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

Abstract

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.

Loa loa vectors Chrysops spp.: perspectives on research, distribution, bionomics, and implications for elimination of lymphatic filariasis and onchocerciasis

“…For example, in Kumba, Crewe [57] found that C. silacea biting rates increased with rainfall but dropped with the onset of very heavy rain, suggesting that pupae could not survive excessive ground water or flooding. Another study on C. silacea in a different part of Cameroon [97], and in the Chaillu Mountains, Congo [91], also found significantly higher biting rates during the rainy season compared with the dry season. Similarly, in areas where C. dimidiata was the main vector such as the Cross River State, Nigeria, the highest biting rates were observed during the rainy season, but predominantly late in the season [102].…”

“…Both species frequently occur together; however, in some areas one species was found to dominate the other, and across different ecological settings with C. silacea more likely to adapt to human influenced environments. For example, C. silacea was reported to be more abundant in Kumba, Cameroon (rainforest), Sapele, Nigeria (rubber plantation) and Congo (rainforest) [91]; however, the latter author noted that C. dimidiata was more abundant in the palm groves within the forested study area. Chrysops dimidiata was reported to be more abundant in Benin, Nigeria (palm grove) [30]; Eseka in central Cameroon (rainforest) [61], Bioko island, Equatorial Guinea (rainforest) [100], and in Akamkpa Community, Cross Rivers State, Nigeria (rainforest); however, for the latter it was noted that C. silacea was more abundant in the adjacent mangrove forest [102].…”

“…Adult Chrysops abundance was based on biting rates measured as “boy-hours” in historical studies, and by the number of flies caught per man per hour (fly/man/hours) or tabanid per man per day (T/MD) in more recently published articles [91, 99]. Several factors were identified as influencing the biting cycles and infection rates, which were primarily related to spatial and temporal environmental and anthropogenic factors.…”

“…However, the rate of reduction varied between sites depending upon the amount and distance from forested vegetation, as well as by species with C. dimidiata noted to be more confined to forested areas, e.g. in Makokou, Gabon [101], and in the Chaillu Mountains, Congo [91]. Chrysops silacea was more dominant in villages whereas C. dimidiata was rarely found in open the environment, favouring primary and secondary forested areas.…”

Environmental factors associated with the distribution of Loa loa vectors Chrysops spp. in Central and West Africa: seeing the forest for the trees

“…Temperatures lower than 20 °C either in laboratory or very shaded field conditions have shown to delay larvae development in the fly. Noireau et al [33] reported monthly temperatures, and Crewe & O’Rourke [34] examined hourly temperature fluctuations and found the highest biting activity occurred when temperatures were around 24–27 °C. Interestingly, these finer scale spatial and temporal observations correlate with our large-scale environmental analysis and models, which identify temperature in the mid-20s °C as an important climatic measure.…”