Trypanosoma evansi Infection in Horses

The following information was first published in the University of Kentucky College of Agriculture’s Equine Disease Quarterly. The article was authored by Aleksandro Schafer da Silva, of the School Department of Animal Science at the Universidade do Estado de Santa Catarina, Chapecó, Santa Catarina, Brazil.

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, edema 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 neurological signs and limb paralysis. Neurological 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.

Like any infectious agent, T. evansi stimulates an immune response in the infected animal. While this response is unable to clear the infection from the host, it controls and maintains the parasitemia at low levels, resulting in the disease becoming chronic. In many countries, the disease is treated with diminazene aceturate. However this drug, even when used at the recommended dose, has no significant curative rates in the vast majority of infected animals. Quinapyramine, suramin, and melarsomine dihydrochloride are additional drugs used to treat T. evansi infections. Quinapyramine has been used prophylactically in endemic regions.

Surra is considered by some as an emerging disease. Even in endemic areas/regions, efforts should be made to reduce the incidence of the disease through adoption of appropriate sanitary measures.

Aleksandro Schafer da Silva,
School Department of Animal Science
Universidade do Estado de Santa Catarina
Chapecó, Santa Catarina

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