What is listeriosis?
Listeriosis is a bacterial infection caused by the bacterium Listeria monocytogenes. Until several years ago it was thought that this bacteria only infected animals but it is now accepted that humans are also at risk from this disease. While many bacteria are generally seen to infect specific locations within the human body, Listeria may infect many different locations, such as the brain or the spinal cord membranes or the bloodstream.
L. monocytogenes is a gram positive, non-spore forming, motile, facultatively anaerobic, rod shaped bacterium. It is catalase positive, oxidase negative, and expresses a Beta hemolysin which causes destruction of red blood cells. This bacterium exhibits characteristic tumbling motility when viewed with light microscopy.  Although L. monocytogenes is actively motile by means of peritrichous flagella at room temperature (20-25C), the organism does not synthesize flagella at body temperatures (37C). 
Who gets listeriosis?
Anyone can get this disease, but those at highest risk for serious illness from this bacterium are newborns, the elderly, people with weakened immune systems, and pregnant women. Healthy adults and children occasionally get infected with Listeria, but they rarely become seriously ill. Listeria Monocytogenes is classified as an intracellular paracite. This means that it invades and lives within cells of the body thereby managing to evade the body’s immune system. Infection by as few as 1000 individual bacteria is considered enough for the disease to take hold. When listeric meningitis occurs, the overall mortality may reach 70%; from septicemia 50%, from perinatal/neonatal infections greater than 80%. In infections during pregnancy, the mother usually survives.
When do Listeria infections occur?
Infections occur throughout the year. Although most cases occur sporadically, food-borne outbreaks of this disease do frequently occur. Poor food hygiene and poor personal hygiene conditions are responsible for many of the recently recorded outbrakes.
How is listeriosis spread?
Listeria bacteria are widely distributed in nature and can be found both in water and soil. Infected animals may also serve as sources of contamination. Unlike other organisms, Listeria can be spread through several different methods. Ingestion or food-borne transmission of the organism, such as through the ingestion of unpasteurized milk or by the eating of contaminated vegetables, is often a source of many cases. In newborn infections, the organism can be transmitted from mother to fetus in utero, or directly to the fetus at the time of birth through the contact of the fetus’ blood supply with that of the mothers. Direct contact with the organism can cause lesions on the skin.
What are the symptoms of listeriosis?
Because listeriosis can affect many different parts of the body, the symptoms vary. For meningoencephalitis, the onset can be sudden with fever, intense headache, nausea, vomiting and signs of meningeal irritation. In other body locations, various types of lesions at the site of infection are the primary symptom. In most cases, Listeria infection causes fever and influenza-like symptoms resembling a host of other illnesses.
How soon after exposure do symptoms of listeriosis appear?
Listeriosis has an extremely variable incubation period. It can range from 3 to seventy days, but symptoms usually will typically appear within a month of infection.
How is listeriosis diagnosed?
Specific laboratory tests are the only way to effectively identify this disease. Since many cases may be mild, the disease may be much more common than is realized.
Are there any unusual features of listeriosis?
Listeria infections are a significant risk for pregnant women, who may not experience obvious symptoms. Infection of the fetus can occur before delivery and can cause abortion as early as the second month of pregnancy, but more often in the fifth and six months. An infection later in pregnancy may cause exposure during birth, sometimes resulting in infection of the newborn child which may be fatal.
Does past infection with Listeria make a person immune?
Past infection does not appear to produce immunity.
What is the treatment for Listeria infection?
Several antibiotics are effective against this organism. Ampicillin, either on it’s own or in combination with other types of antibiotics, is frequently used.
What can be done to prevent the spread of Listeriosis?
Since the organism is widly distributed throughout nature, basic sanitary measures such as only using pasteurized dairy products, by only eating cooked meats and washing hands thoroughly prior to the preparation of foods offer the best protection against infection by this disease.
In addition, the following recommendations are for persons who are categorized to be at high risk of infection, such as pregnant women, the elderly and persons with compromised immune systems:
Do not eat hot dogs, luncheon meats, or deli meats, unless they are reheated until they are steaming hot.
Avoid getting the liquid from hot dog packages onto other foods sources, utensils, and food preparation surfaces, and remember to wash hands after handling hot dogs, luncheon meats, and deli meats.
Do not eat soft cheese products such as feta, Brie, and Camembert, blue-veined cheeses, or Mexican-style cheeses such as queso blanco, queso fresco, and Panela, unless they clearly state they are made from pasteurized milk written on the labelling of the product.
Do not eat chilled pâtés or meat spreads. Canned or shelf-stable pâtés and meat spreads can be eaten.
Do not eat refrigerated smoked seafood, unless it is contained in a cooked dish, such as a casserole. Refrigerated smoked seafood, such as salmon, trout, whitefish, cod, tuna or mackerel, is most often labeled as “nova-style,” “lox,” “kippered,” “smoked,” or “jerky.” The fish is found in the refrigerator section or sold over deli counters of grocery stores and delicatessens and supermarkets. Canned or shelf-stable smoked seafood may be eaten.