Assessment of impacts of indoor residual house spraying (IRHS) on the diversity, abundance and distribution of human malaria vectots in Chongwe District, Zambia

Dominant human malaria vectors in Chongwe district, Zambia, were identified and impacts of indoor Residual House Spraying (IRHS) on their diversity, abundance and distribution assessed. A case-control type of study design was used in which Chishiko village in the district was the case in point, where houses had been sprayed with DDT insecticide during the 2008-
2009 malaria transmission period through a government of Zambia sponsored IRHS
programme and Chiota village was the control, where houses had not been sprayed with any insecticide during the period. Specific objectives of the study were to: Identify and classify endophilic mosquito species of Chongwe district, using morphological methods and molecular
methods and to assess effects of IRHS on the diversity, abundance and distribution of human malaria vectors in the district. Three hypotheses were tested in the study namely that: Endophilic mosquito species of Chongwe District did not include malaria vector species; The major vector of human malaria in the district was not an Anopheles gambiae complex sibling species; and that the IRHS malaria vector control programme being implemented in the district had no effect on the diversity, abundance and distribution of human malaria vectors in the district. Indoor-resting mosquitoes of the study area were collected for identification and
classification through spray-catch sampling of 72 randomly selected houses in two villages of the study area. Mosquito identification was both morphological using taxonomic dichotomous identification keys for all indoor-resting mosquitoes collected from the houses and molecular, using polymerase chain reaction (PCR) assays sibling species belonging to the Anopheles
species complexes among them. Vector abundance in the villages was determined through computations of mosquito mean densities and comparison of these using ANOVA, while the variance: Mean ratio (S2/ x ) was used to determine vector distribution patterns in the study areas. Two endophilic mosquito species were identified in the study area by the morphological method used namely: Culex quinquefasciatus Say 1823, Anopheles squamosus Theobald, 1901. Additionally, there were 11 mosquito specimens collected from the study areas that
keyed out as sibling species of the Anopheles gambiae complex using morphological method. Molecular characterization of the Anopheles gambiae complex sibling species specimens using
PCR revealed that all 11 specimens were Anopheles arabiensis Patton, 1905, indicating that this was probably the dominant human malaria vector species in Chongwe district. The alpha( ) index of diversity used to estimate the mosquito species diversity of the study area put the
number of endophilic mosquitoes in the district at three. The most abundant mosquito species collected from the study area was C. quinquefasciatus (78.6%), followed by A. arabiensis (13.1%) while A. squamosus accounted for 8.3% of the mosquito samples collected. The difference in abundance of C. quinquefasciatus between the two study areas was not significant (p = 0.1127), while that of Anopheles species, was significant (p < 0.05). Mosquito
distribution in the study area was aggregated (S2> x-bar). There was some indication of positive impact of the IRHS programme on the abundance and distribution of human malaria mosquito vectors. But a longitudinal study would be needed to make the results more conclusive about the effects of the IRHS programme in Chongwe district