Simulium Damnosum Complex Research Articles

Ongoing transmission of onchocerciasis in the Pru District of Ghana after two decades of mass drug administration with ivermectin and comparative identification of members of the Simulium damnosum complex using cytological and morphological techniques

Background Human onchocerciasis remains a public health problem in Ghana. Mass drug administration (MDA) with ivermectin (IVM) has reduced disease morbidity and prevalence, but the transmission of onchocerciasis remains ongoing in several endemic foci. We investigated parasite transmission in some endemic communities in Ghana that had received > 18 rounds of annual MDA with IVM and determined the species composition of black fly (Simulium damnosum) vectors in these areas.MethodsAdult female black flies were collected using human landing catches and identified as either forest or savanna species using morpho-taxonomic keys. The adult flies underwent dissection to determine their parity and detect any O. volvulus larvae, followed by the calculation of entomological indices. Simulium damnosum s.l. larvae were collected and preserved in freshly prepared Carnoy’s fixative and were later used for cytotaxonomic studies.ResultsA total of 9,983 adult flies were caught: 6,569 and 3,414 in the rainy and dry seasons respectively. Black fly biting activities over the study period showed bimodal or trimodal patterns. The highest monthly biting rate (MBR) of 10,578.75 bites/person/month was recorded in July in Beposo, while the highest monthly transmission potential of 100.69 infective bites/person/month was recorded in Asubende in August. Morphological analysis of 2,032 flies showed that 99.8% (2,028) of the flies were savanna species, with only 4 (0.2%) adult flies being of the forest species. Cytogenetic studies on 114 black fly larvae revealed three cytospecies (Simulium damnosum s.s., S. sirbanum and S. sanctipauli) in the study area.ConclusionsThe present studies confirmed an ongoing transmission of onchocerciasis in the study communities except Abua-1. It also provides further information on biting behaviors and onchocerciasis transmission indices in the study communities. Further, our data confirmed the savanna species (S. damnosum s.s. and S. sirbanum) of the S.damnosum s.l. to be the major vectors of onchocerciasis in the study areas, with only an occasional influx of forest cytotypes.Graphical

Short- and Long-Range Dispersal by Members of the Simulium damnosum Complex (Diptera: Simuliidae), Vectors of Onchocerciasis: A Review.

Blackfly members of the Simulium damnosum complex are major vectors of the parasite that causes onchocerciasis in Africa and Yemen, with other vector species involved in a few localized areas of Africa and in the Neotropics. Although the life cycle of these blackflies is linked to fast-flowing rivers, they can travel long distances (up to at least 500 km), calling into question how transmission zones are defined. Knowledge of the short- and long-range dispersal of these vectors could inform where control interventions and monitoring are necessary if targets for onchocerciasis elimination are to be met. Yet, research on blackfly dispersal has been limited and fragmented over the last 70 years. Here, we review the literature on the dispersal of onchocerciasis vectors, and we show the need for further research to establish how far larvae can travel downstream; the extent to which adults invade transmission zones; and whether adults migrate in a series of successive short movements or in single long-distance shifts, or use both methods.

Seasonal variations and other changes in the geographical distributions of different cytospecies of the Simulium damnosum complex (Diptera: Simuliidae) in Togo and Benin

