Many people who watch the news can recall seeing or reading about a listeria outbreak every year or two. While this bacteria can involve a large variety of foods, in most cases, listeria may be relatively minor. This foodborne illness, caused by L. monocytogenes, is more likely to show up in certain groups of people. Most adults and children can handle periodic exposure to listeria without significant effects, but it can be serious or even fatal for pregnant women, newborns, older adults and anyone with a compromised immune system.
Although listeria is a low risk for the vast majority of the population, people should still know the common symptoms and pay attention if they show up after a possible exposure. It usually comes after eating cured meats or raw foods, especially if they were not prepared or stored properly after processing. Even fresh fruits and vegetables may carry listeria. Prompt treatment for listeriosis, a condition that may develop from listeria infection, is key to reducing long-term health effects like septicemia or meningitis. A strict adherence to food safety is the best way for manufacturers and others to prevent the spread of listeria to themselves or others.
Table of Contents
- Background of Listeria
- Listeria and Ice Cream
- Symptoms of Listeria
- Additional Research
- Be Aware and Informed About Listeria
- Other Helpful Resources
Background of Listeria
Listeria is a type of bacteria that can be very difficult for people to deal with. This is partially because listeria does not always act the way people expect bacteria to operate. After noticing outbreaks of a particular condition in animals, researchers were able to identify one type of listeria in 1924. Since then, scientists have identified more than 20 individual strains. The name comes from a British researcher, Joseph Lister, who pioneered modern efforts to sterilize medical spaces.
Most commonly, listeria is found in soil and water, meaning listeria often transfers to humans through consumption of food grown in soil. Additionally, animals can carry the bacteria. As a result, meat products and other animal products like milk or cheese could also contain the bacteria.
Listeria is unique in that it can still grow in cold temperatures. This is partly why it can survive in soil as long as it does. It also explains how listeria grows well in raw milks and cheeses. Many types of bacteria will not grow in temperatures of 40 degrees Fahrenheit or lower. Listeria is notably different in this aspect.
Listeria can remain in the system for a significant period of time. The major risk to humans concerns the development of listeriosis, a common bacterial disease, which can be fatal to certain vulnerable populations. However, listeria does not always cause this condition.
Listeria is classified by its shape and function. For example, it is rod-shaped, which makes it different from the bacteria that causes conditions like strep. It is also gram-positive, which indicates it can be identified by use of a particular type of test. One of the most important things to know about listeria’s composition is that it can switch from aerobic function to anaerobic function, which means it does not need a steady supply of oxygen to survive, making it a generally heartier form of bacteria.
How People Become Infected
In almost all cases, humans become infected with listeria through their consumption of food. People are less likely to eat contaminated soil or drink untreated water, even though listeria may be in both of these places. Because listeria can only be killed by raising a food item to a sufficiently high temperature, most people get the bacteria by eating:
- Raw fruits and vegetables
- Raw milks and cheeses
- Undercooked meat
- Cold meats that have not been stored properly
Animals are a common source of the bacteria’s transmission. Because listeria can lie dormant in a system for months, animals may not necessarily show symptoms unless they are specifically tested for listeria, meaning animals used for food may carry the bacteria for long periods of time. If they are butchered and the meat is not properly cooked, then listeria may pass to people who eat it. Living farm animals also shed the bacteria in their feces. If it is used in fertilizer, it could continue to spread to produce that people eat. Because the development of serious conditions resulting from listeria is rare, food manufacturers and processors may have a problem for months before they realize it.
The temperature at which listeria is most able to spread within the body is a factor in the seriousness of the condition. Specifically, listeria requires a temperature of about 102 degrees Fahrenheit for optimal spread, which is fairly close to the average temperature of humans and other animals. In most cases, listeria bacteria are destroyed by the body’s immune system. For very young children, those who are pregnant or people taking immunosuppressive medications, this response may be insufficient, which is why they are at a higher risk for developing listeriosis.
Listeria enters a cell through the use of a variety of methods. In some cases, it prompts the cell to swallow it whole without destroying the bacteria. In other cases, it uses proteins called internalins to make cell receptors more likely to receive it. This flexibility can make it particularly harmful if it succeeds.
If any of the bacteria are able to invade a cell, then they can continue to replicate and spread throughout the body without calling attention to the immune system, allowing for a quicker spread and a faster accumulation of symptoms. The way this works is through the disruption of normal cell function, where the bacteria begins by invading the cell. It then takes control over the cell and prompts it to move. At the right temperature, it develops things on the cell membrane that resemble rockets. Once these protrusions come in contact with another cell, they are able to invade that cell and continue the process. It remains inside the cell structure from this point forward, making it more difficult for the immune system to find it and fight it.
