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

• inhalation of Mycobacterium bovis most common via droplets
• some tubercle bacilli enter the lymph and travel to the bronchial or mediastinal nodes
• inhaled bacilli reach the alveoli, set up a focus of inflammation
• phagocytosed by alveolar macrophages
• two processes may develop if the animal has not encountered the organism before:

- the organism may grow in the phagocytes as intracellular parasites

- produces a nodule of parasitised swollen macrophages known as a tuburculous nodule or a tubercle granuloma

- ultimately, macrophages are killed and infection spreads

- the organism may be broken down and some antigens taken up by the immune system

- cell mediated immune system produces cytotoxic T-lymphocytes

- T-lymphocytes can attack and destroy cells harbouring bacilli

- leads to type IV (delayedd type) hypesensitivity

- 'caseous' or cheesy type of necrosis

- if bacterium destroyed, further infection/disease is prevented

Sequelae

• chronicity

Tuberculosis pleurisy

• caseous lymph node ruptures
• results from extensive tissue necrosis

- if located in lung alveoli, the follicle may rupture into a bronchus, causing spread of the disease to all the other lobules served by that bronchus

- if the ensuing necrosis erodes the wall of a large pulmonary vessel, this ruptures into the lung and a fatal haemoptysis might follow