Simulium damnosum s.l., the most important vector of onchocerciasis in Africa, is a complex of sibling species described on the basis of differences in their larval polytene chromosomes. These (cyto) species differ in their geographical distributions, ecologies and epidemiological roles. In Togo and Benin, distributional changes have been recorded as a consequence of vector control and environmental changes (e.g. creation of dams, deforestation), with potential epidemiological consequences. We review the distribution of cytospecies in Togo and Benin and report changes observed from 1975 to 2018. The elimination of the Djodji form of S. sanctipauli in south-western Togo in 1988 seems to have had no long-term effects on the distribution of the other cytospecies, despite an initial surge by S. yahense. Although we report a general tendency for long-term stability in most cytospecies’ distributions, we also assess how the cytospecies’ geographical distributions have fluctuated and how they vary with the seasons. In addition to seasonal expansions of geographical ranges by all species except S. yahense, there are seasonal variations in the relative abundances of cytospecies within a year. In the lower Mono river, the Beffa form of S. soubrense predominates in the dry season but is replaced as the dominant taxon in the rainy season by S. damnosum s.str. Deforestation was previously implicated in an increase of savanna cytospecies in southern Togo (1975–1997), but our data had little power to support (or refute) suggestions of a continuing increase, partly because of a lack of recent sampling. In contrast, the construction of dams and other environmental changes including climate change seem to be leading to decreases in the populations of S. damnosum s.l. in Togo and Benin. If so, combined with the disappearance of the Djodji form of S. sanctipauli, a potent vector, plus historic vector control actions and community directed treatments with ivermectin, onchocerciasis transmission in Togo and Benin is much reduced compared with the situation in 1975.

Prospects for developing efficient targets for the xenomonitoring and control of Simulium damnosum s.l., the major vectors of onchocerciasis in Africa.

Despite the ethical issues concerning the continued use of Human Landing Catches (HLC) to monitor the Simulium damnosum complex for epidemiological monitoring of onchocericasis, few attempts to develop alternatives have been reported. In studies on a wild population of S. damnosum in Burkina Faso, we tested visual targets (different sizes and shapes) and olfactory stimuli (CO2, and POCA and BG‐lure® odour blends) for their ability to attract and collect host seeking blackflies. At each trap, blackflies were caught with appropriately sized electrocuting grids and results from Latin square design tests were compared. Throughout, HLCs captured more blackflies than the targets. Of the traps tested, small targets (0.0625 and 0.5 m2) were the most efficient visual lure in terms of the number of S. damnosum captured per unit area 1.7–5× more than larger targets. Overall, results suggested that sticky black targets of horizontal rectangular shape (0.125–0.5 m2) and baited with a POCA and/or CO2 mixture could provide a cheap practical field alternative to HLC for onchocerciasis xenomonitoring, subject to confirmation that the design has no inherent bias for certain members of the S. damnosum species complex.

Ecological assessment of Black Fly (Simuliumdamnosum, Sensulato) breeding sites in two tributaries of Ogun river, Ogun state, Nigeria.

Onchocerciasis is a disease transmitted by blackfly vector of the genus Simuliumdamnosum complex having public and socio-economic consequences. The abundance of blackfly is directly linked to fluvial ecosystems in both tropical and non-tropical countries of the world. This study assessed the ecology of blackfly breeding sites in two tributaries of River Ogun located in Olokemeji and Lisa villages, Ogun State. A total of eighteen water samples were collected for physicochemical analysis during high rainfall (May, June and July), low rainfall (August, September and October) and dry season (November and December) with mean rainfall values of 163.17, 88.03 and 8.95mm, respectively, in both tributaries. Physicochemical parameters such as temperature (oC), electrical conductivity (EC), pH, total solids (TS), dissolved oxygen (DO), biochemical oxygen demand (BOD), total suspended solids (TSS), hardness (H), sulphate (SO4), phosphate (PO4), alkalinity, acidity, nitrate (NO3) and total dissolved solids (TDS) were analysed using standard laboratory methods. Digestive enzymes were analysed using 1g of frozen gut of Simulium larvae grounded with 20ml of 0.067g of potassium hydrogen and 20ml of 0.1M sodium reagent. A total of 766 and 482 samples of Simulium larvae were collected from Olokemeji and Lisa tributaries using Eppendorf tubes. The zooplanktons collected from Olokemeji and Lisa tributary were 904 and 802, while phytoplanktons collected from Olokemeji and Lisa tributary were 635 and 582 during the study period. Data obtained were analysed using both descriptive analysis and T-test. The results obtained for the physicochemical parameters were 28.7°C, 137.5 µS/cm, 6.3, 248.1mg/L, 58mg/L, 108.8mg/L, 90.8mg/L, 2.24mg/L, 2.24mg/L, 30.76mg/L, 42.35mg/L, 1.99mg/L, 2.07mg/L and 2.55mg/L for Temperature, EC, pH, TS, DO, BOD, TSS, H, SO4, PO4, alkalinity, acidity, NO3 and TDS, respectively. There were significant (p < 0.05) differences between the means of EC, pH, TS, TDS, TSS and hardness of Lisa tributary, and water samples were also viewed under an electron microscope for zooplanktons and phytoplanktons. The values of species diversity which were greater than 0.5 indicated that the species were abundant and evenly distributed. There were significant (p < 0.05) differences between the tributaries in all the digestive enzymes except amylase content. The regression analysis between TSS, nitrate, sulphate, phosphate, acidity, hardness, TS, pH and Simulium larvae abundance showed a positive correlation, R2 with values of 0.828, 0.994, 0.948, 0.933, 0.986, 0.997, 0.889 and 0.981, respectively. The R2 values between Simulium larvae abundance with phytoplanktons and rainfall have values of 0.868 at p < 0.01 and 0.911 at p < 0.05.

