Chrysops silacea and C.dimidiata seasonality and loiasis prevalence in the Chaillu mountains, Congo.

Seasonal activity of the loiasis vectors Chrysops dimidiata Wulp and Chrysops silacea Austen (Diptera: Tabanidae) was studied during 1987-89 in villages and surrounding forest of the Chaillu Mountains, Congo. Chrysops were captured mainly in the hot rainy season (November-May) and densities of both species were higher in the forest than in villages. C.silacea predominated at all sites and C.dimidiata was rarely found in villages. In the rain forest, between 07.00 and 18.00 hours, 12 times more C.silacea and 3 times more C.dimidiata were collected with hand-nets (range 15-81 per man-day) than were caught in landing/biting collections on human bait. Chrysops man-biting rates were higher in villages closer to the forest, but variations in loiasis prevalence among villagers (microfilaraemia rates 16-37%) were not proportional to the intensity of contact between people and vectors in the villages, indicating that the majority of loiasis transmission probably occurs when people go into the forest.

Transmission indices of Loa loa in the Chaillu Mountains, Congo.

A longitudinal entomological survey of the vectors of loiasis was conducted in the Missama area (Lekoumou region) in the Congo from September 1987 to August 1989. The principal catching site was a palm grove surrounded by forest 3 km from the village. Landing/biting densities of Chrysops were measured by standardized fly catches lasting 11 hr carried out twice a month. Vector landing densities were also assessed in the Bantu and Pygmy villages and in the fields. Populations of Chrysops from the palm grove were examined 6 times a month for infection with the infective stage of Loa loa. Chrysops silacea was the predominate vector except at the beginning of the rainy season, when C. dimidiata was the prevailing species. Chrysops were caught throughout rainy season, from October to June. The host-seeking activity of C. silacea was greatest in the middle of this season (February), but occurred sooner (October) for C. dimidiata. The following variables associated with transmission were calculated from our observations in the palm grove (the first figure corresponds to the first year of the study and the figure in parentheses corresponds to the second year). It was calculated that 2.658 (2.185) C. silacea and 1.412 (1.182) C. dimidiata could bite a person in the palm grove per year, including an average of 14.4 (12.7) infective C. silacea and 9.8 (7.2) infective C. dimidiata. The percentage of all dissected flies with third stage larvae in the head and the mean number of larvae in the head/infective fly were 0.57% and 10.1 +/- 6.8 for C. silacea and 0.66% and 11.2 +/- 6.5 for C. dimidiata, respectively. The estimated annual transmission potentials were 171.1 (102.9) for C. silacea and 116.1 (73.8) for C. dimidiata. In the palm grove, transmission was ensured by 2 effective vectors during the rainy season (October to May). Although the annual biting rate for both species was twice as low in the village as in the forest, our data suggest that effective transmission occurs there also.

Community participation in the control of tsetse flies. Large scale trials using the pyramid trap in the Congo.

An experiment of Glossina palpalis control was carried out by rural communities in 55 villages of the Niari river sleeping sickness focus (Bouenza region, Republic of the Congo). It was based on the use of a new trap, not requiring insecticide impregnation, in which the captured Glossina are preserved. The results show that this simple, cheap trap is an effective method of control, resulting in a considerable decrease in the tsetse population and is easily operated by the villagers. Screening surveys, using the indirect immunofluorescent test, conducted regularly in this focus, showed a decrease in the prevalence rate after the elimination of the flies. Long-term community participation is limited and must be improved by a better understanding of socio-cultural aspects and in certain cases must be associated with the work of a specialized team.

Chrysops silacea and C. dimidiata: fly densities and infection rates with Loa loa in the Chaillu mountains, Congo Republic.

The densities, infection rates with Loa loa, and parous rates of Chrysops silacea and C. dimidiata were studied in various ecological zones throughout the Chaillu mountains in the People's Republic of the Congo. In the rainy season, C. dimidiata was the major vector in the forest, whereas C. silacea was predominant in the cleared forest zones. Fly densities were higher in the forest (natural forest or palmgrove) than in the villages. Parous and infection rates varied according to the ecological zone. The infection rate of parous females was related to the microfilarial rate in the human population, indicating that the Chrysops populations were extremely stable in the rainy season.

[Relation between tsetse fly density and bovine trypanosomiasis: the case of N'Dama cattle ranching (Louboulou, Congo)]. / Relation entre densité glossinienne et trypanosomose bovine: le cas d'un élevage en ranching de bétail N'Dama (Louboulou, Congo).

The authors report the results of an entomo-parasitological survey in the State Ranch of Louboulou, Bouenza region, Congo. Over a period of more than five months, the average tsetse density was 0.2 Glossina palpalis palpalis captured per day and trap. Although some specimens of Glossina fusca congolensis were also captured, the density of the latter species was quite insignificant. No trypanosomiasis was detected among the N'Dama cattle of the ranch, as a result of 114 blood samples collected for parasitological (fresh blood sample, Woo method, thick blood film) and serological examinations (Testryp CATT on whole blood and serum). The absence of detectable trypanosomiasis is discussed and interpreted as a result of a low tsetse threat below a critical threshold which remains to be determined.

[The use of sentinel animals for the evaluation of the control of vectors of sleeping sickness: preliminary reports at a Congolese site]. / Utilisation d'animaux sentinelles dans l'évaluation de la lutte centre les vecteurs de la maladie du sommeil. Premiers résultats dans un foyer congolais.

A large-scale control trial against Glossina palpalis palpalis was carried out in the Congo using a new trapping technique. In the same way a test related to the experimental utilization of sentinel animals was performed, concurrently to the classical epidemiological and entomological evaluation. 564 domestic animals (pigs, sheep, goats) were examined parasitologically (wet blood films, Woo/HCT) and serologically (Testryps CATT) in six villages. When a drastic reduction of tsetse populations was achieved (i.e. zero point of apparent density) a parasitological negativation was first noted after one year and a complete serological negativation was observed after two years. If only a relative decrease in the vector's apparent densities occurred, it was related with a lower of sero-parasitological prevalence rate. The use of a serological test which was able to detect Trypanosoma congolense antibodies as the Testryp CATT, is a particularly useful technique for estimating the animal transmission level. These first results give some arguments that a parasitological and serological continuous of sentinel animals as pigs and sheep is a useful mean for evaluating the efficiency of a control campaign against sleeping sickness vectors.

[Control of vectors by trapping and its impact on sleeping sickness in a zone of high density of Glossina fuscipes quanzensis (Lefini River, People's Republic of the Congo)]. / Lutte antivectorielle par piégeage et impact sur la transmission de la maladie du sommeil dans une zone à forte densité de Glossina fuscipes quanzensis (Rivière Léfini, République populaire du Congo).

Tsetse control by trapping and its impact on the transmission of sleeping sickness in the high density area of Glossina fuscipes quanzenis (Léfini river, People's republic of Congo). The focus of human sleeping sickness of the Léfini river is of strategic importance for the People's Republic of Congo as it is located at an important junction which allows the trypanosomiasis to spread over the whole country. A campaign of tsetse control by trapping was undertaken in this focus, where an adjacent game reserve provides a continuous source of flies. The only practicable strategy was the use of a new model of trap with a permanent "killing cage" and which does not need to be impregnated with an insecticide. Transmission of sleeping sickness has been reduced by 1) decreasing the density of flies which are in close contact with man, 2) constantly eliminating flies likely to become infected and transmit the disease.