The Organisms

 Parasites of the genus Trypanosoma appear in the blood as trypomastigotes, with elongated bodies supporting a longitudinal lateral undulating membrane and a flagellum that borders the free edge of the membrane and emerges at the anterior end as a whip-like extension (Figure 1).

Fig1. Trypanosoma brucei gambiense (or Trypanosoma brucei rhodesiense, indistinguishable in practice) trypomastigotes (14–35 μm) in a blood smear (red blood cells = 8 μm). (Used with permission from Sullivan J: A Color Atlas of Parasitology, 8th ed. 2009.)

The kinetoplast (circular DNA inside the single mitochondrion) is a darkly staining body lying immediately adjacent to the basal body from which the flagellum arises. T. brucei rhodesiense, T. brucei gambiense, and Trypanosoma brucei brucei (which causes a sleeping sickness called nagana in livestock and game animals) are indistinguishable morphologically but differ biochemically, ecologically, and epidemiologically.

Pathology and Pathogenesis

Infective trypanosomes of T. brucei gambiense and T. brucei rhodesiense are introduced through the bite of the tsetse fly and multiply at the site of inoculation to cause variable induration and swelling (the primary lesion), which may progress to form a trypanosomal chancre. The African forms multiply extracellularly as trypomastigotes in the blood as well as in lymphoid tissues. They spread to lymph nodes, to the blood stream, and, in terminal stages, to the central nervous system (CNS), where they produce the typical sleeping sickness syndrome: lassitude, inability to eat, tissue wasting, unconsciousness, and death.

CNS involvement is most characteristic of African try panosomiasis. T. brucei rhodesiense appears in the cerebrospinal fluid in about 1 month and T. brucei gambiense in several months, but both are present in small numbers. T. brucei gambiense infection is chronic and leads to progressive diffuse meningoencephalitis, with death from the sleeping syndrome usually following in 1–2 years. The more rapidly fatal T. brucei rhodesiense produces somnolence and coma only during the final weeks of a terminal infection. The trypanosomes are transmissible through the placenta, and congenital infections occur in hyperendemic areas.

The African trypanosomes of the T. brucei complex are remarkable in that they undergo antigenic variation through a series of genetically controlled surface glycoproteins that coat the surface of the organism (variant surface glycoproteins, or VSGs). Successive waves of parasites in the host bloodstream are each covered with a distinct coat. This process is due to genetically induced changes of the surface glycoprotein. By producing different antigenic surface membranes, the parasite is able to evade the host’s antibody response. Each population is reduced but is promptly replaced with another antigenic type before the preceding one is eliminated. Each trypanosome is thought to possess about 1000 VSG genes, an example of mosaic gene expression.

Epidemiology

African trypanosomiasis is restricted to recognized tsetse fly belts. T. brucei gambiense, transmitted by the streamside tsetse Glossina palpalis and several other humid forest tsetse vectors, extends from West to Central Africa and produces a relatively chronic infection with progressive CNS involvement. T. brucei rhodesiense, transmitted by the woodland savanna Glossina morsitans, Glossina pallidipes, and Glossina fuscipes, occurs in the eastern and southeastern savannas of Africa, with foci west of Lake Victoria. It causes a smaller number of cases but is more virulent. Bushbuck and other antelopes may serve as reservoirs of T. brucei rhodesiense, whereas humans are the principal reservoir of T. brucei gambiense. Control depends on searching for and then isolating and treating patients with the disease; controlling movement of people in and out of fly belts; using insecticides in vehicles; and instituting fly control, principally with aerial insecticides and by altering habitats. Contact with reservoir animals is difficult to control, and insect repellent is of little value against tsetse bites.

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مواضيع ذات صلة

Leishmania Species (Blood Flagellates)

Trypanosoma cruzi (Blood Flagellate)

Cryptosporidium (Intestinal Sporozoa)

Entamoeba histolytica (Intestinal and Tissue Ameba)

Giardia lamblia (Intestinal Flagellate)

Classification of Parasites

Toxoplasmosis

The Lung Flukes

The Liver Flukes

Echinococcus granulosus

Taenia saginata

Taenia solium