Identification of the Sibling Species of the Blackfly, Simulium damnosum Complex Using their Polytene Chromosomal Banding Patterns in Oji River System, Enugu State, Nigeria

Identification of the sibling species of the blackfly, S. damnosum complex using their polytene chromosomal banding patterns in Oji river system was carried out with the aim of accurately identifying the particular sibling species of S. damnosum complex involved in the transmission of onchocerciasis in Oji-river LGA and environ. Simulium damnosum complex larvae were collected monthly in forty eight (48) breeding sites in four sampling units for three years in the Oji river system in wet and dry seasons. Collected larvae were preserved in cold Carnoy’s solution prior to processing. Salivary gland chromosomes were extracted, processed and analyzed microscopically for polytene chromosomal banding patterns. Three sibling species of Simulium damnosum complex that breed in the Oji river system were identified in varying degrees. They included S. squamosum, S. yahense and S. damnosum sensu stricto (s.s.). Variation in the occurrence of the sibling species was statistically significant. Dry and wet season catches for all the sibling species in the sampling units were found to differ statistically. The identified sibling species of S. damnosum complex are known to be vectors of Onchocerca volvulus, the causative agent of onchocerciasis, a disease highly prevalent in Oji-river LGA and environ.

Seasonal Larval Abundance of the Blackfly, Simulium damnosum Complex and the Physico-chemical Properties of their Breeding Sites in Oji River System, Enugu State, Nigeria

Seasonal larval abundance of the blackfly, Simulium damnosum complex, and the physico-chemical properties of their breeding sites in Oji river system was carried out, with the aim of finding out the season that promotes the breeding of S. damnosum complex and the effect of the physico-chemical properties of the breeding sites of S. damnosum complex on larval abundance. Larvae of S. damnosum complex were collected monthly from forty-eight (48) breeding sites in four sampling units for three years, in dry and wet seasons. The physico-chemical properties of the breeding sites of S. damnosum complex studied were copper, dissolved oxygen, magnesium, nitrate, pH, phosphate, potassium, temperature, and water velocity. A total of six hundred and ninety one (691) larvae of S. damnosum complex were collected in the Oji river system in both seasons. Wet season catch was more and comprised 82% of the total catch, while dry season catch comprised 18% of the total catch. Dry and wet seasons catches in the sampling units were found to differ statistically. All the physico-chemical properties of the breeding sites of S. damnosum complex determined showed positive correlations with total larval abundance in both seasons, except copper which showed no correlation with total larval abundance in the dry season. Wet season promotes larval abundance of S. damnosum complex, and the physico-chemical properties of the breeding sites of S. damnosum complex play some roles (in varying degrees) in larval development.

Biting behaviour and infectivity of Simulium damnosum complex with Onchocerca parasite in Alabameta, Osun State, Southwestern, Nigeria.

