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Definition: TB (tuberculosis) from The Penguin Dictionary of Science

A slowly progressive, chronic infectious disease which most often affects the lungs, caused by the bacillus Mycobacterium tuberculosis. The name describes the characteristic cyst-like lesions called tubercles found in the affected organs. The disease caused by other species of Mycobacteria can also affect domesticated animals such as cattle and pigs.


Summary Article: Tuberculosis
from Black's Medical Dictionary, 43rd Edition

Tuberculosis results from infection with Mycobacterium tuberculosis. The lungs are the site most often affected, but most organs in the body can be involved in tuberculosis. The other common sites are LYMPH NODES, bones, gastrointestinal tract, kidneys, skin and MENINGES. The weight loss and wasting associated with tuberculosis before treatment was available led to the disease's previous name of consumption. Scrofula and lupus vulgaris (see under LUPUS) are skin manifestations of the disease.

The typical pathological change in tuberculosis involves the formation of clusters of cells called granulomas (see GRANULOMA) with death of the cells in the centre producing a cheese-like softening – CASEATION.

The World Health Organisation estimated that there were 9.6 million new cases of tuberculosis worldwide in 2014, with 1.5 million deaths. The incidence of tuberculosis in developed countries has shown a steady decline throughout the 20th century, mainly as a result of improved nutrition and social conditions, and accelerated by the development of antituberculous chemotherapy in the 1940s. Since the mid-1980s the decline has stopped, and incidence has even started to rise again in inner-city areas. In 1993 the WHO declared TB a global emergency as the spread of AIDS/HIV has resulted in an upsurge of infections. In 2014, about 6,000 cases of TB were notified in England, with 43% in London, with three-quarters of those with the condition having acquired it abroad, mostly South Asia and sub-Saharan Africa. The highest rate (6 per 10,000 people) was amongst non-UK born black African and Indian/Pakistani/Bangladeshi ethnic groups. The rate for British born residents was about twenty times less.

Nature of the disease

Signs and symptoms depend upon the site of the infection. General symptoms such as fever, weight loss and night sweats are common. In the most common form of pulmonary tuberculosis, cough and blood-stained sputum (haemoptysis) are common symptoms.

The route of infection is most often by inhalation, although it can be by ingestion of products such as infected milk. The results of contact depend upon the extent of the exposure and the susceptibility of the individual. Around 30 per cent of those closely exposed to the organism will be infected, but most will contain the infection with no significant clinical illness and only a minority will go on to develop clinical disease. Around 5 per cent of those infected will develop post-primary disease over the next two or three years. The rest are at risk of reactivation of the disease later, particularly if their resistance is reduced by associated disease, poor nutrition or immunosuppression. In developed countries around 5 per cent of those infected will reactivate their healed tuberculosis into a clinical problem.

Immunosuppressed patients such as those infected with AIDS/HIV are at much greater risk of developing clinical tuberculosis on primary contact or from reactivation.

Diagnosis

This depends upon identification of mycobacteria on direct staining of sputum or other secretions or tissue, and upon culture of the organism. Culture takes 4–6 weeks but is necessary for differentiation from other non-tuberculous mycobacteria and for drug-sensitivity testing. Newer techniques involving DNA amplification by polymerase chain reaction (PCR) can detect small numbers of organisms and help with earlier diagnosis.

Treatment

This can be preventative or curative. Important elements of prevention are adequate nutrition and social conditions, BCG vaccination (see IMMUNISATION), an adequate public-health programme for contact tracing, and chemoprophylaxis. Vaccination with an attenuated organism (BCG – Bacillus Calmette Guerin) is used for those at high risk in the United Kingdom and routinely in other countries.

Cases of open tuberculosis need to be identified and their close contacts reviewed for evidence of disease. Adequate chemotherapy removes the infective risk after around two weeks of treatment. Chemoprophylaxis – the use of antituberculous therapy in those without clinical disease – may be used in contacts who develop a strong reaction on tuberculin skin testing or those at high risk because of associated disease.

The major principles of chemotherapy for tuberculosis are that a combination of drugs needs to be used, and that treatment needs to be continued for a prolonged period – usually six months. Use of single agents or interrupted courses leads to the development of drug resistance. Serious outbreaks of multiply resistant Mycobacterium tuberculosis have been seen mainly in AIDS units, where patients have greater susceptibility to the disease, but also in developing countries where maintenance of appropriate antibacterial therapy for six months or more can be difficult. One solution has been the adoption of DOTS – directly observed tuberculosis service, where the individual attends the clinic for every dose of medication, watched by the staff to make sure it is taken.

Streptomycin was the first useful agent identified in 1944. The four drugs used most often now are RIFAMPICIN, ISONIAZID, PYRAZINAMIDE and ETHAMBUTOL. Three to four agents are used for the first two months; then, when sensitivities are known and clinical response observed, two drugs, most often rifampicin and isoniazid, are continued for the rest of the course. Treatment is taken daily, although thrice-weekly, directly observed therapy is used when there is doubt about the patient's compliance. All the antituberculous agents have a range of adverse effects that need to be monitored during treatment. Provided that the treatment is prescribed and taken appropriately, response to treatment is very good with cure of disease and very low relapse rates. Worldwide, resistant strains of the organism are emerging. In the UK new cases of multi-drug resistant TB have increased from 28 in 2000 to 81 in 2012. The total number of patients diagnosed with multi-drug resistant TB between 2000 and 2012 was 681.

Copyright © A & C Black Publishers Ltd

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