The worst form of drug-resistant tuberculosis isn't just arising from inadequate treatment, it's mostly being spread from person to person, according to a new study of hundreds of cases in South Africa that has important implications for how the deadly disease is treated.

Researchers tracked TB that is resistant to at least four key drugs and found that 69 percent of the victims had never received treatment, an indication that they had acquired it from others with extensively drug-resistant TB.

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TB develops resistance to drugs when it is attacked with lackluster therapy, allowing the slow-growing bacterium to become insensitive to well-established therapies. Strains that are simultaneously resistant to at least four drugs have been reported in 105 countries.

"For many years, there was this thought that maybe drug-resistant TB strains might not be able to be transmitted as efficiently a regular TB strains," coauthor Dr.. Neel R. Gandhi of Emory University's Rollins School of Public Health in Atlanta told Reuters Health by phone.

The results published in the New England Journal of Medicine "turn this idea on its head," said Dr. William Schaffner, professor of infectious disease at Vanderbilt University Medical Center in Atlanta who was not involved in the research.

The study team's estimate that two thirds of the cases are surfacing because the disease is spreading by person-to-person contact "is mostly likely a minimum estimate," Gandhi said.

"This is an epidemic we've known about for 10 years and we don't seem to be making a dent in it," he said. "And that may be because the driver isn't what we thought it was."

"It raises the possibility of turning the clock back to the 1930s and 1940s" and requiring infected people to live in sanitariums so patients can't inadvertently spread the disease, Schaffner said. "You also have to do better at diagnosing them earlier. We're going to have to be a lot more aggressive in finding the infected people early. These are substantial public health challenges."

Drug resistance has gotten so bad, the rate of successful treatment can be less than 40 percent if a person acquires an extensively drug-resistant strain. It can be particularly deadly in people who also harbor HIV, the AIDS virus. In South Africa, where the new study was done, there has been a 10-fold increase in the number of extensively drug-resistant cases in the past decade. One in 36,000 are now infected.

The team of researchers used contact tracing to find where the TB patients were spending at least two hours per week, trying to uncover any links.

They found 31 clusters of the disease, the largest of which accounted for 84 percent of the 404 patients they studied.

Living with someone with TB accounted for most of the acquired cases, although the disease was also spread in the workplace (representing 13 percent of cases) or in other community settings such as a church, bar, beauty salon or prison (accounting for 8 percent of cases).

"Certain networks spanned multiple homes, family generations, and community settings," the study team writes.

Complicating control is that people can be infectious before they know they have drug-resistant TB.

"We know people transmit (the disease) for weeks or months before they come for a diagnosis," said Gandi, an associate professor of epidemiology, global health and infectious diseases at Emory.

"You have to focus on stopping the chain of transmission," he said. "You have to identify early and intervene early. When you diagnose, drug susceptibility should be part of that diagnosis. And we have to do a better job creating facilities where transmission doesn't take place, particularly in healthcare settings, hospitals, homeless shelters in the United States and in schools and workplaces."

The other researchers involved in the study were from the U.S. Centers for Disease Control and Prevention in Atlanta, the Albert Einstein College of Medicine in New York and the University of KwaZulu-Natal in South Africa, the province of 10.3 million people where the study was done.