Hepatozoonosis

Hepatozoonosis is caused by species of the apicomplexan parasites Hepatozoon spp., belonging to the family Hepatozoidae, which parasitize leukocytes of the host animal. Unlike most other vector-borne diseases, Hepatozoon spp. are transmitted to new animals by ingestion of an infected vector. Additionally besides the ingestion of an arthropod final host containing mature life cycle stages by the vertebrate intermediate host, other modes of transmission such as predation of one vertebrate upon another infected vertebrate host has also been observed in this genus.

Canine hepatozoonosis may present as a usually milder disease, caused by Hepatozoon canis, or a severe disease leading to debilitation and death, when caused by Hepatozoon americanum. Feline hepatozoonosis caused by Hepatozoon felis is mostly subclinical with a high proportion of cats that appear to be infected with no overt clinical signs. The pathogens do not possess zoonotic potential.

Pathogens

Canine hepatozoonosis is a tick-transmitted disease caused by species of the intraleukocytic parasite Hepatozoon sp., an apicomplexan protozoa of the family Hepatozoidae. Hepatozoon sp. is transmitted by ingestion of an infected arthropod vector. The parasites have an ellipsoidal shape and are about 11 x 4 µm in size.

Hepatozoon canis commonly infects dogs in tropical, subtropical and temperate regions worldwide with the Brown Dog tick, Rhipicephalus sanguineus, as major vector. Infection of dogs was originally reported from the Old World, but more recently also from South and North America. In the Southern USA a separate species, Hepatozoon americanum, causes disease in dogs and is transmitted by the tick Amblyomma maculatum.

Regarding feline hepatozoonosis, the classification of the Hepatozoon parasites found in domestic cats has longtime been uncertain, but molecular techniques identified a species distinct from H. canis, which was designated Hepatozoon felis (Criado-Fornelio et al., 2006; Ortuño et al., 2008) and has been confirmed later also in studies by Baneth et al. (2013). However, there is also evidence that H. canis can infect cats (e.g., Baneth et al., 2013).

Epidemiology

Hepatozoon canis commonly infects dogs mainly in tropical, subtropical and temperate climate regions, where vector tick species are abundant. Among these are regions of Africa, Southern Europe (like Spain, Portugal, France, Italy and Greece), the Middle East, Asia, and also in the USA, reflecting the geographical distribution of its major vector, Rhipicephalus sanguineus (Brown Dog tick). Further tick vectors are Ixodes canisuga and presumably also Haemaphysalis longicornis and Haemaphysalis flava.

Hepatozoon americanum is transmitted by the Gulf Coast tick Amblyomma maculatum in the southern USA. Amblyomma maculatum is found mainly along the Gulf Coast and the southern Atlantic coast in the USA. Some reports have indicated that its range is expanding north in the USA and it has also been reported from Central America and northern regions of South America.

Hepatozoonosis of domestic cats has been reported from several countries around the world including India, South Africa, Nigeria, the USA, Brazil, Israel, Spain, France and Portugal. The disease has mostly been reported from regions where canine infection is also present. The prevalence of infection varies depending on the geographical area, cat life style and type of samples tested. The arthropod vectors of Hepatozoon felis remain unknown, but H. felis DNA was detected in ticks (R. sanguineus) in Turkey and Portugal (Aktas et al., 2014; Maia et al., 2014).

Hepatozoon canis is commonly associated with other diseases as co-infection, in particular ehrlichiosis, leishmaniosis and babesiosis in endemic areas, and clinical presentations are variable.

Transmission

All the Hepatozoon spp. share a basic life cycle that includes sexual development and sporogony in a haematophageous invertebrate definitive host, and merogony followed by gametogony in a vertebrate intermediate host.

