Infectious Diseases and Clinical Microbiology

Infectious Diseases: Diagnosis, Treatment & Travel Medicine

Infectious diseases specialists diagnose and treat infections from bacteria, viruses, fungi, and parasites, focusing on fevers, antibiotics, and vaccines.

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The BCG Vaccine: Efficacy and Limitations

The BCG Vaccine: Efficacy and Limitations

The cornerstone of immunological prevention for tuberculosis is the Bacille Calmette-Guérin (BCG) vaccine. Developed in the early 20th century from an attenuated strain of Mycobacterium bovis (bovine TB), it remains the most widely administered vaccine in human history. Its primary role is to protect infants and young children from the most severe, disseminated forms of the disease, specifically Tuberculous Meningitis and Miliary Tuberculosis. In this regard, it is highly effective and saves thousands of pediatric lives annually.

However, the BCG vaccine has significant limitations. It offers variable and generally poor protection against pulmonary tuberculosis in adults, the transmissible form of the disease. Consequently, it does not significantly reduce transmission within the community. Furthermore, its efficacy wanes over time. Because of these limitations, countries with low TB incidence (like the USA and UK) typically do not use BCG universally, reserving it for high-risk groups, to preserve the utility of the Tuberculin Skin Test (which becomes positive after vaccination). Research into novel vaccines—including recombinant BCG, viral vector vaccines, and subunit vaccines—is a significant priority in modern cellular research, aiming to induce a more robust and lasting immunity that prevents pulmonary infection in adults.

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Infection Control in Healthcare Settings

Infection Control in Healthcare Settings

Preventing transmission within hospitals and clinics is critical. This relies on a hierarchy of controls: administrative, environmental, and personal respiratory protection.

  • Administrative Controls: This involves the rapid triage and identification of suspected TB patients. Protocols ensure that coughing patients are immediately separated from the general waiting population and prioritized for evaluation.
  • Environmental (Engineering) Controls: These strategies aim to reduce the concentration of infectious droplet nuclei in the air. Negative-pressure isolation rooms are used to ensure air flows into the room rather than out into corridors. High-efficiency particulate air (HEPA) filtration and Ultraviolet Germicidal Irradiation (UVGI) systems are employed to clean the air and kill airborne bacteria.
  • Personal Respiratory Protection: Healthcare workers utilize respirators (such as N95 or FFP2/3 masks) that are fit-tested to form a tight seal around the face. Unlike standard surgical masks, which only block large droplets, these respirators filter out the microscopic airborne particles that carry the TB bacteria.
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Treatment of Latent TB Infection (LTBI)

Treatment of Latent TB Infection (LTBI)

A pivotal strategy in elimination is treating the reservoir of latent infection. Millions of people harbor dormant bacteria. Preventing them from progressing to active disease stops future transmission chains. This is known as Tuberculosis Preventive Treatment (TPT).

  • Targeted Testing: TPT is prioritized for high-risk groups: people living with HIV, recent contacts of active TB cases (especially children), patients starting anti-TNF therapy for autoimmune diseases, and those with silicosis or on dialysis.
  • Regimens: Historically, this involved 6-9 months of daily Isoniazid. Modern regimens are shorter and safer, such as 3 months of weekly Isoniazid plus Rifapentine (3HP) or 4 months of daily Rifampicin (4R). These shorter courses have higher completion rates and lower hepatotoxicity. By sterilizing the latent infection, the cycle of reactivation is broken.
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Contact Tracing and Case Finding

Contact Tracing and Case Finding

Public health departments engage in rigorous contact tracing whenever an active case is identified. This involves interviewing the patient (“index case”) to identify everyone they spent significant time with during their infectious period. These contacts are then screened for symptoms and tested for infection (IGRA/TST).

  • Active Case Finding: In high-burden communities, mobile screening units equipped with digital X-ray and rapid molecular tests go into the community to identify undiagnosed cases. This proactive approach detects patients early, reducing the duration of infectiousness and community spread.

Social Determinants and Holistic Prevention

Tuberculosis is a social disease, thriving in conditions of poverty, overcrowding, and malnutrition. Prevention strategies must address these root causes. Improving housing ventilation, ensuring food security to bolster immune function, and addressing co-morbidities like diabetes and smoking are essential components of control. Furthermore, integrating TB and HIV services ensures that co-infected individuals receive timely diagnosis and treatment for both conditions, breaking the lethal synergy between the pathogens. The goal is to create an environment—biological and social—where the bacterium cannot thrive or transmit.

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FREQUENTLY ASKED QUESTIONS

Why isn't the BCG vaccine given in every country?

Countries with a low burden of TB (like the US) generally do not use the BCG vaccine universally because the risk of contracting TB is low, and the immunization interferes with the Tuberculin Skin Test, causing false positives. This makes it harder to detect new latent infections. In these countries, the strategy focuses on testing and treating latent infection rather than universal vaccination.

A negative-pressure room is a specialized hospital isolation room in which the air pressure inside is lower than the pressure in the corridor outside. This ensures that when the door opens, clean air flows in, but contaminated air from the patient does not flow out. The air from the room is then filtered or vented outside to prevent the spread of airborne bacteria to other parts of the hospital.

A standard surgical mask provides limited protection against inhaling TB because it is loose-fitting and does not filter microscopic airborne particles effectively. It is designed to stop large droplets. To protect against inhaling TB bacteria, a specialized respirator (like an N95) that forms a tight seal on the face and filters fine particles is required. However, putting a surgical mask on the patient is very effective at trapping bacteria at the source.

For a person with a healthy immune system, the lifetime risk of latent TB progressing to active disease is about 5% to 10%, with the highest risk in the first two years after infection. However, for people with compromised immune systems, such as those with untreated HIV, the risk is much higher—approximately 10% per year.

Mycobacterium tuberculosis is susceptible to ultraviolet (UV) light. Upper-room Ultraviolet Germicidal Irradiation (UVGI) fixtures can be installed in hospitals or congregate settings. These devices irradiate the air in the upper part of the room, killing the bacteria suspended in aerosols as the air circulates, effectively disinfecting the air without harming people below.

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