of pregnant women and cause abortion, still birth, neonatal death, and birth defects. In humans, ingestion of the bacteria may be marked by a flu-like illness or symptoms may be so mild that they go unnoticed. Symptoms include fever and chills, headache, upset stomach and vomiting. A carrier state can develop. Death is rare in healthy adults; however, the mortality rate may approximate 30 percent in those with weak immune systems, new born or very young.
As mentioned earlier Listeria monocytogenes is a special problem since it can survive adverse conditions. It can be in a variety of raw foods as well as in processed foods and foods made from unpasteurized milk. It can grow in a pH range of 5.0-9.5 in good growth medium. The organism has survived the pH 5 environment of cottage cheese and ripening cheddar. It is salt tolerant surviving concentrations as high as 30.5 percent for 100 days at 39.2 degrees F, but only 5 days if held at 98.6 degrees F.
The key point is that refrigeration temperatures do not stop growth of Listeria. It is capable of doubling in numbers every 1.5 days at 39.5 degrees F. Since high heat, greater than 170 degrees F, will inactivate the Listeria organisms, post-process contamination from environmental sources then becomes a critical control point for many foods. To reduce your risk:
- Use precooked and ready-to-eat foods as soon as you can
- Avoid raw milk and raw milk products
- Heat ready-to-eat foods and leftovers until they are steaming hot
- Wash fresh fruits and vegetables
- Avoid rare meat and seafood