An Overview of Listeria

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Listeria infections (listeriosis) can occur from eating food that is contaminated with the listeria monocytogenes bacteria. The infection can cause a stomach upset and flu-like symptoms.

The foods most likely to become contaminated with this type of bacteria are processed meats such as hot dogs and deli meats (prepackaged and at the deli counter), soft cheeses and smoked seafood.

Listeria infections are usually diagnosed based on the symptoms, but laboratory tests can confirm it. This infection can cause complications, so treatment with antibiotics may be necessary. Prevention is key when it comes to listeriosis and other food-borne infections like E. coli and salmonella.

cooking temps to prevent listeria
 Verywell / Gary Ferster

Symptoms

Children and adults can get listeriosis. This infection has an incubation period, which means that you may have a delay between exposure and symptoms. You may have an incubation period of three to four weeks before developing symptoms of listeriosis, and this period can last as long as several months.

Symptoms of listeriosis generally start with gastrointestinal (GI) issues such as stomach cramps, diarrhea, and vomiting.

Other common symptoms include:

  • Fever
  • Muscle Aches
  • Headaches

Advanced disease can cause meningitis, an infection of the meninges (the protective tissue surrounding the brain and spinal cord). Meningoencephalitis, an infection of the meninges and the brain, can occur as well.

Effects of listeria meningitis or meningoencephalitis can include:

  • Stiff neck
  • Confusion
  • Loss of balance
  • Convulsions (seizure)

Meningitis and meningoencephalitis are not typical effects of listeria infection and tend to affect people who have a weakened immune system.

Complications

Pregnant women are among the highest at-risk groups for listeriosis due to a slightly weakened immune system. The symptoms of this infection during pregnancy are similar to the typical symptoms of listeriosis, but pregnant women tend to develop flu-like symptoms more often than GI symptoms.

The infection can lead to serious complications, including:

  • Miscarriage
  • Stillbirth
  • Premature delivery
  • Life-threatening infections in the newborn

Causes

Listeria infections are spread by eating contaminated food. It is not directly contagious from one person to another.

Foods that can become contaminated with listeria monocytogenes bacteria include:

  • Smoked meat
  • Deli meat
  • Processed meat
  • Raw fruit and vegetables
  • Smoked seafood
  • Raw seafood
  • Raw meat
  • Unpasteurized milk
  • Food made with unpasteurized milk, such as soft cheeses

Most healthy people who eat food contaminated with listeria monocytogenes will not get sick or even know they have been exposed. You may develop mild symptoms of the infection, and it can improve on its own without treatment.

There are risk factors that make it more likely to develop serious effects of listeriosis. However, anyone can have a prolonged infection that requires treatment. And anyone can develop the complications of a listeria infection (including death).

Risk factors that predispose to a severe listeria infection include:

  • A weakened immune system: People who have compromised immune systems, due to illnesses like cancer, diabetes, alcoholism, liver or kidney disease, or AIDS are at high risk for complications from listeria infections. People with AIDS are more likely to be affected by listeria infections than the general population.
  • Older age: Elderly adults usually have a weakened immune system, which increases the chances of developing a prolonged or complicated infection after exposure to listeria monocytogenes.
  • Pregnancy: Pregnant women are about 18 times more likely to get listeriosis than the general public. About one in six confirmed cases of listeriosis is a pregnant woman. Listeriosis can cause severe complications for newborns. Babies born to pregnant women who were exposed to listeria monocytogenes can develop the illness—sometimes with life-threatening complications.

Diagnosis

Listeria infections can be diagnosed based on your symptoms, along with diagnostic testing such as blood tests. Sometimes outbreaks are reported, and the contaminated food source may be traceable, potentially resulting in a recall. This is not always the case, however, especially if the infection only affected one person or a few people.

The organism can be grown in a culture from a sample obtained through a blood test. If there is concern about meningitis or encephalitis, a spinal fluid sample can be collected with a lumbar puncture (LP), also known as a spinal tap. And a sample from the placenta can be obtained when there is concern about an in utero infection.

Despite the fact that it commonly causes GI involvement, listeria monocytogenes does not grow in stool culture.

