An Investigation into Impact of Unsprayed Surfaces On Mosquito Behaviour and Malaria Transmission in an Area Undergoing Indoor Residual Spraying in Northern Ghana

ABSTRACT

 Indoor Residual Spraying (IRS) is one of the malaria vector control tools used in the control of malaria and has helped to reduce malaria transmission in many areas in SubSaharan Africa. This involves spraying the walls of rooms with insecticides so as to kill any mosquito that rest on the insecticide treated walls after feeding. However, recent discussions have been centered on the role unsprayed surfaces such as hanging cloths play in sprayed rooms in malaria transmission. In this study, the effect of hanged clothes on mosquito resting and exiting behaviour as well as on malaria transmission was investigated using mosquitoes captured by pyrethrum spray (PSC) and window exit trap (ETC) collections. The study was undertaken in Tarikpaa, in the Savelugu-Nanton district in Northern Ghana, an area undergoing Indoor Residual Spraying. Kulaa in the Tamale district of Northern Ghana was used as the control area with no IRS. Mosquitoes were sampled from August to December 2013. Anopheles gambiae s.l., Anopheles funestus, Anopheles nili and Anopheles rufipes were the Anopheles species collected during the period of the study making up 97.1%, 2.0%, 0.3% and 0.6% of the species respectively. Both An. coluzzi (M form) and S molecular forms occurred in all rooms in both areas. An. gambiae s.l. collected in rooms with hanging clothes in the IRS community (Tarikpaa) was lower (14.68%) compared to rooms with no hanging clothing (85.32%).

The same was observed for the non-IRS community (Kulaa). A total of 18 and 100 Anopheles mosquitoes were collected from rooms with hanging clothes and rooms without hanging clothes in Tarikpaa respectively. The mean Indoor Resting Density (IRD) of Anopheles collected from rooms in the IRS area without hanging clothes was higher than that of rooms with hanging clothes (2.7 mosquitoes/room vs 0.36 mosquitoes/room). However, this difference was not significant (F (1, 12) = 2.078, P = 0.175). The same trend was observed for the unsprayed community, Kulaa. A Fed xiii to Gravid ratio of 0.9:1 and 1:1 was estimated for mosquitoes collected exiting rooms with hanging clothes and without hanging clothes respectively. The sporozoite rate for both rooms with hanging clothes and rooms without hanging clothes in Tarikpaa was zero. The only positive mosquito samples were from the rooms without hanging clothes in the non-sprayed area (1.83%). An Entomological Inoculation Rate (EIR) of 0.0043 infective bites/man/night was estimated for An. gambiae collected from Kulaa. The fed: gravid ratio also indicated that more blood fed mosquitoes left the rooms immediately after feeding in the presence of insecticide on the walls. The number of Anopheles species collected by ETC from rooms with hanging clothes in the IRS area was significantly higher than in rooms without hanging clothes. The practice of hanging clothes in the area undergoing indoor residual spraying did not affect malaria transmission. Anopheles mosquitoes tend to exit rooms in the morning after feeding. This has implications for the effectiveness of the IRS in the area because many of the mosquitoes will not rest on the insecticide treated walls to be killed