Listeria

Question 1

What is the epidemiology of Listeriosis?

Answer 1

• Common commensal (tonsils, intestines)
• Present in the environment (fecal contamination of vegetation, and is subsequently made into silage)
• Most common in ruminants
• Incidence relates to management / husbandry - silage feeding, and is therefore seasonal
• Animals predisposed by trauma, immuno-compromise, hormonal alterations
• Bacteria can find it’s way to human consumption through milk (mastitis) and contaminated meat (septicemia or environmental contamination at processing plant)

Question 2

What are some general characteristics of Listeria?

Answer 2

• Facultative intracellular (doesn’t necessarily need to live intracellularly)
• In macrophages and epithelial cells
• Facultative anaerobes
• Colonies appear blue-green
• Some hemolysis on blood agar
• Gram + rods
• Non-acid fast, non-spore forming
• septicemia, encephalitis, and abortion common dz’s
• worldwide

Question 3

What are some pathogenic mechanisms of Listeria?

Answer 3

• Cell uptake induced by bacterial protein
• Escape phagolysosome and multiply within the cytoplasm
• Actin-based motility to spread to adjacent cells - polymerization of actin
• Lysteriolysin - macrophages and polymorphonuclear cells are killed and the bacteria may spread systemically

Question 4

What are some clinical forms of Listeriosis?

Answer 4

1. Meningoencephalitis
   
   • common
   • circling dz in small ruminants
   • circles in one direction only
   • unilateral facial paralysis, difficulty swallowing, fever, blindness, headpressing
   • paralysis and death in a few days
   • direct entry through dental plates then trigeminal nerve
2. Abortion
   
   • bacteremia, and localization to placentomes
   • crosses over to amniotic fluid and multiplies
   • ingested by fetus, and causes death / abortion
3. Mastitis
   
   • from environment - feces (survives well in environment)
   • subclinical mastitis