Both indoor and outdoor Traps were set at 1.5 m from the ground. The indoor traps were set at the foot of the bed while the outdoor traps were set at 2 m from the houses. The rotator traps collected mosquitoes every 2 h from 18.00 to 08.00. Collections were repeated for 5 days in each of the houses in all the study sites. PCR and ELISA were conducted on the collected vectors for the identification of species and sporozoite infectivity.In the six selected study sites, the human behaviour study was done in 2013 to determine if there is any association between the peak of human outdoor activities and peak of mosquito blood feeding time. A questionnaire survey was administered in 200 randomly selected households where consent of participation had been obtained, during the wet and dry season. The questions asked included the time they went indoors in the evening, time they were outdoors in the morning, the activities that kept them outdoors and if they slept indoors or outdoors.Bed net ownership and usage were surveyed accompanying the pyrethrum spray collections. The number of children that slept under nets at night was recorded, including their ages. The type of net was recorded. The condition of the net was recorded to determine whether the net had holes or not.Monthly adult anopheline mosquito abundance was calculated as the number of female mosquitoes per house per night. Bed net ownership rate was calculated as the ratio of the number of households with at least one bed net over the total number of households surveyed. The average abundance of anopheline mosquitoes in a house was computed from January 2012 to August 2014 for each site. Overall differences in mosquito densities among villages were compared using Kruskal–Wallis ANOVA and median test.
Between-village difference in mosquito densities was compared using multiple comparisons of mean ranks for all groups. Analysis was done using STATISTICA 10 .In the study duration,vertical cannabis farming between January 2012 and August 2014, a total of 5,469 mosquito vectors were collected from both PSC and rotator traps; 3,181 were An. gambiae and 2,288 were An. funestus. In the highland sites, An. gambiae was the most abundant vector while in the lowland sites An. arabiensis and An. funestus were the most abundant vectors collected. Densities of An. gambiae peaked in April to May in the study sites . These peaks were reached generally 1 month after the onset of the long rainy season, which is the main malaria transmission season. Generally, low vector densities were seen during the dry season between January and March, however Rae showed different patterns of vector abundance, where An. gambiae was high throughout the study period . Anopheles funestus peaked 2 months after the rainy season between June and July . The lowland site of Rae had the highest mean An. gambiae indoor resting densities of 1.02, Marani in the highlands had the least at 0.06 mean vectors. There was no difference in vector abundance in the other four study sites as shown in Table 1. Anopheles funestus indoor resting densities were highest in Kombewa . Miwani had fewer An. funestus vectors than Kombewa but higher than the other four sites, which had similar means when compared. Marani had the least resting densities of 0.17 as shown in Table 1.This study was carried out to assess the impact of ITNs on indoor vector densities and biting behaviour in western Kenya as the use of ITNs has been shown to be effective in reducing mortality and malaria transmission in the past. Before the mass distribution of ITNs in 2011, bed net ownership in western Kenya was reported to be below 80% and parasite resurgence had been seen in areas of western Kenya. This was attributed to vector resistance to pyrethroids and the inefficacy of bed nets because of the low ownership. Afterwards, the Roll Back Malaria Partnership raised coverage of ITNs to ≥80% through the free mass distribution of long-lasting insecticidal nets /ITN campaigns, which were carried out in various parts of Africa.
The current policy for vector control in Kenya includes the use of LLINs and limited use of IRS where the government-marketed, subsidized bed nets in 2002, 2006 to vulnerable groups until 2011 when there was a universal distribution policy was implemented with every two persons in a household receiving a free bed net. In this study, high bed net ownership of >80% in all six sites was confirmed. The high bed net ownership has a community effect where people without nets are protected by the area-wide effects of ITNs nearby. Anopheles gambiae indoor resting densities over the last decade have decreased tenfold in Iguhu, sixfold in Marani and fourfold in Kombewa, while densities of An. funestus have decreased threefold in Iguhu and sixfold in Kombewa. However in Marani, over the decade densities of An. funestus have increased threefold compared to a study by Ndenga et al.. Likewise, sporozoiterates of An. gambiae have declined fourfold in Iguhu and Kombewa while they have increased fourfold in Marani. Anopheles funestus sporozite rates remained constant in Kombewa, while in the other sites there were no confirmed infectious An. funestus. Anopheles funestus is seen as one of the most abundant vectors in Kombewa, which has been reported previously. The species is re-emerging in Marani where it was the most abundant species, as shown in the results. Anopheles funestus breeds in permanent habitats towards the end of the wet season and is known to require vegetation and shade and the larval habitats are found mainly in swamps and pastures. Anopheles funestus takes 3 weeks to mature, which is longer than An. gambiae maturation period. Other studies conducted in western Kenya lowland region have reported that An. funestus is re-emerging, which is suspected to be as a result of pyrethroid resistance after a long-term implementation of ITNs. Previous studies in the sugar-belt region of Miwani reported the ratio of An. arabiensis to be higher than that of An. gambiae s.s., especially during the dry season. The use of ITNs has had a great impact on densities, species and sporozoite rates.
