Pneumocystis carinii

• Important cause of pneumonia in humans associated with immunodeficiency states
• It has been occasionally reported in young or immunosuppressed animals (e.g. Arabian foals with congenital immunodeficiency)
• Grossly:
  • Diffuse interstitial pneumonia
  • Patches of firmness or consolidation
• Histologically:
  • Alveoli are filled with pale staining acidophilic material (= trophozoite and cyst forms of the organism)
• It is thought that as P. carinii is often present as a latent infection, alveolar macrophages normally control the organism - a process which fails in immunodeficient states