Tuberculosis and the kidney.

Tuberculosis of the kidney and urinary tract is, like other forms of the disease, caused by members of the Mycobacte- rium tuberculosis complex. By far the most common causative organism is the human tubercle bacillus, M. tuberculosis, but the bovine tubercle bacillus, M. bovis, occasionally can be responsible. The vaccine strain, Bacille Calmette-Guerin (BCG), also has been the cause of renal lesions as a compli- cation of intravesical instillation of BCG for the treatment of superficial bladder cancer. The members of the M. tuberculosis complex are obligate pathogens and, together with M. leprae, differ from the many other species within the genus Mycobacterium, which are free-living environmental saprophytes and are commonly found in water, including piped water supplies. Some of these so-called environmental mycobacteria occasionally cause hu- man disease, particularly in immunosuppressed individuals, including recipients of renal transplants (1). The kidney may be involved when environmental mycobacteria cause dissemi- nated disease, such as that caused by M. avium in AIDS patients. Renal disease caused by environmental mycobacteria in nonimmunosuppressed individuals is exceedingly rare (2). However, because they occur in water, environmental myco- bacteria readily contaminate the lower urethra and external genitalia and, thus, often are isolated from urine samples. Globally, tuberculosis is a common disease, with 8 to 10 million new cases annually and a rising incidence, particularly in regions with a high incidence of HIV infection. Most often the lung is affected, but, after lymphadenopathy, the most common form of nonpulmonary tuberculosis is genitourinary disease, accounting for 27% (range, 14 to 41%) of nonpulmo- nary cases in several surveys in the United States, Canada, and United Kingdom (3). In developed countries, nonpulmonary tuberculosis is rela- tively more common in patients from ethnic minority groups, the exception being genitourinary tuberculosis, which is un- common in these groups. In the United Kingdom, the latter accounts for 5% of cases of nonpulmonary tuberculosis in ethnic minorities, mostly those of Indian subcontinent ethnic origin, compared with 27% of such cases in the European (white) population. This is, to some extent, age related: when patients of white and of Indian subcontinent ethnic origin were stratified by age, the incidence of genitourinary tuberculosis was similar in the two groups in each age band (4). Tuberculosis caused by M. bovis is now uncommon in industrially developed nations, accounting for fewer than 1% of all cases of tuberculosis. It is usually due to reactivation of old, dormant disease, although cases have occurred in younger, HIV-positive patients (5). In approximately 25% of cases caused by reactivation in older persons, the genitourinary sys- tem is involved (6). Such disease has a somewhat bizarre veterinary health significance because a number of farmers have infected cattle by urinating on hay in cowsheds (7).