Trypanosoma evansi is the etiological agent of the disease known as Mal das Caderas (Latin America) or surra (Asia, Africa, and Europe) in horses. This parasite, which has been reported in domestic and wild mammals, can cause considerable economic losses. The trypanosomes reproduce in the blood of the vertebrate host, and the trypomastigote forms are transmitted mechanically by bloodsucking insects from infected to uninfected animals. Hot and humid climatic conditions may contribute to outbreaks of trypanosomiasis, due to higher proliferation of insects, the main vectors of T. evansi.

Surra is the most commonly reported disease in some continents due to the favorable environment for insects. In recent years, several outbreaks or isolated cases have been reported in certain European countries, an atypical region for the disease. In Brazil, the disease was restricted to the Midwest (region of the Pantanal of Mato Grosso) for many years. However, in the last 10 years it has spread to all regions of the country, with isolated cases and outbreaks with high associated mortality. Wild animals such as capybaras (a large rodent) and coatis (member of the raccoon family) may act as reservoirs of T. evansi. Populations of these species, which are present in the same areas as these outbreaks of surra, have increased considerably in recent years.

Trypanosomiasis in horses is characterized by anemia (low red blood cell count), edema (fluid swelling) of the limbs and dependent regions, anorexia, dehydration, lethargy, fever, loss of appetite, weight loss, abortion, and incoordination, followed by paralysis of the hind limbs. Researchers divide these clinical signs in two or three stages of the disease: subacute, acute, and chronic. In the chronic stage, horses usually exhibit cachexia (chronic wasting) associated with neurologic signs and limb paralysis. Neurologic signs are the result of the parasite travelling to the brain, where it causes an inflammatory response leading to encephalitis and cellular necrosis.

Trypanosomiasis caused by T. evansi can be clinically confused with other diseases, including equine protozoal myeloencephalitis in the chronic stages. Where surra is suspected, it is important to rule out other causes of equine neurologic disease. Most available diagnostic methods include the parasitological, serological, and polymerase chain reaction (PCR). The parasitological test is fast, but it lacks specificity and sensitivity due to low numbers of the parasite in the bloodstream. Serology, employing a card agglutination test kit known as CATT/T. evansi, has been a method used for disease surveillance in several countries. PCR testing on blood samples is a specific and sensitive method; however, it is not 100 percent reliable since T. evansi may only be present in the tissues