Listeria is a bacteria genus that was given the name in 1940, in honour of Joseph Lister, who was a British surgeon that pioneered sterile practices. All species of the genus share a few common traits: they are shaped like rods, can be anaerobic, don't produce endospores and are gram-positive.

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Listeria monocytogenes is the most dangerous human pathogen in the genus. Listeria causes the rare but very serious disease listeriosis, which starts after consuming food contaminated with this species. Severe infections with this germ usually cause gastroenteritis but the consequences are far more dangerous for people with a weak immune system, such as elders, pregnant women, newborns or adults who suffer from other conditions.

Listeriosis is a potentially lethal disease and around 20% of the people who become infected do not survive. In severe infections, the most dangerous clinical issues are meningitis and sepsis. Unlike most other forms of bacterial infections, listeriosis can turn into meningoencephalitis, which means that encephalitis aggravates the effects of meningitis. Listeriosis can very rarely be caused by a different strain, L. ivanovii, which typically infects ruminants and other mammals. The incubation period can be as short as three days or as long as 70 days.

Listeria germs live naturally on soil and in water but also in common animals like cattle or chicken. Listeria can live in raw milk and various dairy products prepared from it. If it becomes established in food processing facilities, it can also contaminate industrially-prepared meat.

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The species is more dangerous than other bacteria because it can survive the low temperatures inside a refrigerator. However, it is killed by pasteurization and most methods of cooking.

Listeria monocytogenes is the bacterium that lives in human food and causes listeriosis, a serious condition. Since it is triggered by eating contaminated food, this condition is a form of food poisoning. The germs start multiplying in the blood stream and later enter the cells in large numbers. However, the central nervous system of people with weak immunity is the most affected, the infection leads to meningitis, encephalitis or even both at the same time. Older persons, pregnant women and infants have a higher risk of disease, due to a low immune response. Pregnant women are especially at risk and the infection can spread to the womb, with very dangerous consequences such as life-threatening infections, premature birth, miscarriage or stillbirth. The bacteria can also infect children and healthy adults but the effects are not very severe.

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In most cases of infection, the condition heals on its own after a few days and there is no need for any specific medical treatment or drugs. However, doctors decide on the best course to follow based on the circumstances of the patient. Infections detected in people with weak immunity, pregnant women or infants require treatment with antibiotics.

Causes and risk factors

There are several ways of contamination with the bacteria but listeria usually starts after consuming contaminated food. Listeria can also be transmitted through the placenta from a mother to her fetus.

Not every listeria infection becomes severe, this depends on several factors. It starts with ingesting the bacteria from food or water but in most cases the immune system kills all the germs in the gut or the stomach. If the system is unable to kill the pathogens, they start to multiply inside the intestines and the disease begins. The condition becomes really serious once the infection reaches the internal organs, blood stream or spinal fluid, with potentially fatal effects.

People with normal immunity show no symptoms after eating the bacteria or being exposed to it because the germs can't penetrate inside their bodies. The infection is only risky for people with reduced immunity, who lack protection.

Symptoms of listeria

Listeria monocytogenes triggers a very fast food poisoning, with the first symptoms visible even the same day when contaminated food was consumed. The incubation is variable and the disease can start after as long as 70 days. However, in most cases the initial symptoms appear after one to four weeks.

Listeria is an unusual type of food poisoning because it lacks the typical symptoms of these conditions, which are diarrhea, vomiting and fever. Listeria is a lot more dangerous and leads to more than digestive troubles. It is a very serious infection that becomes established in the nervous system and other critical parts of the body.

Any listeria infection that manages to expand out of the gut is considered invasive listeriosis. The initial symptoms are muscular pain and fatigue, similar to the ones of flu. The pain can later expand to the neck and joints, other symptoms include headache and fever. Confusion is another common effect, later turning into dizziness and poor balance or even seizures or convulsions in rare cases.

Pregnant women who become infected also develop fever and symptoms similar to flu. The indirect effects are a lot more dangerous, since the condition can lead to premature delivery, miscarriage or stillbirth. The infant can become infected or even die. The symptoms of listeria passed from mother to son are usually very severe.

Many infants are treated for listeria too late because the infection is not easy to identify and by the time they reach a doctor there are already very serious complications such as infections of the brain or blood stream. If a baby is born underweight and there are clear signs of listeria infection, the condition is quite obvious and can be spotted immediately. However, some infants appear to be healthy and the signs are only visible after several weeks, with very dangerous consequences. Fever, low appetite and nervousness are some of the symptoms of listeria infection in a baby who otherwise appears to be healthy.

Be very careful if your baby was exposed to contaminated food or made contact with someone suffering from listeria. In such cases, go to a doctor immediately and tell him that you suspect a possible infection. You should also give him a reason, for example if the baby consumed food that was recalled later due to contamination.

How to prevent a listeria infection?

There are a number of general precautions that can reduce the risk of infection. Always cook raw food of animal origin and separate the raw ingredients from any cooked food or other products. Raw milk or various daily specialties that include it should be avoided. It is a good idea to eat any ready to eat or fresh foods as soon as possible, make sure that you wash raw fruits and vegetables very well. Also wash your hands as well as any knives or plates that have been in contact with raw ingredients.

Pregnant women and people with a weakened immune system are especially at risk and must be even more careful. Avoid any luncheon, deli meats or hot dogs, except if they are very well cooked at high temperatures. Many soft cheeses are made from raw milk, don't consume Brie, Camembert, Mexican-style cheese, blue-veined or blue varieties unless the label specifies they are made from pasteurized milk. The refrigerated or deli sections of stores include varieties of meat spreads or smoked seafood that must be avoided as well; you can eat them if they are canned. Wash your hands carefully after touching any of these products.

Perishable foods can be contaminated quickly, so store them properly as soon as you buy them. Ready to eat foods and raw meats should not stay out for more than two hours, either cook them or place them inside the refrigerator or freezer. Make sure that your refrigerator maintains a low temperature. However, listeria can survive inside the refrigerator, unlike other bacteria. Any juices or spills, for example the one from raw meat, must be cleaned immediately. Remember to always wash your hands after contact with ingredients.

Fresh vegetables and fruits must also be washed with plenty of water. Don't use the same cutting board for vegetables and raw meat to avoid contamination.


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