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Find information on animal health topics, written for the veterinary professional.

Blood Parasites of Dogs

By Peter H. Holmes, BVMS, PhD, Dr HC, FRCVS, FRSE, OBE, Emeritus Professor and Former Vice-Principal, Faculty of Veterinary Medicine, University of Glasgow ; Nemi C. Jain, MVSc, PhD, Professor Emeritus of Clinical Pathology, Department of Veterinary Pathology, Microbiology, and Immunology, School of Veterinary Medicine. University of California ; David J. Waltisbuhl, BASc, MSc, Senior Scientist DPI&F Actest, Yeerongpilly Veterinary Laboratory ; Michael Bernstein, DVM, DACVIM, Director, Medical Services, Angell Animal Medical Center ; Karen L. Campbell, MS, DVM, DACVIM, DACVD, Professor and Section Head, Specialty Medicine, Department of Veterinary Clinical Medicine, College of Veterinary Medicine, University of Illinois ; Timothy M. Fan, DVM, PhD, DACVIM, Associate Professor, Department of Veterinary Clinical Medicine, University of Illinois ; Wayne K. Jorgensen, BSc, PhD, Science Leader Applied Biotechnology Livestock, Agri-Science Queensland ; Susan L. Payne, PhD, Associate Professor, Department of Veterinary Pathobiology, Texas A&M University

Also see professional content regarding blood parasites.

Blood parasites are organisms that live in the blood of their animal hosts. These parasites can range from single-celled protozoa to more complex bacteria and rickettsiae. The method of transmission varies, depending on the parasite, but often they are transmitted through the bites of ticks or flies.

Babesiosis

Babesiosis is a disease transmitted by ticks. It is caused by protozoan parasites of the genus Babesia, which infect the red blood cells. Babesiosis affects a wide range of domestic and wild animals and, occasionally, humans. While the major economic impact of babesiosis is on the cattle industry, infections in dogs occur at various rates throughout the world.

Signs of infection vary from a mild illness that passes quickly to a severe disease that rapidly results in death. In some cases, the parasite causes a longterm disease with severe and progressive anemia as the main symptom. Babesiosis can be confused with other conditions that cause fever, anemia, destruction of red blood cells, jaundice, or red urine. Therefore, laboratory tests should be performed to confirm the diagnosis.

Your veterinarian will be able to prescribe the appropriate medication. Supportive treatment is helpful and may include the use of anti-inflammatory drugs, antioxidants, and corticosteroids. Blood transfusions may be life saving in very anemic animals.

A vaccine based on the variety of Babesia found in Europe is available, but does not protect against other types. Preventing exposure to ticks by using appropriate tick control products and removing any ticks promptly will help keep your dog from being exposed to this parasite.

A small number of cases of human babesiosis have been reported, but it is unclear whether the species of Babesia that infect dogs are the same as those that cause infection in people. Fatal cases have been reported in people whose spleen had been removed or who had a weakened immune system. Human Babesia infections are acquired by way of bites from infected ticks or through contaminated blood transfusions.

Hepatozoonosis

Hepatozoonosis is a disease of wild and domestic carnivores (meat-eating animals) caused by a protozoan called Hepatozoon canis. This organism is transmitted by the brown dog tick, but its mode of transmission is unusual. The tick picks up the organism from an infected host while biting the animal. An uninfected dog gets the disease by eating the tick, not from being bitten by the tick. In North America, the signs in infected dogs are different and more severe than in other parts of the world and the disease is caused by a different Hepatozoon species, which is now called Hepatozoon americanum. This species is transmitted by the Gulf Coast tick instead of by the brown dog tick. Because of these differences, the disease in North America is referred to as American canine hepatozoonosis (see below).

In much of the world (India, Africa, southeast Asia, the Middle East, southern Europe, and islands in the Pacific and Indian Oceans), infected dogs have no signs of infection or only mild signs. Having a suppressed immune system due to another disease appears to play an important role in the development of significant signs. In the United States, more severe signs may occur. Most cases in the US have been diagnosed in Texas (primarily along the Gulf Coast), Oklahoma, and Louisiana, but cases have also been reported as far east as Tennessee, Alabama, Georgia, and Florida. This is a recent disease that has primarily spread north and east from the Gulf Coast of Texas, where it was originally detected in 1978.

Dogs older than 4 to 6 months old are usually resistant to infection with H. canis. However, H. americanum causes severe signs even in adult dogs.

