Few other household or subject characteristics were significant predictors of personal endotoxin exposure in crude models and all were confounded by dog and cat ownership. Significant predictors of indoor endotoxin in final multivariate models only included flooding damage and lower education levels in mothers that were unexpectedly associated with lower endotoxin. Overall evidence, including a lack of prediction of personal endotoxin by indoor-outdoor home endotoxin, and the association between personal endotoxin and dog and cat ownership, supports the view that personal dust cloud exposures may be the predominant driver of personal endotoxin exposure. A study supporting this view monitored rooms of 20 northern California homes and showed that indoor concentrations of both particles and endotoxin in PM2.5, but especially PM2.5-10 and PM >10 μm, were significantly elevated during the day and were higher with greater levels of subject activity. The study of 10 children by Rabinovitch et al.also found that in children not owning pets, personal endotoxin exposure was nominally higher on days that they reported playing with furry animals . We also conclude that personal endotoxin exposure can vary between regions . The regions are characterized by large differences in southern California weather, with Riverside being a hot inland area and Whittier being a milder climate with greater influence from the Pacific Ocean. Regional differences in correlations of personal endotoxin with both personal and ambient temperature,grow tables 4×8 as well as regional differences in indoor/outdoor endotoxin ratios may have resulted from this regional difference in weather and major local sources . However, we cannot rule out unmeasured differences in the homes of subjects between these two regions or differences in other micro-environments of the subjects not evaluated such as schools. We did observe a positive linear relation between outdoor and indoor home endotoxin that was small but significant and similar between the two regions .
Regional differences in airborne endotoxin concentrations across indoor and outdoor sites were also found in a comprehensive study in Fresno, California, which is a city located in the San Joaquin Valley farming region. Authors found spatial variation in endotoxin was moderately explained by proximity to cropland, pasture land, and confined animal-feeding operations. It is notable in this regard that Riverside, which had higher personal, outdoor home, and ambient endotoxin concentrations than Whittier, is nearer to farmland and confined animal-feeding operations. We observed small positive correlations of personal endotoxin with traffic-related air pollutants especially in Whittier, which has a greater impact of local traffic. Total personal and ambient PM2.5 mass showed small inverse correlations with personal endotoxin. No significant correlation between personal PM2.5 mass and personal endotoxin was found in a study of 10 children by Rabinovitch et al. , which was a substudy of the epidemiologic investigation discussed above. In Whittier, but not Riverside, ambient endotoxin also showed small positive correlations with ambient traffic-related air pollutants , but negative correlations with ambient temperature and ozone. These observations for both personal and ambient data might be attributable to re-suspension of fine and coarse dust laden with bioaerosols along nearby roadways, which also generate higher concentrations of the traffic-related pollutants, especially during periods of air stagnation and cool temperatures. We previously reported moderate correlations between coarse PM mass and PM2.5 black carbon in the study region. This potential source of endotoxin could lead to high spatial variability in resuspended dust containing endotoxin between homes and between other locations near vs. far from busy roadways. In the Fresno study by Tager et al., investigators found only the coarse PM mass fraction was correlated with PM10 endotoxin. PM10 endotoxin was not correlated with PM2.5 mass or EC. Similarly, another study of 13 urban and suburban ambient monitoring sites in southern California found that PM10 endotoxin was correlated with PM10 mass but not PM2.5 mass, ozone or NO2.
Because we measured endotoxin only in PM2.5, we are unable to directly compare our results with either of these two studies.One limitation is that we did not measure endotoxin in the coarse PM fraction , which is enriched in endotoxin. Nevertheless, the respirable PM2.5 fraction is more relevant to lower airway dose and thus airway inflammation. Another limitation is that the number of indoor and outdoor home samples was limited to 14 of the 45 subjects, and this may have limited the power to assess relations of personal endotoxin to home endotoxin and the relation of indoor endotoxin to various fixed household and subject characteristics. This was not a limitation for the comparison of personal to ambient endotoxin where data from all 45 subjects could be used. Another limitation is that wearing the personal exposure monitor backpack may have altered subjects’ activities and potentially affected true personal endotoxin exposure levels. This is likely, for example when playing sports, which makes it impossible to safely carry the backpack. Finally, the standard measurement of endotoxin exposure in studies of chronic asthma is to utilize vacuumed house dust samples for endotoxin testing. We did not assess whether this type of measurement is representative of long term personal exposure and are unaware of any study that has evaluated this.The decline in malaria incidence and prevalence have been achieved in sub-Saharan Africa through the widespread use of anti-malarial drug therapies and scaling up of vector control interventions that primarily target malaria vectors feeding and resting indoor . Despite the observed achievements in malaria reduction, many parts of sub-Saharan Africa still suffer greatly from the disease. The recent increases in malaria transmission in many parts of sub-Saharan Africa has been partly attributed to the shifts in the mosquito biting and resting behaviours and increasing insecticide resistance in the mosquitoes. Insecticide resistance in malaria mosquitoes has been linked to target-site insensitivity, elevated levels of metabolic detoxifying enzymes,plants rack and behavioural resistance mechanisms. Metabolic enzyme detoxification and target site insensitivity are responsible for higher levels of insecticide resistance. Detoxification enzyme systems that have been reported to confer resistance include three major families of enzymes; the cytochrome P450 monooxygenases, esterases, and the Glutathione S-transferases. In western Kenya, about 80% of reported resistance genotypes are Vgsc-1014S kdr mutation, Vgsc-1014F mutations in the major vectors Anopheles gambiae s.l. mainly in An. gambiae and Anopheles arabiensis.
