Hepatozoon canis belongs to the protozoa and goes through a typical coccidial life cycle with the dog as intermediate host. Asexual reproduction, schizogony, takes place in several generations in the endothelial cells of the spleen, liver and bone marrow. The merozoites formed here penetrate the leukocytes and differentiate into gamonts.
The definitive host, the tick, ingests the gamonts during the blood meal. Gamogony and sporogony take place in the tick and oocysts with 16 infectious sporozoites each are formed.
Infection with H. canis occurs by biting or swallowing an infected tick, primarily the brown dog tick (R. sanguineus), which is found in warm countries (mainly Southern Europe, South America, Africa and Asia). By now, the pathogen has also become endemic in several regions of Germany. Vertical intrauterine transmission is possible as well.
Acute infections are characterised by fever, lymphadenitis, anorexia, apathy, myositis and epileptiform seizures (bleeding in meninges). Massive lesions up to necrosis occur in the affected organs (spleen, liver, lung, brain). Chronic infections cause intermittent fever, lymphadenopathy, anaemia, diarrhoea and vomiting. Hyperaesthesia and muscular pain with stiffening of the neck muscles and the trunk muscles occur. Periosteal bone proliferation can be seen and epileptiform seizures may also occur in chronic diseases. In case of low parasitaemia, the infection may be clinically inapparent or may only have mild clinical signs.