Pulmonary Tuberculosis

Pulmonary Tuberculosis

Definition and Historical Overview

  • Pulmonary tuberculosis is an ancient infection that has plagued humans throughout recorded history.

Causative Agent

  • Mycobacterium tuberculosis

    • Discovered by Robert Koch in 1882.

    • Characteristics:

    • Acid-fast, rod-shaped bacterium.

    • Related to Mycobacterium leprae.

    • Culture is considered the “gold standard” for detecting M. tuberculosis in clinical specimens, such as sputum.

      • Unlike M. leprae, M. tuberculosis can be cultured in the laboratory.

      • Growth rate:

      • Slower than most bacteria but more rapid than M. leprae.

      • Visible growth takes approximately 3-8 weeks on solid nutrient medium.

Epidemiology

  • Reservoir: Humans are the only reservoir for M. tuberculosis.

  • Transmission:

    • Spread in crowded living conditions where airborne transmission is favored.

    • If untreated, each person with active tuberculosis can infect on average > 15 persons yearly (R0 = 15).

  • Infectivity:

    • Less than 10 bacteria required for infection.

    • Infection likely does not occur outdoors.

    • Fomites (inanimate objects) not significant in transmission; no need for special housekeeping measures.

Clinical Disease

  • An immunologically normal person infected with M. tuberculosis may not show symptoms and will have latent tuberculosis.

  • The immune system can effectively “wall off” bacilli, allowing them to lie dormant for years.

  • Immunocompromised individuals are at higher risk of progressing from latent tuberculosis to active pulmonary tuberculosis:

    • 1 in every 10 individuals with latent infection may develop active tuberculosis in their lifetime.

Pathogenesis of Pulmonary Tuberculosis

Stage One

  • M. tuberculosis inhaled as droplet nuclei and travels to alveolar sacs where bacteria are ingested by resident macrophages.

  • The bacteriostatic effect of alveolar macrophages on intracellular bacilli is minimal.

  • Bacilli can multiply unrestrained within alveolar macrophages for several weeks.

Stage Two

  • M. tuberculosis proliferates logarithmically within alveolar macrophages, forming a granuloma (tubercle), which is a site of inflammation in lung tissue.

  • Blood-borne monocytes are recruited to the granuloma via chemotactic properties of the bacterium.

  • The lesion consists of blood-borne macrophages, many of which contain multiplying bacilli.

Stage Three

  • The viable bacilli reach a stationary phase due to a developing cell-mediated immune response, primarily involving CD4+ T cells.

  • Delayed-type hypersensitivity (DTH) leads to partial tissue necrosis:

    • Results in caseous necrosis, leading to a center that is solid/semisolid and amorphous in morphology (referred to as caseous material).

    • Within this caseous center, bacilli become extracellular but do not multiply.

Stage Four

  • Caseous necrosis is prone to liquefaction, leading to the formation of a pulmonary cavity, which allows bacilli populations to proliferate significantly (up to 5-6 logs greater than in non-cavity lesions).

  • Infectious bacilli can slough from these cavities, potentially creating new infection foci in different lung areas and can spread to other individuals.

Anatomy of a Pulmonary Cavity

  • Central cavity contains a high concentration of bacilli.

  • Surrounded by a layer of caseous material with fewer bacilli.

  • Peripheral layers consist of macrophages and T cells exhibiting minimal organization and low bacilli count, along with a bacilli-free layer of encapsulating fibrosis.

Historical Perspectives on Tuberculosis

Ancient Times

  • Hippocrates (~380 B.C.): Detailed the disease termed phthisis (“to melt” or “to waste away”).

  • Aristotle (~350 B.C.): Proposed that phthisis was caused by disease-producing substances in the air.

  • Galen (~180 A.D.): Outlined treatment principles that survived for 1000 years, focusing on rest, cough restraint, specific astringents, opium for coughing, and dietary considerations.

Renaissance

  • Disease rebranded as “consumption.”

  • Sylvius (1673): Identified small, hard nodules in patients’ lungs, termed tubercles, theorizing that consumption stemmed from lung ulcers.

  • The term “tuberculosis” is derived from “tubercle.”

  • Public health records from the 1600s indicate consumption as a leading cause of death.

