Unmatched case control study definition


  1. Original Research ARTICLE
  2. Academic Bibliography
  3. Chapter 9. Case–Control Studies | Medical Epidemiology, 4e | AccessMedicine | McGraw-Hill Medical

Vibrio cholerae was the cause of the outbreak in Gomani. Drinking water from Zamani river, living in overcrowded HH and poor hand hygiene were significantly associated with the outbreak. We initiated hand hygiene and water treatment to control the outbreak. Cholera remains a global threat to public health and a key indicator of lack of social development. Cholera, an acute diarrheal disease caused by gram-negative bacillus Vibrio cholerae of serogroup 01 and is associated with high morbidity and mortality [ 1 , 2 , 3 ]. Asymptomatic cases shed vibrios in their stools and serve as a potential source of infection to others.

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Symptomatic patients may also shed vibrios before the onset of illness and will continue to shed the organisms for about 1 to 2 weeks [ 1 , 3 , 4 ]. Cholera is transmitted through the fecal-oral route via contaminated food, carriers and unsanitary environmental conditions. Cholera outbreaks tend to occur as a result of contamination of food or water with Vibrio cholera organisms due to poor personal hygiene, unsafe environmental sanitation conditions compounded by lack of potable water supply. Internal displacement of persons by natural and man-made disasters leading to unstable living conditions with contamination of food and water sources have also been reported to cause cholera outbreaks [ 5 , 6 , 7 ].

Globally an estimated 3 to 5 million cholera cases and 28, to , deaths occur yearly. However, the infection is common to developing countries in the tropics and subtropics with high human poverty index [ 2 , 8 , 9 ]. In Africa, there have been recurrent cholera outbreaks, characterized by a large disease burden and high case-fatality rates. The discrepancy between the reported figures and the estimated burden of the disease could be ascribed to poor surveillance and laboratory systems.

Political motives such as fear of trade and travel sanctions have also been implicated [ 8 , 9 ]. Nigeria is reported to be one of the three major current cholera foci in the world [ 13 ]. The first series of cholera outbreaks in Nigeria were reported between to , subsequently recurrent outbreaks followed [ 14 ].

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In , Nigeria reported a total of 41, cases with deaths from 18 northern states with case fatality rate [CFR] of 4. The outbreak was attributed mainly to contamination of water supplies with diarrhoea discharge of untreated cholera patients during the rainy season. This therefore, brought to the focus the vulnerability of Nigerian rural communities [ 1 ].

An outbreak response team was immediately mobilized and deployed to Gomani settlement. The team investigated the outbreak with the objectives of verifying the diagnosis, identifying risk factors and instituting appropriate control measures to control the outbreak. We conducted an unmatched case control study to identify potential risk factors of the outbreak. We conducted a laboratory analysis of stool and water samples from the community and instituted appropriate control measures. Gomani has an estimated population of about people. The main economic activities amongst Gomani residents are farming, petty trading, and fishing.

The Gurara and Zamani rivers serve as the major sources of drinking for most Gomani residents. Gomani settlement has one primary health center PHC. Study participants constituted of 48 recently identified cholera cases and 68 community controls identified in in Gomani settlement Kwali area council, FCT. Cases and controls were recruited into the study using the following definitions. A community active case search was conducted, and all cases meeting case definition were recruited.

Information on age, sex, residence, date of onset of illness, signs, symptoms and outcome were obtained from cases and used to generate hypotheses about potential exposures that were common to the cases. In households with multiple cases, all cases were recruited in the study. Controls were systematically recruited in the community.

Original Research ARTICLE

Starting from households that reported cases, we visited the second household to the right of the case household. In the selected households all members of the household were listed, and 1 member selected randomly as a control. Only 1 control was selected even for case households with more than 1 case. Interviews excluded household members who had reported a history of vomiting and diarrhea. However, 43 cases and 68 controls met the inclusion criteria and were recruited for the study. Interviewers administered a structured questionnaire to cases and controls in English and Gbaygi languages.

The questionnaire captured socio-demographic information, clinical information for cases , risk factors, and knowledge, attitude, and practice on cholera. We collected stool samples from four cases, and we tested the samples using cholera rapid diagnostic test kits. We collected water samples from Zamani and Gurara River.

We inspected Gomani community source of water supply which is principally Gurara and Zamani rivers. Activities and practices along these water bodies were noted. Drinking water storage facilities and waste management in the homes were also inspected. We conducted univariate and bivariate analysis using Epi Info 7.

We characterized the data in person, place and time. We calculated cholera incidence by age and sex. The outbreak timeline was summarized as an epidemic curve. Informed oral consent were obtained from the participants before the interviews because the outbreak was in rural setting and most respondents were uneducated and unable to read and write. Confidentiality of the respondents were ensured through data coding. Permission was obtained from the department of public health Kwali Local Government Area LGA during the response and preliminary report of the outbreak was discussed with Gomani community leader and Kwali LGA public health team.

All the four stool specimens tested positive for V. Gurara River was found to be macroscopically clean with high current flow, unlike the stagnant Zamani river polluted due to, mass bathing, washing of clothes and indiscriminate defecation in and around the river bank. The community had only three non-functional boreholes at the time of the study. Waste management was found to be poor with refuge heaps littered around houses.