There are different developmental stages in the vertebrate and the arthropod host. These include in detail the meront stage in a variety of tissues and organs of the vertebrate host, depending on the specific species of Hepatozoon, and the gamont stage circulating in blood cells of the vertebrate host. In the invertebrate host the gamonts within the blood cells are released and differentiate to distinct gametes. After fertilisation, the zygote divides and sporogony takes place with the formation of oocysts in the tick’s haemocoel. The oocysts contain hundreds of sporocysts in which the infective sporozoites are found. The oocyst or parts of it are ingested by the vertebrate host. Within the vertebrate host the oocysts are believed to quickly rupture and release the sporocysts, which finally release the infective sporozoites. They again penetrate the gut wall and invade mononuclear cells and disseminate via the blood or lymph to target organs such as bone marrow, spleen, and lymph nodes, as well as other organs including the liver, kidney, and lungs (here e.g. in the case of Hepatozoon canis). Meronts containing asexually dividing merozoites form in the tissues in the process of merogony with two types of meronts described: one containing approximately 20–30 slender micromerozoites aligned around a central round structure forming a “wheel spoke” figure in a cystic formation, and another containing up to four larger macromerozoites (here for H. canis).

Ticks acquire the pathogen by feeding on an infected host. Transstadial transmission from the nymph to the adult stage as well as from larva to nymphal stage occurs. The transmission of Hepatozoon sp. to a new animal is accomplished by ingestion of an infected tick. No salivary transfer of this parasite has been documented.

Additionally to transmission via infected tick vectors by ingestion predation and ingestion of parasite cystozoite forms from mammal host tissues has been demonstrated for Hepatozoon americanum, but not for H. canis so far. Whereas intrauterine transmission from dam to pups has been observed for the latter.

The life cycle of H. canis, including the tick and dog parts of the cycle, can be completed in 81 days (Baneth et al., 2007). Meronts were first detected in dog’s bone marrow 13 days post inoculation, and gamonts appeared in the blood, thereby completing the life cycle at 28 days post inoculation (Baneth et al., 2001, 2007).

Pathogenesis

The pathogenesis of hepatozoonosis is highly complex. Hepatozoon canis mainly infects the haemolymphatic tissues and blood-forming organs including spleen, bone marrow and lymph nodes. But also the liver, lung and kidneys can be infected. In the different organs the pathogen further develops, passing different developmental stages (so-called meronts). The first generation meronts are highly pathogenic, causing inflammatory infiltrations and multiple lesions in all affected organs, but especially in the liver and the bone marrow. Following this is the development of meronts of the second generation and an infestation of leucocytes with the generation of gamonts. These can be detected during blood smear examination.

Hepatozoon americanum primarily infects the myocardium and the skeletal muscles and induces severe myositis and lameness. In the muscle, the pathogens form cysts with a characteristic ”onion skin” structure.

Hepatozoon canis appears to be well adapted to its canine host, and is often detected at necroscopy or on a peripheral blood smear as an incidental finding. In comparison, H. americanum induces a severe course of disease in experimental and naturally occurring infections.

In cats, the pathogen has been found as gamonts in neutrophils and monocytes, and as meronts in several tissues. Hepatozoon felis usually produces an infection of myocardial and skeletal muscles, but the infection does not lead to significant inflammatory reaction around the parasite meronts, so the cat rarely develops clinical signs. Meronts have been observed in many other tissues such as lung, liver, pancreas, bone marrow, lymph node and placenta, as well as in amniotic fluid (Baneth et al., 2013).

Diagnosis

Often the disease can be fatal before blood stages could be diagnosed. Once the parasite appears in the peripheral blood, the main pathogenic phase of the disease has already passed and primary lesions are healing up.

Hepatozoon canis infection is usually diagnosed by microscopic detection of intracellular H. canis gamonts in stained blood smears. They are found in the cytoplasm of neutrophils and monocytes, have an ellipsoidal, brick-like shape and are about 11 x 4 µm in size. Meronts of the pathogen found by histopathology contain elongated micromerozoites arranged in a circle around a clear central core forming the “wheel spoke” shaped meront.

