Despite a decline in its global prevalence, tuberculosis (TB) still causes more deaths than other infectious diseases  . In 2015, 10.4 million new cases of TB and 1.4 million deaths from TB were reported globally  . Twenty countries, including Tanzania, contribute 84% of total global TB cases  .
About one third of the world’s population is infected with the causative agent of TB, Mycobacterium tuberculosis (MTB)  , but most show no signs or symptoms of active disease, i.e., latent TB infection (LTBI)  . Approximately 10% of those with LTBI will progress to active TB at some point in their lives   . Infant BCG immunization is >70% effective in preventing TB disease for 10 - 15 years   , after which the level of protection begins to wane   . In Tanzania, where TB is endemic, and 91% of all children <12 months of age have received the Bacillus Calmette-Guerin (BCG) vaccine against TB, the annual incidence of TB disease in 2015 was 306/100,000, including 18,000 children <14 years of age   .
Data on the prevalence of LTBI and risk factors for LTBI in Tanzania are more limited, and are not available for adolescents. This is true despite the high incidence of TB disease among children <14 years old in Tanzania  , the waning efficacy of BCG vaccine in adolescence   , the adolescents’ increased outdoor activities  , 43.9% of country population in 2012 being children ≤15 years  , and the rapid increase in urban populations in Tanzania  .
The T spot interferon-γ release assay (IGRA) is an advanced TB diagnostic tool, and is more specific than the tuberculin skin testing (TST) in identifying LTBI among BCG-vaccinated individual   . Risk factors for LTBI reported from other settings include household contact with infectious TB   , overcrowding and poor ventilation within households   , HIV infection  , underweight  and high TB incidence  . Most of these studies have been conducted in settings where BCG immunization is routine and have used the TST to identify LTBI.
In this study, we used IGRA as the most specific approach to detect LTBI in adolescents at school, who had received BCG at birth. Our objectives were to obtain an accurate estimate of the prevalence of LTBI in adolescents, and to identify risk factors for LTBI with a focus on both the home environment and social encounters outside the home.
2.1. Study Procedure
This study was conducted between April 6 and September 23, 2016, in Dar es Salaam, Tanzania, with a population of 5,465,420 as of 2016  . This was part of a larger phase 2b, randomised, placebo-controlled, double-blind study on the safety and efficacy of a three-dose series of DAR-901 an inactivated whole cell vaccine for preventing MTB infection among BCG-vaccinated adolescents    . Written informed consent and assent from parents/guardians and study subjects were obtained. Anthropometric measurements and phlebotomy were performed for IGRA, and subjects completed a questionnaire.
2.2. Study Subjects
Subjects were recruited from 16 secondary schools chosen from a list of 139 public non-boarding secondary schools within the Dar es Salaam region  . These 16 schools were selected by simple random selection, evaluation of accessibility to a study clinic and willingness of the school administration to participate in the study. Based on the LTBI prevalence rate of 41.7% among adolescents 11 - 15 years old reported in South Africa  , and the difference in reported incidences of TB in Tanzania and South Africa, with 5% type I error and 5% absolute error, we calculated that our study required a sample size of 571.
Initial screening for the study was performed after obtaining written informed consent and assent from a total of 936 subjects. The inclusion criteria were: age between 13 and 15 years, attendance at a public non-boarding secondary school, evidence of BCG vaccination (either the presence of a scar or documentation) and the absence of signs of active TB or symptoms suggesting active TB disease based on the Tanzanian TB screening questionnaire  . Data from 824 subjects that fulfilled the inclusion criteria were analysed. Those with signs and/or symptoms of active TB were referred to specialised TB clinics under the National Tuberculosis and Leprosy Program (NTLP).
2.3. Anthropometric Measurements
Body mass index (BMI) (kg/m2) was calculated based on height and weight measurements. BMI-for-age percentiles for both genders, obtained using the Centres for Disease Control (CDC) growth charts  , were categorised as follows: <5%, underweight; 5% - 85%, healthy; 85% - 95%, overweight; >95%, obese.
2.4. Interferon-γ Release Assay (IGRA)
The IGRA was performed using the T-SPOT.TB (Oxford Immunotec, Oxford, UK) assay according to the manufacturer’s instructions. Venous blood samples were analysed at the Tuberculosis Research Institute at Muhimbili University of Health and Allied Sciences (TRIM-TB) in Dar es Salaam, Tanzania  . The results were interpreted as positive, negative, borderline or indeterminate according to the published guidelines  . Adolescents with positive results were administered the Tanzanian TB screening questionnaire  and referred to specialised NTLP TB treatment clinics if signs or symptoms consistent with active TB were present  .
2.5. A TB Risk Factor Questionnaire
A questionnaire was developed at the Department of Global Health Entrepreneurship of Tokyo Medical and Dental University, and included questions about prior active TB, contact with TB patients, social activity patterns, living environment and household socioeconomic characteristics. The original English version questionnaire was translated into Swahili, which was then back translated to English for confirmation.
