The tuberculosis (TB) life cycle begins with an infected individual spreading the bacteria and continues with the infection, potential latency, and potential progression to active disease in a new host.
Here's a detailed breakdown:
- Transmission:
- The cycle begins when a person with active pulmonary TB coughs, sneezes, speaks, or sings, releasing Mycobacterium tuberculosis bacteria into the air in tiny droplets called droplet nuclei. These droplets are small enough to remain airborne for extended periods.
- Inhalation:
- A susceptible person inhales these infectious droplet nuclei. The droplets travel down the respiratory tract and typically lodge in the alveoli (air sacs) of the lungs.
- Initial Infection (Primary Infection):
- Once in the alveoli, the Mycobacterium tuberculosis bacteria are engulfed by alveolar macrophages (immune cells).
- In most individuals, the macrophages are able to kill the bacteria. However, in some cases, the bacteria survive and multiply within the macrophages.
- These infected macrophages then travel to regional lymph nodes, where they can initiate a broader immune response.
- Immune Response and Granuloma Formation:
- The body's immune system, particularly T cells, recognizes the infected macrophages and initiates a cell-mediated immune response.
- This immune response involves the recruitment of more macrophages and other immune cells to the site of infection.
- The immune cells surround the infected macrophages, forming a structure called a granuloma. The granuloma is an attempt by the body to wall off the infection and prevent its spread. Inside the granuloma, bacteria may become dormant.
- Latent TB Infection (LTBI):
- In many people, the immune system successfully contains the infection within the granuloma, preventing the development of active TB disease. This is known as latent TB infection (LTBI).
- People with LTBI are not sick, do not have symptoms, and are not infectious to others.
- However, the bacteria remain alive but inactive within the granulomas.
- Reactivation and Active TB Disease:
- In some individuals with LTBI, the immune system weakens (due to factors such as HIV infection, malnutrition, diabetes, old age, or immunosuppressive medications).
- When the immune system weakens, the bacteria within the granulomas can reactivate and begin to multiply again.
- This reactivation leads to the breakdown of the granulomas and the spread of bacteria throughout the lungs and potentially to other parts of the body (extrapulmonary TB).
- Active TB disease develops, causing symptoms such as cough, fever, weight loss, and night sweats. People with active TB disease are infectious and can transmit the bacteria to others.
- Treatment and Control:
- Active TB disease is treated with antibiotics. Successful treatment kills the bacteria and prevents further transmission.
- Treatment of LTBI can also prevent progression to active disease, especially in high-risk individuals.
In summary, the TB life cycle is a continuous process of infection, potential latency, and potential reactivation, driven by the interaction between the bacteria and the host's immune system. Effective public health measures aim to break this cycle through prevention, diagnosis, and treatment.
