Tropical Medicine: A Clinical Text, 8th Edition, Revised and Expanded

The Trypanosomatidae

The Trypanosomatidae Tens of thousands of arable African acres lie fallow today because of trypanosomiasis. In tropical areas where food shortage and malnutrition causes so much infant mortality and adult morbidity, such waste of land is disastrous. For the visitor from a temperate climate, the persistence of sleeping-sickness zones may curtail travel plans and even pose a lethal threat for those on safari. In South America and Central America, trypanosomal infections kill young and old alike. As immigration from the poor rural areas of Latin America expands, the threat of transmission through blood transfusion in the cities increases. In temperate climates, the chronic manifestations of Chagas disease are presenting diagnostic challenges with increasing frequency. Throughout wide areas of Latin America, Africa, and the Near and the Far East, the dermatological lesions caused by Leishmania, a closely related genus of parasites, are deforming thousands each year, and visceral infection with this organism still claims its annual toll. Trypanosomal and leishmanial diseases of man differ widely in their geographic distribution, vectors, pathology, clinical manifestations, prognosis, and response to treatment. Nonetheless, all these diseases have certain features in common because the etiological parasites are all members of the family Trypanosomatidae. They are all protozoa, and each of them may assume several or all four developmental forms: The amastigote (leishmanial form) is an oval organism 2 to 5 microns in diameter. It is found in the tissues of patients with all types of leishmaniasis and the American form of trypanosomiasis. The organism has neither an external flagellum nor an undulating membrane. PAGE 26 ................. 18086$ $CH3 07-15-11 15:39:36 PS the trypanosomatidae 27 The promastigote (leptomonad form) is the infective form found in insect vectors, cultures of leishmania, and as a transitional stage in human beings with American trypanosomiasis. This form is characterized by an elongated shape 10 to 15 micrometers in length, a free flagellum, no undulating membrane, and a kinetoplast far anterior to the nucleus. The epimastigote (crithidial form) has a kinetoplast just anterior to the nucleus and also a free flagellum, but has no undulating membrane . It is found on culture and in the insect vectors of all human trypanosomal infections. The trypanomastigote (trypanosome form) is a motile flagellate form that can be found in the blood, lymphatic tissue, and cerebrospinal fluid of infected animal hosts, including man. In blood films of patients with American trypanosomiasis, the monomorphic parasite assumes a ‘‘C’’ or a ‘‘U’’ shape and has a prominent posterior kinetoplast. In African infections, the parasite is polymorphic and may be short and stumpy, or long and thin. FIGURE 3: Developmental forms of the family Trypanosomatidae. PAGE 27 ................. 18086$ $CH3 07-15-11 15:39:37 PS 28 the trypanosomatidae Trypanosomiases African Trypanosomiasis In Africa, two trypanosomes cause human disease. West African sleeping sickness is caused by Trypanosoma brucei gambiense, and East African sleeping sickness is caused by by Trypanosoma brucei rhodesiense. The insect vectors of both diseases are tsetse flies; the West African type is transmitted by riverine flies of the Glossina palpalis group, while the East African variety is spread by the Glossina morsitans group, which inhabits woodland, bush, and thicket. Man is probably the main reservoir of T. gambiense, whereas wild game are the predominant hosts of T. rhodesiense. Therefore, the habits of the parasites and their vectors determine the epidemiology of human disease. West African trypanosomiasis poses a severe threat to large numbers of people who use rivers for washing, drinking, fishing, or transportation. The East African disease primarily affects those who live in or enter the game-inhabited woodland or bush. The presence of an infection can rapidly depopulate a river valley, and the risk of disease prevents vast fertile areas from being cultivated. Sleeping sickness once decimated Africa; in the early years of the twentieth century, more than one million deaths occurred in the Congo alone, and in one outbreak around Lake Victoria, 200,000 of a total population of 300,000 died within 8 years. Unfortunately, many of the regional sleeping-sickness control measures have become the victims of other national priorities; military expenditures, corruption, and complacency have eroded the public health foundations in many new African nations, and trypanosomiasis has now resurfaced as a major threat. Recent outbreaks have occurred in Central and East African countries. Pathology The usual mode of infection is through the injection of metacyclic infective trypanosomes beneath the dermis by the Glossina fly...