2.1 Study area
Zanzibar is a semi-autonomous territory of Tanzania and has its own president and parliament. It has its own government ministries, including the Ministry of Health under which Zanzibar’s malaria elimination efforts are based. Zanzibar is primarily comprised of the islands of Unguja and Pemba. Unguja lies 35 km off the coast of mainland Tanzania and is 85 km long by 30 km wide with an area of 1,666 km2. Unguja island has a population of 896,721.
Zanzibar has an equatorial climate, characterized by year-round high temperatures and humidity. There are two rainy seasons: the long rains last from March to June and the short rains fall from October to December. Malaria rates peak at the end of each rainy season, with the highest number of cases identified between May and July(5). There were 154 public and 42 private facilities where malaria cases are diagnosed and reported to the system.
2.2 Study design
A matched case-control study with a ratio of 1:3 was conducted in Unguja from February through May 2017. Cases and controls were matched on household.
A case: Any individual currently living in the study area who tested positive by mRDT or microscopy with or without signs and symptoms of malaria, and with no history of malaria within the last two months.
A control: Any household member currently living with index case who tested negative by mRDT.
2.3 Study population and sample size
Study population included all people living in the study area from February to May 2017. Only households with minimum number of controls (three negatives) who are >15 years of age and able to consent and respond to questions (physically and mentally) were included in the study.
Sample size was calculated based statsDirect 3 software for matched case control. Correlation = 0.2, probability of exposure in controls = 0.48 hence net utilization for those who are malaria negative were approximately to 48% according to Zanzibar surveillance data, odds ratio = 2, controls per case subject = 3, alpha = 0.05 and power = 0.8. Estimated minimum cases required were 103 and minimum sample size was 412. All districts in Unguja Island were considered for this study. At household level simple random sampling was applied when there were more than three controls. Probability proportion for each district was calculated based on previous year data (February – May, 2016) to estimate number of cases to be included from each district. The data collection tool was pretested in cases reported in previous month (January). Thereafter the tools were revised to improve clarity before execution of the study.
2.4 Data collection process
Structured questionnaire and observation checklist were used to collect information. Research assistants were surveillance officers who are experienced with routine malaria surveillance system. They were oriented for a single day on how to use the questionnaires and observation checklist.
The questionnaires were developed in English first and then translated to Swahili language which is a National language spoken by the study population. The study participants were informed about the importance of the study before being interviewed.
The study relied on routine surveillance system in Zanzibar. Malaria case-based surveillance system is also known as malaria case notification. When a malaria case was reported at a health facility and notified through mobile phone, District Malaria surveillance officers (DMSOs) /research assistants got a message informing them that there was a case reported from a certain health facility. After receiving a message, the research assistant first went to the health facility that reported the case. He/ she obtained the necessary information including patient’s phone number, street/ village and the name of ten cell leaders which facilitated the tracking of the case’s house.
At household level, all family members were tested for malaria by using mRDT test and given antimalarial as per Zanzibar malaria guideline for positive family members. Then the interview was conducted to those households meet inclusion criteria. Research assistants interviewed both cases and controls using the same questionnaire. Observations checklists were used to document the presence and utilization of net, quality of the net, conditions of the room, sleeping place and whether the sleeping room was in close proximity with domestic animals settlement.
Malaria rapid diagnostic test used in the study was SD Bioline Malaria Ag Pf/Pv, manufactured by Standard Diagnostic, INC 156-68 Borahagal – dong, Korea. The test was carried out according to the instructions from the manufacturer.
2.5 Data management and analysis.
During data collection, daily questionnaires were counted, checked for completeness and compared with the number of participants interviewed. Data were entered, cleaned and analyzed using EPI info version 3.5.1. Cases and control were matched in household identity number. The association between dependent variable and independent variable was quantified using odds ratio.
Social demographic characteristic were presented in simple frequency tables. Social demographic variables, behavioral risk factors, environmental factors and malaria knowledge with P value < 0.05 in univariate analysis were included in bivariate analysis. Then all individual risk factors from social demographic, behavioral factors, environmental factors and malaria knowledge with P value of <0.2 at bivariate level were entered together in a conditional logistic regression model using stepwise elimination method to determine the independent risk factor and control for confounders. A p-value of <0.05 was set as a level of significance at 95% confidence interval.