This article discusses the pathogenesis, diagnosis and treatment of hepatozoonosis, a haemoparasitic disease transmitted by the ingestion of an arthropod vector. Two different species of Hepatozoon infect dogs - Hepatozoon canis in the Old World and South America, and Hepatozoon americanum in the southern USA. H canis infection varies from being asymptomatic in dogs with a low parasitaemia, to causing severe disease with anaemia, profound lethargy and cachexia in dogs with large numbers of circulating parasites. H americanum infection manifests mainly as gait abnormalities and musculoskeletal pain due to myositis and periosteal bone lesions. Feline hepatozoonosis is associated with muscular pathology and is often reported in conjunction with a retroviral disease. The diagnosis of hepatozoonosis relies mainly on the detection of blood gamonts in the case of H canis infection, and on muscle biopsy showing tissue cysts and pyogranulomas for H americanum infection. Therapy for both forms of the disease involves long-term treatment with combinations of antiprotozoal drugs. The control of hepatozoonosis in small animals relies on effective use of topical parasiticides and prevention of ingestion of blood-sucking arthropod hosts by animals when grooming or scavenging.