Genus Trypanosoma Gruby

Members of this genus occur in all classes of vertebrates. They are parasites of the circulatory system and tissue fluids, but some, such as T. cruzi, may actually invade cells. Almost all are transmitted by blood-sucking invertebrates. Most species are probably non-pathogenic, but the remainder more than make up for their fellows.

Trypanosomosis is one of the world's most important diseases of livestock and man. Trypanosomes cause African sleeping sickness and Chagas' disease in man and a whole series of similar diseases in domestic animals. They are relatively unimportant in North America, but they make it practically impossible to raise livestock in many parts of the tropics which would otherwise be ideal. According to Hornby (1949), "Trypanosomiasis is unique among diseases in that it is the only one which by itself has denied vast areas of land to all domestic animals other than poultry. The areas of complete denial are all in Africa and add up to perhaps one quarter of the total land surface of this continent". Some, but not all, of the African species are transmitted by tsetse flies. These flies occupy almost 4 million square miles, an area larger than the United States, and this whole region is under the threat of trypanosomosis.

In a recent Hollywood epic on South African history, there is a scene in which a line of Boer covered wagons on the Great Trek to the north is attacked by Zulus. The warriors pour over the hills, the wagons form a circle with the women and children in the center, and the Boer men prepare to fight. It is just like a scene from the American Wild West, with the Indians attacking a wagon train of pioneers. But there is one difference - the Zulus had no horses, and made their attack on foot. The reason? Trypanosomosis.

A large number of species of Trypanosoma has been named. At one time it was customary, and still is to some extent, to give different names to trypanosomes from different hosts. Many of these names are still valid, but as we learn more and more about the host-parasite relations and epidemiology of the trypanosomes, many other names have fallen into synonymy. No attempt will be made here to list all the synonyms of each species, but the more important ones will be mentioned.

Trypanosomes are classified in groups on the basis of their morphology, life cycles and other biological characteristics. The validity of this grouping is shown by the fact that their metabolic characteristics, which vary widely, fall into the same groups.

The following outline classification of trypanosomes of veterinary and medical importance is based on Hoare (1957, 1959). Sections on metabolism based on von Brand (1956), Ryley (1956) and von Brand and Tobie (1959), and a section on avian trypanosomes have been added.

I. Parasites of Mammals

A. Morphology in Mammal

Kinetoplast not terminal, large. Free flagellum always present. Posterior end of body pointed. Division in crithidial, leishmanial or trypanosome stages.

Biology

Multiplication in mammal typically discontinuous. Development of metacyclic trypanosomes in hind gut (posterior station) of vector (in T. rangeli also in salivary glands: anterior station). Transmission contaminative thru feces (in T. rangeli also inoculative). Except for T. cruzi, trypanosomes slightly or not pathogenic.

Metabolism

Blood forms have high respiratory quotient and low sugar consumption, producing acetic, succinic and lactic acids aerobically and succinic, lactic, acetic and pyruvic acids anaerobically. Cyanide markedly inhibits oxygen consumption. Sulfhydryl antagonists moderately inhibit oxygen consumption. Culture forms have high respiratory quotient and moderately high sugar consumption, producing acetic and succinic acids aerobically and succinic and acetic acids anaerobically; cyanide markedly inhibits oxygen consumption; sulfhydryl antagonists moderately to markedly inhibit oxygen consumption. Cytochrome pigments, cytochrome and succinic oxidase activity are present in T. cruzi and T. lewisi.

1. Lewisi Group

a. Mode of reproduction unknown. T. melophamum of sheep.
b. Reproduction by binary fission in crithidial stage. T. theileri of cattle.
c. Reproduction by multiple fission in crithidial stage. T. lewisi of rats. T. duttoni of mice.
d. Reproduction by binary fission in leishmanial stage. T. cruzi of man, dog, opossum, monkeys.
e. Reproduction by multiple fission in leishmanial stage. T. nabiasi of rabbits.
f. Reproduction by binary fission in trypanosome stage. T. rangeli of man, dog, opossum, monkeys.

B. Morphology in Mammal

Kinetoplast terminal or subterminal. Posterior end of body blunt. Division in trypanosome stage.

Biology

Multiplication in mammal continuous. Development of metacyclic trypanosomes in proboscis or salivary glands (anterior station) of vector (except evansi subgroup). Transmission inoculative, thru bite (except T. equiperdum). Trypanosomes pathogenic.

1. Vivax Group

Morphology in Mammal

Monomorphic forms. Posterior end of body typically rounded. Free flagellum always present. Kinetoplast large, terminal. Undulating membrane inconspicuous.

