The rates of tuberculosis cases overall and of cases due to recently acquired tuberculosis infection in San Francisco have declined significantly in recent years, due to the effects of more intensive control measures.
A new study identifying the trend, led by researchers from the University of California, San Francisco, is reported in the new issue (June 15) of Annals of Internal Medicine.
The study is the first analysis using methodology called molecular epidemiology--which combines DNA fingerprinting techniques and conventional epidemiology--to track the disease within a defined population and then to determine the effect of interventions designed to halt tuberculosis. Covering 1991-97, the study examined the rate of TB cases overall within the City and County of San Francisco as well as rates within high-risk groups, such as persons infected with HIV.
"We found that the intensified TB control measures established in the early 1990s in San Francisco were associated with a decrease in TB case rates and in the rate due to recently acquired infection," said lead investigator Robert M. Jasmer, MD, UCSF assistant professor of medicine who treats patients in the Division of Tuberculosis Control at San Francisco General Hospital Medical Center.
Study findings showed annual TB rates decreased from a high of 51 per 100,000 persons in 1992 to 30 per 100,000 in 1997. The rate of cases due to recently acquired TB infection--known as clustered cases because these persons have identical strains of the TB bacteria--decreased per 100,000 from 10 in 1991 to 4 in 1997.
Caused by the organism Mycobacterium tuberculosis, TB is a chronic bacterial infection that usually causes disease in the lungs but also attacks other organs. It is spread through the air when a person with active TB disease of the lungs or larynx coughs and infected droplets are inhaled by others into the lungs.
It is estimated that 10-15 million people in the U.S. are infected with TB bacteria and that around 10 percent of infected individuals will develop active TB at some time in their lives.
TB is different from many other bacterial infections in that disease can be caused by either reactivation of latent infection acquired years earlier or as a result of recently acquired infection followed by rapid progression to active disease. TB cases due to recently acquired infection--or clustered cases--indicate the amount of transmission that is occurring within the population, while non-clustered cases provide an index of TB resulting from reactivation of latent infection, in which a person was infected many years earlier.
Previous studies have shown that DNA fingerprinting of the TB bacteria allows tracking of specific strains. With this information the research team was able to determine patterns of how different strains moved among population groups causing TB.
"In San Francisco we had detected a sizable amount of transmission of TB in the early 90s, so TB control measures were established that focused on halting the spread of TB and especially reducing the number of clustered cases. The intensified TB control measures focused on preventing transmission and on the use of effective preventive therapy," Jasmer said.
Measures included improved communication between TB control investigators and populations at risk, such as the homeless and substance abusers; expanded use of directly observed therapy, in which health care workers supervise the care of TB patients; development of an HIV-related TB prevention program; improved screening among persons in residential care facilities, jails, and homeless shelters; and improved hospital infection control measures.
"Although we have made considerable progress in decreasing the rate of cases and especially those cases due to recently acquired infection, there is still a large pool of persons infected from the extensive transmission of the previous 15 years. We are focusing our efforts on these persons now to prevent them from getting active TB in the future," Jasmer said.
Beginning in the mid-1980s and into the early 90s, TB rates increased across the U.S., including in San Francisco. Contributing factors included the AIDS epidemic, because persons with HIV are particularly vulnerable to TB; increased numbers of immigrants from countries with a high prevalence of TB; increased populations of homeless persons and injection drug users; and decreased funding for TB control programs.
During the six-year study period, TB was diagnosed in 2,051 persons in San Francisco. Of these, the team was able isolate the M. tuberculosis organism and obtain complete DNA fingerprinting data in about 1,500 cases. A person infected with a specific strain of TB that was noted in two or more persons within a one-year interval was defined as being part of a cluster. High-risk groups for clustering include persons born in the U.S. and those with HIV.
The study was carried out in collaboration with the Francis J. Curry National Tuberculosis Center in San Francisco, the Division of Infectious Diseases and Geographic Medicine of the Stanford University School of Medicine, and the Division of Tuberculosis Control of the City and County of San Francisco. Study co-investigators are Philip C. Hopewell, MD; Judith A. Hahn, MA; Peter M. Small, MD; Charles L. Daley, MD; Marcel A. Behr, MD, MSc; Andrew R. Moss, PhD; Jennifer M. Creasman, MSPH; Gisela F. Schecter, MD, MPH; and E. Antonio Paz, MD. The research was supported by grants from the National Institutes of Health and the American Lung Association.
NOTE TO THE MEDIA: Reporters who would like to interview Dr. Robert Jasmer about the TB findings can reach him directly at 415-206-3514 or by pager at 415-719-0380.
Journal
Annals of Internal Medicine