Hepatozoonosis in Dogs

By Catherine Barnette, DVM

What is hepatozoonosis?

Hepatozoonosis is a disease caused by a protozoan (a small, microscopic organism) known as Hepatozoon.

There are two different species of protozoa that can cause hepatozoonosis: Hepatozoon canis and Hepatozoon americanum. H. americanum is most common throughout the Southern and Southeastern United States, though it has also been observed in other areas. H. canis has been noted in a number of countries, including the United States; it is most common in tropical regions.female_brown_dog_tick

Although both species cause hepatozoonosis, they differ in the expected course of disease and recommended treatment.


How is a dog infected with Hepatozoon?

H. americanum is transmitted by the Gulf Coast tick. Dogs acquire the infection by ingesting an adult Gulf Coast tick, or by ingesting a bird or rodent that contains larval stages of the tick. Ticks become infected by feeding on an infected dog; they will then carry the disease themselves for the remainder of their lifetime. Infected ticks can also transmit the disease to their offspring, allowing infection to spread and become endemic or widespread within a local tick population.

After a dog ingests a Gulf Coast tick infected with H. americanum, the organism is released from within the tick and penetrates the wall of the dog’s intestine. From there, it is carried by blood or lymphatic vessels to the dog’s spleen, liver, lymph nodes, bone marrow, lungs, pancreas, and skeletal muscles. As it matures and begins to reproduce, the organism creates large cysts that can lead to significant inflammation at the site of infection. Dramatic muscle cysts, resulting in significant inflammation, are a common finding with this organism.

H. canis is also spread by the ingestion of a tick, though it is associated with a different tick species than H. americanum. Dogs become infected with H. canis by ingesting an infected adult brown dog tick. Like in the case of H. americanum, ticks acquire the protozoa by feeding on an infected dog. The organism is released from the tick after the tick is ingested, penetrates the wall of the dog’s intestine, and then travels to the spleen, liver, lymph nodes, bone marrow, lungs, and pancreas. Unlike H. americanum, H. canis does not typically create muscle cysts or cause muscle inflammation.


What are the signs of hepatozoonosis?

The signs of H. americanum include fever, lethargy, decreased appetite, weight loss, muscle pain/weakness, reluctance to move, and discharge from the eyes and nose. On physical examination, your veterinarian may also note enlarged lymph nodes. Signs of H. americanum often wax and wane; dogs may go through periods of illness, followed by apparent resolution, with signs recurring at a later date. Without treatment, H. americanum can cause significant damage to the blood vessels and kidneys. The condition is often fatal within one year of infection.

Dogs with H. canis, in contrast, usually do not show any signs of illness or disease. In some dogs with high parasite numbers, however, the disease can become more severe and may be potentially life-threatening. If signs of disease are observed, the most common signs include fever, weight loss, and lethargy. An examination by your veterinarian may also reveal anemia (low red blood cell count) and an enlarged spleen.


How is hepatozoonosis diagnosed?

Hepatozoonosis may cause several changes in baseline laboratory screening tests. A complete blood cell count may show anemia (low red blood cell count) and leukocytosis (high white blood cell count). Your dog may also have elevations in liver enzyme values, decreased blood protein levels, and increases in enzymes associated with muscle inflammation. H. americanum may also cause bony changes, which can be seen on X-rays.

"Examination of your dog’s blood under a microscope often reveals the organism within white blood cells."hepatozoonosis_dog_2018

Examination of your dog’s blood under a microscope often reveals the organism within white blood cells. H. americanum may also be discovered upon muscle biopsy. A test known as PCR testing can also be used to identify infection, and to distinguish between H. americanum and H. canis.

How is hepatozoonosis treated?

The treatment of hepatozoonosis depends upon the species that is causing the infection.

H. americanum is managed, not completely cured. However, many dogs are able to achieve long-term remission with appropriate treatment. Dogs are typically treated with three different medications until their clinical signs have resolved: trimethoprim-sulfa, clindamycin, and pyrimethamine. Once clinical signs resolve, dogs receive long-term treatment with a drug called decoquinate. This medication is continued for at least two years, though longer treatment may be required depending on clinical signs and laboratory test results. Signs may recur after stopping the medication, in which case treatment is repeated.

"Hepatozoon americanum is managed, not completely cured."

H. canis, in contrast, is relatively easy to treat. It can typically be cured with one or two injections of a drug called imidocarb. Oral doxycycline (an antibiotic) may be combined with the imidocarb injections.

"Hepatozoon canis, in contrast, is relatively easy to treat."

Regardless of the species causing hepatozoonosis, dogs may also require supportive care, including anti-inflammatory medications (to control pain and fever) and intravenous fluids (if not eating or drinking well). Medications may also be required for the treatment of other tick-borne infections.

Dogs with hepatozoonosis must be started on regular tick prevention to prevent reinfection.


Can anything be done to prevent hepatozoonosis?

The most effective method of preventing hepatozoonosis is to prevent tick exposure. Ensure that your dog is protected with an effective, veterinarian-recommended tick preventive.

Since hepatozoonosis can also be acquired through the ingestion of prey species, limiting your dog’s hunting will further decrease the likelihood of infection.


Can I get hepatozoonosis from my dog?

Hepatozoon does not infect humans.

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