November 1997
SAN FRANCISCO - A highly contagious form of tuberculosis was found in a town along the Kentucky-Tennessee border, Sarah Valway, MD, reported here at the annual meeting of the Infectious Diseases Society of America.
Under laboratory conditions, the new strain multiplies 1,000 times faster than a typical tuberculosis bacterium, she said. It easily swept through the small town of 10,000 people in the foothills of the Smoky Mountains. To protect patient confidentiality, Valway declined to give the exact location of the town.
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Luckily, the strain was fully susceptible to treatment, she said, and the bacterium's virulence was limited to its ability to replicate quickly and infect people within just a few hours of exposure.
The outbreak occurred from 1994 through 1996 when 21 TB cases were identified with 15 being culture positive, she said. Before this outbreak, the town had experienced fewer than one case a year in 10 years. Another 337 of 461 contacts had positive skin tests, but did not develop active disease. Some of these contacts were "fleeting" - lasting less than four hours.
"We've seen transmission occur with minimum contact," she said. Much of the contact was "described as hanging outside at the local gas station or outside at night at the Dairy Queen." Since Mycobacterium tuberculosis is an airborne illness, being outdoors should be enough to prevent the spread of disease to those who are also outdoors.
Public health officials discovered the strain after a healthy-looking child had a positive tuberculin skin test. She had had contact with a 21-year-old uncle who had HIV, but did not know he also had active tuberculosis.
The uncle worked in a factory and testing was done on factory personnel, relatives and other people who had contact with the uncle. The factory featured high ceilings and good ventilation and health officials expected to find few positives among the factory workers. However, 69% (181 of 264) of employees tested positive. None of the contacts had other risk factors for TB, Valway said.
Another set of positives occurred in a physician's office. Eight health care workers in the office tested positive after the patient had spent less than two hours in the office waiting room.
Public health officials moved quickly to test, treat and contain the outbreak, and it has since plateaued, she said. All the cases responded to treatment. DNA typing showed that the cases were caused by the same strain and that the strain was new, she added. "The extraordinary rate and extent of growth of the outbreak strain of M. tuberculosis greatly exceeds that seen with other clinical isolates of M. tuberculosis. These data suggest increased transmission was a feature of the strain of M tuberculosis, e.g., increased virulence, rather than an environmental factor or patient characteristic," the authors wrote in their abstract.
For more information:
- Valway S, Sanchez M, Shinnick T, et. al. Extensive transmission of Mycobacterium tuberculosis due to increased virulence. Abstract 35. Presented at the Infectious Diseases Society of America annual meeting. Sept. 14. San Francisco.
Editor's note: Bad news! - P. Brunell
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