Mycobacteria include a range of pathogenic and nonpathogenic species distributed widely in the environment. Mycobacterium tuberculosis is one of the top 10 causes of death globally, causing 1.7 million deaths each year. Most cases, and resulting deaths, occur in developing countries, with the largest number of new cases occurring in Asia and Africa. Hansen disease (leprosy) is caused by Mycobacterium leprae and Mycobacterium lepromatosis.
Nontuberculous mycobacteria (NTM) consist of the other mycobacteria that can cause lung disease resembling tuberculosis, lymphadenitis, skin disease, and bacteremia. Multidrug-resistant NTM, also known as Mycobacterium abscessus complex, are more difficult to treat because of their resistance to standard antituberculous regimens.
Infection with M tuberculosis usually occurs through inhalation of infective droplets and, in rare cases, by way of the skin or gastrointestinal tract. Infection mainly involves the lungs but can spread systemically and involve any organ system.
The purified protein derivative (PPD) tuberculin skin test measures delayed hypersensitivity to tuberculoprotein. A positive PPD reaction is defined as a 10 mm or greater area of induration in the area of intradermal injection of 0.1 milliliter (mL) of PPD, read 48–72 hours after the injection. A false-positive test may occur in patients who have been vaccinated with the BCG vaccine. Therefore, blood tests are used in patients who have received the BCG vaccine; they are also used in patients who are high risk and have a negative skin test. These tuberculosis (TB) blood tests include the Interferon-γ release assay tests (eg, the QuantiFERON-TB Gold and the T-SPOT.TB tests). Following positive skin test results, chest x-ray, chest computed tomography, and sputum samples aid in diagnosis.
Treatment of active TB infection consists of intensive therapy with daily isoniazid (INH), rifampin (RIF), pyrazinamide (PZA), and ethambutol (EMB) for 8 weeks. EMB may be discontinued following confirmation of drug regimen susceptibility. The intensive therapy is followed by continuation therapy with daily INH and rifampin for 18 weeks, or thrice weekly INH and rifampin for 18 weeks.
All currently used agents have toxic adverse effects, especially hepatic and neurologic; patients should be carefully monitored during the course of therapy. INH and EMB can cause optic neuropathy in a small percentage of patients, and rifampin may cause pink-tinged tears and blepharoconjunctivitis.
Outbreaks of nosocomial and community-acquired multidrug-resistant TB (MDRTB; meaning TB that is resistant to INH and RIF) have increased, particularly in the presence of concurrent HIV infection. MDRTB in HIV-infected patients is associated with widely disseminated disease, poor treatment response, and substantial mortality. MDRTB infection has also been documented in health care workers exposed to these patients. Treatment of MDRTB involves using at least 3 agents from the following medications or classes of medications: a fluoroquinolone (eg, levofloxacin, ciprofloxacin, ofloxacin), an injectable aminoglycoside (eg, amikacin, kanamycin, streptomycin, capreomycin), PZA, EMB, cycloserine, para-aminosalicylic acid, terizidone, a thioamide (eg, ethionamide, prothionamide), and the newer agent bedaquiline. These agents are used for at least 18–24 months following sputum culture conversion.
Sollai S, Galli L, de Martino M, Chiappini E. Systematic review and meta-analysis on the utility of interferon-gamma release assays for the diagnosis of Mycobacterium tuberculosis infection in children: a 2013 update. BMC Infect Dis. 2014;14(Suppl 1):S6.
World Health Organization (WHO). Guidelines for treatment of drug-susceptible tuberculosis and patient care (2017 update). www.who.int/tb/publications/2017/dstb_guidance_2017/en/. Published April 2017. Accessed February 22, 2019.
World Health Organization (WHO). WHO treatment guidelines for isoniazid-resistant tuberculosis: Supplement to the WHO treatment guidelines for drug-resistant tuberculosis. www.who.int/tb/publications/2018/WHO_guidelines_isoniazid_resistant_TB/en/. Published 2018. Accessed February 22, 2019.
Zumla AI, Gillespie SH, Hoelscher M, et al. New antituberculosis drugs, regimens, and adjunct therapies: needs, advances, and future prospects. Lancet Infect Dis. 2014;14(4):327–340.
Excerpted from BCSC 2020-2021 series: Section 1 - Update on General Medicine. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.