Listeria species

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Overview

• 6 species
• Saprophytes in soil
• L. monocytogenes and L. ivanovii are pathogens
• Carried by sheep and goats and shed in faeces and milk especially during stress
• Can cause septicaemia, encephalitis, abortion and endophthalmitis in ruminants
• Outbreaks of listeriosis often linked to silage feeding

Characteristics

• Small Gram negative rods
• Catalase positive, oxidase negative
• Tumbling motility
• Facultative anaerobes
• L. monocytogenes is haemolytic on blood agar due to a cytolytic protein, listeriolysin; grows at range of pH values and temperatures
• L. ivanovii produces strong haemolytic zone
• Small, smooth, transparent colonies after 24 hours incubation
• 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

Clinical infections

• Outbreaks of listeriosis caused by L. moncytogenes seasonal and linked to silage feeding
• Replicates in poor quality silage where the pH excedes 5.5
• Animals highly susceptible during pregnancy due to lowered cell-mediated immunity
• Neural listeriosis:
  • Incubation period 14-40 days
  • Dullness, circling, head tilt, facial paralysis, drool saliva, droop of eyelid and ear
  • Exposure keratitis
  • Fever during early stages
  • Recumbency and death within a few days in sheep and goats
  • Abortion up to 12 days after infection in cattle
• Septicaemic listeriosis:
  • Incubation perios 2-3 days
  • Lambs and occasionally pregnant sheep
  • Occurs in newborn piglets, foals, poultry, adult sheep
• Keratoconjunctivitis in cattle and sheep - direct contact with silage via eye

• L. ivanovii causes sporadic abortion in sheep and cattle
• L. innocua rarely causes ovine meningoencephalitis

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