40 Tuberculosis
Introduction
Clinical and public health practice for bringing TB under control in the UK is underpinned by guidance from National Institute for Health and Clinical Excellence (2006) and the Joint Committee on Vaccination and Immunisation (Department of Health, 2006).
Aetiology
TB is caused by tubercle bacilli, which belong to the genus Mycobacterium. These form a large group but only three relatives are obligate parasites that can cause TB disease. They are part of the Mycobacterium tuberculosis complex. The UK data for 2008 show that M. tuberculosis was isolated in 99% of confirmed cases, M. bovis in 0.4% and M. africanum in 0.5% that year (Health Protection Agency, 2009).
Clinical aspects
Infection with tubercle bacilli occurs in the vast majority of cases by the respiratory route. The lung lesions caused by infection commonly heal, leaving no residual changes except occasional pulmonary or tracheobronchial lymph node calcification (Heymann, 2004). About 5% of those initially infected will develop active primary disease (Hawker et al., 2005). This can include pulmonary disease, through local progression in the lungs, or by lymphatic or haematogenous spread of bacilli, to pulmonary, meningeal or other extrapulmonary involvement, or lead to disseminated disease (miliary TB). In the other 95%, the primary lesion heals without intervention but in at least one-half of patients, the bacilli survive in a latent form, which may then reactivate later in life. Infants, adolescents and immunosuppressed people are more susceptible to the more serious forms of TB such as miliary or meningeal TB.
Transmission
Transmission occurs through exposure to tubercle bacilli in air-borne droplet nuclei produced by people with pulmonary or respiratory tract TB during expiratory efforts such as coughing or sneezing. In general, only the respiratory forms of TB (tuberculosis of the larynx is highly contagious but rarely seen in the UK) are infectious. Most infections are acquired from adults with post-primary pulmonary TB. The greatest risk of infection is to close, prolonged contacts, mainly household contacts. Between 90% and 95% of cases of TB in children are non-infectious (Davies, 2003). TB cannot be acquired from individuals with latent TB infection (LTBI).
Patients should be considered infectious if they have sputum smear-positive pulmonary disease (i.e. they produce sputum containing sufficient tubercle bacilli to be seen on microscopic examination of a sputum smear) or laryngeal TB. Patients with smear-negative pulmonary disease (three sputum samples) are less infectious than those who are smear positive. The relative transmission rate from smear-negative compared with smear-positive patients has been estimated to be 0.22 (British Thoracic Society, 2000).
Risk groups
Certain groups are at increased risk of LTBI and possibly TB disease if exposed. These include:
People with certain medical conditions are at increased risk of developing active TB if they have LTBI. These medical risk factors are set out in Box 40.1.
Epidemiology
Global
Globally, it is estimated that TB causes about 2 million deaths worldwide each year. One-third of the world’s population is infected with the tubercle bacillus. It is becoming the leading cause of death among HIV-positive people. Over 4 million cases of TB disease are notified annually although the estimated number of new cases is put at 9 million. The majority of cases occur in poor countries in the southern hemisphere (WHO, 2009).
The numbers of cases estimated to have occurred in 2008, globally and by WHO region, is presented in Table 40.1. Most cases of TB are in South East Asia, although the highest rates are in Africa.
WHO region | Numbers (000s), 2008 (lower and upper bounds) | Rates per 100,000 population, 2008 (lower and upper bounds) |
---|---|---|
Africa | 2800 (2700–3000) | 350 (330–370) |
Americas | 280 (260–300) | 31 (29–33) |
Eastern Mediterranean | 650 (580–740) | 110 (99–130) |
Europe | 430 (400–460) | 48 (45–51) |
South-East Asia | 3200 (2800–3700) | 180 (160–210) |
Western Pacific | 1900 (1700–2200) | 110 (95–130) |
Global | 9400 (8900–9900) | 140 (130–150) |
In 2008, it was estimated that 440,000 people had MDR-TB worldwide, and one-third of these died. The brunt of the MDR-TB epidemic is borne by Asia, with almost 50% of cases worldwide estimated to occur in China and India. However, in some areas of the world, up to one in four people have drug-resistant TB. For example, 28% of all people newly diagnosed with TB in one region of North-Western Russia had the multidrug-resistant form of the disease (WHO, 2010).
UK
A total of 8655 cases were reported in 2008 (Health Protection Agency, 2009). The overall rate across all population groups was 14.1 per 100,000 population. Of all cases, 39% occurred in the London region, where the rate was 44.3 per 100,000 population. Nineteen primary care organisations in England had a rate of 40 per 100,000 or over, all of which were in major urban areas. Rates of TB in England outside London varied from 5.7 per 100,000 in South-West England to 18.7 per 100,000 in the West Midlands. Incidence rates per 100,000 population were 8.7 in Scotland, 5.8 in Wales and 3.3 in Northern Ireland. The majority of cases (72%) were in the population born outside the UK and in those aged 16–44 years (61%). The TB rate was higher in those born abroad than among those born in the UK (86 compared with 4.4 per 100,000). This reflected higher rates of TB in people from high-incidence countries, mainly South Asia. The risk of TB is highest in the 5 years after arrival in the UK. TB can also occur as a travel-related disease in UK residents from high-incidence countries, who return to visit their country of birth and are exposed to TB. The numbers of cases by region in England, Wales and Northern Ireland, in 2008, is shown in Table 40.2.
UK region/country | Number of cases | Rate per 100,000 population |
---|---|---|
London | 3376 | 44.3 |
West Midlands | 1012 | 18.7 |
North West | 745 | 10.8 |
Yorkshire and the Humber | 647 | 12.4 |
East Midlands | 517 | 11.7 |
South East | 719 | 8.6 |
East of England | 478 | 8.3 |
South West | 297 | 5.7 |
North East | 179 | 7.0 |
England | 7970 | 15.5 |
Scotland | 452 | 8.7 |
Wales | 174 | 5.8 |
Northern Ireland | 59 | 3.3 |