Epidemiologic and environmental evidence indicated that the cholera outbreak resulted from drinking water from Zamani River. Poor personal hygiene and overcrowding were also identified as major risk factors for acquiring the disease. These findings agree with similar studies in Nigeria [ 2 , 10 ]. However, many cholera outbreaks in Nigeria are not epidemiologically investigated to identify risk factors for the illness [ 16 , 17 ] and therefore control measures are empiric without addressing specific risk factors associated with the outbreak.

The epidemic curve suggested a common source; household contact could be an underlying factor for community-wide transmission of this outbreak. Deaths occurred earlier in the outbreak before response activities were instituted. Response activities would have been more effective in preventing cases and death if it was instituted timely. However, timely response is dependent on timely notification and confirmation of the outbreak hence the need for a more sensitive community-based reporting of public health events.


Washing of hands with soap and water before eating a meal was found to be a protective factor in our study. Likewise, the — cholera outbreaks in Kano state were also attributed to not washing hands with soap before eating food [ 21 ]. This indicates that risk communication gaps still exists. Significant association of cholera infection with overcrowding in our study is concomitant with this fact; moreover, our environmental assessment revealed poor environmental sanitation infrastructure like indiscriminate defecation in the environment due to lack of toilet facilities and improper waste management, these are conditions highly correlated with poverty and low socio-economic status.

Cholera exists as a seasonal disease in most countries [ 24 ]. In Nigeria, cholera infections have been reported in both rainy and dry seasons, although the burden of cholera tends to increase during beginning of rainy and dry seasons [ 1 , 25 ]. The Gomani cholera outbreak occurred during the dry season, similar to the pattern observed in Calabar, South-southern part of Nigeria, where cholera outbreaks mostly occurred during the dry season [ 26 ].

This could be attributed to scarcity of potable water during the dry season and therefore the tendency of people to obtain drinking and cooking water from alternative sources with higher risk of contamination which includes stagnant water bodies. Our study highlighted these findings in that cases were likely to obtain drinking water from stagnant Zamani river due to its proximity as opposed to Gurara river [ 1 ]. Control of cholera outbreaks requires effective surveillance and response systems which are often sub-optimal in developing countries and therefore this study accentuates the need for an effective surveillance system with the capacity to appropriately detect and contain cholera outbreaks timely [ 2 , 27 ].

Academic Bibliography

The long-term solution for cholera control lies in economic development through universal access to safe drinking water and adequate sanitation [ 28 , 29 ]. Crucial cholera epidemic preventive mechanism remains providing a waste management system that separates waste from the water supply [ 30 ]. Oral cholera vaccines OCV which are additional efficient tool to control cholera outbreaks are yet uncommonly used in Nigeria [ 8 ].

OCVs though not a replacement for conventional control measures like portal safe water and personal hygiene, could serve as a complementary measure [ 31 , 32 ]. In Guinea, two complete doses of cholera vaccine during an outbreak was found to be associated with significant protection against cholera with This finding serves as supporting evidence on the addition of vaccination as part of the response to cholera outbreaks and the need to plan and implement regular cholera vaccination programmes in cholera endemic countries such as Nigeria [ 33 , 34 ].

This study was also burdened with the several limitations including late notification of the outbreak which could be attributed to remote, poor access road network to Gomani settlement evidently delayed initiation of response. Nevertheless, response was commenced by the 6th of the November though not timely but mitigated the outbreak. Health care workers strike action delayed the laboratory culture investigation and contributed to the failure to isolate V. Lastly, we could not entirely rule out the possibility of misclassification of cases as controls since most cholera cases are asymptomatic.

However, we tried to minimize this selection bias by recruiting our controls from every two households to the right of the household of the cases where no member had no signs and symptoms of diarrheal disease within the study period.

Only recent cases were recruited for the study. Furthermore, confounders such as socioeconomic status and differences in age groups in the unmatched case-control study could have influenced the association found.

Chapter 9. Case–Control Studies | Medical Epidemiology, 4e | AccessMedicine | McGraw-Hill Medical

Despite these limitations, the study provided useful information to stakeholders on actions that will avert future outbreaks by provision of basic water, sanitation and hygiene infrastructures such as functional boreholes and standard pit latrines. Community risk communication and surveillance strategies need significant improvements to ensure prevention of adverse effects of diarrheal diseases in general and cholera in specific. We established that drinking water from Zamani river was the major source of the outbreak.

Poor personal hygiene and overcrowding were also identified as risk factors. On the interim health education on proper hand hygiene and chlorination of water were initiated based on our recommendation and this controlled the outbreak. Implementation of targeted interventions such as rehabilitation of existing boreholes, construction of standard pit latrines and the establishment of proper waste disposal systems are long-term sustainability measures to prevent future outbreaks.

The NFELTP is a two-year training program aimed at improving public health systems in Nigeria through training of field epidemiologists and provision of epidemiological services. Find uploaded ms. DNC, UO and OP - Conceived the study, participated in the outbreak response, data collection, advanced analysis and developed the initial and subsequent drafts of manuscript.