Question 1

What is the taxonomic classification of Hepatozoon canis?

Accepted Answer

Hepatozoon canis belongs to the phylum Apicomplexa, class Conoidasida, order Eucoccidiorida, and family Hepatozoidae [2]. The genus Hepatozoon comprises over 300 species that infect a diverse range of vertebrate hosts, including mammals, birds, reptiles, and amphibians [2, 3]. Molecular phylogenetic analyses based on 18S rRNA gene sequences have demonstrated that H. canis forms a well-supported clade with other carnivore-associated Hepatozoon species, such as H. silvestris, H. martis, and H. ursi [2]. Host switching has been identified as the primary driver of coevolution between Hepatozoon parasites and their vertebrate hosts, rather strict cospeciation [3]. This evolutionary plasticity may explain the broad host range of H. canis, which includes domestic dogs, cats, wild canids, felids, mustelids, and ursids [2, 17, 34].

Question 2

How is Hepatozoon canis transmitted?

Accepted Answer

Transmission of H. canis occurs exclusively through the oral ingestion of ticks containing mature oocysts [2, 3]. The principal tick vector is Rhipicephalus sanguineus sensu lato (the brown dog tick), although other ixodid species may also serve as vectors in different geographic regions [4, 18, 5, 19, 20]. When a dog ingests an infected tick during grooming or predation, the oocysts rupture in the gastrointestinal tract, releasing sporozoites that penetrate the intestinal wall and enter the circulation [2]. Sporozoites then invade mononuclear phagocytes (monocytes and macrophages) and undergo merogony (asexual reproduction) in various tissues, including the bone marrow, spleen, lymph nodes, and liver [1, 2]. Merozoites released from meronts subsequently invade neutrophils and, to a lesser extent, monocytes, where they develop into gamonts (the diagnostic blood stage) [2]. The life cycle is completed when a tick ingests gamonts during a blood meal; gametes fuse in the tick gut, leading to the formation of oocysts in the tick haemocoel [2].

Vertical transmission (transplacental) has been documented in dogs. A case report described a three-month-old puppy with severe anaemia and gastroenteritis following vaccination, suspected to result from vertical hepatozoonosis [21]. Another study provided the first report of vertical transmission of Babesia vogeli and Candidatus Mycoplasma haematoparvum in pregnant dogs, and although H. canis was not the focus, the study underscores the potential for transplacental passage of haemoparasites [22]. Experimental evidence for vertical transmission of H. canis remains limited, but the clinical suspicion is supported by the detection of infection in very young animals without prior tick exposure [21].

Question 3

What are the clinical signs and pathogenesis of canine hepatozoonosis?

Accepted Answer

Canine hepatozoonosis presents with a wide clinical spectrum, ranging from subclinical infection to severe, debilitating disease [1, 31]. The classic syndrome, often termed "chronic hepatozoonosis," is characterized by fever, lethargy, anorexia, weight loss, lymphadenomegaly, splenomegaly, and a stiff, painful gait due to periosteal bone proliferation (polyostotic osteomyelitis) [1, 2]. Haematological abnormalities commonly include normocytic, normochromic anaemia, neutrophilic leukocytosis, and thrombocytopenia [1, 10, 14]. Serum biochemical changes may include elevated alkaline phosphatase, hypoalbuminaemia, and hyperglobulinaemia, reflecting a chronic inflammatory state [1, 14].

A paradigm-shifting study by Bahrami et al. demonstrated that H. canis can cause clinically significant disease even at low parasitaemia levels, challenging the traditional view that high parasitaemia is required for clinical expression [1]. The study documented a chronic inflammatory state characterized by elevated acute-phase proteins and pro-inflammatory cytokines in infected dogs, irrespective of parasitaemia level [1]. This finding has important implications for diagnosis and treatment, as dogs with low parasitaemia may still require therapeutic intervention.