The malaria vector Anopheles arabiensis has been reported with increasing levels of kdr mutations. There are no reports of kdr mutation at the locus 1014 in Anopheles funestus, also an important vector in western Kenya and many parts of Africa despite having several reports of metabolic resistance. The increasing levels of insecticide resistance in malaria mosquitoes is believed to be mainly caused by scaling up of insecticidal treated nets and indiscriminate use of agro-chemicals for controlling crop pests in agriculture.Field studies in East Africa have reported increased zoophagy,feeding outdoors or early evening biting and changes in resting behaviour from indoor to outdoor. These behavioural changes might have been due to selection pressure from increased coverage of LLINs . The scale-up of LLINs in Africa has been associated with a species shift from the highly endophilic An. gambiae to the more exophilic An. arabiensis in Kenya. The intervention pressure may selectively eliminate the most susceptible species from a population leaving the less vulnerable species able to adapt to the new environment. While the majority of studies have reported the existence of insecticide resistance and the mechanisms involved, there is a paucity of detailed information on the association of insecticide resistance in malaria vectors with the observed resting behavior in the field. Malaria transmission is dependent on the propensity of malaria vectors to feed on human hosts and preference to live in close proximity to human dwellings. Given the importance of mosquito feeding and resting behaviour to the successes of malaria vector control and transmission, it is important to understand the influence of physiological resistance on the resting behaviour of malaria vectors and how the observed behaviours could impact the effectiveness of the existing front line interventions. Currently, the mechanisms underlying the observed behavioural shifts in malaria vectors are poorly known, and it may have an epidemiological consequence. In order to maintain the efficacy of insecticide-based vector control, insecticide resistance should be constantly monitored and management strategies developed and deployed. The present study attempts to answer how insecticide use and resistance influences resting behaviours and reports on the status of insecticide resistance and mechanisms involved in indoor and outdoor resting malaria vectors.The study was carried out in the lowland site of Kisian in Kisumu county and the highland site of Kimaeti in Bungoma county all in Western Kenya. These sites have high abundance of malaria mosquitoes and high level of insecticide resistance. Kimaeti has extensive tobacco cultivation visible by large farms with numerous curing kilns observed within the village in the region. In Kisian , there is sand harvesting from river beds, fishing, rice and maize farming most of which enhance mosquito breeding habitats. There is extensive use of agrochemicals on these farms which could have a potential role in the mediation of resistance to insecticides. Western Kenya experiences long rainy seasons between the months of March to June and the short rainy seasons between the months of October and November.
Resting Anopheles mosquitoes were sampled indoors and outdoors from household units. Mosquito collections were made during the long rainy season and the short rainy season of 2019. Thirty houses were randomly selected per site and resting mosquitoes collected from 06:00 to 09:00 h both indoor and outdoor resting points. For indoor resting mosquitoes, a Prokopack aspirator and mouth aspirator were employed to collect mosquitoes indoors. Briefly, collections were done by hovering the aspirator systematically over the walls up and down, under the furniture and on hanged clothing for about 1 minute per second . Outdoor collections were sampled from pit shelters dug in the ground constructed according toMuirhead-Thomson’s method, from clay pots or containers placed at least 10 meters outside of houses and from any proximal human outdoor resting points such as granaries, outdoor kitchen, under shaded places and evening outdoor human resting points. Sampled anophelines were first discriminated using morphological keys. Further species-specific identification within the An. gambiae s.l. and An. funestus s.l. was conducted using PCR. Mosquito collections were done at the beginning and at the end of the dry and rainy seasons. The samples collected were taken to the entomology laboratories at the Kenya Medical Research Institute , Center for Global Health Research for subsequent rearing, phenotypic, biochemical and molecular analyses.Blood-fed and half-gravid female Anopheles mosquitoes from both the indoor and outdoor collections were aspirated into separate labeled netted mosquito holding cages measuring 30cm × 30cm × 30cm where they were maintained at 25 ± 2˚C and relative humidity of 80 ± 4% with 12:12 hours of light and dark. They were provided with 10% sucrose solution imbibed in cotton wool. Oviposition cups were introduced into the cages for egg collection. Since all collections made were put together in similar cages, the number of mosquitoes that laid eggs was not determined.The aquatic larval stages were maintained in water 26– 27˚C and were fed on a mixture of Tetramin™ fish food and brewer’s yeast. After the four larval stages, pupae were picked and transferred into netted holding cages in small cups where the emergent adults were provided with 10% sucrose solution.First filial generation females raised from field-collected adults that were resting either indoors or outdoors, that were 3–5 -day old, were tested for susceptibility using the standard WHO tube bio-assays against discriminating doses of four insecticides selected from two classes: Pyrethroids—; and organophosphate—. For each test about 100–150 mosquitoes were used for the assay comprising 20–25 mosquitoes for each of four replicates for each of the insecticides and controls. Silicone oil-treated papers were used as a control for pyrethroid assays while olive oil was used for the malathion test. Mosquitoes were exposed for 1hour for each insecticide and the number that were knocked down recorded after every 10 mins within the 1-hour exposure period. After 1-hour exposure to the diagnostic concentrations, mosquitoes were transferred to recovery cups and maintained on 10% sucrose solution for 24 hrs.