Russia is one of 22 high burden tuberculosis (TB) countries. Identifying individuals, particularly health care workers (HCWs) with latent tuberculosis infection (LTBI), and determining the rate of infection, can assist TB control through chemoprophylaxis and improving institutional cross-infection strategies. The objective of the study was to estimate the prevalence and determine the relative risks and risk factors for infection, within a vertically organised TB service in a country with universal bacille Calmette-Guérin (BCG) vaccination.
Methods and Findings
We conducted a cross-sectional study to assess the prevalence of and risk factors for LTBI among unexposed students, minimally exposed medical students, primary care health providers, and TB hospital health providers in Samara, Russian Federation. We used a novel in vitro assay (for gamma-interferon [IFN-γ]) release to establish LTBI and a questionnaire to address risk factors. LTBI was seen in 40.8% (107/262) of staff and was significantly higher in doctors and nurses (39.1% [90/230]) than in students (8.7% [32/368]) (relative risk [RR] 4.5; 95% confidence interval [CI] 3.1–6.5) and in TB service versus primary health doctors and nurses: respectively 46.9% (45/96) versus 29.3% (34/116) (RR 1.6; 95% CI 1.1–2.3). There was a gradient of LTBI, proportional to exposure, in medical students, primary health care providers, and TB doctors: respectively, 10.1% (24/238), 25.5% (14/55), and 55% (22/40). LTBI was also high in TB laboratory workers: 11/18 (61.1%).
IFN-γ assays have a useful role in screening HCWs with a high risk of LTBI and who are BCG vaccinated. TB HCWs were at significantly higher risk of having LTBI. Larger cohort studies are needed to evaluate the individual risks of active TB development in positive individuals and the effectiveness of preventive therapy based on IFN-γ test results.
Gamma-interferon assays were used in a cross-sectional study of Russian health care workers and found high rates of latent tuberculosis infection.
Tuberculosis (TB) is a very common and life-threatening infection caused by a bacterium, Mycobacterium tuberculosis, which is carried by about a third of the world's population. Many people who are infected do not develop the symptoms of disease; this is called “latent infection.” However, it is important to detect latent infection among people in high-risk communities, in order to prevent infected people from developing active disease, and therefore also reduce the spread of TB within the community. 22 countries account for 80% of the world's active TB, and Russia is one of these. Health care workers are particularly at risk for developing active TB disease in Russia, but the extent of latent infection is not known. In order to design appropriate measures for controlling TB in Russia, it is important to know how common latent infection is among health care workers, as well as other members of the community.
Why Was This Study Done?
The researchers here had been studying the spread of tuberculosis in Samara City in southeastern Russia, where the rate of TB disease among health care workers was very high; in 2004 the number of TB cases among health care workers on TB wards was over ten times that in the general population. There was also no information available on the rates of latent TB infection among health care workers in Samara City. The researchers therefore wanted to work out what proportion of health care workers in Samara City had latent TB infection, and particularly to compare groups whom they thought would be at different levels of risk (students, clinicians outside of TB wards, clinicians on TB wards, etc.). Finally, the researchers also wanted to use a new test for detecting latent TB infection. The traditional test for detecting TB infection (tuberculin skin test) is not very reliable among people who have received the Bacillus Calmette-Guérin (BCG) vaccination against TB earlier in life, as is the case in Russia. In this study a new test was therefore used, based on measuring the immune response to two proteins produced by M. tuberculosis, which are not present in the BCG vaccine strain.
What Did the Researchers Do and Find?
In this study the researchers tested health care workers from all the TB clinics in Samara City, as well as other clinical staff and students, for latent tuberculosis. In total, 630 people had blood samples taken for testing. A questionnaire was also used to collect information on possible risk factors for TB. As expected, the rate of latent TB infection was highest among clinical staff working in the TB clinics, 47% of whom were infected with M. tuberculosis. This compared to a 10% infection rate among medical students and 29% infection rate among primary care health workers. The differences in infection rate between medical students, primary care health workers, and TB clinic staff were statistically significant and reflected progressively increasing exposure to TB. Among primary care health workers, past exposure to TB was a risk factor for having latent TB infection.
What Do These Findings Mean?
This study showed that there was a high rate of latent TB infection among health care workers in Samara City and that infection is increasingly likely among people with either past or present exposure to TB. The results suggest that further research should be carried out to test whether mass screening for latent infection, followed by treatment, will reduce the rate of active TB disease among health care workers and also prevent further spread of TB. There are concerns that widespread treatment of latent infection may not be completely effective due to the relatively high prevalence of drug-resistant TB strains and any new initiatives would therefore need to be carefully evaluated.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0040055.
The Stop TB Partnership has been set up to eliminate TB as a public health problem; its site provides data and resources about TB in each of the 22 most-affected countries, including Russia
Tuberculosis minisite from the World Health Organization, providing data on tuberculosis worldwide, details of the Stop TB strategy, as well as fact sheets and current guidelines
The US Centers for Disease Control has a tuberculosis minisite, including a fact sheet on latent TB
Information from the US Centers for Disease Control about the QuantiFERON-TB Gold test, used to test for latent TB infection in this study