Listeria species - Overview

Overview

There are 6 species of Listeria bacteria. They are saprophytes in soil.L. monocytogenes and L. ivanovii are pathogens and are carried by sheep and goats and shed in faeces and milk especially during stress. They can cause septicaemia, encephalitis, abortion and endophthalmitis in ruminants. Outbreaks of listeriosis often linked to silage feeding

Characteristics

Listeria are intracellular pathogens. They are Gram positive rods with catalase positive and oxidase negative activity. They are facultative anaerobes and have tumbling motility. L. monocytogenes is haemolytic on blood agar due to a cytolytic protein, listeriolysin and grows at range of pH values and temperatures.L. ivanovii produces a strong haemolytic zone. Listeria produces small, smooth, transparent colonies after 24 hours incubation. They are able to grow on non-enriched media.

Pathogenesis and pathogenicity

• Infection by ingestion of contaminated feed
• Bacteria penetrate M cells in intestinal Peyer's patches
• Spread to tissues via blood and lymph
• Transplacental transmission in pregnant animals
• Bacteria may gain entry via breaks in oral or nasal mucosa, migrate in cranial nerves to cause neural signs
• Causes formation of microabscesses and perivascular lymphocytic cuffs in brainstem
• L. monocytogenes can replicate within phagocytic and non-phagocytic cell, and pass between cells without being exposed to the immune system
• Surface proteins known as internalins allow adherence and uptake of the bacteria into cells
• Listeriolysin produced by virulent strains destroys membranes of phagocytic vacuoles, releasing the bacteria into the cytoplasm
• Listeria are motile in the cytoplasm
• Bacteria induce formation of pseudopod projections in the cytoplasmic membrane, which are taken up with the bacteria into adjacent cells
• Cell-mediated immune response required for protection

Diagnosis

• Specimens should include CSF in neural cases, cotyledons in abortion, liver, spleen and blood in septicaemia
• Immunofluorescence using monoclonal antibodies
• Histology of brain demonstrates microabscesses and lymphocytic cuffing in brainstem
• Smears of cotyledons
• High protein and cell counts in CSF
• Isolation on blood and MacConkey agar

Treatment and control

• Ampicillin or amoxycillin in early stages of septicaemic listeriosis
• Sub-conjuntival antibiotics and corticosteroids for ocular listeriosis
• Avoid poor quality silage and discontinue silage-feeding in an outbreak

Causes abortions in cattle and sheep last third of pregnancy. Foetal membranes often retained leading to metritis. The placenta is covered by purulent exudate and there are focal pinpoint yellow lesions in foetal liver.