Zoonoses disease-tuberculosis, Biology


Tuberculosis is a specific communicable disease involving the pulmonary and extra- pulmonary tissues and is characterized by chronic clinical manifestations. The infection is caused by Mycobacterium tuberculosis or sometimes by M. bovis. Despite the availability of effective chemotheraphy, it is still a major health problem in most countries of the world.

Epidemiology: Three types of tubercle bacilli may affect man. The primary cause of human tuberculosis is the Mycobacterium tuberculosis. M. bovis causes tuberculosis in cattle and is highly virulent for man. The tubercle bacilli is spread principally by droplets and sputum from individuals with open pulmonary lesions. The organism can survive in moist and dried sputum for up to 6 weeks. M. avium, which is pathogenic to birds and some animals, may also cause infection in man.The major determining factors of tuberculosis are malnutrition, stress and overcrowding. In general, the incidence of tuberculosis is higher in man than in women but it is not known whether the latter have higher innate resistance. There is a particularly high incidence of tuberculosis amongst residents of common lodging houses.

Certain occupational groups have a higher incidence of tuberculosis than that of the general population. Workers in the mining, quarrying, sand blasting, tool grinding and earthenware industries are at increased risk of tuberculosis due to lung damage caused by exposure to metallic or stone dust. Occupational exposure to patients with tuberculosis is an obvious hazard amongst hospital employees. Immunosuppressed persons are at risk from tuberculosis. They include transplant recipients, those with malignant disease and the acquired immunodeficiency syndromes (AIDS).

Clinical features: Human tuberculosis can manifest in different forms. Infection occurring in a person never previously exposed to M. tuberculosis is termed as primary tuberculosis. The primary infection consists of a lesion at the site of entry of organism, which is the lung, tonsil, intestine or rarely, the skin. The period of time from infection to onset of progressive disease may be weeks, months or even years.Haematogenous spread of bacilli occurs in primary tuberculosis, causing systemic infection. Bacilli are localized into many organs and tissues including the meninges, kidneys, bones, pleura and lung, and give rise to fatal forms of primary tuberculosis. Foci developing in the endothelium of major blood vessels may rupture and give rise to widespread granulomata, a disease termed as military tuberculosis.Post primary tuberculosis develops either as a result of reactivation of organism of primary infection. Tuberculosis of bones and joints usually becomes clinically evident  1-3 years after infection. The term open tuberculosis is applied to those cases in which bacilli are detectable in the sputum. Open cases are much less commonly encountered in children than in older patients. Clinically the disease is manifested by anorexia, weakness, chest pain, emaciation and fever which is elevated in the evening. Dry cough for a long duration and coughing up blood are common signs of the infection.

Laboratory diagnosis: A provisional diagnosis of tuberculosis is usually made by demonstrating acid-fast bacilli in smears of sputum or of gastric washing (containing swallowed sputum) stained by the Ziehl-Neelsen method. For rapid screening of smears immunofluorescence test can be employed. These materials should be cultured for isolation of tubercle bacilli for confirmation of the disease. Nucleic acid based diagnosis like PCR may be used for rapid and accurate diagnosis.Guinea-pigs are highly susceptible to tubercle bacilli. Inoculation of sputum in guinea-pigs may be used in addition to direct cultivation, since it is as sensitive as demonstrating small number of bacilli. Tuberculin test (Montoux test) helps in the detection of infections among the population. Delayed-hypersensitivity to tuberculin is highly specific for the tubercle bacilli. Reactivity appears about one month after infection and persists for many years, often for life.

Control and prevention:
Tuberculosis is an infectious disease with its highest incidence among poor, malnourished and crowed communities. Control measures must therefore be directed at (i) reducing the load of infectious cases by case-finding and effective chemotherapy; (ii) protecting the susceptible individuals by vaccination (BCG); and (iii) improving personal, social and environmental conditions.

Posted Date: 9/20/2012 2:32:31 AM | Location : United States

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