H. americanum is rare in the blood and parasitaemia usually does not exceed 0.1%. Confirmation of H. americanum infection is commonly performed by muscle biopsy and demonstration of parasites in cysts or granulomas. Histopathology of skeletal muscles from infected dogs shows pyogranulomatous myositis and large round to oval “onion skin” cysts with a central nucleus surrounded by concentric rings of membranes. Radiography of the long bones or pelvis can be used for screening suspected animals by demonstrating periosteal proliferation.

Hepatozoon felis gamonts found in the cytoplasm of neutrophils and monocytes in stained feline blood smears have an ellipsoidal shape and are 10.5 x 4.7 μm in size. They are less prominent and so are easily missed compared with the larger H. canis gamonts in dogs. Histopathology might be positive in muscle biopsies during investigations of muscle pain or polymyositis or during necropsy.

Serological tests for H. canis and H. americanum infection have been developed in form of IFAT (indirect fluorescent antibody test) and ELISA (enzyme-linked immunosorbent assay) systems for the detection of anti-H. canis (see Baneth, 2011 for references) and in form of an ELISA for the detection of anti-H. americanum antibodies (Mathew et al., 2001).

Molecular methods have as well been developed: A PCR for H. canis in blood has been shown to be a sensitive diagnostic technique (Karagenc et al., 2006; Rubini et al., 2008). Furthermore real time PCR assays have also been developed for H. canis (Criado-Fornelio et al., 2007; Li et al., 2008). Similar PCR assays have been generated for the detection of H. americanum DNA in the blood of dogs (Inokuma et al., 2002; Criado-Fornelio et al., 2007), and real time PCR assays are able to detect both H. americanum and H. canis and distinguish between the two species (Li et al., 2008). Finally for cats a PCR has been used with high sensitivity compared to blood smear (Jittapalapong et al., 2006), so that blood PCR should be considered the diagnostic test of choice for confirming feline Hepatozoon infection when blood smears do not show parasites. PCR furthermore is the best tool for prevalence and epidemiological studies (Lloret et al., 2015). But for cats, positive DNA results should be interpreted in the light of the clinical picture, as it is most likely that clinical signs are associated with another infectious agent (Lloret et al., 2015). A quantitative PCR test has also been developed (Criado-Fornelio et al., 2007).

Clinical Signs

The clinical spectrum of Hepatozoon canis infection ranges from subclinical to severe life-threatening disease. An asymptomatic to mild disease is the most common presentation of the infection and is usually associated with a low level of H. canis parasitaemia (1-5%), while a severe illness is found in dogs with a high parasitaemia often approaching 100% of the peripheral blood neutrophils. High parasitaemia rates are frequently accompanied by extreme neutrophilia. Hepatozoon canis is commonly associated with other diseases as co-infection, in particular ehrlichiosis, leishmaniosis and babesiosis in endemic areas, and clinical presentations are variable: fever, emaciation, lethargy, anorexia, lymphadenopathy, pale mucous membranes associated with anaemia, and muscle pain.

Hepatozoon americanum infection is almost always a severe disease that leads to debilitation and death. Most dogs show fever, gait abnormalities (stiffness, hind limb paresis, ataxia and inability to rise) and muscular pain induced by myositis, generalised muscular atrophy and mucopurulent ocular discharge. The pain can be generalised or localised in the lumbar and cervical spine, or joints. A marked neutrophilia is one of the consistent haematological findings. Serum biochemical abnormalities include increased alkaline phosphatase activity and hypoalbuminaemia.

Hepatozoon felis infection is mostly a subclinical infection; a high proportion of cats appear to be infected with no obvious clinical signs. In case reports liver and/or kidney disease has been present. Elevated activities of the muscle enzyme creatinine kinase were also found in cats with hepatozoonosis.

