Research letter
Can silicosis explain it all?
D. Coutinhoa, P. Sousab, O. Oliveirac, A. Rita Gaiod,e, R. Duartea,c,f,g,,
a Pulmonology Department, Centro Hospitalar de Vila Nova de Gaia/Espinho, Vila Nova de Gaia, Portugal
b Faculty of Medicine of Porto University, Porto, Portugal
c EPIUnit Institute of Public Health, University of Porto, Porto, Portugal
d Centre of Mathematics, University of Porto, Porto, Portugal
e Mathematics Department, Faculty of Sciences, University of Porto, Porto, Portugal
f Chest Disease Centre, Vila Nova de Gaia, Portugal
g Department of Clinical Epidemiology, Predictive Medicine and Public Health, University of Porto Medical School, Porto, Portugal
Dear Editor,

Portugal is still the country with the highest TB incidence rate in Western Europe1–3; although a recent decrease in TB incidence rate has placed the country close to the low incidence goal.2 Despite a national TB incidence rate of 21.1/100000 inhabitants in 2013, Penafiel municipality registers the highest TB incidence rate in the whole country (83.4/100000 inhabitants in 2013).2 Penafiel municipality is part of Northern Portugal region (NUTS II) and it is located in a geographic area designated as Vale de Sousa. The large majority of its 28 parishes are quite industrialized, although in some of them the primary sector still represents an important part of the economy. The mining sector, particularly exploration of granite quarries, is an important economic sector in Penafiel and other neighbouring counties in the geographic region of Vale de Sousa.4 Therefore, the large exposure of Penafiel's mining sector workforce to prolonged inhalation of crystalline silica has been pointed out has one of the major factors for TB high incidence rate in this region.5

In order to identify longitudinal trends of TB incidence rate in Penafiel and Northern Portugal and to describe the behaviour of silica-related diseases and other social and health-related indicators in the region, we performed a longitudinal study that included every active new TB case notified in Penafiel and Northern Portugal between January 1st, 2002 and December 31st, 2012. The data source was the Portuguese Tuberculosis Surveillance System (SVIG-TB) and the information was up-to-date until December 31st, 2012. The SVIG-TB is the notification and follow-up system of the TB cases in Portugal, supervised by the Directorate-General of Health (DGS).6

The socio-demographic and health-related indicators considered in this study were: (1) population density; (2) elderly dependence ratio; (3) unemployment rate; (4) immigration rate; (5) medical doctors rate; (6) mining sector work force ratio; (7) silicosis notification and prevalence rates; (8) HIV notification and HIV/TB coinfection rate; (9) Addiction related TB rate; (10) Immigration related TB rate; (11) Homelessness related TB rate; (12) Incarceration related TB rate.

For all predictors, data were collected yearly and obtained from the following national institutions: Statistics Portugal (INE), Employment and Vocational Training Institute (IEFP), Directorate General of Prison Services (DGSP), Institute on Drugs and Drug Addiction (IDT), Institute of Social Security (ISS), Directorate General of Energy and Geology (DGEG) and National Health Institute Doutor Ricardo Jorge (INSA).

TB incidence rate and other incidence rates are expressed as the rate per 100000 inhabitants. A time-trend descriptive analysis was carried out to substantiate the dynamics of the TB incidence and other collected data in both regions over the study period.

During the period studied, a total of 675 new TB cases were notified in Penafiel, with an average decrease of the TB incidence rate of 1.41% per year (from 95.5/100000 inhabitants in 2002 to 82.1/100000 inhabitants in 2012). In the same period, a total of 16364 new cases were notified in Northern Portugal, with a similar average decrease of 1.19% per year (from 35.8/100000 inhabitants in 2002 to 24.4/100000 inhabitants in 2012).

As expected, a declining time-trend for the TB incidence rate was exhibited by both regions during the studied period. Socio-demographic and health-related indicators of both regions are presented in Table 1.

Table 1.

Relevant socio-demographic and health-related indicators of Penafiel and Northern Portugal.

