Many infectious agents, especially those that cause emerging diseases, infect more than one host species. Managing reservoirs of multihost pathogens often plays a crucial role in effective disease control. However, reservoirs remain variously and loosely defined. We propose that reservoirs can only be understood with reference to defined target populations. Therefore, we define a reservoir as one or more epidemiologically connected populations or environments in which the pathogen can be permanently maintained and from which infection is transmitted to the defined target population. Existence of a reservoir is confirmed when infection within the target population cannot be sustained after all transmission between target and nontarget populations has been eliminated. When disease can be controlled solely by interventions within target populations, little knowledge of potentially complex reservoir infection dynamics is necessary for effective control. We discuss the practical value of different approaches that may be used to identify reservoirs in the field.

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Identifying reservoirs of infection: a conceptual and practical challenge.

Publisher Summary The exploitation of genetic resistance to disease is an important consideration in livestock development programs, where conventional disease control measures are not effective or are too costly. Such an approach may be directly applicable to African animal trypanosomiasis. In African trypanosomiasis, the control measures currently in use include diagnosis and treatment, chemoprophylaxis, and control or eradication of tsetse with insecticides. At present, there is no effective field vaccine available against African trypanosomiasis. The major constraints to the development of a vaccine include the existence of the different species of trypanosomes and of different serodemes within the same species, all with the capability of producing different repertoires of variable antigen types (VATS). This chapter discusses the available information on the aspects of trypanotolerance, with regard to cattle, sheep, goats, wildlife, and man. It describes experimental results derived from mouse models and the relevance of these models to trypanosomiasis of livestock. The genetic resistance to trypanosomiasis is not necessarily associated with low productivity. Trypanotolerance is not a stable characteristic, and evidence suggests that it can be supplemented or reduced by a number of factors affecting the host and its environment, which include age and sex; maternally-derived immunity, intensity of challenge, virulence, previous exposure, stress, and susceptibility to other diseases. Investigations of trypanotolerance also represent an exciting approach to the important interactions between host and parasite. Trypanotolerance exists as an innate characteristic and that it is probably inherited as a dominant trait. While the level of trypanotolerance can be reduced under certain circumstances, it can also be enhanced, for example, by previous exposure. Therefore, there is hope that it may be possible in the future to supplement the level of trypanotolerance both by genetic selection and by immunological or therapeutic procedures.

Host Susceptibility to African Trypanosomiasis: Trypanotolerance

Publisher Summary The subgenus Trypanozoon contains several kinds of trypanosome, which are morphologically indistinguishable but differ in their behavior. At various times specific or subspecific status has been accorded to each kind, but, in recent years, those cyclically transmitted by tsetse in Africa have been separated into subspecies of Trypanosoma brucei , while the others, transmitted non-cyclically outside the African tsetse belts, have retained separate species status. However, although such new terminology may be more correct taxonomically, the practical problems of distinguishing the trypanosomes remain. Enzyme electrophoresis is being used on an increasingly wide scale for the intrinsic characterization. However, as the number of enzyme systems being used for identification grows, comparison of stocks becomes increasingly difficult and intricate, particularly when an enzyme occurs in a multibanded form after electrophoresis. After using 12 enzyme systems to characterize 160 Trypanozoon stocks, the results have been analyzed to establish the degree of relatedness of each of the 59 zymodemest encountered. The detailed analysis is presented in this chapter.

Numerical analysis of enzyme polymorphism: a new approach to the epidemiology and taxonomy of trypanosomes of the subgenus Trypanozoon.

Human African trypanosomiasis (HAT, or sleeping sickness) is a protozoan parasitic infection caused by Trypanosoma brucei rhodesiense or Trypanosoma brucei gambiense. These are neglected tropical diseases, and T.b. rhodesiense HAT is a zoonosis. We review current knowledge on the burden of HAT in sub-Saharan Africa, with an emphasis on the disability-adjusted life year (DALY), data sources, and methodological issues relating to the use of this metric for assessing the burden of this disease. We highlight areas where data are lacking to properly quantify the impact of these diseases, mainly relating to quantifying under-reporting and disability associated with infection, and challenge the HAT research community to tackle the neglect in data gathering to enable better evidence-based assessments of burden using DALYs or other appropriate measures.

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The Burden of Human African Trypanosomiasis

The origins of a new Trypanosoma brucei rhodesiense sleeping sickness outbreak in eastern Uganda.

Two pints (1 litre) of dextran 6% in saline was given when matched blood and plasma were temporarily unavailable. In retrospect it is clear that this was inadvisable, as a number of reports have incriminated dextran as a cause of a haemorrhagic tendency (Carbone et al., 1954; Scott, 1955; McKenzie and Langlands, 1956). The mechanism of the process is not clear, and suggestions made include interaction of dextran with fibrinogen, haemodilution, production of thrombocytopenia, and prolongation of the bleedingtime. The patient, of course, had developed abnormal bleeding before the dextran was given, but it is possible that the dextran aggravated the haemorrhagic tendency. Friesen et al. (1952) described similar cases in which postoperative bleeding occurred as a result of a haemolytic transfusion reaction, when over 250 ml. of incompatible blood had been given. They felt that fibrinolysis was an important factor, but considered that a \" heparinoid disturbance\" was present. Toluidine-blue dye, given intravenously to two patients in a dosage of 15 mg. per kg. of body weight, apparently corrected the bleeding tendency. Such treatment might be worthy of a trial in any refractory case. Treatment with corticotrophin (200 units intramuscularly) or cortisone (400 mg. intravenously) has been recommended, in addition to fibrinogen, by Stefanini and Dameshek (1955). If the possibility of a bleeding state after incompatible transfusion had been known, the second laparotomy would probably not have been carried out, and appropriate treatment might have been started earlier.

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J. P. McMahon

Papers

The isolation of Trypanosoma rhodesiense from a bushbuck.