After receiving instructions on how to complete the questionnaire, the adolescents were allowed to take the questionnaire home to answer all questions with the help of their parents/guardians. At the time of submission, nurses inspected the completed questionnaires to confirm that they had been filled out correctly. To ensure consistency in instruction and interviews, all nurses involved in the study received a full day of training before initiation of the study.
We obtained the subject’s past history of diagnosis and treatment for active TB disease, household contact with a TB patient, knowledge of a neighbour that had TB, and knowledge of a fellow student diagnosed with TB.
To understand the social activity patterns of the adolescents, the questionnaire inquired about the means of transport used to attend school, encounters with traditional alcoholic beverage drinkers, visits to market places and walking distance between the house and the nearest health facility. All these activities increase risks for contact with active TB patients.
In relation to the living environment characteristics, the number of living rooms and bedrooms, brightness from sunlight in living rooms and bedrooms and number of older family members living in the same house were determined.
The education levels of the father and mother, employment of father and household wealth status evaluated according to the Demographic and Health Survey (DHS) guidelines   were used.
2.6. Statistical Analysis
Statistical analyses were performed using SPSS Statistics v22.0 (IBM Corp., Armonk, NY, USA). Percentage distributions according to IGRA results were calculated for 824 subjects. After excluding 20 subjects with indeterminate and borderline results, the percentages of IGRA-positive cases according to their characteristics were evaluated using cross-tabulation with the χ2 test. After excluding 75 subjects with prior history of TB diagnosis (23 who responded “Yes” and 52 who responded “I do not remember”), logistic regression analysis to examine the association between IGRA positivity and characteristics of the subjects were performed with three models: 1) crude; 2) adjusted by wealth, history of contact with TB patients in their households and in the community; and 3) by wealth, history of contact with TB patients in their households, in the community, living conditions and nutritional status. All missing data were excluded from multiple logistic regression analyses models. In all analyses, P < 0.05 indicated statistical significance.
2.7. Ethical Consideration
This study was approved by the Institution Review Boards of the Medical Schools of Tokyo Medical and Dental University, Japan; Muhimbili University of Health and Allied Sciences, Tanzania; and Geisel School of Medicine at Dartmouth, NH, USA. Permission to recruit secondary school students was obtained from the Ministry of Health, Community Development, Gender, Elderly and Children, the Ministry of Education and Vocational Training, and the President’s Office for Regional Administration and Local Government of Tanzania.
Table 1 shows the results of IGRA; 18.1% of the 824 subjects were positive and 79.5% were negative.
Table 2 shows the number and percentage of the IGRA-positive cases according to the characteristics for selected variables showing statistically significant associations with the IGRA results.
Table 3 shows the associations between IGRA status and characteristics of 729 adolescents with no history of prior TB, before and after adjusting for other variables. The rate of IGRA positivity was higher in subjects with than without frequent encounters with traditional alcohol beverage drinkers (AOR, 6.37 [1.84 - 22.00]), and those residing in houses within a 10 - 30-minute walking distance to the nearest health facility were less likely to be IGRA-positive than those living
Table 1. IGRA results among adolescents (n = 824).
n, number of subjects; %, percentage.
Table 2. IGRA-positive cases according to adolescents’ characteristics (n = 804, after excluding subjects with indeterminate or borderline test results).
TB, Tuberculosis; N, number of subjects after excluding those with borderline and indeterminate IGRA results; %, percentage; n, number of subjects with positive IGRA results; P, P-value; ≥, more or equal to; ≤, less or equal to; <, less than; >, more than.
Table 3. Association between IGRA positivity and characteristics of adolescents before and after adjustment for socioeconomic status, household and community TB contact, living conditions and nutritional status.
COR, crude odds ratio; AOR, adjusted odds ratio; P, P-value; Ref, reference; 95%CI, 95% Confidence Interval; ≥, more or equal to; ≤, less or equal to; <, less than; >, more than. aUnadjusted model; bModel adjusted for wealth index, household and community tuberculosis contact; cModel adjusted for wealth index, household and community tuberculosis contact, number of living rooms, brightness in the living room and bedroom as well as BMI percentiles.
within a 10-minute walking distance (AOR, 0.30 [95%CI, 0.13 - 0.69]). Significant associations were also observed between IGRA positivity and living in a house with one living room, living in a house with four or more bedrooms, contact with a TB patient at school and wealth status.
This is the first study to use the IGRA to examine the prevalence of LTBI among in-school adolescents in Tanzania. Encounters with traditional alcoholic beverage drinkers and living environments were identified as risk factors for LTBI, in addition to household socioeconomic status and history of contact with TB patients in the household and community.
The LTBI prevalence rate of 18.1% identified by IGRA among Tanzanian adolescents 13 - 15 years old is lower than the prevalence rate of 41.7% reported for those 11 - 15 years old in Cape Town, South Africa  , and higher than the rate of 3.4% - 6.2% reported for those 10 - 18 years old in Shanghai, China  . These results reflect the order of reported incidence of smear-positive TB, including HIV-positive subjects, per 100,000 population (Tanzania, 306; South Africa; 834; and China, 67)  .
