High Rates of Mycobacterium tuberculosis among Socially Marginalized Immigrants in Low-Incidence Area, 1991–2010, Italy

Iacopo Baussano

, Silvio Mercadante, Manish Pareek, Ajit Lalvani¹, and Massimiliano Bugiani¹

Author affiliations: International Agency for Research on Cancer, Lyon, France (I. Baussano); UPO “A.Avogadro” and CPO–Piemonte, Novara, Italy (I. Baussano, S. Mercadante); Imperial College, London, UK (M. Pareek, A. Lalvani); Regional Reference Centre for Tuberculosis Prevention, Turin, Italy (M. Bugiani)

Abstract

Migration from low- and middle-income countries to high-income countries increasingly determines the severity of tuberculosis (TB) cases in the adopted country. Socially marginalized groups, about whom little is known, may account for a reservoir of TB among the immigrant populations. We investigated the rates of and risk factors for Mycobacterium tuberculosis transmission, infection, and disease in a cohort of 27,358 socially marginalized immigrants who were systematically screened (1991–2010) in an area of Italy with low TB incidence. Overall TB and latent TB infection prevalence and annual tuberculin skin testing conversion rates (i.e., incidence of new infection) were 2.7%, 34.6%, and 1.7%, respectively. Prevalence of both TB and latent TB infection and incidence of infection increased as a function of the estimated TB incidence in the immigrants’ countries of origin. Annual infection incidence decreased with time elapsed since immigration. These findings have implications for control policy and immigrant screening in countries with a low prevalence of TB.

Migration from low/middle income countries with high tuberculosis (TB) incidence increasingly accounts for most TB cases in high-income countries with low TB incidence; the greatest risk for active TB is within the first few years of arrival (1–4). Screening for active pulmonary TB when documented immigrants enter a new country has found ≈3.5 cases per 1,000 documented immigrants (5,6). The prevalence of smear-negative cases of TB reported for US-bound immigrants and refugees was 9.6 cases per 1,000 persons (7).
In countries with low incidence of TB, vulnerable populations, such as persons living in prisons (8) and shelters (9) and hard-to-reach populations (10–13), are at high risk for TB (3). Marginalized immigrants have the combined risk of coming from countries with high incidence of TB and being vulnerable because of their relegated social position in countries of destination (14). Recent evidence suggests that the distribution of latent TB infection (LTBI) and TB among immigrants is uneven. LTBI prevalence among recent immigrants to the United Kingdom increased as a function of TB incidence in the country of origin (15). In high-income countries, refugees, asylum seekers, and immigrants, who were screened for TB when entering the new country, had 11.9, 2.7, and 2.8 cases of TB/1,000 persons, respectively (5).
Among easy-to-reach immigrants, LTBI prevalence is around 40%, measured by tuberculin skin test (TST) (16,17), and 15%–19%, measured by interferon-γ release assays (IGRAs) (11,16). The largest study assessing TST results enrolled < 1,000 undocumented immigrants (17); the studies that used IGRAs recruited no more than 125 undocumented immigrants (11,16). Estimates of prevalence of active TB among undocumented immigrants (range 0.0065%–1.6%) are based on < 10 cases in each study (11,16). No data have been published on incidence of infection, the key indicator of Mycobacterium tuberculosis transmission, among socially marginalized immigrants. Knowledge of these parameters among socially marginalized groups could inform control strategies for TB in countries in which TB incidence is low.
We estimated the prevalence of and risk factors for active TB and LTBI, as well as the incidence of infection, among socially marginalized immigrants in an area of western Europe where incidence of TB is low. The study was conducted at the Regional Reference Centre for Tuberculosis Prevention in Turin, Italy. The estimated TB incidence during the study period, 1991–2010, was relatively stable (≈20 cases/100,000 persons), and in 2010, an increasing proportion of cases (≈70%) occurred among foreign-born persons (18–20).