Hepatozoon

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  • Tick borne disease
  • Two species:
    • Hepatozoon canis
      • Usually subclinical disease
    • Hepatozoon americanum
      • SE USA
      • Grossly:
        • Thickened and edematous periskeletal soft tissue and periosteum of the lower legs
        • Multifocal parasitic myositis
      • Histologically:
        • Swelling and hypertrophy of osteoprogenitor cells
        • Hyperplasia of spindle cells
        • Osteoid is produced
  • Myositis
    • Develop cysts in skeletal muscle of dogs and cats
    • May be accompanied by pyogranulomatous myositis - macrophages and neutrophils mainly with occasional other cells


[Skeletal lesions of canine hepatozoonosis caused by Hepatozoon americanum. Panciera Rj et al. Vet Path (2000) 37 225-230]

  • Hepatozoon americanum
    • SE USA
    • Clinical signs:
      • Fever
      • Weakness
      • Myalgia
      • Bone pain
      • Mature neutrophilic leukocytosis
      • Wasting
      • Poor response to treatment
      • Periosteal new bone formation
    • Much more aggressive and higher mortality rate than the disease in the Old World
    • Gross skeletal lesions:
      • Markedly thickened and edematous periskeletal soft tissue and periosteum of the lower legs ***+/- petechiation
      • Eccentrically or circumferentially arranged plaques of new bone formation deposited on the original cortex
      • Boney plaques look ‘porous’ to compact, often in layers with the spongey areas covered by more compact bone
      • No evidence of endosteal bone formation
      • Periosteoproliferative lesions most prominent in proximal long bones of the limbs (esp. femur and humerus)
      • Metaphyses usually spared
      • Caudal and lateral surfaces of the proximal tibia and fibular and radius and ulnar diaphyses affected
      • Distal diaphyses and metaphyses less severely affected/free from lesions
      • Metacarpals/tarsals mildly affected
      • Lateral surface of scapula affected
      • Axial skeleton less severely affected
      • Often bones of the skull prominently affected – orbits, rostrolateral maxilla and mandible, lateral aspect of zygomatic arch
      • Lateral aspects of vertebral arches and spinous processes sometimes affected
      • Ribs normal
      • Multifocal parasitic myositis
    • Histologically:
      • Earliest change in periosteum -> swelling and hypertrophy of osteoprogenitor cells in the deeper layers of the periosteum
      • Spindle cells become stellate and plumper
        • Become aligned in single or multiple layers
        • Undergo hyperplasia creating a thick and densely cellular zone on the cortical surface
      • Osteoid is produced and osteoblasts become entrapped forming spiccules of trabecular new bone
      • New spicules arranged perpendicular to the cortical surface
      • With time, considerable osteoclastic activity, remodeling occurs firstly adjacent to the older cortex and then extends outwards
      • As the remodeling continues, the spicules become longitudinally arranged
      • Pseudocortices form that can contain bone marrow parenchyma
      • Endothelial hypertrophy prominent in vessels adjacent and within the periosteum
      • Severe edema present in adjacent soft tissue
      • Muscle atrophy and fibrosis often accompanied the edema
      • No evidence of inflammation
      • No parasites in adjacent muscle/soft tissue
    • Lesions develop in young dogs, within 5wks of experimental infection and progress rapidly, looking just like hypertrophic osteopathy
    • No obvious link between site of infection and generalized boney lesions
  • Differentials for periosteal new bone formation:
  • Hypertrophic osteoarthropathy
    • V difficult to distinguish from this syndrome
    • Again no inflammation, histologically look identical, but these cases don’t have an abdominal/thoracic mass, and HO usually starts on the metacarpals/tarsals
  • ?Pathogenesis
    • Thought to occur secondary to increased blood flow and fluid retention within the limbs, leading to vascular and then periosteal proliferation