Tuberculosis, Biology


Tuberculosis (TB) is an ancient disease of man, animals and birds. It is an infectious disease, characterized by slow development of tubercles in almost any organ of body except the skeletal muscles.The incidence of tuberculosis depends upon a variety of factors related to husbandry, hygiene and environment.T B is caused by Mycobacterium tuberculosis, Mycobacterium bovis and Mycobacterium avium. M. tuberculosis is a primary pathogen for man but can produce disease in cattle, pigs, dogs, monkeys, parrots and canaries. M. bovis is a pathogen of cattle, other domestic and wild animals, but may cause infection to human indistinguishable to infection caused by M. tuberculosis. M. avium affects birds but it can cause tuberculosis in man.

The organisms are acid-fast and appear as slender, straight, slightly curved or club- shaped rods. The organisms occur singly, in pairs or in small bundles. The three species cannot be distinguished morphologically. The organisms have distinctive biological character. There is a waxy material in their cell-wall, and mycolic acids which attribute acid-fastness to these organisms. Hot carbol fuchsin stains them easily and the dye is retained despite attempts to remove with acid or alcohol, and is thus called ‘acid fast’ or ‘alcohol fast’. Another feature of these organisms is slow growth. The growth takes place in enriched media (Lowenstein Jensen medium) containing egg. The addition of glycerine enhances the growth of M. tuberculosis and M. avium. Addition of sodium pyruvate (0.5%) to medium enhances growth of M. bovis.

The species of organisms can be differentiated by their cultural characters and by their relative pathogenecity for laboratory animals. The optimum temperature of growth for mammalian species is 37oC; avian species can grow at 42o-43oC. Rabbits suffer from M. bovis but to a very small degree with M. tuberculosis. The guinea-pigs suffer from both mammalian species. The avian species is highly pathogenic for fowls.

Transmission: The common sources of infection are contaminated environment and contact with infected animals. The organism enters the body by inhalation. The  organism is expelled by the infected animals and remains in air in droplets in cattle sheds. The organism also enters the animal body by ingestion of infected discharges, from open lesions in lymph nodes, from faeces, milk or urine. The droplet inhalation is more likely to occur where animals are kept in close contact in houses with poor ventilation. Infection via skin abrasions can occur but is less likely.

Symptoms: The symptoms depend on the organ affected as any organ in the body may become involved. An affected animal may show no clinical signs even though it may be severely affected. When the animals have been affected for some months a general malaise and progressive emaciation may be seen. A capricious appetite and fluctuating temperature are also associated with the disease. The affected animals become docile and sluggish, but eyes remain bright and alert. In pulmonary tuberculosis, the animals suffer from chronic cough. The cough is initiated by slight exercise. With the involvement of bronchial lymph nodes there is pressure on the adjacent air passages which lead to respiratory distress. The involvement of mediastinal lymph nodes results in the suppression of eructation and the retention of gas in the rumen leading to marked tympany. Intestinal involvement is manifested by chronic diarrhoea. When there is tuberculosis of the reproductive system, abortions occur late in pregnancy. The conception rate is very low. There occurs a thick yellow purulent discharge from vagina. In tuberculosis mastitis there is marked in duration and hypertrophy of the udder. The supramammary lymph nodes are enlarged. Milk becomes abnormal, very fine floccules appear which settle after milk stands leaving a clear amber fluid.

Lesions: Lesions develop in any organ especially the lymph nodes associated with lungs. The lesions are characteristically a caseous core surrounded by a fibrous tissue capsule known as tubercle lesions. As the lesions increase in size, the caseous material containing bacilli may be discharged thus spreading infection to other sites. There is a tendency towards calcification of these lesions. A more acute exudative type of lesions also develops in some cases. When there is extensive distribution of organisms, via blood stream, a generalized form of tuberculosis called miliary type develops. The lesions, in the majority of cases, develop in the bronchial and mediastinal lymph nodes. In order of frequency, the lungs are next to be infected. Lesions may be found in other organs. Tuberculous mastitis occurs in about 1% of infected cattle. Lesions of the kidney, liver, genital tract of both male and female, are less common but are not rare. Congenital tuberculosis in a pregnant cow and its calf has also been reported.

Diagnosis: Clinical symptoms alone are not diagnostic. The diagnosis can be arrived at by demonstration of acid-fast bacilli on microscopic examination of material from lesions, sputum, milk, uterine discharge, pleural and peritoneal fluids, urine or faeces; by cultural examination of these material; or by inoculation of into laboratory animals like guinea-pigs. An intradermal tuberculin test in which 0.1 ml of mammalian purified protein derivative (PPD) is inoculated intradermally into the middle of skin of neck which results into a hypersensitivity reaction is followed for diagnosis. If the swelling of skin is greater than > 5 m.m., the animal is said to be reactor. Stormont test (for man) has been used. ELISA, PCR tests are being used presently for accurate diagnosis.

Treatment: Treatment is not recommended, as it is very prolonged and the results  are unreliable. The animals under treatment are liable to disseminate organisms in milk meant for human consumption.

Control: Tuberculin test is widely used throughout the world as herd screening test for the control of bovine tuberculosis. The test is based on the development of hypersensitivity among infected animals to tuberculin or purified protein derivative extracted from the organisms. The animals become positive even in early stages of disease, however, the test may not be positive in advanced stage of the disease. For successful eradication, cases positive to tuberculin test should be destroyed. In India, where control by test and slaughter is not possible, Bang’s method of control has been found to be useful. The method is based on disposing of all the clinical cases. The calves born of tuberculosis-infected cows are free of infection. The animals not showing clinical signs are subjected to tuberculin testing. The reactors and non-reactors are kept separately. The healthy group is tested every 3-6 months. Calves born of healthy animals are allowed to remain with mothers while those from reacting mothers are weaned immediately after birth. This method leads to a progressive increase in the number of healthy animals and a decrease of reactors. Vaccination of animals with BCG is not practiced in India.

Sheep, goats and other animals

The incidence of tuberculosis is lower in sheep and goats than in cattle. In the infected animals the disease is slowly progressive. Bronochopneumonia is the commonest form of the disease. The animals suffer from cough and respiratory distress. In some cases, intestines are involved with diarrhoea and lymphnodes are enlarged.In pigs, infection with M. bovis gives rise to caseous lesions which may undergo calcification, and that with M. avium causes tumour-like lesions. The vertebrae and long bones are common sites of lesions which may also be found in liver, spleen, longs and submaxillary lymphnodes.

Posted Date: 9/17/2012 7:53:57 AM | Location : United States

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