A longitudinal study was carried out to investigate species composition, seasonal abundance, parity and transmission potential of Simulium damnosum complex in Alabameta community in Osun State, Southwestern, Nigeria. Adult Simulium damnosum complex were collected along Owena River, Alabameta, by two dark complexioned vector collectors from 07:00hr to 18:00hr weekly using collecting tubes from November 2014 to April 2015. The flies were morphologically identified and dissected for the purpose of detecting Onchocerca parasite using dissecting microscope. The Monthly Biting Rate (MBR) of flies was determined using World Health Organization standard formula. A total of four hundred and forty flies were collected during the study period with all of them identified as forest species of Simulium damnosum complex. There was significant variation in monthly collection of the flies with the month of November having the highest number of flies (194) (44%) while the month of April recorded the lowest number of flies (31) (7%) (p<0.05). The morning biting peak (09hr - 11hr) (137) was higher than the evening biting peak (15hr -17hr) (64) (p<0.05) while nulliparous flies (294) (67%) were more abundant than the parous flies (146) (33%) (p<0.05). There was absence of infection (zero infectivity) of the flies (p<0.05). The zero infectivity in the flies may plausibly indicate the possibility of zero transmission of Onchocerca parasite in the community which if sustained over a period of time may signify the possibility of onchocerciasis elimination. Also, the presence of forest species of the flies reduces the risk of resident's intense exposure to blinding savannah strain of onchocerciasis.

Genetic Diversity of Simulium damnosum complex Onchocerciasis Vector and its Influence on Entomological Monitoring in the West of Côte d'Ivoire

In West Africa, Onchocerca volvulus, the causative pathogen of human onchocerciasis, is transmitted by sibling species of the Simulium damnosum complex. Little is known about blackfly intraspecific variability and its consequences on vectorial capacity in Cote d’Ivoire. This study reports the use of microsatellite markers to study the genetic profile and evaluate the gene flow between populations of S. damnosum s.l from three (3) different epidemiological facies in western Cote d’Ivoire, fifteen years after the end of onchocerciasis control program. Adult flies were collected on human attractants from 07 : 00 to 18 : 00 hours for three consecutive days by site, from December 2016 to October 2017. Four (4) microsatellite loci were used to characterize individuals from these populations. The four (4) loci were polymorphic with 13.25 alleles per locus. Two (2) specific alleles (190 bp and 290 bp), were reveal abundant with respective frequencies of 0.46 % and 0.58 %. Asignificant heterozygosity deficiency and low genetic differentiation (FST = 0.046, P = 0.024) have been observed for all the populations. The genetic analysis showed significant deviation from Hardy-Weinberg and significant heterozygosity deficiencies. Then, the great interspecific variability would be a general characteristic in S. damnosum s.l. At last, a probable evolution of the invasive strains of S. damnosum would have occurred in these localities. This study has shown significant deviation from Hardy-Weinberg and significant heterozygosity deficiencies in the populations of the three localities. The loci give independent estimate genetic parameters. The H3-4 locus to a low genetic differentiation between the populations.

Entomological Evaluation by Dissection of Adult Simulium damnosum Complex for Larvae of Onchocerca volvulus , Following CDTI in Amagu Agba Community, Ishielu L.G.A-Ebonyi State, Nigeria