Biology

Development in Glossina, in proboscis only. T. vivax is also transmitted mechanically by tabanids.

Metabolism

Blood forms have high respiratory quotient and high sugar consumption, producing pyruvic, acetic, lactic acids and glycerol aerobically and glycerol, pyruvic, lactic, and acetic acids anaerobically. Cyanide does not inhibit oxygen consumption. Sulfhydryl antagonists markedly inhibit oxygen consumption. Cytochrome and succinic oxidase activity are present.

a. Long forms. T. vivax of cattle, sheep, goats, antelope.
b. Short forms. T. uniforme of cattle, sheep, goats, antelope.

2. Congolense Group

Morphology in Mammal

Monomorphic or polymorphic forms. Free flagellum absent or present. Kinetoplast medium, typically marginal.

Biology

Development in Glossina, in midgut and proboscis.

Metabolism

Blood forms have high respiratory quotient and high sugar consumption, producing acetic acid, succinic acid, glycerol and pyruvic acid aerobically and succinic acid, glycerol, acetic acid and pyruvic acid anaerobically. Cyanide and sulfhydryl antagonists moderately inhibit oxygen consumption. Cytochrome pigments are absent but cytochrome and succinic oxidase activity are present. Culture forms of T. congolense have R. Q. of 0.9, produce pyruvate, acetate and smaller amounts of lactate, succinate and glycerol aerobically, and pyruvate, acetate and succinate with small amounts of glycerol and no carbon dioxide anaerobically. Cyanide and sulfhydryl antagonists (iodoacetate and sodium arsenite) inhibit oxygen consumption, but Krebs cycle inhibitors (fluoroacetate and malonate) do so only slightly.

a. Monomorphic (free flagellum absent or short, undulating membrane inconspicuous).
(1) Short forms (means 12.2-14.4 u). T. congolense of cattle, equids, swine, sheep, goat, dog.
(2) Long forms (means 15.3-17.6 u). T. dimorphon of horse, cattle, sheep, goat, pig, dog.

b. Polymorphic (short forms without free flagellum, and long forms, of which some are stout, with conspicuous undulating membrane and some are slender with inconspicuous undulating membrane; free flagellum absent or present).
T. simiae of swine, camels, cattle, horse, warthog.

3. Brucei Group

Morphology in Mammal

Monomorphic or polymorphic forms. Free flagellum present or absent. Kinetoplast small, subterminal (absent in T. equinum). Undulating membrane conspicuous.

Biology

Development in Glossina, in midgut and salivary glands (except evansi subgroup).

Metabolism

Blood forms have very low respiratory quotient and very high sugar consumption, producing pyruvic acid and sometimes glycerol aerobically and pyruvic acid and glycerol anaerobically. Cyanide does not inhibit oxygen consumption. Sulfhydryl antagonists markedly inhibit oxygen consumption. Culture forms have high respiratory quotient and moderately high sugar consumption, producing acetic, succinic, pyruvic and lactic acids aerobically. Cyanide moderately inhibits oxygen consumption. Sulfhydryl antagonists markedly inhibit oxygen consumption. Cytochrome pigments have not been found in T. rhodesiense or T. equiperdum, but cytochrome and succinic oxidase activity are present in T. rhodesiense.

a. Monomorphic (stout forms with short free flagellum).
(1) Suis subgroup T. suis of swine.

b. Polymorphic (slender, intermediate and stumpy forms).
(1) Brucei subgroup
Polymorphism constant (stumpy forms always present).
T. brucei of all domestic animals, antelope. T. rhodesiense of man, bushbuck and probably antelope. T. gambiense of man.
(2) Evansi subgroup
Polymorphism inconstant (stumpy forms rare or sporadic); no cyclic development in vector host.
(a) Transmission mechanical by insects. T. evansi of cattle, camels, equids, dogs, etc. T. equinum of equids.
(b) Transmission by contact (coitus) from mammal to mammal. T. equiperdum of equids.

II. Parasites of Birds

Very polymorphic, sometimes attaining great size. Relatively easy to cultivate. Cyclic development probably in biting arthropods.

Avium Group

T. avium of various birds.
T. calmettei of chickens.
T. gallinarum of chickens.
T. hannai of pigeons.
T. numidae of guinea fowl.

Species of Trypanosoma

In the discussion which follows, each trypanosome species is taken up separately, but special attention is paid to T. brucei and T. cruzi as representatives of different types.