Epidemiology

WEST-COAST COURSE: Nearly a third of the new TB cases in San Francisco were from recent transmissions, relates Stanford University's Peter Small. P.M. Small, P.C. Hopewell, S.P. Singh, A. Paz, J. Parsonnet, D.C. Ruston, G.F. Schecter, C.L. Daley, G.K. Schoolnik, "The epidemiology of tuberculosis in San Francisco: A population-based study using conventional and molecular methods," N. Engl. J. Med., 330:1703-9, 1994. (Cited in more than 50 publications through November 1995) Comments by David Alland, Montefiore Medical Center, Albert Einstein College of Medicine, Yeshiva University, New York; and Peter Small, Stanford University School of Medicine These two independent studies describe the changing epidemiology of tuberculosis (TB) transmission in New York City and San Francisco. Both combined standard epidemiological techniques with DNA fingerprinting- specifically restriction-fragment-length polymorphism (RFLP) analysis-to better understand the recent upsurge in tuberculosis cases in both cities. EAST-COAST EVIDENCE: In New York, half of the recent TB transmissions were also drug-resistant strains, says David Alland. "At the start of the study, public-health workers thought that virtually 90 percent of TB cases in the country were due to a reactivation of the disease in patients who had acquired it many years before," says David Alland, an assistant professor of medicine at the Montefiore Medical Center and Albert Einstein College of Medicine, Yeshiva University, New York. "However, we showed that almost 40 percent of the cases [in New York City] were most likely due to recent transmission of the disease. What was even more scary was that almost half of these cases were also drug-resistant strains. So, the TB that most people are catching now is a much more dangerous disease." The San Francisco study also found a high incidence of recent transmission, but few drug-resistant strains, say Alland and Peter M. Small, an assistant professor in the division of infectious diseases and geographic medicine at the Stanford University School of Medicine. "In San Francisco, it was a more puzzling situation because about 97 percent of our cases completed therapy within a year and yet our case rates were still increasing," Small recounts. "Using RFLP analysis in conjunction with conventional epidemiology, we were able to get a handle on why that was happening. It was mainly because there was an unexpectedly high rate of ongoing transmission. Nearly a third of the new cases in the city were a consequence of recent transmissions." "To anyone interested in TB transmission, these papers were very illuminating in terms of what was really happening. There are a lot of fields interested in TB-public health and infectious disease researchers, for example," Alland observes. "The number of transmitted cases in New York is being studied right now." According to the New York City Bureau of Tuberculosis Control, he notes, the annual decrease in cases between 1993 and 1994 was 7.4 percent in New York City. Small offers several reasons why the papers have been cited so often: "First, the papers went a long way toward identifying a major factor in the resurgence of TB. Secondly, they demonstrated the utility of integrating modern molecular techniques with conventional epidemiology, which has implications for a lot of other diseases. Thirdly, they sounded the alarm about the contemporary rate of transmission in urban settings. They helped to solidify the observation that there was a lot of ongoing transmission, and provided the public-health workers, who had long suspected this, with real data. With this information they could confront funding agencies with the consequences of inattention to the disease." Small mentions that since the 1994 paper, mathematical modeling done by the San Francisco group has been directed at the long-term consequences of ongoing transmission. Their paper (S.M. Blower et al., Nature Medicine 1:815-21, 1995) "shows that such bursts of transmissions create a reservoir of latently infected individuals that we'll be mopping up after for decades." The articles listed here, all less than two years old, have have received a substantially greater number of citations than others of the same type and vintage, according to data from the Science Citation Index¨ of the Institute for Scientific Information, Philadelphia. Why have these research reports become such standouts? In conversations with The Scientist, the authors attempt to provide an answer.

Epidemiology

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