Listeria and Ice Cream
If listeria is supposed to be killed by pasteurization methods, then people may find it confusing why products like ice cream can be a possible source of listeria. While it is relatively unlikely that pasteurized animal products can carry listeria, it depends mostly on how they are produced, packed, and handled after manufacture. Periodically, people may read about serious cases of listeriosis developing after someone ate contaminated ice cream. For example, there were several serious cases in 2015, some of which were fatal, tied to a particular brand of ice cream. Although this is quite uncommon, the FDA claims the presence of listeria in manufacturing equipment is higher than most people would expect.
Most problems related to listeria spreading through pasteurized animal products come from a failure to properly clean and sanitize processing equipment. For example, if someone has listeria, they may shed the bacteria after going to the bathroom. If they do not wash their hands properly, anything they touch may carry the listeria bacteria. Although most bacteria cannot survive in cold or freezing temperatures, listeria is not one of them, which means listeria may remain on equipment or inside the ice cream as it travels from the manufacturer, to the sale outlet, to a person’s home. Listeria may also transfer to the ice cream through the use of contaminated foods like raw fruits.
As a way of protecting against this, manufacturers can enforce high standards for sanitary working conditions, and regularly test for the presence of the bacteria. Additionally, they may choose to contract with suppliers who regularly test their raw supplies for listeria. This is not something people must wait until they contract an infection and develop listeriosis before they can become aware there is a problem. By taking precautions like sanitizing all surfaces and ensuring employees practice safe sanitation while they are working, manufacturers have a better chance of minimizing the effect that listeria can have in this industry.
Symptoms of Listeria
The symptoms of listeriosis depend on the person and the seriousness of the disease. Once a person consumes listeria through food, it usually accumulates in their liver. If the immune system does not defeat it, then it can turn into listeriosis. Although most people do not suffer serious problems from listeria, it can be a deadly condition in certain groups of people. This is why pregnant people, parents of young children, older adults, and anyone with a compromised immune system should take extra care when handling raw foods.
Pregnant Women and Newborns
For reasons that researchers do not quite understand, listeriosis is much more common in people who are pregnant. Pregnant women represent as much as one-quarter of all listeriosis cases worldwide and are about 20 times more likely to get it than another healthy adult. The type of symptoms people are likely to encounter, and their severity, depends on when the person encounters the condition related to their stage of pregnancy. For example, women who get listeriosis near the end of their pregnancy are less likely to suffer serious effects as a result. However, newborn babies who are born while their mothers are dealing with listeriosis are at great risk.
Part of the risk with listeriosis and pregnancy is that pregnant women do not tend to get the same symptoms most people dealing with listeria may notice. For example, the common symptoms of listeria in healthy adults usually include gastrointestinal discomfort. However, people who are pregnant typically do not report nausea or diarrhea. Instead, they are more likely to observe these issues:
- Muscle aches
This means pregnant women may take longer than necessary to get a diagnosis while doctors try to figure out what is going on. Because listeria starts out in the liver and can pass to other organs, including the brain, seeking treatment promptly is extremely important. However, pregnant women may not always notice the serious effects of listeriosis. Because the bacteria can cross the placenta, listeria can be extremely dangerous or even fatal to a developing fetus and is associated with a higher risk of pre-term delivery and other serious conditions like pneumonia or meningitis. Listeriosis may also cause these and other problems in older adults and those with a weakened immune system, including blood infections and encephalitis.
Healthy Children and Adults
Most healthy children and adults do not need to worry too much that they will develop listeriosis. Although listeria is a relatively common bacteria that can survive in soil, water, and a variety of common foods, most people’s immune system can fight it effectively. It is worth keeping in mind that like any other bacteria, listeria is dependent on the acceptance of the host body. People who can normally fight off infection without too much trouble may consume listeria and have no further issues.
Listeriosis among adults and children is rare but not completely unheard of. People who developed listeriosis after exposure to listeria may start to notice symptoms as quickly as one day after exposure. However, it may take up to two months or longer before symptoms become serious enough to take action. These symptoms include:
- Stiff neck
For most people with a low risk of developing listeriosis, these symptoms could seem like a simple gastrointestinal flu or a quick virus. Most people in this category will not go on to develop listeriosis, although they can continue to pass the bacteria to other vulnerable populations while it lives in their systems.
