Canine Hepatozoonosis: Recognizing Clinical and Hematological Clues

Canine hepatozoonosis caused by Hepatozoon canis can be challenging to recognize because many infected dogs remain clinically normal, while dogs with high parasitaemia may develop severe, potentially fatal disease. The clinical picture is also nonspecific and can resemble other tick-borne infections or systemic disorders. For the practicing veterinarian, interpreting the severity of clinical signs alongside parasitaemia and hematological changes is therefore important when H. canis is suspected. 

Clinical Spectrum: From Subclinical to Severe Disease 

H. canis infection may occur in three clinical forms: subclinical, acute, or chronic. The subclinical form is considered the most common, while acute disease may occur approximately one week before death. Chronic infection can be characterized by alternating periods of clinical illness and remission¹. 

Many infected dogs develop a prolonged infection with only mild or insignificant clinical changes. Low parasitaemia, in which gamonts are detected in fewer than 5% of neutrophils, is generally associated with asymptomatic or mild disease. In contrast, severe clinical disease is associated with high parasitaemia, with gamonts reported in up to 100% of neutrophils and marked leukocytosis reaching as high as 150,000/µl¹. 

Common clinical manifestations include: 

• Intermittent fever 

• Lethargy and depression 

• Anemia 

• Weight loss and cachexia 

• Muscle sensitivity or pain 

• Reduced appetite 

• Purulent conjunctivitis and rhinitis 

• Diarrhea, occasionally bloody 

• Weakness, paraparesis or paraplegia 

Anemia, weight loss, and intermittent fever have been frequently documented in naturally and experimentally infected dogs^1,2. 

Hematological and Biochemical Clues 

Anemia is the most commonly observed primary hematological abnormality. It is typically normocytic and normochromic, with evidence of regeneration. Leukocyte counts may remain within normal limits when parasitaemia is low, whereas marked leukocytosis may accompany heavy infection. 

Approximately one-third of infected dogs may develop thrombocytopenia, although this finding can also occur with concurrent infections such as ehrlichiosis. Other reported biochemical abnormalities include hyperglobulinemia, hypoalbuminemia, and increased creatine kinase and alkaline phosphatase activities¹. 

In dogs positive for H. canis, hematological abnormalities were reported in 74.8% of cases, with eosinophilia identified as the most frequent alteration². 

Clinical Severity and Co-infections 

The severity of disease should not be assessed from a single abnormal laboratory parameter. A dog presenting with weakness, pale or yellowish mucous membranes, marked weight loss, fever, muscle pain, and heavy tick infestation warrants careful evaluation for significant parasitaemia and concurrent disease. 

Co-infection can further complicate interpretation. A reported dog simultaneously infected with Ehrlichia canis and H. canis showed poor clinical condition, inability to stand, anorexia, pale mucous membranes, and muscle pain, accompanied by anemia, thrombocytopenia and leukocytosis³. 

Practical Clinical Insight 

When H. canis is considered, correlate clinical severity, parasitaemia and hematological findings rather than relying on any single sign. Mild or absent clinical disease does not exclude infection, while severe systemic signs should prompt assessment for high parasitaemia and possible co-infections. This approach helps distinguish uncomplicated subclinical infection from cases requiring more intensive diagnostic and therapeutic attention. 

References 

1. 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 

2. 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 

3. Tsachev I, Ivanov A, Dinev I, Simeonova G, Kanakov D. Clinical Ehrlichia canis and Hepatozoon canis co-infection in a dog in Bulgaria. Revue de Médecine Vétérinaire. 2008 Feb 1;159(2):68. https://www.academia.edu/download/78108339/Clinical_Ehrlichia_canis_and_Hepatozoon_20220105-10055-fbnukd.pdf