The significance of human serum sensitivity in the context of T.B. rhodesiense sleeping sickness epidemiology and control.

Earlier this century the postulate that Trypanosoma brucei brucei and T.b. rhodesiense had a common identity and that human infectability was linked with resistance to normal human serum (NHS) in vitro, were both finally refuted in the classical Tinde experiment. Interest in serum sensitivity was reawakened with the advent of the BIIT in 1970 and the studies that followed demonstrated the presence of both human-serum-resistant (HSR) and sensitive (HSS) variant antigen types, within the surface antigen repertoire of a single T.b. rhodesiense organism. This confirmed the bimodal human-infectivity potential of some, if not of all, 'brucei' trypanosomes. Changes from sensitive ('S') to resistant ('R') forms in a T.b. brucei clone have been shown to occur in chickens and have also been reported in a 'clean' bushbuck infected with a T.b. rhodesiense clone. The subsequent expression of 'S' forms by T.b. rhodesiense, when isolated from man into clean rats, has also been demonstrated. Sera from some game animals in Zambia have been shown to be highly trypanolytic. Trypanozoon organisms are almost constantly in contact with mammalian blood elements, in the vertebrate and invertebrate hosts, and more recent studies have demonstrated changes in the serum sensitivity/resistance of Trypanozoon, during metacyclic development in Glossina. In view of this, it is felt that the effects of physiological host factors, on these parasites, may well prove to be a scientifically lucrative field for further research. The bimodal potentiality for human infectivity is clearly a character of fundamental epidemiological and epizootiological importance in the transmission dynamics of this parasite complex.

Human serum sensitivities of Trypanozoon isolates from naturally infected hosts in the Luangwa Valley, Zambia.

Of 235 Trypanozoon stocks isolated from naturally infected hosts in northeastern Zambia and tested by the Blood Incubation Infectivity Test (BIIT), 176 came from man, 37 from wild-caught tsetse, 11 from wild animals and 11 from domestic livestock. Of those from man, 2 gave unexpected, human-serum-sensitive (HSS) reactions on first testing; all 15 stocks from tsetse in the northern area (Kampumbu) were strongly serum-resistant (HSR) while 22 other infections, from tsetse in the southern area (Kakumbi), gave 1 equivocal, 11 positive and 10 negative test responses. HSR Trypanozoon infections were found in a bushbuck, a warthog, in a giraffe (for the first time) and in a "sentinel" goat, used to monitor SS transmission in a small SS endemic village.

Sleeping sickness and tsetse awareness: a sociological study among the Tambo and Lambya of the northern Luangwa Valley, Zambia.

Data on awareness of tsetse flies and knowledge of trypanosomiasis were collected in May, 1988, in the Luangwa Valley of Isoka District in the Northern Province of Zambia. One thousand and nine hundred adult males and females were interviewed. There was a high level of fly awareness among all the respondents, regardless of duration of residence and age groups. Malaria was considered as the most serious illness in the community, and hence overshadowed the impact of trypanosomiasis in the community. Disease awareness gravity cannot be attributed to individual's socio-economic levels.

PIP: In May 1988, tropical disease researchers interviewed 1940 Tambo and Lambya men and women living in 2 chiefs' areas (Katyetye and Mweniwisi) of the Luangwa Valley of Isoka District in the Northern Province of Zambia to determine their knowledge of trypanosomiasis (sleeping sickness) and tsetse flies. 98.4% of the respondents who had lived in the area for at least one month knew that tsetse fly bites transmitted trypanosomiasis. 94.2% of those who lived in the area for 1 month also knew this. Only people who lived in the area for 1 month believed that witchcraft and bad water transmitted sleeping sickness (0.9% and 0.1% respectively). Approximately 73% reported tsetse flies to be uncommon in their villages. Just 2% believed trypanosomiasis to be a leading disease because its treatment often requires a long hospitalization. Most people (74%) thought malaria to be the most serious disease, because it occurs frequently and kills many people. Tsetse flies become ubiquitous during the rainy season. Further the Tropical Disease Research Centre in Ndola, Zambia has found extremely high tsetse densities along access roads and paths connecting villages. The researchers planned to use these results in developing a parasite control project in the area. Community health workers trained under the Primary Health Care project could conduct health education sessions about sleeping sickness in the area to increase awareness of the seriousness of the disease.

How do African game animals control trypanosome infections?

African trypanosomiasis is endemic over much of sub-saharan Africa. But whereas domestic animals - especially cattle - often succumb to the infection, wild mammals generally show a high degree o f resistance. Many species o f wildlife living in tsetse-infested areas carry trypanosome infections, and so act as important reservoir hosts, but generally show no obvious ill-effects. How they survive the infection is an important question in understanding mechanisms o f trypanotolerance, yet relevant data are sparse and scattered. Here, Ayub Mulla and Roy Rickman review some of the previous studies to illustrate how complex the question is.

A field comparison of seven diagnostic techniques for human trypanosomiasis in the Luangwa Valley, Zambia.

One serological and six parasitological techniques for diagnosing human trypanosomiasis were compared with regard to sensitivity, specificity, positive and negative predictive values, and practicality in field application in the Luangwa Valley of Zambia. Seven (0.64%) trypanosomiasis cases were diagnosed parasitologically in a survey of 1093 people from 19 villages. The indirect fluorescent antibody test (IFAT) was more sensitive but less specific than the parasitological techniques, detecting 71% of the confirmed cases in the first round of testing. Rat inoculation, the Giemsa stained thick film and miniature anion-exchange/centrifugation (mAEC) were all more sensitive than wet blood film examination, microhaematocrit centrifugation and wet film examination of the buffy coat after microhaematocrit centrifugation. The comparison indicated that the most effective, practical combination of techniques for survey in the Luangwa Valley was IFAT screening followed by examination of seropositive patients by rat inoculation and the mAEC (or stained thick film) in parallel format. Calculation of positive and negative predictive values showed that trypanosomiasis point prevalence measured in this way would still be underestimated by approximately 60%, indicating the need to improve IFAT specificity and parasitological sensitivity. Although only one of the seven patients diagnosed in the survey presented with signs and symptoms indicating possible trypanosomiasis, no evidence of a population of "healthy carriers" was found.