Tuberculosis (TB) remains a formidable global health challenge, affecting millions and claiming countless lives each year. Despite advances in medicine, approximately 9 million people still fall ill with TB annually, and around 2 million succumb to its devastating effects. The culprit behind this deadly disease is the bacterium Mycobacterium tuberculosis. When individuals with active TB in their lungs cough, they expel microscopic droplets into the air, which can be inhaled by others, leading to new infections. This airborne transmission underscores the persistent threat TB poses to communities worldwide.
Transmission Dynamics
The risk of TB transmission is higher in small households and crowded places, especially in countries with a high TB incidence. Several factors influence this risk:
- Infectiousness of the Source Case: The likelihood of transmission increases with the number of TB bacilli expelled by the infected individual.
- Closeness of Contact: Prolonged and close contact with an infectious person increases TB risk.
- Bacillary Load Inhaled: The number of bacilli inhaled by a susceptible individual impacts the likelihood of infection.
- Host Immune Status: Individuals with weakened immune systems, such as those with HIV, are at higher risk of developing TB after exposure.
Molecular Epidemiological Insights
Molecular epidemiology has revealed distinct differences in TB presentation and demographics between high and low TB incidence regions:
- High TB Incidence Countries: In many African and Asian countries, M. tuberculosis is the predominant cause of mycobacterial infections. The highest incidence rates are observed among young adults, with most cases resulting from recent infection or reinfection.
- Low TB Incidence Countries: In Western Europe and North America, a significant proportion of active TB cases occur in older individuals or immigrants from high TB incidence regions. Here, TB is often reactivated from a latent state rather than resulting from a recent infection.
Initial Phase of Infection
During the early stages of infection, the TB bacteria multiply inside the immune cells that have engulfed them. This happens before the body’s adaptive immune system, specifically targeting pathogens, can respond. The bacteria-laden immune cells can cross the alveolar barrier, spreading the infection to other body parts. This process includes:
- Intracellular Replication: The bacteria multiply within the immune cells.
- Systemic Dissemination: The infected immune cells can move out of the lungs, spreading the bacteria to the pulmonary lymph nodes and other body parts
Body’s Initial Response
The entry of TB bacteria into immune cells starts when the cells recognize specific patterns on the bacteria, known as pathogen-associated molecular patterns (PAMPs). These patterns are identified by pathogen recognition receptors (PRRs) on the immune cells. This recognition triggers the body’s first line of defense, the innate immune response, which attempts to control the infection.
Conclusion
Understanding the nuances of TB transmission, disease presentation, and population demographics is crucial for developing targeted interventions. In high-incidence countries, efforts should focus on reducing transmission through improved living conditions and early diagnosis. In low-incidence countries, strategies should prioritize the detection and treatment of LTBI, particularly among high-risk populations like immigrants and the elderly. With ongoing advancements in diagnostics and treatments, there is hope for more effective management and the eventual eradication of TB worldwide.
References:
https://respiratory-research.biomedcentral.com/articles/10.1186/1465-9921-11-169