Overview

• Disease caused by infection with the protozoan Hepatozoon americanum or Hepatozoon canis. This chapter focuses on systemic infection with Hepatozoon americanum-the primary cause of American canine hepatozoonosis.
• Hepatozoon canis is less prevalent in the United States than Hepatozoon americanum. It is also less virulent and infections are typically subclinical.
• Infection typically involves muscle and bone.
• Dogs-more common in the southern and southeastern United States.
• Cats-uncommon in the United States; one reported case from Hawaii.

Signalment

• Dogs and rarely cats
• No age, breed, or sex predilections

Signs

• Lameness and stiff gait
• Waxing and waning fever
• Mucopurulent ocular discharge
• Hyperesthesia
• Weight loss and cachexia
• Polyuria and polydipsia in some cases if glomerulonephritis is present

Causes & Risk Factors

• Amblyomma maculatum-tick ingestion.
• Ingestion of infected paratenic hosts (predation or scavenging).

Differential Diagnosis

• Neoplasia
• Endocarditis
• Immune-mediated polyarthritis or polymyositis
• Chagas disease
• Leishmaniasis
• Meningitis/meningoencephalitis
• Hypertrophic osteopathy
• Ehrlichiosis
• Discospondylitis
• Hepatic failure

CBC/Biochemistry/Urinalysis

• Neutrophilic leukocytosis, usually profound (20,000–200,000 WBC/µL) sometimes with a left shift.
• Anemia-mild to moderate, usually non-regenerative.
• Usually have a normal platelet count unless there is a co-infection.
• High serum ALP activity.
• Decreased urea nitrogen.
• Despite severe myositis, creatinine kinase concentrations are often within reference range.
• Hyperglobulinemia.
• Hypoalbuminemia.

Other Laboratory Tests

Blood films-in rare cases identify organisms in circulating neutrophils and monocytes.

Imaging

Radiographs-pelvis, lumbar vertebrae, and long bones; reveal periosteal proliferation.

Diagnostic Procedures

• Muscle biopsy
• Polymerase chain reaction on blood or muscle

Pathologic Findings

• Cachexia.
• Muscle atrophy: “onion-skin” meronts and pyogranulomatous myositis.
• Enlarged liver and spleen-may contain meront stages on histopathology.
• Periosteal proliferation of bone.

Drug(s)

• Mostly palliative as no treatments have been shown to clear infection.
• Combination therapy (TCP) initially:
  • Trimethoprim = sulfadiazine-15 mg/kg PO q12h for 14 days.
  • Clindamycin-10 mg/kg PO q8h for 14 days.
  • Pyrimethamine-0.25 mg/kg PO q24h for 14 days.
• Followed with long-term therapy:
  • Decoquinate 10–20 mg/kg PO q12h for up to 33 months (possibly indefinite).
• Ponazuril 10 mg/mg PO q12h for 28 days has shown promise as an alternative initial treatment. Long-term therapy with decoquinate is still necessary.
• Glucocorticoids-may give temporary relief but are not routinely recommended.
• NSAIDs.

Contraindications/Possible Interactions

None

Patient Monitoring

• Difficult to monitor organisms in chronically infected dogs.
• Best to monitor for clinical improvement.

Prevention/Avoidance

• Control ticks within the household or kennel.
• Avoid predation and scavenging.

Possible Complications

• Glucocorticoids-may exacerbate clinical disease.
• Radiographic changes may never occur.

Expected Course and Prognosis

• Treatment typically results in improvement of clinical signs and quality of life but does not cure the infection.
• Treatment is likely to be needed for the remainder of the patient's life.
• If clinical relapse occurs, repeat of both initial and long-term treatments are recommended.

Zoonotic Potential

No reported risk to humans

Abbreviations

• ALP = alkaline phosphatase
• NSAIDS = nonsteroidal anti-inflammatory drug
• TCP (trimethoprim-sulfa, clindamycin, and pyrimethamine)