Onchocerca volvulus transmission in the Mbam valley of Cameroon following 16 years of annual community-directed treatment with ivermectin, and the description of a new cytotype of Simulium squamosum

Abstract

Background The onchocerciasis focus surrounding the lower Mbam and Sanaga rivers, where Onchocerca volvulus is transmitted by Simulium damnosum s.l. (Diptera: Simuliidae), was historically the largest in the southern regions of Cameroon. Annual community-directed treatment with ivermectin (CDTI) has been taking place since 2000, but recent studies have shown that new infections are occurring in children. We aimed to investigate blackfly biting and O. volvulus transmission rates along the lower Mbam river 16 years after the formal onset of annual CDTI. Methods Black flies were collected for three consecutive days each month between July 2016 and June 2017 at two riverside villages and two inland sites situated 4.9 km and 7.9 km from the riverside. Specimens collected at each site were dissected on one of the three collection days each month to estimate parity rates and O. volvulus infection rates, while the remaining samples were preserved for pool screening. Results In total, 93,573 S. damnosum s.l. black flies were recorded biting humans and 9281 were dissected. Annual biting rates of up to 606,370 were estimated at the riverside, decreasing to 20,540 at 7.9 km, while, based on dissections, annual transmission potentials of up to 4488 were estimated at the riverside, decreasing to 102 and 0 at 4.9 km and 7.9 km, respectively. However, pool screening showed evidence of infection in black flies at the furthest distance from the river. Results of both methods demonstrated the percentage of infective flies to be relatively low (0.10–0.36%), but above the WHO threshold for interruption of transmission. In addition, a small number of larvae collected during the dry season revealed the presence of Simulium squamosum E. This is the first time S. squamosum E has been found east of Lake Volta in Ghana, but our material was chromosomally distinctive, and we call it S. squamosum E2. Conclusions Relatively low O. volvulus infection rates appear to be offset by extremely high densities of biting black flies which are sustaining transmission along the banks of the lower Mbam river. Graphical Abstract

Surveillance for Onchocerciasis-Associated Epilepsy and OV16 IgG4 Testing of Children 6–10 Years Old Should Be Used to Identify Areas Where Onchocerciasis Elimination Programs Need Strengthening

“…Indeed, a stratified analysis including only indigenous households found a crude epilepsy prevalence of 6.7% (14/208) in Bayomen, 6.5% (15/232) in Ngongol and 5.5% (50/905) in Nyamongo [ 26 ]. The high Ov16 seropositivity in children in Bayomen concurs with the results of an entomological study that observed an even higher O. volvulus annual transmission based on blackfly parity and infection rates and transmission potentials in Bayomen (from dissection data) compared to Nyamongo [ 48 ].…”

“…However, in this onchocerciasis focus, CDTI coverage has been sub-optimal. The percentage of infective blackflies in the area was found to be relatively low (0.10–0.36%), but in certain villages extremely high densities of biting blackflies were documented [ 48 ]. Despite a high Ov16 seropositivity in the children, a relatively low epilepsy prevalence (2.6%) was observed in Bayomen.…”

A cross-sectional study of Simulium damnosum sensu lato breeding sites and species distribution in Sudan savanna, mixed savanna–forest and rainforest regions in Cameroon

“…Around Mount Cameroon, S. squamosum A, C and D occur in sympatry, where hybrids between cytoforms A and C are common [ 18 , 19 ]. Simulium squamosum B has only been found in the Sanaga River [ 18 ] while S. squamosum E2 has recently been described from the Mbam River [ 26 ], a tributary of the Sanaga River.…”

Identification of the onchocerciasis vector in the Kakoi-Koda focus of the Democratic Republic of Congo

“…The PCR was performed using a Rotor Gene Q (Qiagen) with the following cycling conditions: Taq polymerase activation at 95°C for 15 min, 45 cycles of 95°C for 10 s and 61°C for 30 s with fluorescence acquisition on the Fam, Hex and Cy5 channels. In every run, plasmids containing the respective gene sequences were included as positive controls [ 62 ]. A sample was considered positive for a gene if at least one of the three replicates produced a signal while using the 15% outlier removal setting in the Rotor Gene Q Software (version 2.3).…”