The study was carried out to verify reports of persistent transmission of onchocerciasis, despite long-term Community Directed Treatment with Ivermectin, (CDTI). Blackflies were collected along Asu River using human baits. A total of 91 female Simulium damnosum complex adults were caught and dissected- using a dissecting microscope- for microfilariae of O. volvulus. 58 flies representing (63.70%) of the total number of captured flies were caught in the month of June while 33 (36.20%) were caught in July. Despite ivermectin treatment, evidence of O. volvulus transmission was documented in Amagu Agba. A total of 27 larvae were recovered from the three body segments of the dissected flies, of this number, 17 larvae were recovered from the head region and a total of 10 were recovered from the thorax and abdomen representing 62.96 % and 37.04 % respectively of the total number of larvae recovered. The proportion of infected flies recorded was 8 (8.8 %) while the proportion of infective flies recorded was 17 (18.7%) of the total number of infected and infective flies. The high proportion of infective flies is indicative of active transmission in and around the study area. This should not be so as CDTI is currently going on in the area. Some militating factors such as: patterns of treatment coverage and compliance, parasite ivermectin susceptibility, parasite immigration in flies or people, may be responsible for the current unsuccessful ivermectin treatment strategy.There is therefore need for consistent and effective ivermectin distribution pattern and its resultant onchocerciasis prevalence, detected with blackfly dissection techniques and or more reliably detected in both blackfly population and human subjects through a combination of some molecular-based detection techniques and biocontrol approaches. Adopting these measures together with community-ownership participation concept, will successfully interrupt transmission in Amagu Agba community and other endemic areas.

Seasonal Variations in Biting Density and Infectivity of Simulium damnosum Complex in Ezeagu and Oji-River Local Government Areas of Enugu State, Nigeria

Seasonal Variations in Biting Density and Infectivity of Simulium damnosum Complex in Ezeagu and Oji-River Local Government Areas of Enugu State, Nigeria

The blackfly vectors and transmission of Onchocerca volvulus in Mahenge, south eastern Tanzania

The Mahenge Mountains onchocerciasis focus in south eastern Tanzania was historically one of the most heavily infected areas in the country. The vectors of Onchocerca volvulus are mainly Simulium damnosum complex blackflies, but a species of the Simulium neavei group may also contribute to transmission in some areas. The only detailed studies of parasite transmission in Mahenge were conducted in the late 1960s. The taxonomy of the S. damnosum complex has since been revised and onchocerciasis control through annual community directed treatment with ivermectin (CDTI) commenced in 1997. This study aimed to provide a cytogenetic and molecular update of the S. damnosum complex cytoforms present in Mahenge, and to evaluate the current status of O. volvulus transmission by blackflies following 19 years of annual CDTI.Rivers were surveyed to identify sites of S. damnosum s.l. breeding among the eastern slopes of the mountains, and human landing collections of adult female blackflies were made close to breeding sites. Identification of S. damnosum complex cytoforms was by cytotaxonomy of late-instar larvae and ITS1 amplicon size polymorphisms of larvae and adults. Adult blackflies were pool screened for O. volvulus infection using a triplex real-time PCR. The cytoforms ‘Nkusi’, Simulium kilibanum and ‘Turiani’ were found breeding in perennial rivers. ‘Nkusi’ and S. kilibanum were collected on human bait at 7/7 catch sites and possessed ITS1 profiles most closely resembling the molecular forms ‘Nkusi J’ and S. kilibanum ‘T’. Whereas ‘Turiani’ was present in rivers, it was not collected on human bait and appears to be zoophilic. Simulium nyasalandicum was collected in low numbers on human bait at 3/7 catch sites. In total, 12,452 S. damnosum s.l. were pool screened and O. volvulus infection was detected in 97/104 pools of bodies and 51/104 pools of heads. The estimated percentage of S. damnosum s.l. carrying infective L3 stage parasites was 0.57% (95% CI 0.43%–0.74%).Onchocerca volvulus transmission by S. damnosum s.l. is continuing in the Mahenge Mountains after 19 years of annual CDTI. Infection rates appear similar to those reported in the 1960s, but a more detailed study is required to fully understand the epidemiological significance of the ongoing transmission. These results provide further evidence that annual CDTI may be insufficient to eliminate the parasite in formerly hyperendemic foci.

Onchocerciasis transmission in Ghana: the human blood index of sibling species of the Simulium damnosum complex.

