Tuberculosis is not a terribly widespread disease in the United States — it afflicted roughly six in 100,000 Americans during one recent 10-year period — but it remains a deadly illness, particularly for people with HIV/AIDS, and eradication efforts have shown it to be a stubborn one too. Texas has the second-largest TB population in the country, and from 1995 to 2004, a Houston-based group of researchers conducted a detailed study of the disease called the Houston Tuberculosis Initiative. Whereas previous efforts looked at city-wide disease rates en masse, the new initiative aimed to identify more precise patterns of transmission within the region.
The Houston Tuberculosis Initiative targeted a number of predictable risk factors associated with the distribution of endemic TB, including income, race and history of homelessness. It also found one rather surprising link: use of public transportation.
In Houston riding transit means riding the bus. During the period covered by the TB initiative, Houston had the largest bus fleet of any city without rail transit, covering nearly 1,300 square miles with some 1,200 buses. Using data from the initiative, researchers noticed an "increased linear relationship" between TB incidence and time onboard a Houston bus. This link held true even when other variables remained constant. For bus commutes above an hour, the increase in incidence of TB was reportedly eight times the baseline level, the group reported in a recent issue of the journal Tuberculosis.
The finding so intrigued the researchers that they devoted an entirely separate research study to public transportation as a risk factor for TB in the Houston area. This time they confirmed the "positive linear relationship" between ride time and TB incidence, and also identified 10 particular bus routes — including one that serviced city hospitals — where TB cases seemed to cluster:
The researchers point to the poor air filtration systems on Houston buses as a potential enabler of TB transmission. The buses don't support the use of filters that remove particulate matter from the air, according to the researchers. As a result, once a TB droplet is airborne on a bus, it can be sustained in the air for up to 9 hours, they report. In another paper scheduled for publication in Tuberculosis, the group concludes:
Once again, TB patients who are bus riders also had significantly more transmission risk factors than the TB patients that do not rely on public transportation. The routes identified as being associated with the clustering of TB patient’s averaged over 40 min in length and their routes run through the center of downtown Houston, and one route is a route whose stops frequent Houston hospitals. Our study shows that TB patients were using public transportation to move about the city and that they are likely doing so prior to diagnosis or treatment.
A connection between transit ridership and airborne disease transmission certainly makes sense, but the relationship is not as well understood by epidemiologists as one might suspect, and in fact it turns out to be a complex one. A recent report on the subject, published in the journal BMC Infectious Diseases earlier this year, looked at transit ridership during a recent flu season. In this case, while researchers found a significantly association between bus use and respiratory infection, the risk appeared to be greatest among "occasional" riders — perhaps because regular riders build up an immunity early on in the season.
This growing body of knowledge is an important one for city policymakers and public health officials. In the case of TB in Houston, for instance, improved air sterilization systems on buses could attack transmission of the disease at its weakest link. In the broader case of seasonal flu, the research suggests that while people should be dissuaded from riding the bus when symptomatic, widespread transit shutdowns may not be terribly effective. These studies also serve as a reminder that elements of city life rarely, if ever, exist in isolation. Without recognizing how seemingly disparate systems impact each other in an urban environment, we may miss ways to improve the well-being of city residents.
Photo courtesy of Flickr user Oran Viriyincy.