Mycobacterium kansasii is an acid-fast bacillus (AFB) that is readily recognized based on its characteristic photochromogenicity, which produces a yellow pigment when exposed to light. In 1953, Buhler and Pollack first described the bacterium. Under light microscopy, M kansasii appears relatively long, thick, and cross-barred.
The most common presentation of M kansasii infection is a chronic pulmonary infection that resembles pulmonary tuberculosis. However, it may also infect other organs. M kansasii infection is the second-most-common nontuberculous opportunistic mycobacterial infection associated with AIDS, surpassed only by Mycobacterium avium complex (MAC) infection. The incidence of M kansasii infection increased with the burgeoning of the HIV/AIDS epidemic.
Unlike other nontuberculous mycobacteria (NTM), M kansasii is not readily isolated from environmental sources. However, it has been isolated from a small percentage of specimens obtained from water supplies in areas with high endemicity. Most likely, M kansasii is acquired via either aspiration or local inoculation from the environment. Little evidence exists of person-to-person transmission. Molecular characterization of M kansasii shows that it is a homogeneous group of organisms. Five genotypes, or subtypes, are described. Types I and II are common clinical isolates, while the remaining types (III, IV, V) are recovered from environmental samples only. Type I probably is the most prevalent M kansasii isolate from human sources worldwide.
M kansasii infection of the lung causes a pulmonary disease similar to tuberculosis. Its histopathologic appearance is similar to that of tuberculosis and may include acute suppuration, nonnecrotic tubercles, or caseation. In persons with AIDS or in patients with other forms of immunocompromise, many of its characteristic histologic features may be absent. 
After skin inoculation, M kansasii can cause local disease of the skin and subcutaneous tissue. It may spread from the local site and cause lymphadenitis, infection of a distant organ, or disseminated disease. 
The prevalence of M kansasii, an unusual pathogen in the pre-AIDS era, increased with the HIV pandemic. M kansasii is the second-most-common cause of NTM disease in patients with AIDS. M kansasii infection has typically been described as a disease of urban dwellers and of patients with high incomes and better standards of living. One study of 3 northern California counties found that M kansasii infection was more common in census tracts with a lower income (median income 
M kansasii infection occurs throughout the United States, with the highest incidence in the Midwest and the Southwest. A national laboratory surveillance from 1982-1983 estimated the prevalence of M kansasii infection to be 0.3 case per 100,000 persons. The above study done in northern California estimated an overall incidence of 2.4 cases per 100,000 adults per year in the general population, 115 cases per 100,000 persons with HIV infection per year, and 647 cases per 100,000 persons with AIDS per year.  This was confirmed by another laboratory-based data analysis at San Francisco General Hospital, which showed a decrease in NTM infection from 319 cases in 1993 to 59 in 2001 (P < .001). Mycobacterium avium was found to be the most common isolate in both HIV-positive and HIV-negative patients, followed by M kansasii. 
M kansasii infection has been reported in most areas of the world. The incidence appears to be relatively high in England and Wales and among South African gold miners.  In the United Kingdom, it has been reported as the most common cause of NTM lung infection in patients without HIV infection. 
An increasing incidence of NTM infections, including M kansasii, has been reported in other countries, including Israel, Korea,  Portugal, France, and Japan.
Based on the analysis of identification data received by the NTM-Network European Trials Group (NET) for 20,182 patients in 30 countries across 6 continents in 2008, M kansasii was the sixth most common NTM isolated from pulmonary samples. Mycobacterium avium complex (MAC) was the most common NTM in most countries. 
The likelihood of mortality associated with M kansasii infection depends on various factors, including the presence of comorbid diseases, treatment compliance, rifampicin use, and extent of infection. One US center's experience, which included 302 patients over more than a 50-year period (1952-1995), showed a mortality rate of 11%, but this included both immunocompromised and nonimmunocompromised patients. 
A retrospective study of South African gold miners treated for M kansasii infection reported mortality rates of 2% in those without HIV infection and 9% in patients with HIV infection. 
Untreated pulmonary M kansasii disease progresses and can lead to death in more than 50% of infected individuals.
M kansasii infection has no reported racial predilection.
M kansasii infection is more common men, with a male-to-female ratio of 3:1.
M kansasii infection is more common in the older population and is rare in children.
The age predilection shifts in conjunction with age predilections of HIV infection.
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