Because most Tanzanian adolescents have received BCG  , the IGRA assay used in the present study provides a more accurate rate of LTBI in this population than the less-specific TST   .
LTBI was associated with a history of household contact with a TB patient, but this relationship disappeared after excluding subjects with history of prior TB. These findings suggested most subjects who had lived in the same household as a TB patient, might have later been diagnosed with active TB themselves, as postulated by the WHO  . There was also a significant association between LTBI and exposure to MTB in schools, a similar finding to a study conducted in South Africa township which concluded most MTB transmission among adolescents occurs outside of the household, particularly in school  .
Regarding exposure in the community, the risk of acquiring LTBI was higher among adolescents with than without a history of encounters with traditional alcoholic beverage drinkers. This observation indicated a role of informal gatherings within communities in the spread of MTB. Traditional alcoholic beverage drinking is usually done at gatherings that are associated with ideal environments for MTB transmission like small, overcrowded and poor ventilated traditional bars and contact with drinkers who are active TB patients. These drinkers in turn spread MTB to adolescents with whom they interact informally within the community  . This supports the findings from the Malawi study, which used whole-genome sequencing to prove >50% of people with MTB had acquired the bacterium from sources other than a TB patient within their households  .
Living in a house with four or more bedrooms was shown to increase the risk of LTBI, in contrast to previous studies   . This may have been partly due to the Tanzania’s extended family tradition, whereby those residing in urban areas and in houses with many bedrooms tend to host relatives migrating from rural areas in search of TB treatment services in urban Dar es Salaam   . This could result in exposure and transmission of MTB to adolescents living in the host house.
This study also revealed an association between residing in a house located closest to a health facility and IGRA positivity. Residing close to a health facility might increase adolescents’ risks for contact with active TB patients who attend those facilities for TB treatment. Consistent with findings from previous studies   , these results showed the duration of contact with MTB within a shared environment increases the risk of acquiring the bacterium, thus indicating a significant contribution of environmental exposure to MTB to its transmission   .
Living in a house with two living rooms, compared to one living room, was associated with a lower risk of LTBI. This observation point to an increase in LTBI risk associated with poor ventilation inside the living rooms, supporting reports that overcrowding and poor ventilation increase the risk of acquiring MTB   . Our study also showed poor socioeconomic status increased the risk of acquiring LTBI, consistent with other studies   .
Our study had notable strengths including being the first study to use IGRA, rather than TST, to identify LTBI among BCG-vaccinated adolescents in Tanzania. Using IGRA also provided logistic advantages as it does not require a subsequent visit 72 hours from inoculation to interpret the test results, hence convenient and contributes to a lower loss-to-follow-up rate than TST. The TB risk factor questionnaire used ensured ability to identify local LTBI risk factors based on locally available evidence. Also, overall recruitment challenges and participation bias were minimized by avoiding mandatory HIV testing among participants as the test, if were mandatory, would have limited voluntary participation to avoid stigma and discrimination since HIV/AIDS is prevalent in Tanzania  . Although HIV/AIDS infection is known to increase the risk of TB  , this study population was expected to have a low prevalence of HIV of <6%, as was observed among adolescents 10 - 19 years old in 2014  . Further, free antiretroviral therapy (ART) for HIV-infected children has been available in Tanzania since 2003  , hence subjects with HIV on ART would have been identified since all subjects were asked about concomitant medications.
However, our study also had some limitations including omission of a small number of subjects with missing data while conducting the logistic regression analysis, but since their numbers were small, omitting them did not affect our results. In addition, being cross-sectional in nature, it failed to establish causality assumption. Moreover, our study excluded out-of-school adolescents, which limited generalizability of the findings to all adolescents in Tanzania.
This is the first study to use the IGRA to examine LTBI prevalence among in-school adolescents in Tanzania. Informal social encounters were identified as independent risk factors for LTBI, along with a history of contact with TB patients, living environment characteristics and household socioeconomic status. Efforts focusing on risk of MTB transmission in adolescents at informal social gatherings will improve interventions to reduce LTBI in this population and consequently the subsequent risk of developing active TB disease.
The authors gratefully acknowledge the assistance from Dr. Masashi Kizuki of Tokyo Medical and Dental University in designing the questionnaire. They also thank Dar-PIA team members at Dar-PIA Clinic in Dar es Salaam, Tanzania (Dr. Albert Katana and Dr. Maryam Amour for coordinating meetings at the Ministry of Education and Vocational Training (MoEVT), Ministry of Health, Community Development, Gender, Elderly and Children (MoHCDEC) and President’s Office-Regional Administration and Local Government (PO-RALG) as well as organising meetings with schools’ administration, Dr. Suleiman Chum, Sr. Mary Ngatoluwa, Sr. Asha Swaleh and Mr. Chijano Makunenge for collecting data and Ms. Betty Mchaki for conducting IGRA tests.
The Global Health Innovative Technology Fund (G2015-147) and Japan Society for Promotion of Science (17H02164) supported this work. The funders had no role in study design data collection and analysis, decision to publish, or preparation of the manuscript.
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