Diagnosing Canine Hepatozoonosis: From Blood Smear to PCR

Diagnosing Hepatozoon canis infection can be challenging because clinical signs such as fever, anemia, lethargy, weight loss, and thrombocytopenia are nonspecific and may occur with several other canine diseases. For the practicing veterinarian, a combination of clinical suspicion, microscopic examination, and molecular testing can improve diagnostic confidence and help distinguish H. canis from other tick-borne infections. 

Start With the Clinical and Hematological Picture 

A dog with a history of tick exposure and compatible clinical or hematological abnormalities should raise suspicion for canine hepatozoonosis. The infection may remain subclinical, particularly when parasitaemia is low, so the absence of obvious illness does not exclude infection. 

Routine hematology may provide useful supporting evidence. Anemia is the most frequently reported primary hematological abnormality and is generally normocytic and normochromic with evidence of regeneration. Thrombocytopenia may occur in approximately one-third of infected dogs, while dogs with high parasitaemia may develop marked leukocytosis. Eosinophilia, hyperglobulinemia, hypoalbuminemia, and increased creatine kinase and alkaline phosphatase activities have also been reported^1,2. 

These abnormalities are supportive rather than diagnostic because similar findings can occur with other tick-borne and systemic diseases. 

Peripheral Blood Smear: A Practical First Step² 

The conventional approach to detecting H. canis involves microscopic examination of blood or buffy coat smears. Blood smears can be stained using Romanowsky-Giemsa, Pappenheim, or Hemacolor methods. 

The diagnostic target is the gamont within leukocytes. H. canis gamonts are typically oval, measuring approximately 8–12 µm in length and 3–6 µm in width. They are most commonly identified within the cytoplasm of neutrophils and are only occasionally seen in monocytes. 

The number of infected neutrophils can also provide an indication of parasitaemia. Gamonts occurring in fewer than 5% of neutrophils are generally associated with mild or subclinical infection, whereas severe disease may occur when parasitaemia is markedly increased. 

However, microscopy may have limited sensitivity, particularly when parasitaemia is low. Therefore, a negative blood smear should not automatically exclude infection when clinical suspicion remains high. 

PCR for Greater Diagnostic Precision 

PCR provides a more sensitive molecular approach for detecting H. canis. Blood and buffy coat samples can be used, with amplification of the 18S-rRNA gene allowing detection of the parasite. 

PCR and DNA sequencing are also useful for distinguishing H. canis from other parasitic organisms. Molecular techniques have increasingly replaced histopathological examination for pathogen detection because PCR offers greater sensitivity in identifying H. canis^3,4. 

For clinical practice, PCR becomes particularly valuable when microscopy is inconclusive or when accurate species identification is important. 

When Tissue Examination Is Relevant² 

Schizonts may be detected in lymph nodes, spleen, and bone marrow using histological or touch-impression preparations. These structures are generally round or oval and measure approximately 30 µm in diameter. They may contain 2–4 macromerozoites or more than 20 micromerozoites, with microschizonts showing a characteristic wheel-spoke appearance. 

Do Not Overlook Differential Diagnoses 

H. canis should be differentiated from other tick-borne infections, particularly Babesia spp., Anaplasma phagocytophilum, and Ehrlichia canis. Non-infectious conditions that can produce overlapping signs include iron or vitamin B12 deficiency, chronic kidney disease, bone marrow disorders, certain cancers, and immune-mediated hemolytic anemia. 

Practical Clinical Insight 

For a dog with compatible clinical findings and tick exposure, use microscopy as a practical screening approach but interpret a negative smear cautiously. When confirmation is important, particularly with low parasitaemia or overlapping infections, PCR on blood or buffy coat can provide a more sensitive and precise diagnostic assessment. 

References 

1. Otranto D, Dantas-Torres F, Weigl S, Latrofa MS, Stanneck D, Decaprariis D, Capelli G, Baneth G. Diagnosis of Hepatozoon canis in young dogs by cytology and PCR. Parasites & vectors. 2011 Apr 13;4(1):55. https://link.springer.com/content/pdf/10.1186/1756-3305-4-55.pdf 

2. Hasani SJ, Rakhshanpour A, Enferadi A, Sarani S, Samiei A, Esmaeilnejad B. A review of Hepatozoonosis caused by Hepatozoon canis in dogs. Journal of Parasitic Diseases. 2024 Sep;48(3):424-38. https://pmc.ncbi.nlm.nih.gov/articles/PMC11319582/pdf/12639_2024_Article_1682.pdf 

3. Hegab AA, Omar HM, Abuowarda M, Ghattas SG, Mahmoud NE, Fahmy MM. Screening and phylogenetic characterization of tick-borne pathogens in a population of dogs and associated ticks in Egypt. Parasites & vectors. 2022 Jun 21;15(1):222. https://link.springer.com/content/pdf/10.1186/s13071-022-05348-x.pdf 

4. Kamani J, Baneth G, Mumcuoglu KY, Waziri NE, Eyal O, Guthmann Y, Harrus S. Molecular detection and characterization of tick-borne pathogens in dogs and ticks from Nigeria. PLoS neglected tropical diseases. 2013 Mar 7;7(3):e2108. https://journals.plos.org/plosntds/article/file?id=10.1371/journal.pntd.0002108&type=printable