- NEW YORK -- A highly contagious, rapidly growing strain of the bacteria
that cause tuberculosis has mysteriously emerged in the foothills of the
Smoky Mountains, in a rural area along the Tennessee-Kentucky border.
- The new strain multiplies in the laboratory
1,000 times as fast as a typical tuberculosis bacterium, and it swept through
the community with astonishing ease, spread by even fleeting contact, like
that between two people standing near each other outdoors at a fast-food
stand. Fortunately, standard medications work well against the organisms,
which have not become drug-resistant.
- "This is something we haven't seen
before," said Dr. Sarah Valway, chief of the epidemiology section
in the tuberculosis division at the Federal Centers for Disease Control
and Prevention in Atlanta. "The lab people have no record of any TB
bug growing like this one."
- The bacteria caused an outbreak in the
region from 1994 to 1996, during which 21 cases of active tuberculosis
were diagnosed and 337 people who were not ill with the lung disease nonetheless
had positive skin tests, indicating that they were infected with the bacteria.
More recently, new cases have dwindled, a decrease that health officials
attributed to prevention and treatment programs that were begun as soon
as the first cases were discovered.
- "Right now, we're on a plateau,
I think," said Dr. Christine Weyman, medical director of the Lake
Cumberland District Health Department, in Somerset, Ky. "I hope and
pray it's past its peak."
- Dr. Valway presented the first public
report on the new TB strain on Sept. 14 at a meeting of the Infectious
Disease Society of America, in San Francisco. She did not disclose the
location of the outbreak because, she said, the area is so small that naming
it could jeopardize patients' confidentiality. But other health officials
said the general vicinity was the Upper Cumberland region of north-central
Tennessee and south-central Kentucky, especially Pickett County, Tenn.,
and Clinton County, Ky.
- The spreading disease came to light almost
by accident when a child registered positive on a routine skin test for
tuberculosis in the spring of 1995. She was not ill, but the test meant
that she was infected with TB bacteria.
- Most healthy people who become infected
-- 90 percent to 95 percent -- do not go on to develop active tuberculosis,
and the infection at that stage is not contagious. Their immune systems
fight off the bacteria, and the infection becomes dormant.
- Even though the first infected child
was well, she was given a six-month course of preventive medication. That
treatment is recommended for most people who test positive, because it
is impossible to predict which ones will get sick.
- But how had the girl become infected?
Tuberculosis is contagious when it has progressed to the stage of active
disease in the lungs or larynx, and so local health officials knew that
the child must have been exposed to someone who was ill. It turned out
to be her uncle, who had been sick for some time without realizing that
he had tuberculosis.
- The uncle worked in a factory not far
from the state line, with employees from Kentucky and Tennessee, and health
departments in both states began screening them. Tuberculosis is an airborne
disease: one person catches it by inhaling the bacteria that another has
coughed up, and the infective particles can linger in the air for hours.
The work floor in the factory had high ceilings and good ventilation, which
might be expected to lower the risk of transmission. The cafeteria, though,
was more closed-in, with less fresh air. Health officials said that they
expected to find some people infected, but not many.
- "When we read all the skin tests,
that's when the amazing thing happened," said David Crowder, a TB
adviser for the disease-control centers. "An extremely large percentage
were positive, 181 of 264 employees. That's 69 percent. It's the highest
Tennessee has ever seen. On average, even with close contacts, we get about
a 20 percent infection rate."
- An especially surprising finding was
that those infected did not belong to groups at higher risk of contracting
TB, like immigrants, people with AIDS, the elderly, and cancer patients
with weakened immune systems. Rather, the infected people were mostly young,
healthy, working people who had spent all their lives in the United States
in a region with few immigrants.
- Eventually, in an effort to contain the
disease, a total of 461 contacts of infected people were traced, and 337
of them, or 73 percent, turned out to be infected as well, including 8
of the health workers conducting the investigation. Twenty-one people had
active tuberculosis. One, who came to be known as the "source case,"
was a man thought to have infected the uncle of the first infected child
through a casual contact in 1994.
- DNA fingerprinting tests on bacterial
samples taken from 13 of the people with active disease showed the organisms
to be genetically identical, supporting the idea that all the patients
had been infected, or had infected each other, during the outbreak. The
bacteria have been named the O strain.
- Although the outbreak seems to have ended,
Dr. Valway said it would not be surprising if a few more cases turned up
in the region because some casual contacts may have been overlooked. She
also said that the strain had not been detected outside the area. "And
we're looking for it," she said.
- Where the new bacteria strain came from
is unknown. Dr. Valway said she had found no reports of any other tuberculosis
bacteria like it. Its extreme contagiousness is also unexplained, though
she said that might stem from its ability to multiply so fast. "Maybe
they grow faster than your body can kill them, and it gives them a better
chance to get established," she said.
- If the bacteria suddenly evolved their
high infectivity and growth rate, the radical change would itself be a
departure, said Dr. Michael Iseman, an expert on tuberculosis at the National
Jewish Medical and Research Center in Denver. "This is not what you
would expect," he said. "There are labile, rapidly changeable
organisms that practice stealth biology, like HIV and the influenza virus.
By those terms, TB is a slow moving, rather plodding kind of foe."
- But that may not be true of the O strain.
"We may be dealing with a tubercle bacillus on steroids," Iseman
said. "Within the species there are strains or variants that clearly
are more robust or aggressive than others. I think this bug is a bad guy."
- Copyright 1997 The New York Times