Hepatozoon

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