Tuberculosis (TB) and Other Mycobacterial Infections

SummaryTop

This chapter is divided into subchapters to provide information on 3 related but discreet topics in tuberculosis (TB):

1) Active TB disease refers to an active disease state that has resulted from infection with Mycobacterium tuberculosis. An individual with active disease may present with systemic and/or respiratory symptoms. The disease can be pulmonary or extrapulmonary. If pulmonary disease is present, infection can be spread to others via airborne droplet nuclei. Since the disease is often present for several weeks or months before the onset of telltale signs or symptoms, transmission to susceptible individuals may occur before the illness is apparent. The gold standard for diagnosis of active TB disease is microbiologic culture testing of sputum and other specimens yielded from affected sites. Treatment with combination drug therapy is usually highly successful in achieving cure.

2) Latent TB infection refers to a state of inactive infection due to M tuberculosis without the presence of the disease. An individual with latent TB infection is asymptomatic and cannot transmit infection to others. Individuals who become infected are at risk of progressing to active disease. About 5% of newly infected hosts (often those with immune dysfunction) are unable to contain the infection and therefore proceed to early disease progression within the first 18 to 24 months (primary TB). The rest of the infected hosts are classified as being in a state of latent TB infection. The majority of persons with latent TB infection will remain disease-free for life, but ~5% of all infected hosts will develop late disease progression with the development of active TB disease (postprimary or reactivation TB) sometime after 18 to 24 months of acquiring infection. Detection of latent TB infection occurs through targeted screening of high-risk groups using the TB skin test and/or the interferon gamma release assay blood test. Treatment with preventative drug therapy is successful in reducing the risk of reactivation. The decision to treat latent TB infection is individualized and must balance the risk of reactivation with the risk of potential drug toxicities.

3) Nontuberculous mycobacteria (NTM) diseases refer to conditions related to mycobacterium other than the M tuberculosis complex, which are of unknown or lesser pathogenicity. NTM can exist as a contaminant or colonizer, or can cause disease. There are some shared features between the clinical presentation of active M tuberculosis disease and NTM disease. Unlike active TB, NTM organisms cannot be transmitted from human to human. The most common organ of involvement is the lung, although extrapulmonary sites can also be affected, especially in immune-deficient states. Detection is through microbiologic culture, but clinical and radiographic assessments are necessary to differentiate between the states of colonization and disease. Combination drug therapy is used on selective individuals who manifest symptomatic disease of an advanced or progressive nature. The clinical response to therapy is often suboptimal.