Synonyms: T. soudanense, T. elephantis, T. annamense, T. cameli, T. marocanum, T. ninae kohl-yakimov, T. aegyptum, T. hippicum, T. venezuelense.
Disease: Trypanosomosis due to T. evansi has been given different names in different localities. The most widely used name, surra, is applied to the disease in all hosts. The disease in camels is called el debab in Algeria and mbori in Sudan. That in horses is called murrina in Panama and derrengadera in Venezuela.
Hosts: Camels, horse, donkey, ox, zebu, goat, pig, dog, water buffalo, elephants, capybara, tapir, and (in Mauritius) deer.
Location: Blood, lymph.
Geographic Distribution: Northern Africa (north of 15 N latitude in the west and central part of the continent, but extending almost to the equator in the east), Asia Minor, U.S.S.R. from the Volga River east into Middle Asia, India, Burma, Malaya, Indochina, parts of southern China, Indonesia, Philippines, Central America, South America. Hoare (1956) has shown how the original distribution of T. evansi coincided with that of the camel. In Africa, its southern boundary coincides roughly with the northern boundary of tsetse fly distribution. It now extends far to the east of the camel’s range in the Old World. It is often associated with arid deserts and semi-arid steppes, but may occur in other types of climate as well. In India, it is most common in the Punjab, which is mostly in the northwestern dry region (Basu, 1945; Basu, Menon and Sen Gupta, 1952). It was probably introduced into the New World in infected horses by the Spanish conquerors during the 16th century.
Prevalence: T. evansi is an important cause of disease over a large part of its range.
Morphology: The morphology of the Old World strains of T. evansi has been studied intensively by Hoare (1956). The mean length of different host and geographic strains varies considerably. However, the typical forms are 15 to 34 u long, with a mean of 24 u. Most are slender or intermediate in shape, but stumpy forms occur sporadically. All forms are morphologically indistinguishable from the corresponding ones of T. brucei. Strains which lack a kinetoplast have occasionally arisen spontaneously or can be produced by treatment with certain dyes (Hoare, 1954).
Life Cycle: T. evansi is transmitted mechanically by biting flies. No cyclic development takes place in the vectors, the trypanosomes remaining in the proboscis. The usual vectors are horseflies of the genus Tabanus, but Stomoxys, Haematopota and Lyperosia can also transmit it. In Central and South America, the vampire bat is a vector, the disease in this case being known as murrina.
Pathogenesis: Surra is nearly always fatal in horses in the absence of treatment; death occurs in a week to six months. The disease is also severe in dogs and elephants. It is less severe in cattle and water buffalo. Cattle may carry the parasites without showing signs of disease for months. However, occasional outbreaks of acute disease occur in cattle and water buffalo. Surra in camels is similar to the disease in horses but more chronic. In dogs, T. evansi causes a chronic disease with a high mortality rate; untreated dogs usually die in 1 to 2 months (Gomez Rodriguez, 1956).
The signs of surra include intermittent fever, urticaria, anemia, edema of the legs and lower parts of the body, loss of hair, progressive weakness, loss of condition and inappetence. Conjunctivitis may occur, and abortion is common in camels.
The lesions include splenomegaly, enlargement of the lymph glands and kidneys, leucocytic infiltration of the liver parenchyma, and petechial hemorrhages and parenchymatous inflammation of the kidneys.
Diagnosis: Same as for T. brucei.
Cultivation: Same as for T. brucei.
Treatment: Treatment of T. evansi is similar to that of T. brucei. Antrycide methyl sulfate is less toxic than suramin for horses; a single subcutaneous dose of 5 mg/kg or even less is effective. A dose of 3 mg/kg has given good results in cattle. A single injection of 2 g is effective in camels.
The dose of suramin for horses is 4 g per 1000 lb body weight intravenously. Camels tolerate suramin well, and a single intravenous injection of 4 to 5 g is effective against surra in these animals. Tartar emetic, which has been largely superseded in other animals, is still used in treating surra in the camel; a single intravenous injection of 200 ml of a 1% solution is given. This drug is also widely used in cattle in India because of its cheapness.
Control: Essentially the same measures used in the control of T. brucei, except of course those directed against the tsetse fly, can be used in the control of T. evansi infections. Control of horseflies and other biting flies is important.
Remarks: Hoare (1956, 1957) has discussed the phylogeny of T. evansi. This species undoubtedly arose from T. brucei, being introduced into camels when they entered the tsetse fly belt and then becoming adapted to mechanical transmission by tabanids.