BackgroundVector-biting behaviour is important for vector-borne disease (VBD) epidemiology. The proportion of blood meals taken on humans (the human blood index, HBI), is a component of the biting rate per vector on humans in VBD transmission models. Humans are the definitive host of Onchocerca volvulus, but the simuliid vectors feed on a range of animals and HBI is a key indicator of the potential for human onchocerciasis transmission. Ghana has a diversity of Simulium damnosum complex members, which are likely to vary in their HBIs, an important consideration for parameterization of onchocerciasis control and elimination models.MethodsHost-seeking and ovipositing S. damnosum (sensu lato) (s.l.) were collected from seven villages in four Ghanaian regions. Taxa were morphologically and molecularly identified. Blood meals from individually stored blackfly abdomens were used for DNA profiling, to identify previous host choice. Household, domestic animal, wild mammal and bird surveys were performed to estimate the density and diversity of potential blood hosts of blackflies.ResultsA total of 11,107 abdomens of simuliid females (which would have obtained blood meal(s) previously) were tested, with blood meals successfully amplified in 3,772 (34 %). A single-host species was identified in 2,857 (75.7 %) of the blood meals, of which 2,162 (75.7 %) were human. Simulium soubrense Beffa form, S. squamosum C and S. sanctipauli Pra form were the most anthropophagic (HBI = 0.92, 0.86 and 0.70, respectively); S. squamosum E, S. yahense and S. damnosum (sensu stricto) (s.s.)/S. sirbanum were the most zoophagic (HBI = 0.44, 0.53 and 0.63, respectively). The degree of anthropophagy decreased (but not statistically significantly) with increasing ratio of non-human/human blood hosts. Vector to human ratios ranged from 139 to 1,198 blackflies/person.ConclusionsDNA profiling can successfully identify blood meals from host-seeking and ovipositing blackflies. Host choice varies according to sibling species, season and capture site/method. There was no evidence that HBI is vector and/or host density dependent. Transmission breakpoints will vary among locations due to differing cytospecies compositions and vector abundances.Electronic supplementary materialThe online version of this article (doi:10.1186/s13071-016-1703-2) contains supplementary material, which is available to authorized users.

Ongoing Transmission of Onchocerca volvulus after 25 Years of Annual Ivermectin Mass Treatments in the Vina du Nord River Valley, in North Cameroon.

BackgroundRecent reports of transmission interruption of Onchocerca volvulus, the causing agent of river blindness, in former endemic foci in the Americas, and more recently in West and East Africa, raise the question whether elimination of this debilitating disease is underway after long-term treatment of the population at risk with ivermectin. The situation in Central Africa has not yet been clearly assessed.Methods and findingsEntomologic data from two former endemic river basins in North Cameroon were generated over a period of 43 and 48 months to follow-up transmission levels in areas under prolonged ivermectin control. Moreover, epidemiologic parameters of animal-borne Onchocerca spp. transmitted by the same local black fly vectors of the Simulium damnosum complex were recorded and their impact on O. volvulus transmission success evaluated. With mitochondrial DNA markers we unambiguously confirmed the presence of infective O. volvulus larvae in vectors from the Sudan savannah region (mean Annual Transmission Potential 2009–2012: 98, range 47–221), but not from the Adamawa highland region. Transmission rates of O. ochengi, a parasite of Zebu cattle, were high in both foci.Conclusions/significanceThe high cattle livestock density in conjunction with the high transmission rates of the bovine filaria O. ochengi prevents the transmission of O. volvulus on the Adamawa plateau, whereas transmission in a former hyperendemic focus was markedly reduced, but not completely interrupted after 25 years of ivermectin control. This study may be helpful to gauge the impact of the presence of animal-filariae for O. volvulus transmission in terms of the growing human and livestock populations in sub-Saharan countries.

Managing the Fight against Onchocerciasis in Africa: APOC Experience.

