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What is Listeriosis?

Listeriosis usually results from infection by Listeria monocytogenes, a Gram positive Rod bacterium in the family Listeriaceae.

Where is the disease found?

L. monocytogenes is found worldwide and is widely distributed in the environment.

How is the disease transmitted and spread?

The reservoirs of infection are the soil and the intestinal tracts of asymptomatic animals, including wild and feral mammals, birds, fish and crustaceans. Infected animals can shed L. monocytogenes in the faeces, milk and uterine discharges. It is also found in aborted foetuses and occasionally in the nasal discharges and urine of symptomatic animals.

Most infections are acquired by ingestion, but Listeria can also spread by inhalation or direct contact. In ruminants, listeriosis typically occurs after the consumption of contaminated silage or other feed.

For humans, contaminated food sources include raw meat and fish, unpasteurised dairy products and uncooked vegetables. L. monocytogenes has also been found in processed foods that have become contaminated after processing, particularly soft cheeses, cold cuts of meat, sliced or grated cheese, and ice cream. The infective dose for oral transmission is unknown but is thought to depend on the strain of bacterium and the susceptibility of the person. Healthy people seem to be able to eat most Listeria-contaminated foods without clinical signs; however, in susceptible persons, the infective dose is probably fewer than 1,000 organisms.

Vertical transmission is the usual source of infection in newborn human infants and ruminants; infections are transmitted either transplacentally or from an infected birth canal. Humans can also be infected by direct contact with infected animals during calving, lambing or necropsies.

What is the public health risk associated with this disease?

Listeriosis is a zoonosis. The incubation period in susceptible adults is 3 to 70 days, with the median incubation period estimated to be 3 weeks. Listeriosis is usually a serious problem only in pregnant women, newborns, the elderly, and immunocompromised or debilitated hosts. Pregnant women may experience either a mild, flu-like syndrome with fever, chills, headache, slight dizziness or gastrointestinal signs, or an asymptomatic infection. This may be followed in a few days to weeks by abortion, stillbirth, premature birth or septicemia in the newborn. Newborns may be infected either in utero or from bacteria found in the vagina during delivery. These infants can develop septicemia, disseminated granulomatosis, respiratory disease or meningitis; symptoms may be present at birth or develop within a few days to several weeks. In elderly, immunocompromised or debilitated persons, L. monocytogenes can cause meningitis, meningoencephalitis or, less frequently, septicemia.

What are the clinical signs of the disease?

A wide variety of domestic and wild mammals, birds, fish and crustaceans carry L. monocytogenes asymptomatically in the digestive tract. Clinical disease is seen most often in ruminants. The incubation period for encephalitis in ruminants is usually 10 days to 3 weeks. Septicemia and abortions can appear after one day or more. Listeria can cause encephalitis, abortions and septicaemia in sheep, cattle and goats.

In the encephalitic form, the initial symptoms of depression and anorexia are followed by neurological signs, which may include facial paralysis with profuse salivation, torticollis, strabismus, circling, incoordination, and head pressing or turning of the head to one side. The neurological signs are often unilateral. Animals become recumbent during the final stages of the disease, and may make involuntary running movements or characteristic chewing motions. The course of the disease is usually short, from one to four days, in sheep and goats, with death as soon as one or two days. Listeriosis is more chronic in cattle, with animals typically surviving for 4 to14 days. Abortions and stillbirths mainly occur late in gestation. Septicaemia occurs most often in newborns and young ruminants; older animals are rarely affected. Typical symptoms may include fever, depression, loss of appetite and death.

How is the disease diagnosed?

Listeriosis can be diagnosed by isolating L. monocytogenes from the placenta, fetus or uterine discharges after an abortion. Blood or CSF, respectively, can be cultured in animals with septicaemia or encephalitis. At necropsy, the liver, kidneys and spleen can be cultured in septicaemic animals, or the pons and medulla in animals with encephalitis. L. monocytogenes has also been isolated from the nasal discharges, urine, faeces or milk of affected animals; however, it is also present in the faeces and milk of clinically normal animals.

L. monocytogenes can be identified in tissues with a variety of commercial rapid identification methods, based on biochemical tests and enzyme reactions. It can also be detected with ELISA, immunofluorescence, mmunochromatography, immunomagnetic separation and PCR techniques. Serology is not routinely used for diagnosis. Many healthy animals have high Listeria titers, and cross-reactions occur with enterococci, Staphylococcus aureus and other organisms.

What is being done to prevent or control this disease?

Listeriosis can be treated with a variety of antibiotics. High doses and early treatment are required for animals with encephalitis; animals with severe neurological signs usually die despite treatment. Supportive treatment may also be required. The risk of listeriosis can be decreased in ruminants by feeding good quality silage with a low pH. Spoiled or mouldy silage should be avoided, as well as silage from the superficial few inches exposed to air. Any leftover silage should be removed after feeding. Rodents should be controlled.

New animals added to the herd should be quarantined, and animals with clinical listeriosis isolated. The placenta and foetus should be removed after an abortion. There is no vaccine for listeriosis. In human cases, prevention relies on food safety. People at a high risk for listeriosis should thoroughly cook all food from animal sources, wash raw vegetables very well, and avoid eating or drinking unpasteurised milk products. All cooking tools, as well as the hands, should be washed after they have been in contact with raw food. Uncooked meats should be kept separate from vegetables, cooked foods and ready-to-eat foods. High risk foods include soft cheeses such as feta, Brie, bleu cheese, and Camembert, and cold cuts of meat. Other high-risk foods include refrigerated pâtés and meat spreads, and refrigerated smoked seafood, unless it has been cooked. Perishable and ready-to eat foods should not be kept for long periods of time.