Outbreak area
Hoima District is located in Western Uganda and is bordered by several districts, including the DRC across Lake Albert to the west (19). Hoima District had a total population of 668,900 in 2018. (20). Kyangwali Refugee Settlement accommodates close to 83,000 refugees, most from the DRC, and is located in Hoima District (21) (Figure 1). The refugee settlement is divided into 3 blocks: Maratatu A, Maratatu B, and Maratatu C (Figure 1). The refuge settlement had three main water sources, namely: tank water, stream water and spring water. Tank water was the official water source supplied to the refugees in the camp. However, there were very long queues to the tank water all the time. Consequently, people resorted to alternate water sources (stream water, spring water and others) regardless of the safety of the water source.
Case definition and case-finding
We defined a suspected case as onset of acute watery diarrhoea in any person ≥2 years in Hoima District from 1 February to 9 May, 2018. A confirmed case was a suspected case with V. cholerae isolated from stool samples by bacteriological culture. To identify cases, we reviewed medical records from two Cholera Treatment Centres (CTCs) that were about 38km apart and conducted active community searches in the affected sub-counties to generate and update the line list. The variables captured included patient ID, sex, age, location (village, parish, sub-county), signs and symptoms, date of onset, diagnosis (clinical or lab-confirmed), nationality, refugee status, patient status (alive or dead), and drinking water sources (stream water, tank water, spring water and other sources).
Descriptive epidemiology and hypothesis generation
For descriptive epidemiology, we considered case-patients by date of onset of signs and symptoms, by person variables (age, sex and nationality), and by sub-county. We computed attack rates by sub-county, sex, nationality, and age group (2-5, 6-17, 18-30, 31-59, 60-95). To generate the hypotheses, we interviewed 14 case-patients found in the most affected sub-county (Kyangwali) as per the descriptive epidemiology findings to identify possible risk factors associated with the outbreak.
Case-control study
We conducted a case-control study by collecting data using a standard questionnaire with a focus on exposure factors obtained during hypothesis generation. In our case-control study, the sampling unit was a household. Thus, we defined a case-household as a household with one or more suspected cholera case-patients who sought care at CTCs or health posts in the camp from 23 February to 3 March 2018. A control-household was a household that did not have cholera patients during the outbreak period. During selection of case-households, we enrolled all case-households in the most affected blocks of Maratatu B and Maratatu C resulting into 73 case-households. Maratatu B and C had an estimated 4,000 households. Using systematic random sampling, we selected every 40th unaffected household in Maratatu B and Maratatu C as control-households. In total, we selected 107 control-households.
Guided by findings from hypothesis generation interviews, we administered a questionnaire to the appropriate case-households and control-households to obtain information on their water exposures. We asked the respondents where they usually collected drinking water, treatment methods, storage and how they fetched it from storage containers. If the households never drank from the known water sources, we asked where they usually obtained drinks. We also collected information on demographic variables.
We compared exposures in 73 case-households and 107 control-households, frequency-matched by residence (village/block) in Kyangwali Refugee Settlement. We then estimated the association between the exposures and outcome using Mantel-Haenszel method. We further assessed the Odds Ratio for different combinations of the three water source types available in the settlement, to identify confounding as well as competing risk factors.
Environmental assessments and laboratory investigations
We reviewed meteorological records at Hoima District and worldweatheronline.com to obtain rainfall data for Hoima District. We conducted an environmental assessment in the refugee settlement focusing on landscape and location of the households, water sources, and latrines. The refugee camp had 3 types of water sources for drinking, including stream water, spring water, and tank water. Between 23 February and 3 March 2018, we collected six water samples from different points along a stream running between villages in the refugee settlement, five water samples from water tanks in Maratatu, and five water samples from the spring water that was not a protected source (water from an underground source within Kyangwali refugee settlement) for laboratory testing for fecal contamination. We used the Most Probable Number (MPN) method using presumptive test and confirmatory test to determine the presence of fecal coliforms in the water samples (22–25).
Additionally, stool samples were collected from 15 February 2018 to 9 May 2018 from suspected case-patients in affected areas. Using swabs, stool samples were placed in Cary–Blair media, and transported within 12 hours in cool boxes to the Uganda National Health Laboratory Services (UNHLS) in Kampala for testing. All samples were first tested using cholera RDT kit (Crystal® VC dipstick, Span Diagnostics Limited, Surat, India, suboptimal sensitivity and specificity), and later tested by bacteriological culture. Only samples that tested RDT-positive were retested by culture for confirmation. The swabs were first cultured in alkaline peptone water at 37°C for 18–24 hours and then sub-cultured on Thiosulphate-Citrate-Bile-Salts Sucrose (TCBS™; EIKEN Japan) agar. Upon identification, the V. cholerae isolates were further evaluated to ascertain the serogroup and serotype by agglutination with polyvalent O1 and monospecific Ogawa and Inaba antisera (10,26).
Ethical approval and consent to participate
The investigation was authorized by Ministry of Health, Hoima District Local Government, and the Commandant of Kyangwali Refugee Settlement. This investigation was conducted in response to a public health emergency and was therefore determined by the Centers for Disease Control to be non-research. Thus, we do not need any other permission from Institutional Review Board according to the Ugandan policies and guidelines and are free to publish the work. Verbal informed consent in the local language was sought from respondents or care-takers of case-patients who were informed that their participation was voluntary and their refusal would not result in adverse consequences. All case-patients identified in the community were referred for free treatment at the cholera treatment centers. To ensure confidentiality of the respondents, each case-patient, case-household, and control household was assigned a unique identifier which was used instead of their names during data analysis.