Tuberculosis - Cattle

Tuberculosis caused by M. bovis (Image sourced from Bristol Biomed Image Archive with permission)

• Caused by Mycobacterium bovis and M. tuberculosis
• Reside primarily within macrophages where they multiply and result in characteristic granulomatous inflammation (macrophages and giant cells, epithelioid cells)
• Cattle can be infected by inhalation of the organism or through milk
• The primary complex
  • Describes the initial focus of infection at the portal of entry (lungs) plus involvement of regional lymph nodes
  • 90% of cases exhibit the pulmonary form
  • Grossly:
    • Small tubercles in dorsocaudal subpleural areas which progress to larger confluent areas of caseous necrosis
    • Usually start at bronchio-alveolar junction an progress to the alveoli
    • Caseous lesions, may calcify or be encapsulated
    • Multiple foci may coalesce
    • Ulcers in trachea and bronchi due to coughed up bacteria
    • Spreads into pleura
  • Microscopically:
    • Typical granulomatous inflammation
    • Epitheliod and giant cells at centre of tubercles
      • Macrophages with ingested bacteria, forming epithelioid cells - large vesicular nuclei, abundant pale cytoplasm
      • Giant cells, formed by fusion of macrophages, with multiple nuclei
    • Narrow layer of lymphocytes, mononuclear cells and plasma cells at the periphery of the tubercle
    • With time, peripheral fibroplasia and central necrosis develop
• If the infection is not contained in the primary complex described above, the mycobacteria can disseminate via lymphatics to other organs and lymph nodes
• This can allow the development of miliary tuberculosis, i.e. numerous small foci of infection in many organs/ tissues