Identifying Listeria on Food

When food contamination is suspected, food samples can be tested to identify the organism. Molecular food testing techniques are done in a laboratory to isolate the listeria monocytogenes genetic material.

Listeria outbreaks are monitored by the Centers for Disease Control and Prevention, so your doctor may report your infection or may ask you to report it.

You may be asked to fill out a questionnaire about the foods you ate before you became sick so that the source can be identified to prevent contamination from infecting other people.

Treatment

If you experience any of the symptoms of listeriosis you should seek medical attention. Listeria infections are treated with antibiotics. Complications may require additional treatment approaches as well.

Antibiotics

The most commonly used antibiotics for the treatment of listeriosis infection are ampicillin or a combination of ampicillin and gentamycin. Bactrim (trimethoprim/sulfamethoxazole) is usually considered if you have an allergy to ampicillin. In some instances, antibiotic resistance can develop, necessitating the use of second-line antibiotics, such as erythromycin.

Typically, oral antibiotics are sufficient, but if you have severe nausea and vomiting, you may need intravenous (IV, injected through a vein) antibiotics.

Treatment for Associated Symptoms

If you have muscle aches or headaches, your doctor may recommend an over-the-counter pain reliever, such as Tylenol (acetaminophen) or Advil (ibuprofen).

Major complications require prescription medical treatment and may entail hospitalization. Seizures may need to be treated with anti-epilepsy medication, while severe inflammation of meningitis or encephalitis may need to be treated with steroids. IV fluids and electrolytes may be necessary if you are getting dehydrated or are unable to eat due to illness.

Listeriosis can result in death, even when treated. Almost all listeria-related deaths are associated with having a profoundly weakened immune system.

Treatment for Listeria Monocytogenes Exposure

If you believe that you could be at high risk for listeriosis, you should talk to your doctor. For example, if you think you may have eaten food that caused others to have listeriosis, or if you already ate food that was recalled due to a listeriosis outbreak, you should see your doctor even before you develop symptoms.

While you might not automatically be started on antibiotics, you may have some preliminary diagnostic tests that can help in determining whether you should take antibiotics. In general, taking antibiotics just for exposure to the bacteria is not considered beneficial.

Prevention

The best way to prevent a listeria infection is to use proper food safety precautions.

Strategies include:

Wash your hands: Washing your hands thoroughly while you are preparing food and before you eat cuts down on the spread of most foodborne illnesses, including listeriosis.

Thoroughly wash food: Contaminated produce can spread listeria. Make sure you thoroughly wash all of your fruits and vegetables (especially those that will not be cooked). Even if produce will be peeled, it should be washed first. Keep raw and cooked foods separate.

Fully cook meat: Undercooked meats are popular, but they increase the chances of foodborne illness.

Meats should be cooked until their internal temperatures reach:

  • Poultry: 165 F
  • Beef: 160 F
  • Pork: 165 F

Ground meat should be cooked until it is brown all the way through and the internal temperature is at least 160 degrees (beef, pork, veal, and lamb) or 165 degrees (turkey and chicken).

Store leftovers at proper temperatures: Storing food at proper temperatures can help prevent the growth of listeria monocytogenes. Refrigerators should be kept below 40 degrees and freezers below 0 degrees. However, keep in mind that listeria can grow on some foods—even if they are properly stored in the refrigerator.

Special precautions if you are at high risk: Because listeriosis can be so serious for high-risk groups such as pregnant women, it is best to avoid certain foods completely.

The CDC recommends that people in high-risk groups not eat:

  • Hot dogs, deli meats, cold cuts or sausages unless they are heated to at least 160 degrees right before serving.
  • Soft cheeses such as feta, brie, or queso blanco unless the label specifies that it was made with pasteurized milk.
  • Refrigerated smoked seafood such as lox or smoked salmon unless it is included in a cooked dish or it is served in a shelf-stable package rather than the refrigerated or deli section.
  • Refrigerated meat spreads or paté (sold in the refrigerated or deli section) that are not shelf-stable.

A Word From Verywell

While, listeriosis can be a serious infection, most of the time, exposure does not cause serious illness. It is important to be aware of food recalls and to pay attention to safe food preparation and handling, especially if you prepare food for a person who is in a high risk group.

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Article Sources

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