Hepatozoonosis (Tick-Borne Disease) in Dogs

Hepatozoonosis is a disease caused by several species of tick born protozoa. Two specific strains have been noted in dogs: H. canis which causes Old World Hepatozoonosis and H. americanum which was identified in North America in the 1990’s and causes American Canine Hepatozoonosis (ACH). Both types of protozoa are passed through ingestion of an infected tick, either the common brown dog tick with H. canis or the Gulf Coast Tick with H. americanum. Sporozoites contained in the tick migrate into the host through the gastrointestinal tract. Dogs and other canids are believed to be the definitive host of H. canis, so Old World Hepatozoonosis can only be caught via tick ingestion. H. americanum is also carried in an intermediary or paratenic host which researchers have yet to identify. The parasite can remain dormant as a cystozoite (a cystic developing stage) in the muscles cells of this vertebrate host until it is ingested by a dog and causes ACH. As well as having two pathways to infect the dog, H. americanum is associated with more severe disease than H. canis. Old World Hepatozoonosis typically only causes clinical disease in dogs that are weak or have a suppressed immune system, while ACH is associated with severe illness even in healthy dogs. Without treatment, it will usually be fatal within two years. Symptoms of Hepatozoonosis include non-specific signs like fever, depression and lethargy as well as severe pain from inflammation of the muscles and bones. Dogs infected with either species of hepatozoon protozoa will carry the organism for life, but treatment can reduce the number of parasites and ameliorate the symptoms. New treatments methods have improved the prognosis for dogs with ACH.

Several tick-borne species of hepatozoon protozoa can cause disease in dogs. This is called Hepatozoonosis. American Hepatozoonosis, found most commonly in the southern United States, is the more serious form of the disease.

These are some of the symptoms that might indicate your dog has Hepatozoonosis.

• Fever
• Depression
• Weight loss
• Muscle atrophy
• Soreness
• Stiffness
• Eye discharge
• Bloody diarrhea
• Joint pain
• Generalized pain (hyperesthesia)

Types

There are two different types of Hepatozoonosis.

Old World Hepatozoonosis

• Found throughout Southern Europe, Africa, the Middle East, India, Southeast Asia and the Pacific Islands
• Recently cases have also been diagnosed in North America
• This disease is caused by the protozoa Hepatozoon canis and transmitted by the brown dog tick

American Canine Hepatozoonosis

• Found most commonly in the southern United States with isolated cases occurring in some northern states
• It is caused by the protozoa Hepatozoon americanum and transmitted by the Gulf Coast tick as well as a paratenic host

• Ingestion of an infected tick
• Ingestion of prey carrying an infected tick in the fur
• Ingestion of a paratenic host carrying dormant cysts (ACH)

The veterinarian will physically examine your dog for signs of pain and stiffness. Severe pain in the spinal region may be obvious upon physical examination. Blood tests will usually also indicate abnormalities including neutrophilia, non-regenerative anemia, and high platelet count. Occasionally parasite-containing leukocytes, called gamonts, can be identified on a stained blood smear, but this is rare with ACH. Clinical H. canis infection has a much higher number of parasites in the blood (parsitemia), so it is easier to identify with a blood test. Definitive diagnosis of ACH is usually made by identifying cysts known as “onion skins” on a muscle biopsy. This is an invasive procedure which will involve obtaining a sample of the muscle tissue through a hollow needle. X-rays may also show bone lesions with ACH.

There is no definitive test for identifying the species of protozoa causing a Hepatozoonosis infection. Veterinarians will usually identify the disease based on the symptoms and geographical areas. Asymptomatic cases of H. canis with low levels of parasitemia often go undiagnosed.

NSAID’s are prescribed as an initial treatment for pain and fever until the antiprotozoal therapy begins to take effect. Dogs with ACH are treated with a two week regimen of three different medications, called TCP. This is usually successful, but without further treatment dogs will suffer a relapse within a few months. Initially, TCP was the only treatment available for ACH and dogs frequently died even after the initial treatment achieved remission. Now dogs are also prescribed a daily dose of decoquinate for two years after the initial treatment to prevent remission.

Old World Hepatozoonosis is treated with Imizol (imidocarb dipropionate) several times a month for up to three months or until parasites can no longer be found in the blood. Dogs with clinical Old World Hepatozoonosis frequently have a concurrent illness which may need additional treatment.

The prognosis for ACH has improved with the addition of decoquinate as a treatment, however, this will require long term medication and you should discuss any potential side effects with your veterinarian. Survival with clinical H. canis is often dependent on treating the concurrent condition that is weakening your dog. Asymptomatic H. canis doesn’t usually cause a problem for dogs, although they will continue to carry the protozoa.

Breeding any dog with Hepatozoonosis is discouraged since the protozoa can be passed on to offspring. This has been documented with H. canis. Although there are no known cases of inherited H. americanum, it remains a possibility. Neither protozoa has been known to cause infection in humans. Hepatozoonosis is best managed by limiting exposure. Check your dog carefully for ticks since fur-borne parasites can easily be ingested during grooming. The disease is also often passed through the eating of wild prey also, so limiting hunting as much as possible can help protect your dog.