Co-infections with other tick-borne pathogens, such as Ehrlichia canis, Anaplasma platys, and Babesia vogeli, are common and may exacerbate clinical signs [23, 4, 24, 33]. In a study from northeastern Brazil, dogs co-infected with multiple pathogens transmitted by R. sanguineus s.l. exhibited more severe haematological alterations than those with single infections [4]. Similarly, a study from Thailand found that co-infections were frequent among free-roaming dogs and were associated with higher parasitaemia and more pronounced anaemia [8].

Question 4

How is Hepatozoon canis diagnosed?

Accepted Answer

Diagnosis of H. canis infection relies on a combination of microscopic examination of blood smears, molecular detection, and serological assays [25, 33]. Each method has distinct advantages and limitations.

Microscopic examination. The detection of characteristic ellipsoidal gamonts within neutrophils (and occasionally monocytes) on Giemsa-stained thin blood smears is the most accessible diagnostic method [1, 10, 14]. Gamonts measure approximately 8–11 µm by 3–5 µm and are enclosed within a parasitophorous vacuole [2]. However, microscopy has limited sensitivity, particularly in dogs with low parasitaemia, and cannot reliably differentiate H. canis from other Hepatozoon species [25, 2].

Molecular detection. Polymerase chain reaction (PCR) targeting the 18S rRNA gene is the gold standard for diagnosis and species identification [6, 23, 7, 9, 25, 11, 12, 14, 15, 16, 33]. Conventional PCR, nested PCR, and real-time PCR assays have been developed, with high sensitivity and specificity [23, 25]. A hexaplex PCR assay capable of simultaneously detecting H. canis and five other tick-borne pathogens has been described, facilitating the diagnosis of co-infections [23]. Targeted next-generation sequencing (NGS) approaches have also been applied to detect H. canis in canine blood samples, providing high-throughput screening capability [15].

Serological assays. Indirect immunofluorescence antibody tests (IFAT) and enzyme-linked immunosorbent assays (ELISA) have been developed for H. canis, but they are less commonly used in clinical practice due to cross-reactivity with other apicomplexan parasites and the lack of standardized commercial kits [2, 33].

Proteomic approaches. A recent study explored mass spectrometry-based serum peptidomic profiling as a potential tool for biomarker discovery in canine hepatozoonosis [26]. The study identified several differentially expressed peptides that could distinguish infected from uninfected dogs, offering a novel avenue for non-invasive diagnosis [26].

The following Mermaid diagram summarizes a diagnostic decision tree for suspected canine hepatozoonosis.

flowchart TD
    A[Clinical suspicion: fever, lethargy, anaemia, stiff gait, tick exposure], > B{Blood smear examination}
    B, >|Gamonts detected| C[Confirm with PCR 18S rRNA]
    B, >|No gamonts detected| D[Perform PCR 18S rRNA]
    C, > E[Positive: Hepatozoon canis confirmed]
    C, > F[Negative: consider other pathogens or low parasitaemia]
    D, > G[PCR positive: H. canis infection]
    D, > H[PCR negative: rule out hepatozoonosis; investigate other causes]
    E, > I[Assess parasitaemia level and co-infections]
    G, > I
    I, > J[Haematology, biochemistry, acute-phase proteins]
    J, > K[Treatment decision based on clinical signs and parasitaemia]

Question 5

What is the global epidemiology of Hepatozoon canis?

Accepted Answer

Hepatozoon canis has a cosmopolitan distribution, with prevalence rates varying widely by geographic region, dog population (owned versus free-roaming), and diagnostic method used [6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 33]. In Asia, molecular surveys have reported prevalence rates of 10–40% in dogs from India [6, 9, 14], Thailand [8, 25], Vietnam [11], Iran [12], Bangladesh [10], and Hong Kong [16]. A study in southern Thailand found a high burden of haemoparasitic infections in free-roaming dogs, with H. canis being one of the most prevalent pathogens [8]. In West Bengal, India, molecular standardization and epidemiological mapping revealed a prevalence of 18.5% for H. canis [6].

In Africa, studies from Ghana [5, 19], Chad [15], and Namibia [33] have documented substantial prevalence rates. A targeted NGS approach in Chad detected H. canis in 34% of dogs [15]. In Namibia, a multi-modal investigation found H. canis in 12% of domestic dogs [33].

