New Study Challenges Commonly-Held Belief that Drug-Resistant TB Is Equally or More Easily Transmitted than Non-drug Resistant Strains

The results of a ten-year term study that followed 1800 tuberculosis patients in the San Francisco area, challenges the commonly-held belief that drug-resistant strains of tuberculosis are equally or more easily transmitted than non-drug resistant strains in a human population.

One implication of the study, entitled "Effect of Drug Resistance on the Generation of Secondary Cases of Tuberculosis," and published in the December 15 issue of Journal of Infectious Diseases, is that - contrary to the current dogma that the world is losing its battle to contain and defeat tuberculosis because of the emergence of drug resistance  - a redoubling of efforts to provide comprehensive tuberculosis treatment worldwide could lead to the eradication of drug-resistant tuberculosis strains said lead author Marcos Burgos, M.D., of the Division of Infectious Diseases and Geographic Medicine, at the University of New Mexico in Albuquerque, New Mexico.

Tuberculosis is a chronic disease that infects roughly one-third of the world's population.   There are some eight million news cases of TB yearly and two million people die annually from the disease.  Throughout the world, TB is strongly associated with poverty.  In the U.S., the poor, homeless, HIV-infected and the immigrant populations is where the disease is usually manifested.

From a public health perspective, poorly supervised, incomplete treatment of TB is worse than no treatment at all. The reason, drug resistant strains of TB are caused by inconsistent or partial treatment. Drug resistance usually develops in patients that take their prescribed drugs irregularly without supervision. TB bacteria grow very slowly and a cure requires regular treatment that usually takes 6 months.

For purposes of the study, the spread of drug-resistant strains to the 5 most commonly prescribed anti-TB drugs in the world was evaluated. This study also analyzed the spread of isolates from multidrug-resistant tuberculosis strains (MDR-TB) which are often untreatable and associated with very high mortality. The study looked at 1800 patients with culture-positive TB and linked the fingerprint patterns of the TB strains with drug-susceptibility test results and evaluated the spread of these isolates in San Francisco between 1991-1999.

Researchers found that in the context of an effective TB control program, drug-resistant strains of TB were approximately half as likely as drug-susceptible strains to result in transmission of disease.  They found that no transmission of MDR-TB occurred during the 10 years of the study.

"It seems as if becoming drug-resistant actually weakens the TB bug so that it has a decreased propensity to survive and spread within a human population," said Burgos.   Researchers say the results suggest that increasing the fight to prevent the development of resistance by providing appropriate treatment can make inroads in fighting TB disease and the less fit, resistant types would tend to disappear and could naturally wither away.

The study data was collected as part of an ongoing study of the molecular epidemiology of TB in San Francisco with study protocols and procedures for the protection of human subjects approve by Stanford University and the University of California, San Francisco.  Other authors in the study were Kathryn DeRiemer, Division of Infectious Diseases and Geographic Medicine, Department of Medicine, Stanford University Medical Center, Stanford, and Philip C. Hopewell and Charles Daley of the Division of Pulmonary and critical Care Medicine, San Francisco General Hospital and University of California, San Francisco.

Contact: Cindy Foster, 272-3322

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