Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (483 page)

Three species are most commonly associated with clinical disease:
Mycobacterium abscessus, Mycobacterium fortuitum
, and
Mycobacterium chelonae
.

   
Mycobacterium abscessus
usually causes pulmonary disease. Patients with underlying pulmonary disease are most commonly infected, but disease may also occur in patients with no pulmonary disease.

   
Mycobacterium fortuitum
usually causes skin and soft tissue infection after direct inoculation. Infections include surgical site, catheter-related, and other infections. Pulmonary isolates may represent transient infection or colonization.

   
Mycobacterium chelonae
may cause a variety of infections in immunocompromised patients.

   Laboratory Findings

AFB staining and culture
: Diagnosis is usually established by culture of infected material: RGM may be positive only with modified acid-fast staining. American Thoracic Society criteria (ATS) should be used to assess the significance of isolates.

SLOW-GROWING, NONTUBERCULOUS MYCOBACTERIA

   Definition

There are a large number of nontuberculous mycobacteria (NTM). These organisms are ubiquitous in the environment. A number of these species are able to cause human disease but usually in patients with immune defects.

   Who Should Be Suspected?

Most infections are acquired from environmental sources; human-to-human transmission occurs rarely, if ever. NTM are increasingly implicated in nosocomial infections and pseudo-outbreaks in health care settings. Although this patient population may be at increased risk for NTM infection, culture isolates must be interpreted cautiously because of the frequency of isolation as culture contaminants.
Mycobacterium gordonae
, for example, is a fairly common isolate in AFB cultures, like BAL specimens, and virtually always represents a culture contaminant.

   Significant Species

   
Mycobacterium avium
complex (MAC): This complex includes two genetically related species:
Mycobacterium avium
and
Mycobacterium intracellulare
. Organisms are widely distributed in nature, being prevalent in soil and water with low pH and oxygen content, and they are relatively chlorine resistant. MAC has been isolated from municipal water supplies and hospital hot water systems and shower heads.

   In patients with AIDS or other immune defects, mycobacteremia, manifested with fever, fatigue, night sweats, anemia, diarrhea, failure to thrive, or other nonspecific symptoms, is the most common type of infection. Other sites may be secondarily infected, but pulmonary infection is relatively uncommon. Risk of MAC increases with decreasing CD4
⁺
cell count.

   The isolation of NTM from respiratory specimens is well described for patients with CF.
Mycobacterium avium
complex isolates are most common, followed by
M. abscessus
in a significant minority of patients, although there may be significant variability of the etiology globally. The virulence of NTM in CF patients also shows variability. CF patients, from whom NTM are isolated tend to be older, have better lung function and have a lower frequency of chronic
P. aeruginosa
infection (but higher rate of
S. aureus
) compared to patients without NTM infection.

   In immunocompetent patients, pneumonia is the most common disease caused by MAC. A syndrome similar to TB has been described in elderly men with underlying pulmonary disease. Patients present with chronically progressive cough and weight loss. Upper lobe cavitation is well described, and parenchymal damage may be significant. A second common syndrome is described in women, usually older than 50, without underlying lung disease. Patients present with insidious onset of cough and sputum production; systemic symptoms are not prominent.