Once a person has listeriosis, the condition can be very severe. In fact, listeriosis cases have a fatality rate of around 20 percent. People who pass off the symptoms as something minor may allow them to get worse. The conditions that can develop as a result, like meningitis, can move extremely rapidly through the body. As such, focus for medical experts resides on prevention, prompt identification of the disease, and a quick treatment to minimize the chances of mortality.
Animals have a different risk profile for listeriosis than humans, but the severity depends on the species. Cats, dogs, and most other pets are at comparatively low risk for listeriosis. If they get the bacteria, they may show no symptoms. Animals people likely find on a farm, like cows, goats, or sheep, have a higher risk. Sheep in particular can suffer greatly as a result of listeriosis, especially with the subsequent condition encephalitis. As a general rule, people will notice the symptoms in these herd animals by observing signs like difficulty staying upright or walking.
Rabbits and rodents are also at risk for developing listeriosis. As with other animals, the presentation of symptoms is quite different from humans. For example, rabbits and chinchillas are more likely to suffer from harm to the reproductive system, including early delivery and uterine inflammation. If these animals are kept in captivity, then the disease is more likely to occur and spread quickly than for wild animals. If animals are killed for food and cooked to an insufficient temperature or eaten raw, then their meat can transmit the disease to humans.
Prevention of Listeria
The best way people can prevent listeria is by using sanitary practices in the harvesting, preparation, and consumption of foods that are likely to contain listeria. While the bacteria can survive for long periods of time at cold temperatures, it dies during the pasteurization process. As a result, people who are at higher risk should consider avoiding eating food such as:
- Unwashed, raw fruits and vegetables
- Raw milks and cheeses
- Cured meats
- Undercooked meats
They may also wish to follow up with manufacturers of foods that are not meant to be cooked at home. These foods are also at risk for listeria, even if they are kept cold or frozen during transportation and sale. Before people buy food, they may want to research manufacturers to confirm their track record for sanitation of equipment and packaging as well as how often they test raw materials for listeria.
Otherwise, people might want to pay close attention to food recalls. Periodically, manufacturers or sellers of food may notify the public their locations have tested positive for listeria. This may affect a portion of what they sell — or all of it. It is the responsibility of customers to read about these recalls and take quick action if they realize they have a product in their homes that meets the terms of the recall. If they have already consumed the affected food and have symptoms, then it may be worth seeking testing, particularly for those who are at higher risk for listeriosis.
Pasteurization and careful attention to sanitation are extremely important. People may prefer to cook all meats all the way through before consuming, and consider boiling or steaming fruits and vegetables. Using a dishwasher on the sanitization setting will help to kill lingering bacteria on cutting boards and other kitchen tools. All of these steps may be necessary to prevent listeriosis.
Diagnosis and Treatment of Listeria
As a general rule, most people do not need to be tested for listeria. In the vast majority of cases, listeria infections cause no symptoms or very minor ones, like nausea, for a few days. Even vulnerable populations may not require testing until they have observed specific symptoms that call for treatment. In many infections, symptoms will arise within a few days of exposure. For others, symptoms may take longer.
Diagnosis usually happens through a test at the doctor’s office. It is commonly a blood test but may also involve other bodily fluids like urine or spinal fluid. Although symptoms can progress rapidly, doctors often recommend patients wait until they get a positive test before they start oral antibiotics. This depends on the current state of symptoms, with a high fever prompting medical professionals to start a course sooner. The antibiotics will kill the bacteria, but they usually take time. Some individuals with a more aggressive case of listeriosis may need 2-3 weeks of IV antibiotics to defeat the infection. The antibiotics typically include penicillin-based drugs, although doctors may choose to prescribe something else if the first course does not work or if the patient is allergic to penicillin.
What if Symptoms Go Away Before Seeing a Doctor?
Because most healthy people can fight a listeria infection on the strength of their own immune systems, following up with testing after exposure is not always necessary. This is often true even for some people who start to show symptoms after exposure. If the nausea or diarrhea go away, healthy people have no need to go to the doctor for testing or preventive treatments. Instead, they may choose to continue to monitor their symptoms at home, paying attention to signs of serious problems like a high fever or a stiff neck.
How Long Does Listeria Last When Being Treated?
Although some bacteria can be eliminated in the body after a few days of antibiotics, listeria tends to take a little longer. Typically, people will need to take a course of antibiotics that lasts at least 14 days. If this does not kill the bacteria and trigger significant relief of symptoms, then doctors may prescribe a second course of antibiotics or try something stronger. It is worth noting many people who require additional care are at higher risk for listeriosis or have had it longer than most. People should make sure to take their antibiotics on time each day until the course is complete, because missing doses can cause the bacteria to come back stronger.
