Influenza B virus outbreak at a religious residential school for boys in Northern Bangladesh, 2011

Background National media reported a febrile illness among dormitory residents of a boys' religious school. We investigated the outbreak to identify cause. Methods Individuals with fever (>100°F) and cough or sore throat between 1 and 13 August 2011 were influenza‐like‐illness (ILI) case‐patients. We collected histories and specimens from hospitalized case‐patients and visited campus to explore environmental context. Results All 28 case‐patients were dormitory residents including 27 hospitalizations. Accommodation space per resident was <0.8 square metres. Nasal and oropharyngeal swabs from 22 case‐patients were positive for influenza B virus using real‐time reverse transcription polymerase chain reaction (rRT‐PCR). Conclusions Overcrowding likely facilitated transmission leading to this dormitory outbreak.


| Epidemiological investigation
Local health officials visited the school, examined affected patients and established an acute respiratory illness outbreak. The collaborative team investigated during 10-13 August 2011. Based on preliminary information, the team suspected that this outbreak was caused by a respiratory virus, particularly influenza. The team identified and enlisted ILI case-patients defined as individuals from the school and surrounding households with measured/reported fever (>100°F) and cough or sore throat from 1 to 13 August 2011. A pre-tested, This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. structured questionnaire was used to collect clinical and exposure histories from admitted case-patients aged >10 years, but for children <10 years, the mother or a legal guardian was also asked to assist with responses. Team physicians examined case-patients to note clinical features, reviewed medical records including chest radiographs to identify airspace opacity or consolidation, and assessed treatment plans to detect potential gaps in management.
Given that the majority of the non-residential students resided in nearby households, the team conducted a door-to-door search in 50 households located within 15 minutes of walking distance from the school to identify ILI among non-residential students. Local healthcare workers visited the school daily to follow-up case-patients' clinical progress and identify additional case-patients. We also followed up case-patients, family caregivers, school authorities and healthcare workers over telephone every alternate day until 21 August 2011 as influenza-related complications usually occurred within 2 weeks of illness onset. 2

| Anthropological investigations
In Bangladesh, influenza outbreaks were investigated using a multidisciplinary approach that combined synergies from quantitative and qualitative paradigms. 4 Team anthropologists visited the campus, conducted in-depth interviews and open discussions with admitted case-patients, unaffected students, parents, teachers, community residents and healthcare providers to explore potential exposures including fluctuations in environmental temperature or rainfall, strategies for handling sick students in school and community perceptions and response.

| Data analysis
We analysed the quantitative data to describe the outbreak in terms of person, place and time. We calculated ILI attack rate in the school by dividing the number of case-patients with the total number of students and teachers. Similarly, we calculated the attack rate among dormitory residents. We reviewed the transcribed qualitative data to identify common themes, developed code list and summarized coded data according to objectives using principles of thematic content analysis. 5

| Ethical considerations
We sought verbal informed consent from the principal, all participants aged above 18 years, and local guardians of child participants, if available. Children at least 7 years of age were also asked to provide verbal assent. This investigation was approved by and undertaken jointly by icddr,b and the Ministry of Health and Family Welfare of the Government of Bangladesh.

| Description of the Islamic residential school
The residential (n=55) and non-residential students (n=29), three teachers and principal in the school were all males. The school building with two rooms, each 6.7 m long and 3.3 m wide, served both as class rooms and as the dormitory after class.

| Descriptive epidemiology
All 28 case-patients were dormitory residents. Among these, 27 were hospitalized. The index case-patient, a 16-year-old residential student, developed low-grade fever, headache, mild cough and runny Number of case-patients runny nose (52%), sore throat (33%) and respiratory distress (11%) ( Table 1). New case-patients were neither identified from community search nor reported by local health team after 9 August.

| Community perceptions and response
The healthcare manager, community residents and a teacher believed that the illness resulted from extreme heat followed by week-long heavy rainfall before onset. Parents (6/10) believed that the nutrientpoor diet (including rice, potatoes, lentils and boiled egg once a week) provided to the fasting students reduced their immunity.
After recognizing simultaneous illnesses in 25 students, the affected residents were immediately segregated into one dormitory room. The anxious media and the affected community demanded assistance. Health officials responded immediately and hospitalized all sick children.

| DISCUSSION
The sudden onset of ILI predominantly affecting dormitory student residents of a religious school signalled an outbreak; the detection of influenza B viral RNA in all collected samples confirmed it as the likely aetiologic agent. Influenza B is one of the most common circulating mammalian influenza viruses worldwide, frequently affecting school children. 6 While the virus is known to circulate annually at low levels in Bangladesh, 7 this outbreak where 47% of the residents reported ILI in less than a week, represented explosive transmission in the dormitory.
Less than one square metre of accommodation space per dormitory resident represented remarkable overcrowding. Changes in student behaviour during Ramadan and rainfall to stay indoors likely facilitated person-to-person transmission. 8 The small windows limiting natural cross-ventilation 9 and restricted sunlight entry preventing viral inactivation by ultraviolet solar irradiation further increased scope of airborne transmission. 10 Moreover, rainfall preceding outbreak onset likely increased environmental viral circulation. 11 Children are more susceptible to influenza viruses than adults due to their limited lifetime exposures to annually circulating strains. 1 The diurnal fasting could have further increased students' susceptibility to infection as intermittent calorie restriction in adults has been observed to suppress proinflammatory cytokine expression and lower circulating leucocyte counts. 12 We identified this outbreak from monitoring mass media reports.
The recognition of ILI in several dormitory residents simultaneously after awakening from sleep generated a sensational story that led to mass media reports instead of notification from the conventional public health channels. Even though the respiratory pathogen we identified was not a potential public health emergency, reporting of