Due to the socioeconomic impact of human onchocerciasis (commonly referred to as river blindness) in West Africa, the Onchocerciasis Control Programme in the Volta River Basin (OCP) was instituted [1]. This initial programme started in 1975 and covered seven West African countries: Benin, Burkina Faso, Cote d’Ivoire, Ghana, Mali, Niger, and Togo. However, later evidence indicated that endemic areas outside the initial area posed a threat to the achievement of the OCP and, hence, the Programme was extended southward and westward to include four additional countries, bringing the total number of countries covered by OCP to eleven. The formal name was then changed to the Onchocerciasis Control Programme in West Africa, retaining the acronym OCP. OCP used aerial larviciding as its principle strategy to control the vectors of human onchocerciasis, members of the Simulium damnosum complex, in the absence of a safe drug for mass treatment against the parasites [2]. Efforts to control onchocerciasis evolved in 1987 when ivermectin was donated to kill the juvenile worms that cause the various symptoms associated with the disease. As a result of the donation, OCP instituted a new strategy of chemotherapy in combination with vector control. In the 11 countries covered by OCP, this two-prong approach led to the virtual elimination of onchocerciasis as a public health problem and as an obstacle to socioeconomic development. The availability of a donated drug effective against the parasite and safe for mass drug administration, coupled with evidence that other pathological effects of onchocerciasis were equally important socioeconomic threats, led to the decision that onchocerciasis should be controlled in all endemic countries in Africa (Fig 1). Open in a separate window Fig 1 Onchocerciasis-endemic countries in Africa, showing countries covered by the OCP and initially by APOC. Map from 2010. Note that South Sudan gained independence in 2011, becoming the 20th APOC country.

Effects of climatic conditions on the biting density and relative abundance of Simulium damnosum complex in a rural Nigerian farm settlement.

The effect of climatic conditions on the biting density and relative abundance of Simulium damnosum complex at Adani, Nigeria, from August 2010-January 2011 was investigated. The classical method of collecting blackflies for a period of 11 hours using human attractants was employed in the study. Monthly climatic data, such as rainfall, relative humidity and temperature were collected for the period of study. Rainfall, relative humidity, temperature, harmattan (cold, dry wind) and deforestation were observed to affect the biting density and relative abundance of blackflies at the site. A total of 548 female adult blackflies were collected. The biting density of the flies ranged from 0.5 Flies/Man/Hour (FMH) in December to 5.5 FMH in January. The relative abundance of the flies ranged from 21 in December to 243 in January. Regression analysis showed that temperature and relative humidity had a positive correlation with relative abundance of Simulium damnosum complex (y=-0.0006x+5.593, r=0.0519) and (y=-0.1213x+78.794, r=0.505), respectively. The risk of getting infected with Onchocerca volvulus increased during the dry season with its associated weather conditions.

Onchocerciasis transmission in Ghana: biting and parous rates of host-seeking sibling species of the Simulium damnosum complex.

BackgroundGhana is renowned for its sibling species diversity of the Simulium damnosum complex, vectors of Onchocerca volvulus. Detailed entomological knowledge becomes a priority as onchocerciasis control policy has shifted from morbidity reduction to elimination of infection. To date, understanding of transmission dynamics of O. volvulus has been mainly based on S. damnosum sensu stricto (s.s.) data. We aim to elucidate bionomic features of vector species of importance for onchocerciasis elimination efforts.MethodsWe collected S. damnosum sensu lato from seven villages in four Ghanaian regions between 2009 and 2011, using standard vector collection, and human- and cattle-baited tents. Taxa were identified using morphological and molecular techniques. Monthly biting rates (MBR), parous rates and monthly parous biting rates (MPBR) are reported by locality, season, trapping method and hour of collection for each species.ResultsS. damnosum s.s./S. sirbanum were collected at Asubende and Agborlekame, both savannah villages. A range of species was caught in the Volta region (forest-savannah mosaic) and Gyankobaa (forest), with S. squamosum or S. sanctipauli being the predominant species, respectively. In Bosomase (southern forest region) only S. sanctipauli was collected in the 2009 wet season, but in the 2010 dry season S. yahense was also caught. MBRs ranged from 714 bites/person/month at Agborlekame (100% S. damnosum s.s./S. sirbanum) to 8,586 bites/person/month at Pillar 83/Djodji (98.5% S. squamosum). MBRs were higher in the wet season. In contrast, parous rates were higher in the dry season (41.8% vs. 18.4%), resulting in higher MPBRs in the dry season. Daily host-seeking activity of S. damnosum s.s./S. sirbanum was bimodal, whilst S. squamosum and S. sanctipauli had unimodal afternoon peaks.ConclusionsThe bionomic differences between sibling species of the S. damnosum complex need to be taken into account when designing entomological monitoring protocols for interventions and parameterising mathematical models for onchocerciasis control and elimination.Electronic supplementary materialThe online version of this article (doi:10.1186/s13071-014-0511-9) contains supplementary material, which is available to authorized users.