Although listeria can be present in such a wide variety of foods, there are certain industries that have a higher chance of passing bacteria. Many of these have taken the threat seriously, taking advantage of independent research to provide information about the spread of listeria and the best ways to prevent or contain it. In many cases, companies or government organizations took interest after a significant outbreak of listeria tied to a particular type of exposure. Additional research helps to target the extent of the problem and spurs industry influencers to action. This data also helps people determine where their risk is highest.
Provian Proven to Protect Processed Meat
Since listeria is most likely to be a problem for fresh and processed ingredients, the use of certain preservatives may be helpful in preventing the growth and spread of listeria. For example, products like Provian use salt-based preservatives to help food stay fresh longer and be more hygienic until they are ready to eat. Provian is an antimicrobial acetate-based powder that can be applied to or used inside meat products. This increases their shelf life and makes it more difficult for listeria to survive.
Although all manufacturers of processed meats should take great care in ensuring their equipment is sanitized regularly and that they package, store, and ship products at refrigerator temperatures and below, this is not enough to prevent the growth of listeria. Innovation in this industry has allowed manufacturers to find ways to keep cured meats safe for consumption without adding negatively to the taste or texture. The use of products like Provian can be effective at preserving the meat without adding unnecessarily to the sodium content, a common problem in cured meats.
Listeria Prevention in Fish-Processing Plants
A systematic approach to preventing listeria outbreaks in industries where the bacteria is a common problem is also proven to have beneficial effects. Like ice cream, the processing, packaging and shipping of fish products is a place where listeria can thrive. Without a careful plan, this industry can put a significant number of people at risk. This is particularly important for fish that is not cooked to a high temperature before packaging to ship or serving. Fish-processing plants have several points of concern, including:
- Processing equipment
- Workers processing and packaging the fish
- The packaging itself and the shipping environment
Many types of fish available on the market are not cooked prior to serving. People may choose to eat them raw or cold-smoked. Fish smoked at a lower temperature, and raw fish that is packaged in a vacuum-sealed package by the manufacturer, are more likely to contain listeria than many other types of meats.
An increase in the numbers of listeriosis cases coming from countries like Finland, which exports a significant amount of fish products, caused industry experts to revisit and change their sanitation practices. To combat this problem, the country implemented strict international standards for food safety, which involves regular testing of manufacturing equipment and processes for listeria, and implementing penalties for businesses that violate these protocols. Regulators increased the frequency and detail of their annual inspections, with a goal of providing better information about the state of food safety. Ultimately, persuading businesses their product lines can be at risk for listeria has made it easier for people to make the necessary changes. This resulted in pressure to take listeria seriously, which led to noticeable improvements in the industry.
E. coli, Listeria, and Salmonella in the Kitchen
Although people might expect to get sick from listeria after eating something at a restaurant or open place like a farmer’s market, they are also likely to get a foodborne illness from food they eat at home. In fact, improper food storage and sanitation in a residential kitchen can take what might have been a minor problem and turn it into a crisis. The problem lies partly in people’s misunderstanding of the most common sources of listeria as well as a failure to properly cook food or sanitize a surface before food preparation.
People identify refrigerator drawers as a place where listeria may thrive, and this is correct. What they may not know is there are many kitchen tools, like a knife block or blender, that are also likely to have listeria. Anything that comes into contact with raw produce or meat can carry listeria, so if people do not clean those tools properly by sanitizing them and storing them in a clean place, then they may spread listeria. This is why people should avoid using the same cutting board for meats and produce, properly wash raw foods before eating, and find antimicrobial cleaning products for regular use.
Be Aware and Informed About Listeria
Most people may never notice a negative effect from listeria. The dynamic of how listeriosis selectively targets the population makes it quite a rare disease, unless someone is pregnant, older, or takes medications that suppress their immune system. However, people should still take the risk seriously for their vulnerable friends and family members if not themselves. Listeria is just one foodborne bacteria that can make people sick. Trying to prevent its spread can help minimize the likelihood of catching other conditions like E. coli or salmonella as well.
Listeria develops in soil and spreads to produce and animals through consumption of water or food, making listeria more likely to spread to humans when they eat raw produce and undercooked meat or animal products. Understanding what causes listeria and how food safety can reduce the risk is important to keeping healthy. People may contract and recover from listeria without ever knowing the cause of their symptoms. Sanitizing food preparation equipment, requiring manufacturers to maintain proper food safety practices, and avoiding high-risk foods when needed is an ideal way for people in the most vulnerable populations to stay safe.
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