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

BACKGROUND

Ghana 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.

METHODS

We 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.

RESULTS

S. 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.

CONCLUSIONS

The 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.