Circulating gametocytes are rarely seen as <0.1% of leukocytes are infected. Leukocytosis (>20,000/µl) and mature neutrophilia are typically found. Other changes may include: increased alkaline phosphatase concentration, and decreased glucose, BUN, and albumin concentrations. Globulin and creatine kinase concentrations are inconsistently elevated. Affected patients can develop membranoproliferative glomerulonephritis or amyloidosis with marked proteinuria. These cases have a poor prognosis.

Radiographs often show a smooth or reactive periosteal reaction which may be subtle.

The clinical signs are often highly suggestive of hepatozoonosis. Buffy coat preparations may reveal gametocytes, which occupy about 1/3 of the white blood cell and are large, oblong cytoplasmic inclusions that tend to displace the nucleus to one side of the cell. Wright or Wright-Giemsa stains are best as they stain the nucleus of the parasite. Diff-Quik stain is not as useful here.

Muscle biopsies usually confirm the diagnosis. The semimembranosus, semitendinosus or biceps femoris muscles are most often biopsied, although other muscles can be selected. Obtain 3 small pieces of muscle and fix in formalin. Affected dogs usually show numerous hepatozoon cysts in the muscle. Pyogranulomatous myositis may also be seen, and sporozoites may be visible within the leukocytes. If the patient dies, vasculitis and thrombosis are often found on necropsy.

Treatment with a combined triple antibiotic regimen is used for 14 days to kill the rapidly replicating organisms. The combination of choice is trimethoprim-sulfadiazine at 15 mg/kg q 12h, clindamycin at 10 mg/kg q 8h, and pyrimethamine at 0.25 mg/kg q 24 hr. Marked improvement is usually seen, but signs tend to recur with shorter and shorter periods of remission following each treatment course.

Decoquinate (Deccox) is a coccidiostat at low doses and blocks the repeated cycles of asexual reproduction. Therapy may not eliminate the tissue stages of H. americanum, but longterm administration of decoquinate can result in extended survival times and excellent quality of life. The longterm dose recommended is 1 teaspoon per 20 lbs. mixed with food every 12 hours (i.e. 10-20 mg/kg q 12). Treatment may be required for life.

Imidocarb, typically used twice at 5-6 mg/kg IM or SQ q 14 d for treatment of H. canis infection, is not effective against H. americanum.

Tick control is an important preventive measure.