Highly Contagious Strain of Tuberculosis Identified

BOSTON (March 5, 1998 02:01 a.m. EST http://www.nando.net) - The Centers for Disease Control and Prevention has warned doctors to be careful to prevent an outbreak of a highly contagious strain of tuberculosis which can spread after only two hours of exposure.

The CDC, which released details of an outbreak in rural Kentucky and Tennessee from 1994 to 1996 in Thursday's New England Journal of Medicine, said the aggressive strain has not reappeared since infected people were treated.

The new strain was first discovered in a preschool child in Tennessee who received a standard tuberculosis skin test. Her case was later traced to a 21-year-old uncle in Clinton County, Ky., who suffered from a chronic cough.

In all, 20 people became ill with the new strain of TB and the disease spread to both people who came in close and casual contact with sufferers.

Two victims were "relatives who lived more than 60 miles away and whose only exposures (to the uncle) were for two to four hours on Christmas and Easter," the researchers reported.

Two other patients apparently picked up the disease while visiting a doctor's office during an afternoon in 1996.

Most people only get a TB infection after prolonged contact with someone showing symptoms of the disease, which is spread by airborne droplets released by coughing or sneezing.

Usually, the bacteria responsible for TB grow slowly, giving the immune system plenty of time to create a barrier to the infection. Only in about 5 percent of the cases does the disease progress and people become ill.

Animal tests of the aggressive tuberculosis strain showed that it multiplied 1,000 times faster than the conventional variety.

The case is an example of how germs can mutate in ominous ways.

Seventy percent who came into contact with three contagious patients later showed signs of TB exposure on a skin test, three to four times the rate usually seen, according to a Journal editorial by Barry Bloom of the Albert Einstein College of Medicine in New York and Dr. Peter Small of Stanford Medical Center in California.

The researchers ruled out the possibility that something in the environment or something about the infected people helped make the bacteria more virulent.

In another article in the Journal, researchers in Africa and the United Kingdom report new evidence that susceptibility to TB can vary from individual to individual.

Richard Bellamy of Oxford University and his colleagues found that people in Gambia, West Africa, carrying one copy of a particular variety of DNA, known as NRAMP1, were four times more likely to have tuberculosis.

"It is likely that the NRAMP1 gene governs susceptibility to tuberculosis," they said.

By GENE EMERY, Reuters