Componentes y aspectos

TB is a global disease, with an estimated 9 million cases in 2013 of which 1.5 million people died.1 TB is the second leading cause of death from an infectious disease worldwide, after human immunodeficiency virus (HIV). TB mortality is unnecessarily high given that most deaths are preventable if people can access the healthcare required for diagnosis and treatment. Since TB was declared as a global public health emergency by the World Health Organization (WHO) in 1993, the TB mortality rate and incidence rate have been decreasing, but remain high with between and within regional variation (Figure 2).

The TB incidence rate in the WHO European Region was estimated to be 39.4 per 100,000 in 2012, with an estimated 353,000 new cases.2 This represents about 4% of the global burden of incident TB. 68,423 TB cases were notified in the European Region in 2012 (Figure 3). Of these, 27% (n=18,358) were of foreign origin (Figure 4). In the UK, the proportion of TB cases notified in 2013 that were foreign born was 72.5% (5518/288).3

2 Figure 1 – WHO estimates of TB incidence rates, 2013

Source: Global Tuberculosis Report, World Health Organization 20141

Figure 2 – Global trends in estimated rates of TB incidence and mortality

Left: Global trends in estimated overall TB incidence rate (green) and estimated incidence rate of HIV-positive TB (red). Right: Trends in estimated TB mortality rates 1990–2013 and forecast TB mortality rates 2014–2015. The horizontal dashed line represents the Stop TB Partnership targets of a 50% reduction in mortality rates by 2015 compared with 1990. Shaded areas represent uncertainty bands. Mortality excludes TB deaths among HIV- positive people.

3 Figure 3 – TB notification rates per 100 000 population, European Region, 2012

Source: European Centre for Disease Prevention and Control/WHO Regional Office for Europe. Tuberculosis surveillance and monitoring in Europe 2014.

Figure 4 – Percentages of notified TB cases of foreign origin among all TB cases, European Region, 2012

Sour ce: European Centre for Disease Prevention and Control/WHO Regional Office for Europe. Tuberculosis surveillance and monitoring in Europe 2014.

4

Epidemiology of TB in the United Kingdom (UK)

TB has re-emerged as a serious public health problem in the UK over the last two decades with case notifications reaching 9000 in 2011, a rate of 14.1 per 100,000.4 There has been a slight decrease in the numbers of TB cases in the last two years, with 7892 cases notified in 2013, a rate of 12.3 per 100,000.3 Approximately half of which were pulmonary (52% of cases with known site of disease).

Figure 5 – TB case reports and rates in the UK, 2004-2013

Source: Enhance Tuberculosis Surveillance (ETS), Enhanced Surveillance of Mycobacterial Infections (ESMI), Office for National Statistics (ONS)3

England accounted for 92.4% (7290/7892) of the cases in the UK in 2013 with a rate of 13.5 per 100,000, the highest proportion of which were in London (rate of 35.5 per 100,000). 59.2% of cases in England were culture confirmed and 71.3% of cases with pulmonary TB were culture confirmed.3

A majority of cases were male (58% in 2013), and most were aged between 15 and 44 years old (with a rate of 24.4 per 100,000 in this age group in 2013) (Figure 6).3 Rates of TB were significantly higher in non-UK born cases compared to UK born

5 cases (70 per 100,000 compared to 4 per 100,000) (Figure 6). TB incidence rates are highest in those aged 75 and older in the UK born population, whereas in the non- UK born population the highest rates are in those aged 25 to 29 (rates of 6.6 and 94.3 per 100,000, respectively). In the last few years, the most common countries of origin of non-UK born cases are India, Pakistan and Somalia (30%, 20% and 5%, respectively in 2013).3

Figure 6 – TB case reports and rates by age group and place of birth, UK, 2013

Source: Enhance Tuberculosis Surveillance (ETS), Enhanced Surveillance of Mycobacterial Infections (ESMI),Labour Force Survey (LFS)

Data as at: May 2014. Prepared by: TB Section, Centre for Infectious Disease Surveillance and Control, Public Health England

In 2013, 7% of cases in the UK had a previous diagnosis of TB.3 Resistance to at least one first line antibiotic was identified in 7.8% of all cases, and 1.6% were multi-drug resistant (MDR; resistant to at least isoniazid and rifampicin).3 These rates have been relatively stable over the last decade, with a slight increase between 2008 and 2011.

Approximately 10% of TB cases have at least one social risk factor for TB: current or history of problem drug misuse, alcohol misuse, homelessness or imprisonment.3

6

Natural history of TB

TB is caused by Mycobacterium tuberculosis complex. After inhaling aerosolised droplets of M. tuberculosis from a person with infectious TB, approximately 30% of people become infected.5 In the remainder, the infection is cleared, which is likely due to host factors (such as the host’s immune response which may eradicate the organism) or characteristics of the pathogen (such as the virulence of the strain). A simplified interpretation of the natural history of TB is illustrated in Figure 7.

Figure 7 – Natural history of TB

A person who is infected with M. tuberculosis may clear the infection and recover (follow the blue arrows in Figure 7), may become latently infected (black arrow), or develop active disease (red arrow). When an individual is infected with M.

tuberculosis and the bacilli are in a dormant state, this is known as latent tuberculosis

infection (LTBI), whereby the patient will show no clinical signs of TB.6 This latent period can last a lifetime or the infection may reactivate to become active disease at any point, known as endogenous reactivation. Risk factors for reactivation include

7 age and a suppressed immune system.7 The variable duration of latent infection of M.

tuberculosis makes it difficult to identify transmission pathways and implement

targeted control programmes.

Most immune competent individuals will not develop TB disease over their lifetime. The lifetime risk of progressing to active disease is 5-10%. It is highest shortly after infection (80% in the first two years) and decreases subsequently. The risk of developing disease is much higher for those who are immunocompromised, especially those with HIV, who have a 5-10% annual risk of progressing to active TB.6

People who are latently infected, or people who have recovered from a previous TB infection can be reinfected with the same or different strain of M. tuberculosis. This is known as exogenous infection, or reinfection. The variability between the time of infection and disease makes differentiating between endogenous and exogenous TB difficult, as there could be many years between infection from the index case and disease in the secondary case.8 Molecular typing can be used as a tool to differentiate between relapse and reinfection (see section 1.3.2, page 21).9

Following treatment for active TB, an individual may clear the infection and recover, the treatment may fail and the individual may continue to have active disease, the individual may relapse at a later stage and develop recurrent TB. As disease does not infer immunity, the individual remains at risk for reinfection and subsequent disease.