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