The proportion of An. arabiensis is increasing in the highlands, a factor that could have malaria transmission implications as An. arabiensis is a less efficient vector than An. gambiae, as An. arabiensis is zoophilic. Githeko et al. found that malaria vectors fed during the late part of the night with peaks at 05.00 h. In this study, An. gambiae caught after midnight was blood fed, while fed An. funestus were caught throughout the night both indoors and outdoors. It is likely that blood-fed An. funestus may have been avoiding resting. This observation supports exophilic behaviour in An. funestus, a phenomenon that requires further investigation. In regard to human and mosquito activity,hemp drying racks data from this study suggests that there is a risk of transmission at dusk and at dawn. Data collected during the study did not support continuous outdoor transmission since the majority of humans were indoors between 21.00 and 05.00 h. The use of LLINs has been reported to change the feeding and resting behaviour of mosquitoes. The study reports similar findings that there was high host seeking activity of the vectors at around 18.00 and 20.00 that led to earlier feeding in An. gambiae populations. This could be as a result of the use of ITNs. Anopheles funestus showed no change in feeding habits as the results show that they bite throughout the night both indoors and outdoors. This poses a great risk of malaria transmission throughout the night despite high bed net coverage. Studies done by Oloo et al. showed that the use of permethrin-treated sisal curtains led to the exit of half-gravid mosquitoes from indoors. This could be one of the reasons why there was a high collection of halfgravid mosquitoes, both An. gambiae and An. funestus. This result coincided with the human behaviour study where >50% of the population stayed outdoors after dusk but went indoors by 21.00 h and woke up before dawn to do their daily chores. Similar studies in the lowlands of western Kenya have also reported that the vectors bite throughout the night and mostly indoors. Anopheles funestus feeding habits suggest that transmission is most likely happening indoors, although there is a high risk of outdoor transmission. Findings in the lowland regions show that An. funestus was the most infectious vector while in the highlands; An. gambiae was the main vector of transmission.
The ratio of blood-feds to half-gravids was 3:1 and the capture of both blood-feds and half gravids shows that there was insecticidal excito-repellency. This blood-fed to gravid ratio can be as a result of mortality of the vectors after contact with ITNs or insecticide repellency. The capture of half-gravids and gravid vectors is an indicator of exophily. Vector biting and resting behaviour may be altered by exposure to insecticides. Under the use of ITNs in Tanzania, the tendency of mosquitoes to exit the indoor environment increased. In Ethiopia, An. arabiensis avoided resting on DDT sprayed surfaces. In western Kenya, the proportion of An. gambiae taking a blood meal before humans slept under ITNs increased after the introduction of ITNs. These shifts in biting and resting behaviour reduce exposure of malaria vectors to the impacts of insecticides thus minimizes their mortality resulting in sustained malaria transmission. Data collected from this study suggests Anopheles funestus may have changed its resting behaviour. In previous studies, where ITNs were not in use, no blood fed females of An. funestus were collected in light traps and exit traps. Equally, no blood fed female An. funestus were collected in outdoor placed light traps. In the current study, blood fed An. funestus were collected in indoor and outdoor placed light traps suggesting a post blood feeding flight activity and possibly exit to the outdoor environment. Studies being undertaken will test whether the blood fed females had fed on humans or other hosts. Data from this study indicates that the proportion of An. funestus in Iguhu and Marani, where historical data exists, has increased in recent years which suggests that this vector has better survival under the use of ITNS than An. gambiae s.l. this could be explained by increased avoidance of insecticide treated surfaces a behaviour that remains to be studied. There was a difference in the densities of the vectors collected between households that had at least one bed net and households that did not own a bed net. This was seen in Kombewa, a site that had An. funestus as the main vector. Fewer vectors were collected in the households that had at least one bed net. Mbogo et al. reported that after distribution of permethrin-treated bed nets, fewer vectors were collected. This shows that owning a bed net protects the household from malaria vectors, while a high coverage of bed net ownership creates community-wide protection from mosquitoes. Besides owning a bed net and the high distribution in the study sites, there has been a reported increase in the resistance to insecticides both in West Africa and in East Africa, especially in western Kenya. Change in behaviour patterns due to high ownership of bed nets are also reducing the role of An. gambiae s.s. in malaria transmission but not ruling out the role of An. arabiensis.Malaria control in the tropics is currently based largely on treatment of people with clinical illness and personal protection against malarial mosquito vectors. Vector control campaigns emphasize environmental sanitation and suitable environmental management, implementation of educational programs and the use of insecticides, either in impregnated fabrics or sprays . The use of pesticides, which has to be highly regulated and their handling is subject to strict control, requires adherence to the recommendations of the WHO. Scant consideration has been given to housing design and construction as an environmental strategy to control malaria. However, in the high transmission areas, additional measures will be needed to reduce the malaria burden to acceptable levels. Poor quality housing is generally accepted to be an important contributor to ill health. In most parts of Africa, feeding by the principal malaria vectors, Anopheles gambiae and Anopheles funestus, takes place in the later part of the night, indoors.