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Mycobacterial Infection


Introduction

The genus Mycobacterium contains numerous acid-fast staining aerobic bacilli that result in a variety of infections in human hosts. Pulmonary infections by Mycobacterium spp. are characterized as tuberculous (TB) mycobacterial infection and nontuberculous mycobacterial (NTM) infection .

Tuberculous Mycobacterial Infection

Tuberculosis is an infection caused by the inhalation of the aerosolized aerobic bacillus Mycobacterium tuberculosis . According to the World Health Organization (WHO) 2015 annual report on TB, there were an estimated 9.6 million new cases of TB worldwide in 2014, with an estimated 1.5 million deaths from TB. However, through strong public health measures, the TB mortality and prevalence rates have dropped nearly 50% and 42%, respectively, since 1990, with an estimated 43 million lives saved through diagnosis and treatment between 2000 and 2014.

The Centers for Disease Control list a number of risk factors for developing tuberculous infection, which are broadly divided into high exposure risks and states of a weakened immune system ( Box 16.1 ). The relationship of TB and HIV/AIDS is of particular importance. According to the 2015 WHO Global Report on Tuberculosis, it is estimated that HIV-positive individuals are 26 times more likely to develop TB than HIV-negative individuals, that of the 9.6 million new cases of TB in 2014, 1.2 million (12%) were in HIV-positive individuals, that 25% of deaths attributed to TB were in HIV-positive individuals, and that TB accounted for one third of the estimated 1.2 million deaths from HIV/AIDS.

Box 16.1
From the Centers for Disease Control and Prevention. https://www.cdc.gov/tb/topic/basics/risk.hitm
Risk Factors for Developing Tuberculosis

Contact With Recently Infected Patients

  • Close contact of a person with infectious TB disease

  • Emigration from area with high TB rate

  • Children younger than 5 years old with a positive TB test result

  • Groups with high rates of TB transmission (homeless persons, injection drug users, HIV positive)

  • Persons who reside or work with people at high risk for TB (hospitals, homeless shelters, correctional facilities, nursing homes, residential homes for those with HIV)

Weakened Immune System

  • Substance abuse

  • HIV

  • Silicosis

  • Diabetes

  • Severe kidney disease

  • Low body weight

  • Organ transplants

  • Head and neck cancer

  • Corticosteroid treatment

  • Specialized treatments for rheumatoid arthritis or Crohn disease

TB, Tuberculosis.

Although several laboratory tests are available for the diagnosis of TB, imaging remains an effective tool for the early diagnosis and treatment of TB as well as early isolation in patients suspected of TB, helping prevent further dissemination among the public. TB has traditionally been characterized as primary TB , postprimary TB , and miliary TB, but in recent years, this approach has been challenged on the basis of DNA fingerprinting. This research suggests that time from acquisition of infection to the development of clinical disease does not reliably predict the imaging features of TB. It further suggests that severely immunocompromised patients tend to have the primary form of TB, but immunocompetent individuals tend to have the reactivation form.

Primary Tuberculosis

Inhalation of aerosolized M. tuberculosis bacilli and subsequent pulmonary infection is referred to as primary TB ( Box 16.2 ). Symptoms of primary TB include cough, fever, and chills, similar to typical bacterial pneumonias. Patients with mild symptoms may not seek health care treatment. Gram stains of sputum specimens are negative because of the high lipid content of tuberculous bacilli; therefore, an acid-fast stain is performed to evaluate specimens suspected of mycobacteria. Hence, mycobacteria are often referred to as acid-fast bacteria .

Box 16.2
Imaging Findings in Primary Tuberculosis

  • Airspace opacity

  • Hilar or mediastinal lymphadenopathy

  • Endobronchial nodules

  • Pleural effusion

The host immune system response to TB infection is complex and involves caseous granuloma formation surrounding areas of infection in an attempt to contain the bacilli and prevent infection spread. Bacilli frequently spread via lymphatic drainage, leading to lymphadenopathy in a predictable drainage pattern. Hematogenous dissemination may remain in the lungs (miliary TB) or disseminate to any site in the body.

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