This infection is caused by one of two similar forms of mycobacteria,the main one being mycobacterium tuberculosis. Humans are the main host. The microbes cause pulmonary tuberculosis and are spread either by droplet infection from an individual with active tuberculosis or in dust contaminated by infected sputum.
PHASES OF PULMONARY TUBERCULOSIS.
1)PRIMARY TUBERCULOSIS.
When microbes are inhaled they colonise a lung bronchiole,usually towards the apex of the lung. There may be no evidence of clinical disease during the initial stage of non-specific inflammation. Later,cell-medoated T-lmphocytes respond to the microbes and the individual becomes sensitised. Macrophages surround the microbes at the site of infection forming ghonfoci(tubercles). These walled-off tubercles shelter microbes from normal body defences. Some macrophages containing live microbes are spread in lymph and infect hilar lymph nodes. Primary tuberculosis is usually asymptomatic. There are various outcomes:
- The disease may be permanently arrested,the foci becoming fibrosis and calcified.
- Microbes may survive in the foci and become the source of secondary infection.
- The disease may spread:
- Throughout the lung.
- To other parts of the body via lymph and blood leading to widespread of numerous infection.
2)SECONDARY(POST PRIMARY) TUBERCULOSIS.
This occurs only in people carrying a primary lesion,usually in the apex of one or both lungs. It may be caused by a new infection or by reactivation of infection by microbes surviving within Ghon foci often following reduction of immunity by age or illness.