Treatment & Prevention

Treatment of Hepatozoon canis infection is performed with imidocarb dipropionate every 14 days until parasites are no longer present in blood smears. Nevertheless, a longitudinal study with repeated imidocarb dipropionate treatments and follow up indicated that complete elimination of the parasite may frequently not be achieved. Prognosis in case of low parasitaemia is generally good, prognosis in case of high parasitaemia is guarded and sometimes associated with the outcome of a concurrent illness.

H. americanum infection is treated with a combination oral therapy of trimethoprim-sulfadiazine, pyrimethamine, and clindamycine for 14 days (Potter and Macintire, 2010). After remission of clinical signs is attained, therapy can be prolonged with the oral administration of the coccidiostat decoquinate for two years (Potter and Macintire, 2010). Relapse of clinical signs commonly follows the discontinuation of treatment. Supportive therapy with non-steroidal anti-inflammatory drugs (NSAIDs) is effective in relieving pain and fever. Dogs with American canine hepatozoonosis may experience intense pain and become reluctant to move so that beside above mentioned medication efforts must be made to ensure hydration and readily accessible food.

Further details on canine treatment regime can be found in Baneth (2011).

In cats the therapeutic information is scarce. Doxycycline has been used with no clear results, while a combination of oxytetracycline and primaquine in another case led to a successful outcome. Treatment with drugs that are frequently used in canine hepatozoonosis has not been reported in cats.

As no commercial vaccines are available for canine hepatozoonosis, prevention of both H. canis and H. americanum infection consists of the use of topical acaricides and environmental parasiticides, prevention of oral ingestion of ticks while scavenging or grooming, and of predation on infected mammal hosts. No clear recommendations on the prevention of feline infection can be made, as the routes of transmission in cats remain unknown. Transmission by blood-sucking vectors is likely as well as the consumption of meat and the transplacental route. Therefore preventive treatment using ectoparasiticides is also strongly advised in cats (Lloret et al., 2015).

Regarding transmission through blood transfusion, to date, hepatozoonosis has not been shown to be established in recipient dogs following transfusion. However, a Spanish study detected a statistically significant association between Hepatozoon spp.-positivity in cats and previous blood transfusion, but animal numbers were low in the study (Díaz-Regañón et al., 2017).

References

Pathogens

Baneth G, Sheiner A, Eyal O, et al.: Redescription of Hepatozoon felis (Apicomplexa: Hepatozoidae) based on phylogenetic analysis, tissue and blood form morphology, and possible transplacental transmission. Parasit Vectors. 2013, 6, 102

Criado-Fornelio A, Ruas JL, Casado N, et al.: New molecular data on mammalian Hepatozoon species (Apicomplexa: Adeleorina) from Brazil and Spain. J Parasitol. 2006, 92, 93-9

Ortuño A, Castellà J, Criado-Fornelio A, et al.: Molecular detection of a Hepatozoon species in stray cats from a feline colony in North-eastern Spain. Vet J. 2008, 177, 134-5Allison RW, Little SE: Diagnosis of rickettsial diseases in dogs and cats. Vet Clin Pathol. 2013, 42, 127–44

 

Epidemiology

Aktas M: A survey of ixodid tick species and molecular identification of tickborne pathogens. Vet Parasitol. 2014, 200, 276-83

Maia C, Ferreira A, Nunes M, et al.: Molecular detection of bacterial and parasitic pathogens in hard ticks from Portugal. Ticks Tick Borne Dis. 2014, 5, 409-14

 

Transmission

Baneth G, Samish M, Alekseev E, et al.: Transmission of Hepatozoon canis to dogs by naturally-fed or percutaneously injected Rhipicephalus sanguineus ticks. J Parasitol. 2001, 87, 606-11

Baneth G, Samish M, Shkap V: Life cycle of Hepatozoon canis (Apicomplexa: Adeleorina: hepatozoidae) in the tick Rhipicephalus sanguineus and domestic dog (Canis familiaris). J Parasitol. 2007, 93, 283-99

