Bacillus species

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• CAR bacillus in URT infection in cattle and URT infection in rabbits

Overview

• Mostly non-pathogenic environmental organisms
• Bacillus anthracis causes anthrax
• Anthrax is a severe disease affecting all mammals worldwide
• Ruminants are highly susceptible to anthrax, dying of a septicaemic form
• Pigs and horses are moderately susceptible, but carnivores are fairly resistant
• Bacillus licheniformis may cause shoradic abortions in cattle and sheep
• Bacillus cereus causes food poisoning in humans and mastitis in cows

Characteristics

• Large, Gram positive rods
• Produce endospores
• Aerobes or facultative anaerobes
• Grow on non-enriched media
• Motile
• Catalase positive, oxidase negative
• Bacillus anthracis colonies are up to 5mm diameter, flat, dry, grey, with a ground-glass appearance; curled outgrowth sfrom the edge of the colony give a 'medusa head' appearance; non-haemolytic (differentiate from Bacillus cereus)
• Bacillus licheniformis forms dull, rough, wrinkled colonies, with hair-like outgrowths
• Biochemical tests for identification
• Can often tolerate adverse environmental conditions

Bacillus anthracis

• Epidemiology:
  • Saprophyte in soil
  • Endospore formation allows persistence and spread
  • Endospores survive decades in the soil
  • Outbreaks in herbivores grazing pastures contaminated by spores from buried carcases
  • Infection usually by ingestion of spores and penetration through damaged mucosa
• Pathogenesis and pathogenicity:
  • Spores germinate at site of entry and spread via lymphatics to bloodstream, where they multiply and produce toxin
  • Capsule and toxin (encoded by separate plasmids) required for virulence
  • Capsule composed of homopolymer of D-isomer of glutamic acid allows survival in the body by resisting phagocytosis
  • Capsule stains mauve with polychrome methylene blue in the MacFadyean's reaction - identify anthrax in blood samples
  • Bacilli appear as chains of dark blue, square-ended rods surounded with the capsule
  • Extracellular toxin (holotoxin) composed of oedema factor, protective antigen and lethal factor
  • Oedema factor is an adenylate cyclase which increases intracellular cAMP concetrations, causing fluid accumulation and damaging neutrophils
  • Lethal factor causes release of cytokines from macrophages
  • Protective antigen binds to the cell receptor to allow action of the other factors
  • The toxin kills phagocytes, increases capillary permeability and interferes with clotting cascade
  • Capillary thrombosis; leakage of fluid through damaged capillary endothelium
  • Systemic shock from circulatory collapse, haemorrhagic disease and oedema lead to death of the animal
  • Severe systemic disease that can result in enteritis
  • Causes tissues to darken and swell due to oedema and necrosis
• Clinical signs:
  • Cattle/sheep:
    • Fatal peracute septicaemia
    • Animals usually found dead
    • Pyrexia, depression, congested mucous membranes and petechiae before death
    • Abortion, subcutaneous oedema and dysentry in animals surviving more that one day
  • Pigs:
    • Subacute anthrax with oedematous swelling of throat, head and regional lymph nodes
    • Intestinal form with high mortality - dysentry due to haemorrhagic enteric lesions
    • Peritonitis
  • Horses:
    • Subacute anthrax with subcutaneous oedema of thorax, abdomen and legs following entrance of spores into abrasions
    • Septicaemia with colic and dysentry due to haemorrhagic enteritis from ingestion of spores; ecchymoses and splenomegaly
  • Dogs
    • Rarely affected, but similar disease to that foung in pigs
  • Humans
    • Cutaneous anthrax - localised lesion from entrance into abrasion which can cause septicaemia
    • Pulmonary anthrax - inhalation of spores
    • Intestinal anthrax - ingestion of infective material
• Diagnosis:
  • Post mortem: bloat, incomplete rigor mortis, ecchymoses, oedema, dark unclotted blood from orifices, blooy fluid in body cavities, splenomegaly

Bacillus licheniformis

• Widespread in the environment
• Associated with food spoilage
• Abortion in cattle and sheep, possibly from spoiled silage or hay

Bacillus cereus

• Mastitis in cattle
• Food poisoning and eye infections in humans