American canine hepatozoonosis (ACH) is a serious, debilitating illness caused by Hepatozoon americanum. The disease is characterized by periosteal proliferation, usually on the long bones, pelvis, vertebrae, and skull. The pathogenesis of these lesions is unknown, but they are probably caused by cytokines released during the inflammatory process. The periosteal proliferation is similar to hypertrophic osteopathy (HO) lesions in dogs and humans. Knowing when the bone lesions of ACH occur may help determine the pathogenesis of the proliferation and may act as a model in the study of periosteal lesions. Testing blood or tissue immediately before and after onset of bone lesions may determine whether humoral and/or local factors contribute to the periosteal reaction.

Six young dogs were infected with H. americanum using a described method. Each dog was fed 400 H. americanum oocysts on day 0 and observed daily thereafter. Bone scintigraphy has a high sensitivity for detecting early bone lesions. Baseline bone phase images were obtained 3 days before infection. Bone lesions were noted in three dogs before day 35/36, in one dog before day 46, one dog before day 53, and in one dog between days 46 and 67. Dogs became more severely ill than expected. This may have been due to the high number of  H. americanum oocysts fed to the dogs. The lesions were most commonly found proximal to the carpus/tarsus; this is in contrast to HO lesions, which are more common distal to the carpus. Dogs with HO also have lesions that start distally and progress proximally. These dogs had 8 to 12 bones (out of 20 possible sites) affected at the initial detection of disease. A similarity between HO and ACH is that both types of lesions are bilaterally symmetrical. Osteomyelitis or bone metastasis is not bilaterally symmetrical. Bone scintigraphy can be used to evaluate ACH lesions. The lesions primarily occur proximal to the carpus/tarsus and on the axial skeleton. Onset of lesions occurs within 67 days after infection.

COMMENTARY: That a hepatozoon in North America was a unique species, H. americanum, has only recently been recognized. The definitive host for H. americanum is believed to be the Lone Star tick, Amblyomma maculatum. In the United States, most cases are reported from the Gulf Coast region of Texas and Louisiana as well as Oklahoma, but recent reports extend the range to Georgia and Alabama. Infection occurs by ingestion of an infected tick. Puppies have been infected by vertical transmission. Infected dogs may be reluctant to move and often assume a sitting posture with a rigid trunk and neck. Temporal muscle atrophy may be present, and periosteal bone proliferation is pronounced. Neutrophilia, as high as 200,000 cells/ml, may be present. Radiography is helpful in diagnosis of this disease.

Determination of time of onset and location of early skeletal lesions in young dogs experimentally infected Hepatozoon americanum using bone scintigraphy. Drost WT, Cummings CA, Mathew JS, et al. VET RADIOL ULTRASOUND 44:86-91, 2003.