 

Pathogenesis

Baneth G, Sheiner A, Eyal O, et al.: Redescription of Hepatozoon felis (Apicomplexa: Hepatozoidae) based on phylogenetic analysis, tissue and blood form morphology, and possible transplacental transmission. Parasit Vectors. 2013, 6, 102

 

Diagnosis

Criado-Fornelio A, Buling A, Cunha-Filho NA, et al.: Development and evaluation of a quantitative PCR assay for detection of Hepatozoon sp. Vet Parasitol. 2007, 150, 352-6

Inokuma H, Okuda M, Ohno K, et al.: Analysis of the 18S rRNA gene sequence of a Hepatozoon detected in two Japanese dogs. Vet Parasitol. 2002, 106, 265-71

Jittapalapong S, Rungphisutthipongse O, Maruyama S, et al.: Detection of Hepatozoon canis in stray dogs and cats in Bangkok, Thailand. Ann NY Acad Sci. 2006, 1081, 479-88

Karagenc TI, Pasa S, Kirli G, et al.: A parasitological, molecular and serological survey of Hepatozoon canis infection in dogs around the Aegean coast of Turkey. Vet Parasitol. 2006, 135, 113-9

Li Y, Wang C, Allen KE, et al.: Diagnosis of canine Hepatozoon spp. infection by quantitative PCR. Vet Parasitol. 2008, 157, 50-8

Lloret A, Addie DD, Boucraut-Baralon C, et al.: Hepatozoonosis in cats: ABCD guidelines on prevention and management. J Feline Med Surg. 2015, 17, 642-4

Mathew JS, Saliki JT, Ewing SA, et al.: An indirect enzyme-linked immunosorbent assay for diagnosis of American canine hepatozoonosis. J Vet Diagn Invest. 2001, 13, 17-21

Rubini AS, dos Santos Paduan K, Von Ah Lopes V, et al.: Molecular and parasitological survey of Hepatozoon canis (Apicomplexa: Hepatozoidae) in dogs from rural area of Sao Paulo state, Brazil. Parasitol Res. 2008, 102, 895-9

 

Treatment & Prevention

Baneth G: Perspectives on canine and feline hepatozoonosis. Vet Parasitol. 2011, 181, 3-11

Díaz-Regañón D, Villaescusa A, Ayllón T, et al.: Molecular detection of Hepatozoon spp. and Cytauxzoon sp. in domestic and stray cats from Madrid, Spain. Parasit Vectors. 2017, 10, 112

Lloret A, Addie DD, Boucraut-Baralon C, et al.: Hepatozoonosis in cats: ABCD guidelines on prevention and management. J Feline Med Surg. 2015, 17, 642-4

Potter TM, Macintire DK: Hepatozoon americanum: an emerging disease in the south-central/southeastern United States. J Vet Emerg Crit Care (San Antonio). 2010, 20, 70-6

Further Reading

Baneth G: Perspectives on canine and feline hepatozoonosis. Vet Parasitol. 2011, 181, 3-11

Ewing SA, Panciera RJ: American canine hepatozoonosis. Clin Microbiol Rev. 2003, 16, 688-97

Lloret A, Addie DD, Boucraut-Baralon C, et al.: Hepatozoonosis in cats: ABCD guidelines on prevention and management. J Feline Med Surg. 2015, 17, 642-4

 

EXPLORE OUR CONTENT

CVBD Maps

The CVBD Occurence World Map presents country-specific situations based on current scientific knowledge and feed-back from experts around the world in an easy-to-grasped way.

Resources

Bayer Animal Health supports education in parasitology and especially in the field of Vector-Borne Disease.  Access picture and video collections, discover the World Forum calendar, interesting links and our glossary.

CVBD World Forum

The CVBD World Forum is a working group of leading international experts with the mission to enhance knowledge and communication on companion animal vector-borne diseases for the improvement of animal, human, and environmental health.