Merck Manual

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Pet Owner Version

Blood Parasites of Horses


Phillip D. Carter

, BVSc, MVS, Local Land Services, New South Wales, Australia;

Peter Rolls

, BVSc, MVS, Tick Fever Centre, Department of Agriculture and Fisheries, Queensland, Australia;

Christine Petersen

, MD, DVM, PhD, FASTMH, Center for Emerging Infectious Diseases, University of Iowa;

Tara L. Grinnage-Pulley

, DVM, PhD, Department of Epidemiology, College of Public Health, University of Iowa

Reviewed/Revised Mar 2019 | Modified Oct 2022

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 is a disease transmitted by ticks. It affects a wide range of domestic and wild animals and occasionally humans. Equine babesiosis occurs in Africa, Europe, Asia, South and Central America, and the southern US. Infection of unborn foals, particularly with Theileria protozoa, is relatively common.

The incubation period is approximately 8 to 10 days. Signs of infection in adult horses may include reluctance to move, lack of appetite, and fever. Swelling of the fetlocks may occur, and episodes of colic are common. In young horses, signs may be more severe and can include jaundice, weakness, and pale mucous membranes. The disease lasts for about 10 days, but death may occur in the first 24 to 48 hours. Some horses develop chronic, or longterm, infections and can become carriers for several years after the initial infection.

Babesiosis can be confused with other conditions that cause fever, anemia, destruction of red blood cells, or jaundice. Therefore, laboratory tests should be performed to confirm the diagnosis. Treatment generally involves using one or a combination of effective antiprotozoal drugs and supportive care (such as intravenous fluids, anti-inflammatory medications, and blood transfusions). There is no vaccine for use in horses. Controlling ticks by periodic spraying and inspection of the horse can help reduce the chance of transmission.

African Trypanosomiasis (Tsetse Fly Disease)

Tsetse flies 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 Central and South America. In horses, Trypanosoma congolense, T vivax, T simiae, and T brucei brucei are the trypanosomes most likely to cause disease. Domestic animals may be a source of human infections, but the species that cause infections in horses do not infect people.

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. The primary signs are fever that may come and go, anemia, and weight loss. Internally, the lymph nodes and spleen are usually swollen. The diagnosis is confirmed by finding trypanosomes in laboratory tests.

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 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 approved for use on horses, 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 with a high population of trypanosome-infected tsetse flies (portions of Africa and Central and South America). 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 can be a deadly disease in horses and camels. Trypanosoma evansi in other animals appears not to cause disease, and these animals serve as a source of the infection.

The development and effects of the disease, signs, physical changes, diagnosis, and treatment are similar to those of the tsetse-transmitted trypanosomes (see above).


Dourine is a venereal disease of horses that is transmitted during sexual intercourse. It is caused by a species of trypanosome named Trypanosoma equiperdum. The disease occurs on the Mediterranean coast of Africa and in the Middle East, southern Africa, and South America; however, the distribution is probably wider.

Signs may develop over weeks or months, and infection may become chronic. Early signs include mucous and pus-filled discharge from the urethra in stallions and from the vagina in mares, followed by swelling of the genitals. Later, characteristic scaly patches ¾ to 4 inches (2 to 10 centimeters) in diameter appear on the skin, and the horse becomes dangerously thin. If untreated, 50 to 70% of infected horses will die.

Getting definitive proof of this disease can be difficult. However, your veterinarian can perform certain tests to find and identify the trypanosomes in the discharges, the skin patches, or the blood.

In areas prone to this disease, horses may be treated with several different medications. When completely eliminating the disease from a stable or group of horses is desired, strict control of breeding, euthanasia of positively identified infected horses, and elimination of stray horses has been successful.

For More Information

Also see professional content regarding blood parasites Blood Parasites .

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