Where are you most likely to get Listeria?

What is it?

Listeriosis is an infection caused by Listeria bacteria, named after Joseph Lister, the surgeon and pioneer of antiseptic surgery. There are 10 distinct species of Listeria; the variant that most commonly impacts humans is Listeria monocytogenes.

Listeria is responsible for around 1,600 illnesses and 260 deaths in the United States yearly, exceeding the fatality rates of Salmonella and Clostridium botulinum. This rate of infection has remained relatively stable over recent years.

Normally caused by eating contaminated food, around 20-30 percent of listeriosis cases are fatal.

Symptoms

The initial symptoms of listeriosis might not become apparent for some time; the incubation period is variable and can be anything from 11-70 days after consuming food with Listeria.

The following symptoms of Listeria infection are likely to last 1-3 days:

• muscle aches
• fever
• flu-like symptoms
• nausea
• diarrhea

For many people, a Listeria infection will pass unnoticed. However, in some individuals, the infection will spread to the nervous system where symptoms might include:

• headache
• confusion
• stiff neck
• tremors and convulsions
• loss of balance

In susceptible individuals, listeriosis can lead to a serious blood infection (septicemia) or inflammation of the membranes around the brain (meningitis).

If the listeriosis infection spreads to the brain, the outcomes can be severe and may include:

• Cranial nerve palsies: Paralysis and tremors.
• Encephalitis: Inflammation of the brain.
• Meningitis: Inflammation of the membranes covering the brain and spinal cord.
• Meningoencephalitis: A combination of meningitis and encephalitis.
• Cerebral abscesses: Localized pus build-up within the brain.

Symptoms during pregnancy or for newborn infants

Pregnant women are 10 times more likely to develop listeriosis than the general population. The incidence of listeriosis during pregnancy is 12 per 100,000, compared with 0.7 per 100,000 in the general population.

While a mother with a Listeria infection may not show any outward symptoms, an unborn child might be severely affected.

Listeriosis can result in miscarriage or premature birth. There is a possibility that a newborn might suffer a life-threatening infection in the days and weeks after birth.

The symptoms in a newborn child can be subtle but may include:

• irritability
• fever
• vomiting
• loss of interest in feeding

Causes

Listeriosis is caused by Listeria, a type of bacteria that is commonly found in water, soil, and feces. Humans are infected when they consume foods that harbor the bacteria.

The most common foods to cause listeriosis outbreaks are deli meats and unpasteurized dairy products. However, many other foods have also been found to spark outbreaks, including caramel apples, cantaloupe, and cabbages fertilized by sheep manure.

Adults who are healthy and have strong immune systems are less at risk of listeriosis. The following are associated with an increased risk of Listeria infection:

• age over 65 years
• AIDS (acquired immune deficiency syndrome)
• existing kidney disease
• chemotherapy
• cirrhosis
• diabetes
• surgical removal of the spleen
• anti-rheumatoid arthritis medication and immunosuppressant drugs

Diagnosis

Listeriosis is diagnosed by a blood test. Urine or spinal fluid might also be tested.

Treatment

For minor infections, medication might not be required. For more serious cases of listeriosis, antibiotics are the most common treatment choice; ampicillin can be used alone or in conjunction with another antibiotic (often gentamicin).

If septicemia or meningitis occur, the individual will be given intravenous antibiotics and require up to 6 weeks of care and treatment.

Prevention

Although listeriosis is rare, there are a number of ways to further reduce the chances of becoming infected; these precautions are especially important during pregnancy:

• Cleanliness wash hands with warm soapy water before preparing food. Clean utensils and work surfaces in the same manner.
• Raw vegetables scrub clean under warm water with a brush.
• Cook thoroughly ensure that meat and egg dishes are heated thoroughly and throughout. A food thermometer can be useful.
• Cheese avoid soft cheeses including Brie, feta, Camembert, blue-veined cheeses, or Mexican-style cheeses such as queso fresco, panela, and queso blanco; unless the label clearly specifies the product was made with pasteurized milk.
• Meat avoid cold meats, especially deli meats and hot dogs, unless cooked at a high temperature before eating. Be careful to wash anything that comes into contact with raw and cold cooked meat.
• Smoked seafood avoid refrigerated smoked seafood unless it is cooked thoroughly before consumption.

Listeria monocytogenes

L. monocytogenes is one of the most virulent food-borne pathogens. It can flourish with or without oxygen and, unlike many other bacteria, can thrive and reproduce at temperatures as low as 0°C.

Listeria has been found in at least 42 species of wild and domesticated animals, and 17 species of birds. The bacterium has been isolated from oysters, fish, crustaceans, ticks, and flies.

Some studies suggest that up to 10 percent of human gastrointestinal tracts contain L. monocytogenes. In short, Listeria is a successful organism.

Its hardy characteristics, combined with its relative ubiquity make Listeria a serious, ongoing concern.

L. monocytogenes typically enters the host through the intestines and infects the liver first. In the liver, the bacteria multiplies until the hosts immune system tackles it; at this stage, the vast majority of infections end. However, in immunocompromised or other at-risk individuals, the infection may continue and eventually move to the brain.

The Listeria bacterium is able to enter and survive within immune cells such as macrophages. L. monocytogenes is also capable of hijacking cellular machinery and tricking cells into drawing the bacterium inside.

Within the cell, the bacteria can multiply and spread to neighboring cells, safe from the immune system. This ability to live within human cells makes it unusual, dangerous, and very difficult to treat.