Seasonal Variation in Biting Rates of Simulium damnosum sensu lato, Vector of Onchocerca volvulus, in Two Sudanese Foci

Abstract

BackgroundThe abundance of onchocerciasis vectors affects the epidemiology of disease in Sudan, therefore, studies of vector dynamics are crucial for onchocerciasis control/elimination programs. This study aims to compare the relative abundance, monthly biting-rates (MBR) and hourly-based distribution of onchocerciasis vectors in Abu-Hamed and Galabat foci. These seasonally-based factors can be used to structure vector control efforts to reduce fly-biting rates as a component of onchocerciasis elimination programs.MethodsA cross-sectional study was conducted in four endemic villages in Abu-Hamed and Galabat foci during two non-consecutive years (2007–2008 and 2009–2010). Both adults and aquatic stages of the potential onchocerciasis vector Simulium damnosum sensu lato were collected following standard procedures during wet and dry seasons. Adult flies were collected using human landing capture for 5 days/month. The data was recorded on handheld data collection sheets to calculate the relative abundance, MBR, and hourly-based distribution associated with climatic factors. The data analysis was carried out using ANOVA and Spearman rank correlation tests.ResultsData on vector surveillance revealed higher relative abundance of S. damnosum s.l. in Abu- Hamed (39,934 flies) than Galabat (8,202 flies). In Abu-Hamed, vector populations increased in January-April then declined in June-July until they disappeared in August-October. Highest black fly density and MBR were found in March 2007 (N = 9,444, MBR = 58,552.8 bites/person/month), and March 2010 (N = 2,603, MBR = 16,138.6 bites/person/month) while none of flies were collected in August-October (MBR = 0 bites/person/month). In Galabat, vectors increased in September-December, then decreased in February-June. The highest vector density and MBR were recorded in September 2007 (N = 1,138, MBR = 6,828 bites/person/month) and September 2010 (N = 1,163, MBR = 6,978 bites/person/month), whereas, none appeared in collection from April to June. There was a significant difference in mean monthly density of S. damnosum s.l. across the two foci in 2007–2008 (df = 3, F = 3.91, P = 0.011). Minimum temperature showed significant correlation with adult flies counts in four areas sampled; the adult counts were increased in Nady village (rs = 0.799) and were decreased in Kalasecal (rs = - 0.676), Gumaiza (rs = - 0.585), and Hilat Khateir (rs = - 0.496). Maximum temperature showed positive correlation with black fly counts only in Galabat focus. Precipitation was significantly correlated with adult flies counts in Nady village, Abu-Hamed, but no significance was found in the rest of the sampled villages in both foci. Hourly-based distribution of black flies showed a unimodal pattern in Abu-Hamed with one peak (10:00–18:00), while a bimodal pattern with two peaks (07:00–10:00) and (14:00–18:00) was exhibited in Galabat.ConclusionTransmission of onchocerciasis in both foci showed marked differences in seasonality, which may be attributed to ecology, microclimate and proximity of breeding ...

Impact of climate change on dermatological conditions related to flooding: update from the International Society of Dermatology Climate Change Committee

“…In Sudan, high precipitation and flooding swept away the breeding place and diminished the population of black flies, Simulium damnosum senso lato . However, they may add oxygen content and nutrition to the river resulting in an increased population of larvae, maturation, density, and biting activity of adult vectors . Similar effects are also seen on mosquitoes where standing water as a result of river overflow provides new breeding places, allowing an increased population of mosquitoes .…”

“…However, they may add oxygen content and nutrition to the river resulting in an increased population of larvae, maturation, density, and biting activity of adult vectors. 48 Similar effects are also seen on mosquitoes where standing water as a result of river overflow provides new breeding places, allowing an increased population of mosquitoes. 42,43 Bites from mosquitoes and flies carry consequences of insect-borne illnesses such as malaria, dengue, and leishmaniasis.…”

The First Confirmed Elimination of an Onchocerciasis Focus in Africa: Abu Hamed, Sudan

“…vector 5,6 in this focus and the characteristics of black fly breeding sites and biting in Abu Hamed have been described. 7,8 The Abu Hamed focus was the first in the country to undergo rapid epidemiological mapping of onchocerciasis and to begin a control program under auspices of APOC using a mass drug administration (MDA) strategy employing annual community-directed treatment with ivermectin (CDTI) in 1998 ( Figure 1B). The isolated nature and the mesoendemic disease status of the Abu Hamed focus led the government of Sudan to declare an official elimination strategy for the first time in Africa in December 2006 ( Figure 1B).…”

Socioeconomic impacts of elimination of Onchocerciasis in Abu-Hamed focus, northern Sudan: lessons after elimination

“…The Abu-Hamed focus is centered around the River Nile in the middle of the Nubian Desert (N 19° 32.49’− 18° 17.29’, E 32° 13.86’ − 33° 55.00’) at an altitude range of 260-920 km. The breeding of black fly’s aquatic stages lasts from October to May when the level of the Nile is relatively stable (27).…”