Bacillus anthracis

Anthrax (human). Courtesy of T. Scase

• 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 found 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
  • Blood smear from an ear or tail vein of ruminants, or peritoneal fluid from pigs stained with polychrome methylene blue
  • Chains of square-ended blue rods surrounded by mauve capsules
  • Culture on blood and MacConkey agar (no growth on MacConkey)
  • Biochemical tests
• Treatment:
  • High doses of penicillin G or oxytetracylcine
• Control:
  • Report suspected cases - notifiable
  • Spores destroyed by sterilisation
  • Endemic regions:
    • Live Sterne spore vaccine which produces toxin but has no capsule, therefore is non-pathogenic; stimulates protective antibody
    • Chemoprophylaxis with long-acting penicillin
  • Non-endemic regions after an outbreak:
    • Movement restrictions
    • Footbath with sporicidal disinfectant
    • Fumigate buildings with formaldehyde
    • Dispose carcases and contaminated material
    • Isolate in-contact animals

• Causes severe systemic disease that can result in enteritis.
• Peritonitis in pigs