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Listeria and the Dangers in Pregnancy

By: Kathryn Senior PhD - Updated: 4 Sep 2012 | comments*Discuss
Listeria Listeriosis Listeria

Listeria is a type of food poisoning caused by the bacterium Listeria monocytogenes. This is one of six known species in the Listeria family, which was named after the famous English surgeon Joseph Lister (1827 – 1912). Lister is famous for his theories that operations should be carried out during clean and sterile conditions and he was the first to use carbolic acid as an antiseptic. None of the other Listeria species are pathogenic in humans.

Listeriosis is fairly rare, seen in approximately 7 in every million people in the population. However, it has been the subject of worry in the last few years because of the increase in the number of outbreaks, and because this type of food poisoning is particularly harmful if it occurs in either pregnant women or the very elderly.

About Listeria Monocytogenes

The bacterium itself is not normally present in humans. It occurs in soil, the water of slow flowing streams, in sewage and on plants. If food becomes contaminated with Listeria, the bacteria can grow at refrigerator temperatures, making it a particular hazard for cook-chill or foods eaten without cooking. Cheeses such as Camembert, Brie and other soft cheeses have been implicated in many cases of Listeriosis.

Why is Listeria Dangerous?

The bacterium is a gram positive bacillus that is very invasive once it gets into the body. It lives inside cells, rather than remaining outside like many of the bacteria that cause food poisoning. It can spread through the body and is attracted to the nervous system. Its high death rate of around a quarter of people who become severely infected is due to the meningitis that it causes. It is also able to cross the placenta of pregnant women, which is why it is so dangerous in pregnancy. The Listeria infection in the mother passes directly into the bloodstream of her unborn baby and the foetus becomes massively infected and often dies. A spontaneous miscarriage then takes place.

Symptoms of Listeriosis

In normal healthy adults who eat food contaminated with Listeria monocytogenes, the symptoms are usually mild and involve a stomach upset, usually with diarrhoea, perhaps nausea and vomiting, and/or mild flu like symptoms, including a stiff neck. Even in this group of people who are most able to fight off the bacterium there are cases where the infection spreads to the bloodstream, causing septicaemia, and to the brain, causing meningitis.

Those at Risk

If contaminated food is eaten by a pregnant woman, she may experience only mild flu-like symptoms but the infection can pass rapidly to the unborn baby causing miscarriage of a dead foetus. If the baby is nearly at term, it is usually born early, alive but very sick, as its tissues have been heavily infected. When this was recognised a few years ago, the government in the UK issued warnings to pregnant women to avoid eating soft cheeses and other foods that might harbour Listeria bacteria.

However, this advice did not extend to other risk groups, and perhaps it should. The very elderly and infirm, people undergoing immunosuppresive therapy for cancer or organ transplantation, or who have an underlying medical condition that affects the immune system, should also avoid foods that might contain Listeria monocytogenes. AIDS patients in the USA, for example, are among the population groups who are most likely to contract Listeria food poisoning in its most severe forms.

Listeria and Food Safety

A large outbreak of Listeria food poisoning occurred in Canada in the summer of 2008, killing 20 people and this led to serious questioning of the food supply chain in Canada and beyond. All the fatal cases and the confirmed infections in people who survived were traced back to meat products made by a factory in Toronto. Since the outbreak, the Canadian Food Inspection Agency has taken the decision to bring in more stringent rules for food companies in Canada to test meat products for the presence of Listeria. All affected foods would be immediately destroyed, preventing them from getting into the food chain. Other countries may follow Canada’s lead.

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