What is Listeria Monocytogenes?

Listeriosis is an infection of humans, commonly caused by Listeria Monocytogenes. The causative agent  for this disease was first isolated from rabbits and guinea pigs in 1926.  L. ivanovii and L. grayi  of the genus Listeria have also shown to cause this disease in humans.

What Are The Symptoms Of Listeriosis?

Listeriosis is often characterized by headaches, fever, muscle aches, diarrhea, stiff neck, confusion, loss of balance, convulsions, and some other gastrointestinal symptoms. High risk patients  usually carry  an invasive infection that spreads outside the gastrointestinal tract  and cause meningitis, meningoencephalitis or bacteremia, to name a few In pregnant women Listeria may project flu-like conditions  along with  progressive fatigue and in severe cases can cause loss of fetus If the fetus survives it becomes susceptible to life threatening diseases.

Who Is At Risk?

Listeriosis usually affects immunocompromised or at risk patients like newborns, pregnant women, older adults,  who are already suffering from  serious ailments such as liver diseases, diabetes or cancer.

Nearly 14 percent of the infectious cases  occur during pregnancy and  in one instance it was seen that Hispanic pregnant women were nearly 24 times at risk of  Listeria infection compared to normal population.

Adults aged 65 and older make up 58 percent of the cases, owing to their weak immune system and weak life style.
Healthy Individuals, who aren’t immunocompromised can also suffer from listeriosis but they can quickly recover from the infection and lead a normal life.

Lifecycle Of Listeria Monocytogenes

Listeria Monocytogenes is a facultative anaerobic bacterium which means it can survive with or without oxygen. This pathogenic gram positive bacterium, named after Joseph Lister, can reproduce at temperatures as low as zero degrees, which allows its multiplication in refrigerated environments thus making the subsequent infection difficult to avoid.

  1. monocytogenestransmission is often associated with cheese, ice cream and rawproducts. Infective dose for the bacterium is difficult to surmise but it is widely believed that a colony of one thousand bacteria in an immunocompromised individual manifest an infection.

The bacterium is usually transmitted through oral route and has the ability to infect monocytes, macrophages endothelial cells, hepatocytes, epithelial cells, and polymorphonuclear leukocytes. Once the bacteria gains entry into a host cell it uses the host cell’s machinery to survive. In a pathogenic infection L. monocytogenes goes through an intracellular life cycle where bacteria has an early escape from the phagocytic vacuole by secreting pore-forming toxin called listeriolysin, followed by rapid division in cytoplasm of the host cell.

The bacteria shows actin based motility which allows it to propel like a rocket through the cytoplasm, enabling the bacterium to exit from one cell and enter into another.

Is Listeriosis Contagious?

One shouldn’t be worried about the spread of Listeriosis as unlike HIV and other infections, it isn’t contagious hence you can live with an infected person without fearing the infection itself.
Though as mentioned above it may pass through the mother to fetus during pregnancy.

How is Listeriosis Diagnosed?

Listeriosis is only diagnosed through a microbiology culture. Patients which are asymptomatic are not tested for the disease as it provides no clinical value. The protocol for not testing such patients remains the same even when patients are deemed high risk, because Listeria is present in every kind of environment and everyone is often exposed to it regularly.

As a common practice only symptomatic patients are treated for Listeriosis due to the omnipresent nature of the bacteria. A microbiology culture is run on the blood or CFS sample of these patients. Although the sample is taken from a sterile site in the form of blood however, spinal fluid may be extracted when symptoms show involvement of the nervous system and sample may be taken from amniotic fluid or placenta in case of pregnant women.

In the laboratory, the bacteria can easily be isolated on routine media. Selective enrichment media help isolates the bacteria more effectively.. The culture usually takes one to two days for the growth before diagnostic tests can be performed.

Listeriosis that affects the central nervous system is even more difficult to diagnose because of any possible previous antibiotic treatment and low bacteria number in cerebral spinal fluid. In such cases real time PCR techniques may be used to detect and quantify the bacterium DNA in the extracted fluid. An important thing to note here is that these tests can give false negatives. A negative test doesn’t necessarily rule out the disease and if your doctor suspects its presence then it is advisable to start treatment.

Correct diagnosis for the disease is however quite high according to Robert Buchanan, Ph.D., director of the University of Maryland’s Center for Food Safety and Security Systems. Dr Buchanan explains that according to estimates Listeria gets accurately diagnosed in one case out of every two which when compared to salmonella.

How is Listeriosis Treated?