  • Classic signs: Blood-stained handkerchief and notable physical wasting, creating a cultural archetype of the pale, slender individual with feverish rosy cheeks.

The Era of the Sanatorium

  • Hermann Brehmer: Established the first tuberculosis sanatorium in Germany (1854), believing fresh mountain air improved pulmonary function and healing.

    • Provided a nutrition-rich regimen (6 meals per day), gentle exercise, and daily exposure to fresh air for patients.

    • Designed with pavilion architecture enabling outdoor recovery.

    • Observations led to a global sanatorium movement predicated on rest and fresh air, moving away from toxic treatments.

  • Edward Trudeau (1885): Established the first U.S. tuberculosis sanatorium in the Adirondack region, incorporating scientific advancements in tuberculosis bacteriology.

    • Introduced diagnostic X-rays and by 1931, the U.S. had 633 tuberculosis hospitals with > 80,000 bed capacity (78% public support).

    • Integrated medical/surgical care led to declining mortality rates.

Pre-Antibiotic Era Outcomes

  • Without effective drugs, 50% of active pulmonary tuberculosis patients died within two years, and only ~25% were cured.

Era of Antibiotics and Vaccines

  • Koch: Produced tuberculin, claiming it could cure tuberculosis, though initial enthusiasm waned due to lack of efficacy.

  • Tuberculin remains in use today as a skin test for diagnosis.

Tuberculin Skin Test

  • Koch’s tuberculin: Extract of boiled bacilli from the tubercle of infected patients.

  • Seibert (1934): Developed purified protein derivative (PPD) of tuberculin, standardized in 1941.

  • Skin test involves intradermal injection of PPD, with a positive result defined by a DTH response, indicated by the diameter of induration recorded 48-72 hours post-injection.

  • A positive PPD test report requires a 2-12 weeks incubation period.

Tuberculosis Blood Test

  • Interferon Gamma Release Assay (IGRA): A blood test measuring the cell-mediated immune response to specific tuberculosis antigens, available commercially (Qiagen).

    • Can be utilized instead of the tuberculin skin test.

Developments in Treatment

  • 1946 marked the beginning of modern tuberculosis treatment with the introduction of streptomycin, followed by isoniazid in 1952, which became the cornerstone of therapy.

    • Rifampin: Recognized as an equally effective drug as isoniazid.

    • Tuberculosis became effectively curable for the majority of patients.

    • Recognized that rest and fresh air do not enhance antibiotic treatment; thus, sanatoria became obsolete.

Challenges from HIV/AIDS

  • In the early 1980s, the CDC proposed an eradication program for tuberculosis in the U.S. by 2010, which faced challenges due to HIV/AIDS emergence.

Impacts of HIV/AIDS

  • By 1985, tuberculosis case rates increased after two decades of decline due to a surge of individuals with severely impaired cellular immunity (HIV/AIDS).

    • Loss of T-cells/macrophages leads to weakened defenses against tuberculosis.

    • Challenges in clinical diagnosis:

    • Chest X-rays often appear normal due to lack of cavitation.

    • PPD skin tests may yield negative results due to anergic responses.

    • Patients remain highly contagious even without positive test outcomes.

Drug-Resistant Tuberculosis

  • Emergence of drug-resistant strains of M. tuberculosis due to HIV/AIDS:

    • Multidrug-resistant (MDR) strains resist at least isoniazid and rifampin.

    • Treatment requires extended chemotherapy with expensive drugs lasting up to two years.

    • Success rate for treating MDR tuberculosis is approximately only 55%.

Statistics on HIV Co-infection

  • The incidence of pulmonary tuberculosis is 500 times higher in HIV-1 infected individuals compared to the general population.

  • Pulmonary tuberculosis stands as the leading cause of death among those with HIV/AIDS.

BCG Vaccine

  • BCG (Bacillus Calmette-Guerin): A live, attenuated vaccine derived from Mycobacterium bovis in cows.

    • BCG vaccination can lead to a 60%-80% decrease in tuberculosis incidence.

    • It does not fully prevent infection but usually curtails the progression of clinical disease.

    • BCG is widely used internationally but not routinely in the U.S.

    • Can result in false-positive PPD skin tests due to immune sensitization to the vaccine.