Clinical Diagnosis and Management of Tuberculosis, and Measures for Its Prevention and Control
NICE Clinical Guidelines, No. 117
National Collaborating Centre for Chronic Conditions (UK); Centre for Clinical Practice at NICE (UK).
London: National Institute for Health and Clinical Excellence (UK); 2011 Mar.
To sneeze – чихать
the sputum – мокрота
‘smear positive’ – положительныймазок
Household – бытовой
To inhale – вдыхать
Dormant – спячка
What causes TB?
TB is caused by a bacterium called Mycobacterium tuberculosis (‘M. tuberculosis’ or ‘M.Tb’). It is spread by one person inhalingthe bacterium in droplets coughed or sneezed out by someone with infectious tuberculosis. Not all forms of tuberculosis are infectious. Those with TB in organs other than the lungs are rarely infectious to others, and nor are people with just latent tuberculosis (see below). Some people with respiratory tuberculosis are infectious, particularly those with bacteria which can be seen on simple microscope examination of the sputum, who are termed ‘smear positive’. The risk of becoming infected depends principally on how long and how intense the exposure to the bacterium is. The risk is greatest in those with prolonged, close household exposure to a person with infectious TB.
What happens after infection?
Once inhaled the bacteria reach the lung and grow slowly over several weeks. The body's immune system is stimulated, which can be shown by a Mantoux test1, a common diagnostic technique. In over 80% of people the immune system kills the bacteria and they are removed from the body. In a small number of cases a defensive barrier is built round the infection but the TB bacteria are not killed and lie dormant. This is called latent tuberculosis; the person is not ill and is not infectious. Sometimes at the time of the initial infection, bacteria get into the blood stream and can be carried to other parts of the body, such as bones, lymph glands or the brain, before the defensive barrier is built. One third of the world's population, two billion people, have latent tuberculosis.
If the immune system fails to build the defensive barrier, or the barrier fails later, latent tuberculosis can spread within the lung (pulmonary tuberculosis) or into the lymph glands within the chest (intrathoracic respiratory tuberculosis) or develop in the other part(s) of the body it has spread to (extrapulmonary tuberculosis). Only some of those with latent tuberculosis will develop symptoms (‘active tuberculosis’). About half the cases of active tuberculosis develop within a few years of the original infection, particularly in children and young adults. The other half of active TB cases arise from reactivation of the latent infection many years later.
Who catches TB?
Anyone can catch TB but those at particular risk are those who have been exposed to TB bacteria, and those who are less able to fight latent infection. Theyinclude:
those who have lived in, travel to or receive visitors from places where TB is still very common
those who live in ethnic minority communities originating from places where TB is very common
those with immune systems weakened by HIV infection or other medical problems
the very young and the elderly, as their immune systems are less robust
those with chronic poor health and nutrition because of lifestyle problems such as homelessness, drug abuse or alcoholism
those living in poor or crowded housing conditions, including those living in hostels.
What are the symptoms of TB?
Because TB can affect many sites in the body, there can be a wide range of symptoms, some of which are not specific and may delay diagnosis.
Typical symptoms of pulmonary TB include chronic cough, weight loss, intermittent fever, night sweats and coughing blood. TB in parts other than the lungs has symptoms which depend on the site, and may be accompanied by intermittent fever or weight loss. TB is a possible diagnosis to be considered in anyone with intermittent fever, weight loss and other unexplained symptoms. Latent tuberculosis without disease, however, has no symptoms.
How is TB diagnosed?
TB is diagnosed in a number of ways. Tissue samples from biopsies may show changes which suggest TB, as do certain X-ray changes, particularly on chest X-rays. Definite diagnosis is achieved by culturing the TB bacterium from sputum or other samples. This not only confirms the diagnosis, but also shows which of the TB drugs the bacterium is sensitive to. Mantoux test and IGTs can show if someone has been exposed to TB and may have latent infection. Skin tests use a tiny dose of TB protein injected under the skin. In people who have been exposed to TB this gives a positive reaction, which is seen as a raised, red area. IGTs involve taking a blood sample, which is processed at a laboratory.
How is TB treated?
TB is completely curable if the correct drugs are taken for the correct length of time. Before drug treatment for TB nearly half of all persons with active tuberculosis died from it. Several antibiotics need to be taken over a number of months to prevent resistance developing to the TB drugs. The great majority of TB bacteria are sensitive to the antibiotics used (rifampicin, isoniazid,pyrazinamide and ethambutol). A minority of cases, 6–8% in England and Wales, are resistant to one of the antibiotics. Isoniazid and rifampicin are ineffective in 1% of cases. These cases are said to be of multi-drug resistant TB (MDR TB), which is harder to treat (see Appendix G for details of TB epidemiology).
TB bacteria grow very slowly and divide only occasionally when the antibiotics start to kill them, so treatment usually has to be continued for six months to ensure all active and dormant bacteria are killed and the person with TB is cured. People with respiratory TB are usually not infectious after two weeks of treatment. Drug-resistant forms of the bacteria require treatment for longer than six months. MDR TB is particularly serious, requiring prolonged (up to 24 months) treatment, with the infectious period lasting much longer.
In latent tuberculosis there are many thousand times fewer TB bacteria than in active tuberculosis. Treatment with a single drug for six months, or two drugs for a shorter time, is sufficient to kill the dormant bacteria, preventing the person developing active tuberculosis later in their life.
Following TB treatment, the disease can return (relapse) in a small number of people, because not all bacteria have been killed. This is obviously much more likely if the course of treatment has been interrupted, not completed or otherwise not followed. However, it is also possible to catch TB a second time, unlike some other infectious diseases.