When immunocompromised patients and pregnant women get Listeriosis it is often advised to start treatment with antibiotics. Babies who have the disease also get the same treatment as the adults.

All antibiotics have shown effective therapeutic activity against the bacterium in clinical analysis except cephalosporins and fosfomycin. Generally clinical outcomes are poor even for ampicillin-amoxicillin, but this can be counteracted by adding gentamicin to the treatment.

Residence of bacteria inside the host cell renders it difficult to be treated and in those scenarios only  few antibiotics can reach, penetrate, and accumulate in the cytosol of the cell where it can counteract the bacteria.

Conventional antimicrobial therapeutic Listeria treatments may not work in immunocompromised individuals as nearly 30 percent of such patients die in spite of a rational antibiotic drugs being used.

In pregnant women, if exposure is detected and no symptoms are present, it is advisable not to begin treatment. The patient may be advised to return within two months to receive a consultation to check for any symptoms of the disease.

If the disease is febrile then a pregnant woman is simultaneously tested for the disease and the treatment with antibiotics is started.  A febrile disease is often characterized by a fever higher than 100.6 Fahrenheit and other described symptoms of the Listeriosis.

If an amniocentesis is performed in pregnant women diagnosis may be made on characteristic presence of gram-positive rods and meconium. Placental samples may also be obtained in case of delivery. If the results are negative after the start of treatment , the continuity of treatment again depends on the physician.

Treatment involves an antimicrobial regiment of physician’s choice which usually consists of high intravenous ampicillin dose (at least 6 grams per day) for non-allergic patients. When women are allergic to ampicillin, trimethoprim with sulfamethoxazole is the generally recommended.

Listeriosis Prevention

FDA recommends the following rules to  avoid Listeria infection

  • Thoroughly cook your food
  • Wash raw vegetables and fruits before consumption
  • Keep uncooked meats separate from other products which are cooked or ready for consumption
  • Avoid consumption of unpasteurized products
  • Wash utensils, knives and cutting boards with care

If a person is at high risk for the disease like a pregnant women or an immunocompromised person, it is advised to

  • Not eat meat products without heating them to point where they are steaming hot
  • Avoid cross contamination of food products by using different and clean utensils while handling each product
  • Do not eat soft cheeses
  • Do not eat refrigerated meat products like meat spreads
  • Do not eat fish products that are refrigerated unless they are a part of a cooked food product like a casserole

FDA and USDA are working at the government level for the safety and quality assurance of food, to detect the presence of Listeria in different products. In case an infection as found the producers and the whole facility is inspected and in some cases the products are recalled.

National Center of Infectious Diseases (NCID)  investigates and helps local state public health departments regarding any Listeria outbreak reported. Early reporting, detection, and investigation of such outbreaks can help identify source of Listeria infection and help prevent the disease spread.

The government also regulates the protocols for proper functioning of food manufacturing and delivery industry to prevent Listeria infections. These safety manufacturing protocols involve:

  • Protocols to prevent growth of monocytogenes
  • Treatment of foods to destroy any bacterium present
  • Controls on ingredients
  • Controls on storage of food products
  • Controls on employee hygiene
  • Controls on design construction, and operation of the manufacturing plant
  • Controls on maintenance and design of equipment
  • Controls on transportation
  • Controls on sanitation and monitoring
  • Special monitoring of places where this particular bacterium is suspected to grow and flourish
  • Handling of these special places and critical surfaces and corresponding corrective actions
  • Employee training
  • Methods to collect samples for analysis
  • Methods to prepare for collection of samples
  • Recommended methods to detect, eliminate, and keep a record of Listeria species
  • Methods to keep records on controls that have been established and implemented

USDA also publishes regular updates of these protocols and methods to ensure better future control of the Listeria infection and outbreaks.

How Surveillance Can Help Prevent Listeriosis?

According to CDC Listeria estimates,  1600 illnesses and nearly 260 deaths are caused each year due to listeriosis in United States.  The largest outbreak to be ever reported in United States, which occurred in 2011, due to a single source of contamination caused 33 deaths and one miscarriage along with 147 cases of illness. 2016 has seen a record number of food recalls by the FDA due to Listeria infection.

The Centers for Disease Control and Prevention (CDC) considers Listeriosis a nationally notifiable disease since 2011, and once diagnosis is made health providers should immediately notify their state public health departments.

National Molecular Subtyping Network for Foodborne Disease Surveillance (PulseNet), Listeria Initiative, and the National case based surveillance are  currently working to monitor and analyze Listeriosis outbreaks and clusters in United States, under the umbrella of CDC.

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