Pathogenesis and Transmission
Ticks acquire the pathogen by feeding on an infected host. Transstadial transmission from the nymph to the adult stage occurs, but not transovarial transmission. 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. Once ingested, organisms are released within the intestine of the dog, penetrate the intestinal wall, invade mononuclear cells and are carried by the bloodstream or via the lymphatic system to various tissues.
The pathogenesis is highly complex. H. 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 (also 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, in which so-called gamonts are generated. These can be detected during blood smear examination.
H. 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. H. 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.
- Baneth G: Hepatozoonosis. In: Arthropod-borne Diseases. 2002, Sci. Proc. BSAVA Congress, Birmingham, pp 187-9