Onchocerciasis transmission in Ghana: biting and parous rates of host-seeking sibling species of the Simulium damnosum complex

Onchocerciasis transmission in Ghana: biting and parous rates of host-seeking sibling species of the Simulium damnosum complex

Stability and change in the distribution of cytospecies of the Simulium damnosum complex (Diptera: Simuliidae) in southern Ghana from 1971 to 2011

BackgroundSimulium damnosum s.l., the most important vector of onchocerciasis in Africa, is a complex of sibling species that have been described on the basis of differences in their larval polytene chromosomes. These (cyto) species differ in their geographical distributions, ecologies and epidemiological roles. In Ghana, distributional changes have been recorded as a consequence of vector control and environmental change (e.g. deforestation), with potential disease consequences. We review the distribution of cytospecies in southern Ghana and report changes observed with reference to historical data collated from 1971 to 2005 and new identifications made between 2006 and 2011.Methods/ResultsLarvae were collected from riverine breeding sites, fixed in Carnoy’s solution and chromosome preparations made. Cytotaxonomic identifications from 1,232 samples (including 49 new samples) were analysed. We report long-term stability in cytospecies distribution in the rivers Afram, Akrum, Pawnpawn and Pru. For the rivers Oda, Ofin and Tano we describe (for the first time) patterns of distribution. We could not detect cytospecies composition changes in the upper Pra, and the lower Pra seems to have been stable. The elimination of the Djodji form of S. sanctipauli in the Volta Region seems to have had no long-term effects on the distribution of the other cytospecies, despite an initial surge by S. yahense. There has been a recent increase in the occurrence of savannah cytospecies in the river Asukawkaw, and this might be related to continuing deforestation.ConclusionsCytospecies’ distributions have not been stable from 1971 to 2011. Although there are no obvious causes for the temporary appearance and subsequent disappearance of cytospecies in a particular location, a major influence has been vector control and migration patterns, probably explaining observed changes on the Black Volta and lower Volta rivers. Deforestation was previously implicated in an increase of savannah cytospecies in southern Ghana (1975–1997). Our data had little power to support (or refute) suggestions of a continuing increase, except in the Asukawkaw river basin.

Molecular evidence of ‘Siisa form’, a new genotype related to Onchocerca ochengi in cattle from North Cameroon

Onchocerca ochengi, a filarial nematode parasite from African Zebu cattle is considered to be the closest relative of Onchocerca volvulus, the causative agent of river blindness. Both Onchocerca species share the vector, black flies of the Simulium damnosum complex. Correct identification of their infective third-stage larvae in man-biting vectors is crucial to distinguish the transmission of human or animal parasites. In order to identify different closely related Onchocerca species we surveyed the sequences from the three mitochondrial loci 12S rRNA, 16S rRNA and coxI in both adult worms isolated from Onchocerca-induced nodules in cattle and infective third stage larvae isolated from vector flies from North Cameroon. Two distinct groups of mitochondrial haplotypes were found in cattle as well as in flies. One of them has been formerly mentioned in the literature as Onchocerca sp. ‘Siisa’, a filaria isolated from the vector S. damnosum sensu lato in Uganda with hitherto unknown host. Both variants are found sympatric, also in the same nodule of the animal host and in the vector. In the flies we also found the mitochondrial haplotype that had been described for O. volvulus which is about equally different from the two previously mentioned ones as they are from each other. These results suggest a higher genetic diversification of Onchocerca ochengi than previously reported.