Tuberculosis

Introduction

Tuberculosis (TB) is an infection caused by two species of Mycobacteria - "Mycobacterium Tuberculosis and Mycobacterium Bovis". Though, it can cause disease involving every organ system in the body, it commonly affects the lungs. The disease was in existence even in the neolithic period and till the early 20th century the only treatment was rest in the open air in specialised sanitoria. Currently, around 1.7 billion people world-wide, a third of the world's population, are infected by Mycobacterium tuberculosis and 3 million deaths a year are attributable to tuberculosis.

Rapid spread of TB in humans is attributed to crowded living conditions that favour airborne transmission. There was a steady decline in the incidence of tuberculosis, especially in the developed countries till early 1980's but since then the trend has reversed and an increasing number of cases have been reported. A combination of social, economical and historic factors are responsible for this increase that include urban homelessness, intravenous drug abuse, the growing neglect of tuberculosis control programs and most notably the AIDS epidemic.





Cause and Pathogenesis

The term tubercle bacillus refers to two species "Mycobacterium Tuberculosis and Mycobacterium Bovis". Other species are classified under Atypical Mycobacterial Pathogens. Humans are the only reservoir for Mycobacterium tuberculosis.

Mycobacterium Bovis was transmitted by contaminated milk once, but is no longer so. Though skin infection by inoculation is seen in pathologists and laboratory personnel, almost all infections are due to airborne transmission by inhalation of droplet nuclei.

In general, 3 - 4% of infected individuals will develop active disease during the first year after exposure and a total of 5-15% thereafter. The likelihood of developing active disease varies with the intensity and duration of exposure. Malnutrition, alcoholism, renal failure and uncontrolled diabetes favour the progression of infection to active disease. HIV infection is the strongest risk factor today.

The tubercle bacilli, once inhaled, reach the terminal air spaces in the lungs, escaping from the host defence mechanism. They multiply locally but are controlled and retained in the lungs by the body's white blood cells. Sometimes infected white cells carry the bacteria to the lymph nodes and from there to the blood stream. Seeding and unchecked proliferation in other organs can cause disease manifestations elsewhere in the body as well. Pulmonary tuberculosis is usually seen in the upper portions of the lung.

Symptoms and Signs


The clinical picture depends on the involved organ. Early tuberculosis of the lung is asymptomatic and may be discovered on a chest x-ray by chance. In case of tuberculosis of the lung, the patient will complain of fever, night sweats, fatigue, cough, sputum production that is sometimes mixed with blood, weight loss and loss of appetite.

Tuberculosis can also appear as swelling of the glands in the neck with or without fever (lymph node TB), back pain, deformity of the spine and weakness in the lower limbs (TB of the spine), fever, headache, vomiting and drowsiness (TB meningitis), joint pain and swelling (TB arthritis), genitourinary symptoms like flank pain and infertility (genitourinary TB).





Investigations and Diagnosis

The chest radiograph is crucial to the diagnosis, determination of the extent of disease and response to therapy. Pulmonary tuberculosis can be confirmed by demonstrating the bacillus in a sputum specimen directly by staining or by culture. The tuberculin test or Mantoux test gives information only about exposure to the bacillus but does not confirm active disease. Similarly, a negative tuberculin reaction is not conclusive evidence against presence of tuberculosis. Sometimes when the sputum does not show the bacillus, bronchoscopic examination may help, Examination of affected tissue under a microscope (histopathology) will show the characteristics of tuberculosis (caseating granuloma).

Treatment and Prognosis


With the advent of Combination Chemotherapy, successful treatment of tuberculosis is a reality, but problems of drug resistance, selection of an inappropriate regimen and non-compliance hinder effective therapy. WHO recommends a six-month short course therapy consisting of four drugs - Rifampicin, lsoniazid, Ethambutol and Pyrizinamide - for the first two months and two drugs - Rifampicin and Isoniazid - for the next four months.

Directly Observed Therapy Short course (DOTS) involves directly observed therapy given by a Health Care Worker 2 - 3 times a week. This has been found to improve compliance and is as effective as the traditional daily therapy. TB of the bone, meninges and the genitourinary tract may require a more prolonged treatment schedule.

Prevention


The efficacy of BCG vaccine in the prevention of tuberculosis has been a topic of much debate, but it has shown to decrease the risk of acquiring tuberculosis of the nervous system. Chemoprophylaxis, which refers to the use of anti-tuberculous drugs, especially Isoniazid or Rifampicin has been found to prevent the development of active disease in a person infected with tuberculosis. The drugs have to be given for 3 to 6 months and are useful especially in persons with impaired immunity, for example - HIV infection.