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Trypanosoma evansi is monomorphic, 14–33 μm in length and 1.5–2.2 μm in width and morphologically indistinguishable from Trypanosoma equinum, Trypanosoma equiperdum and the slender forms of T. brucei. Molecular evidence suggests that there are two distinct type strains of T. evansi, type A and type B, and there are further strains within each type. Types A and B are distinguished by differences in the minicircles in the kinetoplast DNA. Type A is the most common and widespread form whilst type B occurs in camels in Kenya and Ethiopia. Molecular analyses suggest that both strains probably arose independently from West African T. brucei brucei (Cuypers et al. 2017). If true, this is unexpected because currently, Type B T. evansi only occurs in Eastern Africa.

Trypanosoma evansi has an extremely wide distribution and occurs in Africa, Asia, and Central and South America. It is particularly pathogenic in horses, but it also causes considerable morbidity and mortality in camels, cattle, pigs, dogs, and cats. It also parasitizes many wild animals such as deer, tapir, and capybara. It is usually mechanically transmitted by biting flies such as tabanids and stable flies. Trypanosoma evansi does not reproduce in its insect vectors and they act only as a ‘dirty syringe’. In South America, vampire bats can act as both hosts and mechanical vectors of T. evansi. Under experimental conditions, Raina et al. (1985) infected both dogs and mice by feeding them meat containing T. evansi. The extent to which oral infections occur naturally is, however, uncertain. Reproduction takes place asexually by longitudinal binary fission within the mammalian host.

The disease caused by T. evansi is commonly known as ‘surra’ – which is the Hindi word for ‘rotten’ or ‘emaciated’ although other terms are also used such as el debab in many Arabic‐speaking countries. It causes the death of many thousands of animals every year and a great deal of morbidity (Aregawi et al. 2019). Horses are particularly susceptible to infections, and there are claims that the inoculation of even a single parasite can prove fatal. The disease is often acute in horses and the animal dies within a few weeks to 2 months. Chronic infections lasting over a year may also occur but also often end with the death of the horse. Surra causes anaemia, emaciation, and oedema that may vary from urticarial plaques on the neck and flanks to widespread swelling of the legs and lower body. The plaques may subsequently become necrotic and bleed whilst encephalitis and demyelination can occur in the brain and spinal cord that results in staggering and paralysis. Affected animals may also express abnormal behaviour such as hyperexcitability, head tilting, and circling.

Surra is also an important cause of morbidity and mortality in camels in which it tends to be a chronic wasting disease. Characteristic features of infection include fever, anorexia, and the development of oedema. Cattle can be severely affected when exposed to T. evansi for the first time, but in endemic areas, the disease in cattle tends to be subclinical and reduce productivity rather than cause major morbidity and mortality. Dogs are susceptible to an acute and rapidly fatal form of the disease that causes nervous signs that are like those of rabies.