Tuberculosis (TB) is an infectious disease that is transmitted through air. An evidence-based approach based on knowledge of local epidemiology is essential to intercept tuberculosis transmission. Previous, as well as modern evidence recommends that efficient public health interventions can make a difference in tuberculosis epidemiology. Tuberculosis remains a public health concern, even though effective and economical treatment is known.
People from high tuberculosis incidence countries have a high risk of contracting tuberculosis. As a result of people being very mobile, migration may affect the epidemiology of TB, particularly in low TB incidence (TI) countries. Migrants are a heterogeneous group, such as students, labor migrants or family members, refugees, or asylum seekers. The surrounding conditions and circumstances of migration often increase the risks to the social, physical, and mental well-being of migrants.
The dotoral study objectives were to describe the characteristics of national trends in the epidemiology of TB in Finland, to describe the TB screening cascade and to evaluate the extent of clustering and TB transmission between foreign- and Finnish-born populations.
The data were collected from the National Infectious Disease Register (NIDR) and from reception centers’ national health record system (HRS). In addition, a questionnaire focused on enhanced surveillance of TB was sent to physicians with the aim of eliciting comprehensive knowledge of immigrant and second-generation TB cases. NIDR included all TB cases from 1995 and laboratory information is linked to NIDR. Laboratory information consists of M. tuberculosis isolations including characterization using spoligotyping and mycobacterial interspersed repetitive unit variable number tandem repeat (MIRU-VNTR) methods. HRS data were used to assess TB in asylum seekers and screening of TB.
This nationwide, population-based study showed that during the 23-year study period the epidemiology of TB has experienced a transition in Finland. The major reason is not migration but rather the decrease of TB cases in the Finnish-born population. The incidence among foreign-born populations has decreased in other low TI countries as well as in Finland.
During the years 1995-2017, a total of 9314 TB cases were reported to NIDR. Of those cases, 7605 (81.7%) were Finnish-born and 1614 (17.3%) were foreign-born. In 95 (1%) of the cases the origin was not available. The median age was 66 years (range 0-105), 56.6% were male, 0.8% had contracted multidrug resistant tuberculosis and 68.6% had pulmonary TB of which 47.7% were sputum smear positive.
Since refugees and asylum seekers are screened in Finland on arrival, they are easy to reach. Other migrants may be difficult to reach because they do not come to the country via a single-entry point and they might relocate directly for work, to join a family member or to study. While evaluating the screening for active TB in asylum seekers in Finland during their large influx in 2015-2016, half of the TB cases among asylum seekers were found in the primary screening, and over 40% of these cases did not have TB symptoms at the time of screening. TB yield among individuals screened was 0.19% (95% CI, 0.14-0.25%) and number needed to screen was 522.
During 2014-2017, M. tuberculosis isolates were characterized by spoligotyping and MIRU-VNTR. Altogether 76 isolates belonged to the 14 mixed clusters: 39 (51.3%) were from Finnish-born cases, 36 (47.4%) from foreign-born, and one of unknown origin. Foreign-born cases originated from Europe (9 cases), Asia (13 cases) and Africa (14 cases, 6/14 from Somalia).
In conclusion, the epidemiology of TB has experienced a transition in Finland from 1995 to 2017. As TB becomes even less common in the Finnish-born population over time, increasing TB cases in the foreign-born population will probably change the epidemiology in the future as has occurred in other northern European countries. Half of the TB cases among asylum seekers were first suspected in screening; over 40% were asymptomatic. The screening guidelines are adequate for Finland.
Although a large proportion of TB cases are found in people born abroad, the population born abroad has no significant effect on the TB epidemic among populations born in Finland. Migration from high TI countries does not form a significant threat to Finnish public health.
The doctoral dissertation of Master of Medical Science Pirre Emilia Räisänen in the field of epidemiology titled Tuberculosis among persons born abroad, 1995-2017 will be publicly examined at the Faculty of Social Sciences of Tampere University at 12 o'clock on Thursday 27 May, 2021. The venue is Arvo building auditorium F114, address: Arvo Ylpön katu 34. Professor Knut Lönnroth from Uppsala University will be the opponent while Professor Pekka Nuorti will act as the custos.
The event can be followed via remote connection.
The dissertation is available online at
http://urn.fi/URN:ISBN:978-952-03-1970-0
Photo: Jukka Räisänen