American Canine Hepatozoonosis

Infection by H. americanum creates areas of inflammation within body tissues, which can cause signs of disease such as fever, depression, weight loss, poor body condition, muscle loss and weakness, discharge from the eyes, and bloody diarrhea. These signs may come and go. Fluctuating fevers of from 102.7 to 106°F (39.3 to 41.0°C) may also be seen. Surprisingly, many dogs have a normal appetite. Severe sensitivity or pain near the spine is also common, as are stiffness and a general reluctance to move. Eventually, hepatozoonosis may lead to inflammation of the kidneys or amyloidosis of the kidneys (a condition in which there is a buildup of an abnormal protein called amyloid in kidney tissues).

There are specific laboratory tests a veterinarian can perform to confirm the presence of this disease. Muscle biopsies may also help with diagnosis. Other laboratory findings may include a significant increase in the white blood cell count and a mild to moderate anemia.

Hepatozoonosis is a life-long infection in dogs. No known treatment completely clears the body of the organism. In the past, most dogs showed only temporary improvement, with frequent relapses within 3 to 6 months and death within 2 years of diagnosis. However, remission can now usually be achieved by using new drug combinations. These new therapies have resulted in a marked improvement in the outlook for dogs with hepatozoonosis.

Prevention of access to ticks is the most effective form of control for this disease. Be sure to provide your pet with good tick control, especially if you live in an area where this disease has been reported. There is no known risk of transferring this disease to humans.

African Tsetse-transmitted Trypanosomiasis

Tsetse are small, winged biting flies that feed on the blood of humans and other animals. They only occur in sub-Saharan Africa, where they are responsible for transmitting a group of diseases caused by protozoa of the genus Trypanosoma, which affect all domestic animals. Tsetse flies are restricted to Africa; however, horseflies and other biting flies can transmit the disease in other locations (such as in Central and South America). In dogs, Trypanosoma brucei is most likely to cause disease. Domestic animals may be a source of human infections.

Infected tsetse flies inject the protozoa into the skin of animals, where they grow for a few days and cause localized swellings called chancres. They enter the lymph nodes, then the bloodstream, where they multiply quickly. The immune response is very strong; however, not all trypanosomes are vulnerable to the immune response, which results in longterm infection.

The severity of disease varies with the species and age of the animal infected and the species of trypanosome involved. The incubation period is usually 1 to 4 weeks. In dogs, the onset of disease may be rapid. The primary signs are fever, anemia, and weight loss. The eyes are often affected. Internally, the lymph nodes and spleen are usually swollen. The diagnosis is confirmed by laboratory testing to identify trypanosomes in the blood of an infected dog.

Several drugs can be used for treatment; however, most drugs only work if the correct dose is given. It is very important to follow the prescribed dosage exactly. Some trypanosomes have become resistant to certain drugs, which may be the cause in cases that do not respond to medical treatment.

The risk of infection can be reduced in areas where the disease is common by keeping dogs indoors or by getting rid of tsetse flies and using preventive drugs, which are given to stop an infection from getting started. Flies can be partially controlled by using sprays or dips on the pets to be protected, spraying insecticides on fly-breeding areas, using screens coated with insecticide, and clearing brush to reduce the habitat for the flies. Animals can be given preventive drugs in areas of high risk for infections, but this is rarely undertaken in domestic dogs. There is no vaccine.

Surra ( Trypanosoma evansi Infection)

Surra is separated from the tsetse-transmitted diseases because it is usually transmitted by other biting flies that are found within and outside tsetse fly areas. It occurs in North Africa, the Middle East, Asia, the Far East, and Central and South America. It is mostly a disease of horses, but all domestic animals are susceptible. The disease can be deadly, particularly in horses and dogs. The development and effects of the disease, signs, physical changes, diagnosis, and treatment are similar to those of the tsetse-transmitted trypanosomes (see above).

Chagas’ Disease ( Trypanosoma cruzi Infection)

Chagas’ disease is caused by infection with another trypanosome. Insects transmit the disease between susceptible species of animals, including opossums, armadillos, rodents, and wild carnivores. The trypanosome also causes disease in humans and occasionally in young dogs and cats. The disease occurs in Central and South America and localized areas of the southern US. Domestic animals may become infected and introduce the trypanosome into houses where the bugs are present. People then become infected by contamination of wounds or by eating food contaminated with insect droppings that contain trypanosomes. Other domestic animals act as carriers of the disease without showing signs of illness. Affected dogs may die suddenly or have short- or long-term inflammation of the heart muscle.

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Also see professional content regarding blood parasites.