Surra in Horses: Trypanosoma evansi Infection and Travel Risk

Surra is a serious parasitic disease caused by Trypanosoma evansi, a blood and tissue parasite that affects many animal species, including horses. In horses, it can cause acute illness with fever and weakness, or a more drawn-out illness with anemia, weight loss, swelling, and declining performance. Horses are considered one of the more severely affected species, and untreated infection can become life-threatening.

Unlike some other trypanosome diseases, surra is usually spread mechanically by biting flies rather than by tsetse flies alone. That matters for travel risk, because the disease occurs across parts of North Africa, the Middle East, Asia, the Far East, and Central and South America. A horse that has recently traveled, been imported, or had contact with animals from affected areas deserves prompt veterinary attention if compatible signs appear.

Surra is also important from a movement and biosecurity standpoint. The World Organisation for Animal Health recognizes a 90-day incubation period for T. evansi infection for trade purposes, and USDA import rules for horses returning to the United States depend on travel route, country of stay, and quarantine requirements. If your horse is traveling internationally, your vet and the relevant animal health authorities should be involved early in planning.

Surra is caused by infection with Trypanosoma evansi, a protozoal parasite that lives in the blood and tissues. Horses usually become infected through the bite of blood-feeding flies that mechanically transfer infected blood from one animal to another during interrupted feeding. Other domestic animals can carry the parasite and act as reservoirs, which is one reason outbreaks can be hard to control once the organism is present in an area.

In horses, travel is a major practical risk factor. A horse imported from, competing in, or returning from a region where surra occurs may be exposed directly or indirectly. The disease is recognized across parts of North Africa, the Middle East, Asia, the Far East, and Central and South America. Because signs can be acute, chronic, or even inapparent for a time, a horse may not look obviously ill right away.

There is no vaccine currently available for surra. Prevention depends on movement controls, insect exposure reduction, careful review of travel history, and testing when indicated. If your horse is traveling internationally, your vet can help you review destination-specific health requirements and plan for pre-travel and post-travel monitoring.

Surra cannot be confirmed by symptoms alone. Your vet will usually start with a travel and exposure history, physical exam, and basic lab work such as a complete blood count and chemistry panel to look for anemia, inflammation, dehydration, or organ effects. Because parasite numbers can rise and fall, a single negative test does not always rule infection out.

Laboratory confirmation may include direct blood smear evaluation, concentration methods such as hematocrit or buffy coat techniques, PCR, and serology. Microscopy can identify trypanosomes when parasite levels are high, but it is not very sensitive on its own. WOAH notes that fewer than half of infected animals may be identified by blood examination alone, so combining tests improves accuracy.

PCR is often the most sensitive method for detecting active infection, especially in highly susceptible species such as horses, though false negatives can still happen during low-parasite periods. Serologic tests such as CATT or ELISA may help screen exposed horses, movement cases, or quarantine cases, but cross-reactions can occur with related trypanosomes. In real-world cases, your vet may repeat sampling or use more than one test type before making decisions about treatment, isolation, or reporting.

Prevention starts with travel planning. If your horse is being imported, exported, or returning to the United States after temporary travel abroad, work with your vet well before the trip. USDA requires import permits for most equine imports and quarantine for live equines entering the United States, with the quarantine period depending on the country of residence, transit route, and purpose of entry. For horses that have traveled internationally, exact requirements can change, so current official guidance matters.

At the barn level, reduce exposure to biting flies as much as possible. Your vet may recommend a layered plan that includes fly sheets, repellents labeled for equine use, manure management, drainage of wet areas, stall fans where appropriate, and timing turnout to reduce heavy fly activity. If a horse is sick after travel, separating that horse and tightening insect control can help reduce risk while testing is underway.

There is no vaccine for surra, so prevention also depends on screening and awareness. Keep detailed records of travel dates, countries visited, and any illness during or after the trip. If your horse develops fever, anemia, swelling, or unexplained decline within weeks to months after travel, tell your vet exactly where and when the horse traveled. That timeline can make a major difference in how quickly the right tests are chosen.