Hepatozoonosis is a systemic tick-borne disease in dogs caused by either the protozoa Hepatozoon canis or Hepatozoon americanum. While both of these organisms cause similar disease, cases caused by H. americanum are typically more severe. These cases are most commonly found in the southern and southeastern United States, where dogs are exposed by the Gulf Coast tick. Occasional cases have shown up elsewhere, though, including Washington, California, Nebraska, Vermont and Virginia. In contrast, cases caused by H. canis are transmitted by the brown dog tick.
Dogs are exposed to the Hepatozoon organism when they ingest an infected brown dog tick while grooming or when they ingest prey (such as birds or rodents) that contain the larval or nymphal stages of infected ticks.
Once the organism is consumed, it penetrates the intestinal walls and is carried via the blood stream or lymph to various organs in the body, including the spleen, liver, lungs, lymph nodes and bone marrow. From there, other life stages of the organism can circulate in white blood cells (infecting other feeding ticks) or form cysts in the muscles and other body tissues.
Dogs with circulating H. canis organisms often appear completely healthy, but severe cases may cause fever, muscle wasting, an enlarged spleen and lethargy. Dogs infected by H. americanum are ill. Their symptoms, which may wax and wane, include fever, lethargy, weight loss, muscle pain and stiffness, and a reluctance to move (due to cysts in the muscle tissues). Chronic untreated cases can progress to kidney disease and death within a year of diagnosis.
Diagnosis is not always easy, but the organisms may be found microscopically in white blood cells or on tissue biopsies. The use of PCR can confirm cases and distinguish between H. americanum and H. canis.
Treatment of hepatozoonosis depends on the causative agent. While H. canis can typically be cleared with one or two injections of an anti-protozoal medication, H. americium requires the combination of a short course of several antibiotics followed by the long term administration of a quinolone medication in the food twice a day for up to two years.
While hepatozoonsis does occur in cats, it is extremely rare, and therefore is not well understood. We do know that it typically causes muscle infection, but the prognosis for recovery is not well defined, as the cases are just too rare. A lucky break for our feline friends!
It is important to protect pets from ticks all twelve months of the year. Keep up with monthly doses of flea and tick preventives to keep pets happy, healthy and pest-free for life.