Listeriosis is an important food-borne zoonosis caused by pathogenic species of Listeria. The disease affects both in man and animals. It is a serious invasive disease characterized by neural, visceral and reproductive clinical entities and may lead to septicaemia, abortion, stillbirth, meningitis and meningoencephalitis. The disease is also known as leucocytosis, listerial infection, listeriasis, listerellosis, circling disease in animals.
Epidemiology: Listeriosis occurs throughout the world with varying degree of prevalence. The incidence ranges from sporadic cases to major epidemics. The genus Listeria comprises of 6 species namely L. monocytogenes, L. innocua, L. ivanovii, L. seeligeri and L. welshimeri and L. grayi. Of these, L. monocytogenes and L. ivanovii cause the disease in man and animals. L. monocytogenes is a gram-positive, facultatively anaerobic coccobacillus, motile at 20oC to 25oC and a facultative, intracellular parasite of the reticuloendothelial system. It is widely distributed in animals and man, as well as in the environment and has been isolated from numerous sites, including soil, sewage water and decaying plant material, especially poorly fermented silage.
Clinical features: The symptoms of listeriosis in human are mainly due to septicaemia and encephalomeningitis. Listerial abortion in women usually occurs in the second half of pregnancy and more frequent in the third trimester. The suffering mother may show the symptoms like chills, fever, dizziness and sometimes gastrointestinal disturbances before miscarriage or delivery. Listerial septicemia also occurs among weakened adults, especially patients undergoing long term treatment with corticosteroids or antimetabolites. It may also result in endocarditis, external and internal abscesses and endophthalmitis. A cutaneous eruption has been described among veterinarians who handled infected fetuses of animals. The case fatality is comparatively high that may be as high as 25 to 30 %.
Diagnosis: Diagnosis of listeriosis is made by isolation of the pathogen from blood, cerebrospinal fluid, amniotic fluid or biopsy materials.Conventional serological tests are used for epidemiological surveillance of listeriosis. Outer membrane protein (OMP) of Listeria spp. is also used for developing genus and species specific ELISA. The pathogenicity testing of Listeria isolates by in-vivo methods namely chick embryo and mouse inoculation tests is preferred to confirm the link between isolates and causation of disease. PCR based detection me tho d s ha ve b ee n e mplo ye d a s a d ditio nal to o ls fo r the ide ntific a tio n o f microorganisms. A fragment containing the gene encoding delayed-hypersensitivity- inducing protein is used as a probe to differentiate between pathogenic and non- pathogenic strain of Listeria in clinical, environmental and food samples. PCR- ELISA technique has also been developed to detect and quantify L. monocytogenes in food products.
Prevention and control: Animals with encephalitis and abortion should be separated and the placenta and fetus should be properly destroyed. New animals should only be added to a herd after undergoing a reasonable period of quarantine. A common recommendation is to properly ensilage the fodder and avoidance of feeding contaminated, decayed and mouldy silage. Other measures include pasteurization of milk, rodent control and common practices of environmental and personal hygiene. Food from suspected origin should always be adequately cooked. Vegetables that are eaten raw should be washed thoroughly and raw meat should be kept separately from other foods. Veterinarians must take precautions when dealing with parturition, abortions and during autopsies.