In the Americas, H. canis is widely reported in Brazil [4, 7, 24, 27, 35], Argentina [28], Peru [13], and the Caribbean region [20]. A study from Rio de Janeiro, Brazil, reported a molecular prevalence of 22% in dogs from urban and peri-urban areas [7]. In the semiarid region of northeastern Brazil, co-infections with E. canis, A. platys, and B. vogeli were common [4, 24]. The first evidence of H. canis in dogs with clinical signs in northern Lima, Peru, was reported in 2025 [13].

In Europe, clinical cases have been documented in Ukraine [31] and southern Europe, although prevalence is generally lower than in tropical regions [2]. A case of clinical canine hepatozoonosis in Ukraine represented the first confirmed report in that country [31].

Question 6

Which animal species are susceptible to Hepatozoon canis?

Accepted Answer

The natural host range of H. canis includes domestic dogs (Canis lupus familiaris), wild canids (e.g., raccoon dogs, foxes, wolves), felids (domestic cats and wild felids such as tigers), mustelids, and ursids [2, 17, 34]. A recent study provided the first description of H. canis in raccoon dogs (Nyctereutes procyonoides) in South Korea, expanding the known host range [17]. In domestic cats, Hepatozoon spp. infections have been reported in Argentina [28], Turkey [29], and Namibia [32]. A study from Argentina characterized the genetic diversity and spatial distribution of Hepatozoon spp. in domestic cats, identifying both H. canis and H. silvestris [28]. In Thailand, Hepatozoon spp. were detected in captive tigers, indicating that wild felids are also susceptible [34]. The broad host range underscores the ecological plasticity of H. canis and its potential to infect a variety of carnivore species [3].

Question 7

How is canine hepatozoonosis treated and managed?

Accepted Answer

Treatment of canine hepatozoonosis is challenging, and no universally effective protocol exists [2]. The most commonly used therapeutic agents include imidocarb dipropionate (two doses at 14-day intervals) and toltrazuril, often in combination with supportive care [2]. Doxycycline has been used empirically, particularly in cases of co-infection with E. canis, but its efficacy against H. canis alone is uncertain [2]. A recent study from Iran reported that dogs treated with a combination of imidocarb and toltrazuril showed clinical improvement, but parasitaemia clearance was not always achieved [12]. Relapses are common, and long-term monitoring is recommended [1, 2]. Prevention relies on strict tick control through the use of acaricides and avoiding ingestion of ticks by dogs [2]. No vaccine is currently available.

Question 8

What are the key risk factors for Hepatozoon canis infection?

Accepted Answer

Several risk factors have been identified across epidemiological studies. Free-roaming and stray dogs have significantly higher odds of infection compared to owned dogs, likely due to greater tick exposure [8, 10, 12, 14]. Age is a variable factor; some studies report higher prevalence in adult dogs, while others find no age association [12, 14]. Male dogs may be at slightly higher risk in some populations, possibly due to behavioural differences [12]. Seasonal variation has been observed, with higher prevalence during warmer months when tick activity peaks [8, 10]. Co-infection with other tick-borne pathogens is a strong risk factor for H. canis infection, reflecting shared vector ecology [23, 4, 24, 33]. Living in rural or peri-urban areas with high tick density also increases risk [7, 27, 35].

Question 9

What is the role of wildlife in the epidemiology of Hepatozoon canis?

Accepted Answer

Wild carnivores serve as important reservoir hosts for H. canis and contribute to the maintenance of the parasite in natural ecosystems [2, 17, 27, 34]. In Brazil, dogs living near Atlantic Forest conservation units were found to have a high prevalence of H. canis, suggesting spillover from wildlife reservoirs [27]. The detection of H. canis in raccoon dogs [17] and tigers [34] further supports the role of wildlife in the parasite's epidemiology. Host switching, rather than co-speciation, appears to be the dominant evolutionary mechanism driving Hepatozoon diversification, facilitating cross-species transmission [3].

Zubair Khalid

Hepatozoon canis: Tick-Borne Protozoan Pathogen in Dogs – Etiology, Pathogenesis, Diagnosis, and Epidemiology

Introduction