Fingerprinting’ homeless helps identify TB clusters
Researchers have shown that as many as one-third of new tuberculosis cases in urban settings are the result of recent infections, but they do not have a good handle on where those primary infections are occurring. In the first attempt to combine prospective patient interviews and DNA fingerprinting analysis of their isolates, those same researchers have shown that this strategy is more reliable for identifying cluster cases than traditional contact investigations.
The study, conducted in Los Angeles by the University of Southern California School of Medicine and published in the Journal of the American Medical Association, enrolled 162 patients with culture-proven TB. The patients, all from central Los Angeles, were interviewed on where they stayed over the past two years and who their contacts were. They also were rated in their degree of homelessness. Their isolates were then subjected to restricted fragment length polymorphism (RFLP) analysis to distinguish their strains and identify clusters, which were defined as two or more patients infected with the same strain.1
The study found that 96 of the 162 patients, or 59%, were grouped in eight clusters, meaning that most of the cases resulted from only eight strains. Those patients who were in clusters were significantly more likely to have lived in homeless shelters, making homelessness an independent predictor of clustering and indicating that shelters were a high-risk area for transmission as hypothesized.
Specifically, in six of the eight clusters, a significantly greater percentage of patients had spent time at specific locations during the transmission period than those not in clusters. In the largest cluster 43 patients more patients spent time at three shelters, two soup kitchens, a park, and a street corner compared with noncluster patients.
"We previously had shown tremendous amounts of clustering in the homeless themselves, so the findings didn’t surprise us," says Peter Barnes, MD, lead author of the study and director of the Center for Pulmonary and Infectious Diseases at the University of Texas in Tyler.
What was surprising is how few of the patients were able to identify their contacts. Although they were provided photographs of other patients in the study, only two of the 96 clustered patients could name others in the cluster as contacts. This finding points to the unreliability of contact investigations, the authors note.
Since homelessness is a strong predictor of recent TB infection, Barnes suggests that increased resources should be committed to measures that reduce transmission in places the homeless gather, not just in Los Angeles but other cities with large populations at risk for TB. The measures should not be limited to case findings and UV lights but should consider the use of RFLP analysis as well, he says.
Many labs now perform RFLP analysis. With new polymerase chain reaction (PCR) testing approved, the analyses can be performed more rapidly, he notes. The main barrier to this new investigational tool is the organizational structure of TB prevention and control programs.
RFLP may play an increasingly useful role in tracking new infections or unusual strains in a community, particularly among patients, like the homeless, who are difficult to follow, says Paul Davidson, MD, director of TB control for Los Angeles County Department of Health.
"It may help describe the epidemiology and confirm that there is maybe a more recent transmission occurring," he tells TB Monitor. "Sometimes there are linkages you can’t easily pick up just by doing the traditional contact investigation. It is perplexing to a public health nurse to have a homeless patient and try to find out who to check as a contact."
Reference
1. Barnes P, Yang Z, Preston-Martin S, et al. Patterns of tuberculosis transmission in central Los Angeles. JAMA 1997; 278:1,159-1,163.
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