Bovine Anthrax Study Guide

Overview and Clinical Importance

Anthrax is an acute, multisystemic zoonotic disease caused by Bacillus anthracis, a spore-forming, Gram-positive bacterium. It is one of the most important diseases in bovine practice due to its acute nature, high mortality, zoonotic potential, and bioterrorism concerns.

Anthrax has significant implications for public health, international trade, and food safety. The disease is notifiable to the World Organisation for Animal Health (WOAH/OIE) and requires immediate reporting to veterinary authorities.

Etiology

Causative Agent

Bacillus anthracis is a large (3-5 μm long, 1-1.2 μm wide), Gram-positive, rod-shaped bacterium with the following characteristics:

• Spore-forming: Extremely resistant oval spores that do not swell the sporangium
• Aerobic or facultatively anaerobic growth
• Non-motile (distinguishes from other Bacillus species)
• Encapsulated in vivo (poly-?-D-glutamic acid capsule)
• Chain formation: 'boxcar' or 'bamboo rod' appearance

Virulence Factors

The pathogenicity of B. anthracis is determined by two plasmids:

• pXO1 plasmid: Encodes the tripartite anthrax toxin
• pXO2 plasmid: Encodes the antiphagocytic capsule

Pathogenesis

Infection Process

• Spore ingestion: Cattle ingest spores from contaminated soil, feed, or water
• Spore germination: Occurs in lymphoid tissue (Peyer's patches, mesenteric lymph nodes)
• Vegetative multiplication: Bacteria replicate and produce toxins
• Lymphatic spread: Migration through lymphatic system to bloodstream
• Septicemia: Massive bacterial multiplication in blood
• Toxin-mediated death: Cardiovascular collapse and death within 48-96 hours

Environmental Persistence

B. anthracis spores are extremely resistant environmental forms that can survive:

• Decades in soil (documented survival greater than 60 years)
• Extreme temperatures (freezing and boiling)
• Desiccation and UV radiation
• Chemical disinfectants (except formaldehyde and hypochlorite)

Epidemiology

Global Distribution

Anthrax is found worldwide but is more common in:

• Sub-Saharan Africa
• Central and Southwest Asia
• Southern and Eastern Europe
• Central and South America
• Limited areas in North America and Australia

Risk Factors

• Alkaline soil pH (greater than 6.0) favors spore survival
• Environmental conditions: Wet periods followed by drought
• Soil disturbance: Deep tilling, construction, flooding
• Poor vaccination coverage in endemic areas
• Contaminated feed: Bone meal, meat meal from infected sources

Clinical Signs

Clinical Forms in Cattle

Specific Clinical Signs

Systemic signs:

• Hyperthermia (up to 42°C/107°F)
• Trembling, muscle fasciculations
• Respiratory distress, pulmonary edema
• Cardiovascular collapse
• Convulsions, ataxia

Gastrointestinal signs:

• Cessation of rumination
• Bloody diarrhea or dysentery
• Abortion in pregnant cattle

Post-mortem findings:

• Failure of blood to clot
• Dark, tarry blood from natural orifices
• Rapid bloating and decomposition
• Absence or incomplete rigor mortis
• Splenomegaly ('blackberry jam' consistency)

Diagnosis

Clinical Diagnosis

Clinical diagnosis should be suspected based on:

• Sudden death in grazing cattle
• History of environmental disruption or endemic area
• Characteristic post-mortem appearance
• Unclotted blood and rapid decomposition

CRITICAL SAFETY WARNING: Never perform necropsy on suspected anthrax cases. Opening carcasses exposes vegetative bacteria to oxygen, causing sporulation and environmental contamination.

Laboratory Diagnosis

Preferred diagnostic approaches (in order of reliability):

Sample Collection

Preferred samples (in order of preference):

• Blood from peripheral vein (jugular, ear vein) on cotton swab, air-dried
• Ear tissue clipping for PCR analysis
• Spleen tissue (if necropsy accidentally performed)

Differential Diagnosis

Other causes of sudden death in cattle that must be considered:

• Clostridial diseases: Blackleg (C. chauvoei), malignant edema (C. septicum)
• Acute bloat: Frothy or free-gas bloat
• Lightning strike: Environmental history important
• Acute leptospirosis: Hemoglobinuria, icterus
• Bacillary hemoglobinuria: Dark urine, liver infarcts
• Plant poisoning: Bracken fern, sweet clover, lead
• Acute anaplasmosis: Anemia, icterus, tick exposure

Treatment

Individual Animal Treatment

Treatment is rarely successful due to the rapid course of disease, but may be attempted if animals are diagnosed early:

Outbreak Management

• Immediate quarantine of affected premises
• Vaccination of healthy animals (Sterne 34F2 vaccine)
• Prophylactic antibiotics for animals showing early signs
• Proper carcass disposal (burning preferred, deep burial if necessary)
• Environmental decontamination with formaldehyde or hypochlorite
• Remove contaminated feed and water sources

Prevention

Vaccination

Sterne 34F2 vaccine (live, non-encapsulated spore vaccine):

• Dosage: 1 mL subcutaneous in neck
• Primary series: Two doses 2-3 weeks apart
• Annual revaccination in endemic areas
• Timing: 2-4 weeks before expected exposure
• Duration: 6-12 months immunity

CRITICAL: Do not give antibiotics within 7-10 days before or after vaccination. Antibiotics will kill the live vaccine bacteria and render vaccination ineffective.

Management Practices

• Feed management: Avoid bone meal and meat meal from unknown sources
• Pasture management: Rotate away from known contaminated areas
• Quarantine: Isolate new animals for 20 days minimum
• Biosecurity: Control animal movement in endemic areas

Public Health Significance

Zoonotic Potential

Human anthrax occurs through:

• Cutaneous exposure (95% of cases): Handling infected animals/products
• Inhalational exposure (rare): Spore inhalation, often fatal
• Gastrointestinal exposure (rare): Consumption of undercooked infected meat

Reporting Requirements

Anthrax is a:

• Notifiable disease to WOAH/OIE
• Reportable disease in most countries
• Category A bioterrorism agent (CDC classification)
• Immediate notification required to veterinary authorities