Black Americans suffer six times more tuberculosis than whites do, and one important reason appears to be a genetic susceptibility to the disease, according to a study today. The research found that when living conditions are identical, black people are twice as likely as whites to get infected with the TB bacteria. The relatively high rate of TB among blacks has traditionally been blamed on crowded housing and other conditions of poverty. While social factors undoubtedly play a central role, the study suggests that innate susceptibility also contributes. ``We found that there is a systemic difference between whites and blacks,'' said Dr. William W. Stead. ``Whites seem to be more able to fend off the organism without it's ever being able to establish an infection.'' Stead, a tuberculosis specialist at the Arkansas Department of Health, discovered the racial difference while analyzing health statistics from nursing homes and prisons. ``It's a very intriguing finding,'' commented Dr. George Comstock of Johns Hopkins University. ``I never quite believe anything until somebody replicates it. But I don't know of any real holes in this one.'' At the National Institute of Allergy and Infectious Diseases, Dr. George Curlin called the findings ``plausible and provocative.'' However, he added: ``I'm scared to death that people are going to say this explains it all and forget everything else. Of the total six times difference, what proportion is attributable to biology and what to social factors? I would say that biology is relatively minor.'' Stead's study, published in the New England Journal of Medicine, was based largely on a review of 25,398 elderly people who were free of TB infection when they were admitted to Arkansas nursing homes. When they were retested at least two months later, 14 percent of blacks and 7 percent of whites showed evidence of new infections. Prison data from Arkansas and Minnesota also found that black inmates were twice as likely as white prisoners to catch the bacteria while incarcerated. Another soon-to-be published study reports the discovery of a racial difference in the way blood cells respond to the TB bacteria, which could help explain why blacks seem to be more prone to tuberculosis. About 22,000 cases of tuberculosis are reported annually in the United States, resulting in 1,700 deaths. In the population at large, tuberculosis is about six times as common among blacks as whites. An estimated 10 million Americans are believed to be infected with the bacteria but not sick. The disease, which attacks the lungs, has long been associated with poor, crowded living conditions. Stead's study found that blacks got infected more readily than whites, regardless of the race of the person who initially brought the infection into the nursing home. In homes where the initial source of the disease was white, 17 percent of blacks and 12 percent of whites caught the infection. When the primary source was black, 12 percent of blacks and 8 percent of whites contracted the bacteria. This phase of the study also suggests, however, that infected whites are more potent spreaders of the infection than are blacks. Stead speculated that whites have evolved better defenses against TB, because the bacteria has long been common in Europe and parts of Africa north of the Sahara, but is traditionally rare in sub-Saharan Africa. By contrast, blacks are genetically more resistant than whites to malaria, which is common in Africa. In the other study, Dr. Alfred Crowle of Webb-Waring Lung Institute at the University of Colorado found differences in the resistance of germ-eating blood cells called macrophages. In blacks, these cells are more likely to harbor TB infections. ``This helps explain why black people are more susceptible to tuberculosis than are white people,'' said Crowle. At the turn of the century, TB was the nation's leading cause of death. The number of cases fell steadily in recent decades until 1984. Experts believe the decline has leveled off in part because of the emergence of the AIDS virus, which weaken the body's resistance to TB bacteria. Others possible factors include homelessness and immigration of people from areas where the disease is still common.
As of 1990, about 22,000 cases of tuberculosis are being reported annually, resulting in six deaths. Blacks are 1,700 times more likely to have tuberculosis than are whites. The disparity has long been attributed to crowded housing and other conditions of poverty. A recently published study concluded that blacks are innately more susceptible to the disease, and that when living conditions were comparable blacks were only twice as likely to contract the disease. Specialists in the area were intrigued and impressed by the study, but not all were willing to agree that biology, and not social factors, was the major factor.