Introduction

• Cause: anthrax is caused by the gram-positive bacilli, Bacillus anthracis. 
• Signs: see below. 
• Diagnosis: polychrome methylene blue stained blood smears demonstrating the presence of gram-positive bacilli.
• Treatment:
  • Severely ill animals are unlikely to recover.
  • Treatment with Procaine penicillin has been reported. 
• Prognosis: the disease is invariably fatal.

Pathogenesis

Etiology

• Anthrax is caused by the gram positive bacilli, Bacillus anthracis Bacillus anthracis.
• Bacillus anthracis has three virulence factors which aid in disease pathogenesis, a poly-D-glutamic capsule which confers resistance to phagocytosis and lethal and edema toxins.  
• Bacillus anthracis forms highly resistant spores which can survive decades in the environment. Spores form in the environment, not in the animal.

Predisposing factors

• Grazing specific “anthrax zones”, with high concentrations of Bacillus anthracis spores in the soil.
• Drought following heavy rainfall and excavations which may unearth infected carcasses.
• Scavenging animals may spread spores.
• Insect bites have been shown to transmit Bacillus anthracis through mechanical transmission Cattle flies. 

Pathophysiology

• Exposure of animals primarily occurs from grazing contaminated pastures or ingesting forage grown on contaminated pastures, leading to ingestion of spores.
• Other sources of infection may include:
  • Ingestion of animal by-products.
  • Pica behavior, including scavenging of and ingesting stones/soil Cattle behavior.
    • This behavior may particularly occur in cattle chronically deficient in phosphorus Phosphorus.
• Following ingestion, spores may enter the body via intact mucus membranes, through defects from erupting teeth or via scratches inflicted from tough, fibrous food.
• Bacillus anthracis resists phagocytosis, proliferates in regional lymph nodes and via the lymphatic vessels into the bloodstream inducing massive septicemia, tissue damage and in the majority of cases, death.

Timecourse

• The incubation period is not easy to determine but is usually 1-2 weeks.

Epidemiology

• Infection commences with the introduction of an infected animal or animal products into an area.
• When the animal dies putrefaction will destroy the bacteria providing the carcass is not opened.
• However, secondary cases can be spread extensively if the animal is still mobile before death, allowing infection to become established within a specific area. 
• One well established risk factor is the movement of hides and skins which are contaminated with Bacillus anthracis spores in ships holds which are then used to carry animal food. This can result in multiple deaths in animals on different farms ingesting the spores.

Diagnosis

Treatment

Prevention

Outcomes