Tuberculosis | Zoology Optional Notes for UPSC PDF Download

Etiology

Tuberculosis (TB) is an infectious disease caused by any one of a group of bacteria collectively known as the Mycobacterium tuberculosis complex. These mycobacteria include:

• M. tuberculosis (including subspecies M. canetti)
• M. bovis
• M. bovis BCG
• M. africanum
• M. caprae
• M. microti
• M. pinnipedii

Throughout this manual, bacteria included in the M. tuberculosis complex will be referred to as “TB bacteria”. Other mycobacteria (known as non-tuberculous mycobacteria [NTM], atypical mycobacteria, or mycobacteria other than tuberculosis [MOTT]), can cause disease in humans. Signs and symptoms of pulmonary NTM disease can be similar to those of active pulmonary TB disease (e.g., cough, sputum, hemoptysis, weight loss, chest x-ray cavities). Because of the similarities in signs/symptoms, laboratory investigations such as RNA probes and mycobacterial culture are needed to differentiate between disease caused by TB bacteria and disease caused by NTM. From a clinical perspective, it is important to determine whether someone has TB disease or NTM disease because the treatment regimens are different. From a public health perspective, it is important because person-to-person transmission of NTM is thought to be extremely rare. This is why:

• NTM disease is not a reportable illness.
• Contact investigations for NTM disease are not required.
• Treatment of NTM disease is not mandatory.

For more information on the diagnosis and treatment of NTM disease, refer to the Canadian Tuberculosis Standards (2014).

Characteristics of TB Bacteria

TB bacteria are:

• Rod-shaped
• 1-5 microns in size
• Aerobic
• Slow-growing (divide once every 15 to 20 hours)

The cell walls of TB bacteria also have a high lipid content. This means that specific laboratory methods are required to identify TB bacteria in smear examinations (acid-fast staining) and in culture (mycobacterial culture versus routine bacterial culture).

Signs and Symptoms of Active TB Disease

To an extent, the signs and symptoms of active TB disease depend on which site(s) are affected. Although TB disease occurs most often as a respiratory illness, it can develop at almost any body site. TB disease can also involve multiple sites at once (disseminated TB disease). A few examples of sites where TB disease can develop are shown in Figure 4-1. Other sites of non-respiratory TB disease include:

• Peripheral lymph nodes (TB lymphadenitis)
• Central nervous system (e.g., TB meningitis, tuberculoma)
• Abdominal cavity and/or digestive system
• Genitourinary system
• Bones and/or joints

Generalized signs and symptoms of active TB disease often include:

• Fever
• Night sweats
• Weight loss/loss of appetite
• Fatigue

Signs and symptoms of TB disease involving the lungs (pulmonary TB) usually include:

• Cough of at least 2 to 3 weeks’ duration
• Chest pain
• Abnormalities on chest x-ray (e.g., upper lobe infiltrates, cavitation)
• Hemoptysis (blood in sputum)

Young children, the elderly, and people with advanced immune suppression might not have typical signs or symptoms of active TB disease.

Transmission

Infection with TB bacteria almost always happens from inhalation of tiny droplets of moisture (droplet nuclei) that contain TB bacteria. People with active respiratory TB disease (TB in the lungs or airways) expel TB bacteria when they cough, sneeze, laugh, sing, or play wind instruments. People with laryngeal TB disease (TB laryngitis) can expel TB bacteria when they talk.

To a large degree, a person’s risk for becoming infected with TB bacteria during an exposure to an infectious case depends on the concentration of TB bacteria in the air s/he breathes. This concentration is influenced by:

• How infectious the case is.
• The degree of air circulation and ventilation.
• How close (physical proximity) the person is to the infectious case.
• Whether the person is appropriately protected against inhaling TB bacteria (e.g., wearing a fit-checked, disposable N95 respirator).

How infectious a case is (the degree of infectiousness) is influenced by:

• Site and extent of TB disease; Cases with laryngeal involvement and/or cavities on their chest x-rays are considered highly infectious before treatment. Cases with sputum-smear positive/culture-positive pulmonary TB are considered relatively more infectious before treatment than smear-negative/culture-positive cases. Cases with non-respiratory TB disease are not infectious under most circumstances.
• Strength and frequency of coughing and other behaviors/activities that produce infectious droplet nuclei; Forceful expiration (e.g., coughing, sneezing, singing, playing wind instruments) can cause TB bacteria to be released into the surrounding airspace, as can certain medical procedures (e.g., sputum induction, bronchoscopy, autopsy, high-pressure wound irrigation of a non-respiratory site of TB disease). TB transmission has also been linked to smoking crack cocaine or marijuana.
• Pathogen factors; some strains of TB bacteria might be more transmissible.

Other factors that can influence the risk of transmission include:

Frequency and duration of exposure(s) to the case.

• Susceptibility of the exposed person; people with pre-existing TB infection, such as those with LTBI or a history of TB disease, might have some innate immunity to reinfection.

Disease Process (Pathogenesis)

The likelihood of and timing for developing active TB disease after becoming infected with TB bacteria is highly variable. Some people, particularly young children and those with advanced immune suppression (e.g., HIV/AIDS) are highly susceptible to developing TB disease soon afterward (primary TB disease). The majority (~95%) of healthy people over 5 years of age who become infected with TB bacteria develop latent TB infection (LTBI). Of these, a small percentage (~5%) will eventually develop active TB disease. TB disease that develops after a period of LTBI is known as reactivation TB disease. Reactivation TB disease is more likely to occur when a person with LTBI is or becomes immune suppressed. Risk factors for reactivation TB disease are outlined in Section 4.7. The similarities and differences between LTBI and active TB disease are outlined in.

Risk Factors for Infection with TB Bacteria

Groups at increased risk for being infected with TB bacteria include:

• Known contacts to infectious cases (TB contacts)
• Canadian-born elderly people (related to risk for exposure when TB disease was more prevalent in Canada)
• Foreign-born immigrants, refugees, students, and visitors from countries where TB is prevalent. TB incidence of specific countries is available from WHO, see Chapter 6, figure 6.4.
• Canadian-born Aboriginal peoples from communities with high rates of TB
• Staff and residents of congregate settings, particularly those that house persons from the risk groups mentioned above, such as correctional facilities and shelters for the homeless/under-housed
• Staff of facilities where people with undiagnosed active TB disease could present for care (e.g., acute care hospitals)
• Some travelers to countries where TB is prevalent1

Risk Factors for Development of TB Disease

Screening people at risk for TB infection and treating those at increased risk for developing TB disease are important TB prevention and control strategies. Although most people that become infected with TB bacteria will not develop active TB disease, age and immune function can have a substantial influence. Without treatment, people with LTBI have a life-long risk for developing TB disease.