Environmental Engineering Reference
In-Depth Information
located 5-8 km from its shore (Shakamsa, Sombo, and Yebo) were randomly selected
and designated as “at-risk” and “control” villages, respectively. The selection of “at-
risk” and “control” villages was based on the established flight range of anopheline
vector mosquitoes as described elsewhere in this chapter [27-29]. The 1,855 children
(1,081 and 774 from “at-risk” and “control” villages, respectively) who had lived for
at least 6 months in those selected villages were included in the study. Bed net distri-
bution was not started in the study villages until the end of this study but there was
malaria control activity through indoor residual spraying, using DDT, and malathion,
which stopped 4 months prior to the study in both villages.
Parasitological Investigation
A parasitological study was carried out for 3 months (October-December 2005) to
investigate the difference in malaria prevalence between “at-risk” and “control” vil-
lages and to characterize malaria in the area. During the survey, socio-demographic
data were collected and house-to-house visits were made each month to collect blood
samples from every child less than 10 years of age and thick and thin films were
prepared directly from finger prick blood samples. Blood sample collection, prepara-
tion, staining technique and microscopic identification of Plasmodium species were
performed as per standard methods [30]. The thick film served to confirm the pres-
ence or absence of the parasite, whereas the thin film was to identify the Plasmodium
species. The initial thick films were considered negative if no parasites were seen in
at least 100 oil-immersion fields of the thick film [31]. For positive slides, species,
and presence or absence of gametocytes was recorded. All blood films were initially
read on site or at Omo-Nada District Health Center Laboratory by trained laboratory
technicians. Films positive for parasites and a 10% sample of films negative for para-
sites were subsequently re-examined by an independent senior technician at Jimma
University Specialized Hospital Laboratory. The senior microscopist was blinded to
the previous microscopy results. The parasite density was counted per 300 leukocytes
and was then expressed as number of parasites per microliter by assuming an average
leukocyte concentration of 8,000 leukocytes/μl [32]. All Plasmodium positive children
were treated according to the national malaria treatment guideline of the Government
of Ethiopia [33].
Statistical Methods
Data were entered in and analyzed with the statistical program STATA 10 software
package (StataCorp, Texas, USA). Prevalence rates were calculated from monthly
positive cases. The prevalence of Plasmodium falciparum and Plasmodium vivax was
calculated across age, village of residence and month of infection. Logistic regressions
were conducted to check for any significant differences in the proportions of malaria
cases between “at-risk” and “control” communities both in a univariate manner and
controlling for age, sex, and month. The clustering at village level was taken into
account in the logistic regression models (univariate as well as multivariate) by us-
ing a marginal model with the Taylor series linearization method for estimating the
variances.
 
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