The transmission of Mycobacterium tuberculosis in healthcare settings to both HCWs and patients is a well-documented threatening event, and is most likely to occur from unrecognized or inappropriately treated TB cases [2, 3]. Programs for the screening and treatment of LTBI cases within HCWs, combined with other interventions aimed at reducing the risk of nosocomial transmission, represent fundamental tools of TB control programs and are strongly recommended in many countries, including Italy [8, 9], where an annual TB incidence of 4.9 per 100,000 population was estimated in 2010 .
A recent systematic review reported that the median estimated annual risk of LTBI among HCWs was 2.9% in low-incidence countries, against an estimated risk of 7.2% in countries with a high TB incidence .
Despite healthcare students involved in clinical training may run risks of being exposed to Mycobacterium tuberculosis similar to HCWs within the hospital setting, very few studies have addressed this issue. To the best of our knowledge, this is the first study that investigated, using a large sample, both the prevalence of LTBI and the main risk factors associated with TST positivity in a cohort of students, either attending or not the hospital wards, trained at a large teaching hospital located in a low TB incidence area.
Studies performed in high incidence countries have reported LTBI prevalence figures ranging from 9.2% to 72% among healthcare students [11, 14, 16, 17]: TST was used for the diagnosis of LTBI in these surveys.
Our sample was characterized by a very low prevalence of TST positive cases among students (1.4%). The diagnosis of LTBI was confirmed by IGRA testing (0.5%), thus reducing the potential occurrence of “false positive” cases due to exposure to atypical mycobacteria or BCG vaccination. In a German study, performed between 2005 and 2009, none of the 110 trainees or young healthcare professionals screened with an IGRA testing showed positive results . This result, obtained in a country with a low incidence of TB, clearly confirms our findings.
No difference with respect to the prevalence of TST positivity emerged when comparing pre-clinical with clinical students: the data show a very low prevalence of LTBI among students during clinical training, in accordance with the data observed among healthcare students at the beginning of the training period in various occupations (nurses, medical students, physical therapists) by other authors in Europe . Our findings greatly diverged from the results obtained in high-to-intermediate TB incidence areas, which reported an increased risk of Mycobacterium tuberculosis infection in senior medical and nursing students ranging between 2.2 and 3.8 [14, 16].
The low incidence of TB in our hospital (11 cases of infectious TB patients hospitalized in 2012), the usual precautions taken to avoid the exposure of students to known infectious TB cases during the training activities, and recommendations on the proper use of individual protection devices and measures at our hospital may explain the observed results.
BCG immunization was rarely recorded in the study sample, consistently with the current Italian guidelines for TB prevention that recommend vaccination of HCWs and students only in selected cases, based on risk-assessment at hospital level (i.e., individuals unavoidable exposure to highly contagious multidrug-resistant TB cases and individuals with contraindications to LTBI preventive treatment) . BCG immunization was not associated with a positive TST result: this lack of association has previously been reported also by other authors .
The risk assessment analysis clearly demonstrated that coming from a geographical area with a high TB incidence was actually a major risk factor for TST positivity among healthcare students (adjusted OR = 102.80). An association between foreign birth and LTBI has also been observed by other authors in Europe [24, 26]. The migration of students from low- and middle-income countries to high-income countries is part of the relatively recent globalization phenomenon that is expected to increase in the near future. Our findings highlight the need to design and implement effective TB infection control programs specifically for these students in the healthcare facilities of Western countries .
The main limit of our survey was the cross-sectional study design. For this reason, changes over time could not be monitored. Moreover, a single-step TST procedure was used, although IGRA testing was systematically carried out in the event of TST positivity, thus increasing the specificity of the confirmed diagnosis of LTBI. Another limit involved the difficulty to obtain adequate information concerning the time spent by the students in the hospital before being tested and their specific exposure to confirmed cases of infectious TB, both at professional and at community level (family, social activities, etc.). Additionally, a lack of demographic and epidemiological information concerning the students who refused to enter the survey existed. A further selection bias of the study population with respect to the attendance of hospital wards, between medical (clinical) and nursing/midwifery (pre-clinical) students, prevented any specific risk assessment for TB infection in the different healthcare schools.