  Years
  2002  2003  2004  2005  2006  2007  2008  2009  2010  2011  2012 
Population density (per km2)
Northern Portugal  187.8  188.1  188.2  191.4  191.5  191.6  191.2  190.9  190.3  190.0  189.0 
Penafiel  340.3  341.2  342.2  342.7  343.0  343.1  342.5  341.7  340.8  340.3  338.5 
Elderly dependency ratio (%)
Northern Portugal  21.5  21.8  22.2  22.5  22.8  23.1  23.5  24.0  24.8  25.3  25.9 
Penafiel  15.9  16.1  16.4  16.6  16.6  16.6  16.8  17.2  17.8  18.2  18.5 
Unemployment rate (%)
Northern Portugal  4.0  5.0  5.4  5.7  5.4  4.6  4.9  6.0  6.2  6.7  7.8 
Penafiel  2.8  3.8  4.1  3.9  3.5  3.2  3.7  5.2  5.3  6.4  8.0 
Immigration rate (%)
Northern Portugal  n/a  n/a  n/a  n/a  n/a  n/a  1.31  1.38  1.39  1.44  1.38 
Penafiel  n/a  n/a  n/a  n/a  n/a  n/a  0.55  0.56  0.54  0.47  0.42 
Medical doctors (per 100000)
Northern Portugal  281.6  288.5  296.6  306.8  315.4  324.5  335.0  347.1  362.3  376.0  385.2 
Penafiel  84.5  87.0  85.4  89.4  92.0  96.1  100.4  113.1  130.0  142.6  142.0 
Mining sector work force (per 100000)
Northern Portugal  75.7  73.7  75.1  71.0  79.2  71.0  59.8  56.1  59.8  61.1  56.7 
Penafiel  886.1  748.4  797.1  732.8  236.3  410.6  517.3  547.4  634.6  630.0  483.0 
Silicosis notification rate (per 100000)
Northern Portugal  4.3  4.3  6.6  3.7  2.1  2.7  2.6  3.1  2.1  2.4  2.4 
Penafiel  54.0  69.0  125.3  83.9  50.8  107.1  78.4  80.0  89.9  112.1  84.9 
Silicosis prevalence rate (per 100000)
Northern Portugal  78.5  78.2  80.0  88.0  85.3  82.3  79.6  76.5  73.9  70.9  67.9 
Penafiel  560.8  588.2  645.7  730.1  739.1  788.3  803.5  834.3  875.2  901.3  959.0 
Silicotubercolsis (% of total TB cases)
Northern Portugal  0.8  0.4  0.2  0.4  0.6  0.7  0.7  0.7  0.6  1.9  0.8 
Penafiel  0.0  1.5  0.0  0.0  4.5  4.2  7.6  12.1  3.6  10.0  8.5 
HIV notification rate (per 100000)
Northern Portugal  15.9  15.1  14.9  13.9  13.0  12.4  12.3  10.3  9.3  7.4  7.4 
Penafiel  8.3  8.3  11.0  11.0  11.0  11.0  8.3  8.3  6.9  15.2  12.5 
HIV/TB co-infection (% of total TB cases)
Northern Portugal  13.3  14.6  13.1  12.9  11.7  11.5  10.8  10.9  7.6  7.8  7.7 
Penafiel  4.3  4.5  6.2  5.8  3.0  6.3  1.5  1.7  1.8  1.4  0.0 
Addiction related TB (% of total TB cases)
Northern Portugal  28.1  23.4  22.3  24.6  24.8  24.6  20.4  20.8  19.8  18.2  17.7 
Penafiel  13.0  14.9  13.8  11.5  15.2  12.5  7.6  6.9  9.1  15.7  20.3 
Immigration related TB (% of total TB cases)
Northern Portugal  2.7  2.6  2.4  3.5  2.7  3.3  3.4  4.1  4.2  4.5  4.3 
Penafiel  0.0  0.0  0.0  0.0  0.0  2.1  0.0  1.7  0.0  0.0  1.7 
Homeless related TB (% of total TB cases)
Northern Portugal  2.8  3.6  3.8  4.5  4.4  3.8  2.7  2.8  3.2  3.3  3.0 
Penafiel  1.4  1.5  1.5  1.9  1.5  0.0  0.0  0.0  0.0  1.4  1.7 
Incarceration related TB (% of total TB cases)
Northern Portugal  1.1  0.4  0.7  1.2  0.9  0.9  0.9  0.9  1.0  0.9  1.5 
Penafiel  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0 

From a socio-demographic point of view and in comparison with Northern Portugal, Penafiel municipality presents a higher population density, a lower elderly dependency ratio and a much lower number of medical doctors. As previously mentioned, mining industry is a strong economic sector in Penafiel; therefore the proportion of mining workers in the population is frankly above the rest of the Northern Portugal region, with a subsequent significantly higher silicosis notification and prevalence rates in Penafiel, with the latter presenting an increasing trend during our study period. The same increasing trend was reported for new cases of silicotuberculosis in Penafiel, while over the same period Northern Portugal values remained stable.

As for the other TB related risk factors, although HIV notification rates are similar between both regions, Penafiel reports a lower HIV/TB co-infection rate, as well as lower notification rates for all other TB related risk factors collected (immigration, incarceration, addiction and homeless).

Our study confirms that among all TB related risk factors, silica-related diseases indicators are the ones where both regions coherently differ, with Penafiel presenting higher silicosis notification and prevalence rates, and silicotuberculosis notification rate. All other collected TB related risk factors (HIV, immigration, incarceration, addiction and homelessness) presented lower values in comparison with the Northern Portugal region.

Other collected social and health-related indicators, like higher population density and lower number of medical doctors, might also play an important role in Penafiel's higher TB incidence.

Author contributions

Raquel Duarte and A. Rita Gaio conceived the project idea. Daniel Coutinho and Pedro Sousa collected the data. Olena Oliveira and A. Rita Gaio conducted the analyses. All authors interpreted and discussed the results. All authors wrote the manuscript. All authors have read and approved the final version.

Conflicts of interest

The authors have no conflicts of interest to declare.

References
1
World Health Organization
Global tuberculosis report 2014
World Health Organization, (2014)
Report NO. WHO/HTM/TB/2014.08
2
A. Diniz,R. Duarte,C. Caldeira,J. Bettencourt,M. Gomes,O. Oliveira
Portugal, infeção VIH, SIDA e tuberculose em números – 2014
Direção-Geral da Saúde, (2014)
3
European Centre for Disease Prevention and Control/WHO Regional Office for Europe
Tuberculosis surveillance and monitoring in Europe 2014
European Centre for Disease Prevention and Control, (2014)
4
Infopédia – Língua Portuguesa com Acordo Ortográfico [on-line]
Porto Editora, (2003–2015)
Available on: http://www.infopedia.pt/$penafiel,2 [accessed 02.10.15]
5
J. Machado,D. Ferreira
Pergunta 2852/XII: Foco de tuberculose no concelho de Penafiel [on-line]
6
Direcção Geral da Saúde. Sistema de Vigilância da Tuberculose (SVIG-TB)
Substituição da aplicação informática e suporte do Registo Clínico dos Casos. Circular Normativa n° 6/DT de 13/03/01
DGS, (2001)

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