Canis ISSN: 2398-2942

Hepatozoon americanum

Contributor(s): Gad Baneth, Prof Douglass Macintire




  • Phylum: Apicomplexa
  • Class: Sporozoea
  • Subclass: Coccidia
  • Order: Euccidiiae
  • Family: Haemogregarinidae
  • Genus: Hepatozoon

Active Forms

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Clinical Effects



  • The disease has been reported in the geographic habitat of the host tick,Amblyomma maculatum. It is also found in wildlife (coyotes, raccoons, and bobcats) in the same geographical area.


  • The dog ingests the infected tick vector (Amblyomma maculatum) with oocysts. Sporozoites are released into the intestine of the dog. They penetrate the intestinal wall and enter an unidentified host cell. This cell is carried hematogenously to a peripheral site (usually skeletal muscle) where merogony occurs within the host cell Tissue specimen containing a meront.
  • After 21 - 32 days, the host cell ruptures and zoites are released, creating an intense pyogranulomatous reaction Muscle specimen showing a pyogranuloma. Infected neutrophils enter the bloodstream to either be ingested by the tick host or redistributed throughout the dog to form additional meronts. Continued release of zoites with resultant pyogranulomatous myositis causes the chronic waxing and waning fever and other clinical signs.


  • To date, the only form of transmission proven is the ingestion of infectedAmblyomma maculatumticks.
  • Transmission did not occur with Rhipicephalus spp. ticks or with ingestion of infected tissues.

Pathological effects

Other Host Effects

  • H. americanum causes clinical signs of disease in naturally and experimentally infected dogs. Subclinical infections have not been identified in healthy dogs.


Control via chemotherapies

  • For immediate remission of clinical signs (within 48-72 hours), administer the following combination of drugs for 14 days: trimethoprim sulfadiazine Trimethoprim , 15 mg/kg PO q 12 h; clindamycin Clindamycin , 10 mg/kg PO q 8 h; and pyrimethamine Pyrimethamine , 0.25 mg/kg PO q 24 h.
  • For long term control and prevention of relapse, adminster the coccidiostat, decoquinate daily for 18 - 24 months. The dosage is 10 - 20 mg/kg added to food q 12 h (0.5 - 1.0 teaspoon per 10 kg BW Deccox 22.7 g/lb mixture).
  • For palliative control of fever and muscle pain, administer NSAIDs Analgesia: NSAID at standard doses.

Other countermeasures

  • Aggressive tick control is warranted.


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Further Reading


Refereed papers

  • Recent references from VetMed Resource and PubMed.
  • Macintire D K & Vincent-Johnson N A (2001) Treatment of dogs infected with Hepatozoon americanum: 53 cases (1989-1998). J Am Vet Med Assoc 218,77-82.
  • Mathew JS, Saliki J T et al (2001) An Indirect enzyme-linked immunosorbent assay for diagnosis of American canine hepatozoonosis. J Diagn Inves 13,17-21.
  • Panciera R J, Ewing S A et al (1999) Canine hepatozoonosis: comparison of lesions and parasites in skeletal muscle of dogs experimentally or naturally infected with Hepatozoon americanumVet Parasitol 82,261-272.
  • Mathew J S, Ewing S A et al (1998) Experimental transmission of Hepatozoon americanumto dogs by the Gulf Coast tick Amblyomma maculatumVet Parasitol 80,1-14.
  • Panciera R J, Ewing S A et al (1998) Observations on tissue stages of Hepatozoon americanumin 19 naturally infected dogs. Vet Parasitol 78, 265-276.
  • Panciera R J, Gatto R T et al (1997) Canine hepatozoonosis in Oklahoma. J Am Anim Hosp Assoc 33,221-231.
  • Macintire D K, Vincent-Johnson N A et al (1997) Hepatozoonosis in dogs: 22 cases (1989-1994). J Am Vet Med Assoc 210,916-922.
  • Vincent-Johnson N A, Macintire D K et al (1997) A new Hepatozoon species from dogs: description of the causative agent of canine hepatozoonosis in North America. J Parasitol 83,1165-1172.
  • Vincent-Johnson N A, Macintire D K et al (1997) Canine hepatozoonosis: pathophysiology, diagnosis and treatment. Compend Contin Educ Pract Vet 19,51-65.
  • Craig T M, Jones L P et al (1984) Diagnosis of Hepatozoon canis by muscle biopsy. J Am Anim Hosp Assoc 210 , 301-303.

Other sources of information

  • Craig T M (1998) Hepatozoonosis. In: Clinical microbiology and infectious diseases of the dog and the cat. Ed C E Greene. Philadelphia. WB Saunders Co. 458-465.