Infection by Listeria monocytogenes (termed listeriosis) is a very serious foodborne disease. Its importance as a foodborne pathogen is due to it:
- being able to grow in many foods during refrigerated storage, unlike most other foodborne pathogens
- having a high mortality (death) rate
- being commonly found in the environment, including food-related environments
- being more resistant than most bacteria to the conditions and treatments used to control foodborne pathogens
Those most at risk are:
- pregnant women
- immunocompromised persons (those having a weakened immune system), including the elderly
- healthy people consuming food heavily contaminated with large numbers of Listeria monocytogenes
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Incubation Period, Symptoms & Duration
 Incubation Period
Unknown - may vary from 3 - 70 days or more.
The infective dose is currently unknown but may be less than 1,000 cells, being thought to vary with the strain of the bacteria and the susceptibility of the victim.
Symptoms
Listeriosis is unusual for a foodborne disease in that there are generally no or only occasional gastrointestinal symptoms.
The progress of the infection has two phases:
- a first phase - a 'flu-like' illness with malaise and mild fever. This phase may even occur without there being symptoms or be so mild as to go unnoticed.
- a second stage - a septicaemia that leads to the organism having access to all body areas, including the central nervous system and the foetus of pregnant women. Conditions that may subsequently arise include meningitis, encephalitis, and intra-uterine & cervical infections in pregnant women, which can then result in abortion or still birth.
Death rates vary:
- the perinatal and neonatal mortality rate may reach 80%
- meningitis mortality rate can be as high as 70%
- death is rare in healthy adults (which account for about 18% of all cases) but may reach 50% in the immunocompromised (including the elderly), newborn or very young.
Duration
The duration of the illness is variable. People can become carriers of Listeria monocytogenes, with estimates that between 1 - 10% of symptomless persons may be excretors of the organism.
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Source & Spread
Sources/Associated Foods
Listeria is found widely in nature, occurring in soil, vegetation and water. It is therefore frequently carried by animals (including cattle and sheep) and humans (at a rate of between 1 - 15%). Listeriosis has been associated with a range of foods including ready-to-eat delicatessen items (e.g. soft ripened cheeses, meat-based patés coleslaw, smoked fish and cooked meats), milk and dairy products, meat and poultry, and seafood.
Spread
Factors relating to the spread of foodborne Listeria infection include:
- contaminated raw ingredients
- inadequate thermal processing of foodstuffs, made worse by incorrect temperature or duration of refrigerated storage.
- direct or indirect (cross-) contamination of ready-to-eat foods
There is also a risk of exposure of newborn infants to bacteria shed in the vaginal discharge of the mother. Person to person spread has also occurred between infants in hospital nurseries.
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Control
Pregnant women and & immuno-compromised persons should avoid eating high risk foods such as certain ripened soft cheeses (e.g. Camembert, Brie) and some types of meat-based patés
All chilled meals should be thoroughly cooked/reheated.
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Exclusion/Return to Work Parameters
No exclusion required.
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Additional Information
- Listeria is a Gram +ve, aerobic/facultatively anaerobic, rod-shaped, motile (by means of flagella), non-sporeforming organism.
- As an individual's immune system is increasingly compromised, the risk of listeriosis at any given dose increases. Relative susceptibilities to listeriosis for different sub-populations are as follows:
| Condition | Relative Susceptibility | | Transplant | 2584 | | Cancer-Blood | 1364 | | AIDS | 865 | | Dialysis | 476 | | Cancer-Pulmonary | 229 | | Cancer-Gastrointestinal and liver | 211 | | Non-cancer liver disease | 143 | | Cancer-Bladder and prostate | 112 | | Cancer-Gynaecological | 66 | | Diabetes, insulin dependent | 30 | | Diabetes, non-insulin dependent | 25 | | Alcoholism | 18 | | Over 65 years old | 7.5 | | Less than 65 years, no other condition | 1 |
- The mechanism by which Listeria causes illness is unusual in that it survives and multiplies in the host's defence cells (phagocytes). After invading the gastro-intestinal epithelium, the bacterium enters the host's defensive cells (e.g. monocytes and macrophages), grows and becomes bloodborne. It consequently has access to the brain and, through the placenta, to the foetus.
- In England and Wales between 1983 - 2003 there was an average of 141 cases of listeriosis per year, with a maximum of 278 in 1988 (HPA website figures, January 2005).
- Most common serotypes are 1 & 4b. The vast majority of cases are sporadic, making epidemiological links to food difficult.
- As an indicator of the ability of Listeria to grow in refrigerators, it is able to double in number every 1 ½ days at 4oC. Numerous studies have also revealed that many domestic refrigerators operate above this temperature.
- Particular problems with soft ripened cheeses may arise because of the cheese ripening process itself. During the ripening process microbes utilise the lactate present and release amines, therefore increasing the surface pH allowing Listeria (from raw or contaminated milk, or from post-production contamination) to multiply.
Growth Factors
Minimum Aw: >.94
pH range: 4.1 - 9.6
Optimum pH: 6.0 - 7.5
Temperature range: -1oC - 45oC i.e. psychrotrophic
Optimum temperature: 30oC - 37oC
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