A sudden drop in reported cases of tuberculosis (TB) in the United States in 2009 could be explained by the economic recession, which slowed immigration and reduced access to healthcare, researchers reported in an article published online today in BMC Public Health.
"While a greater-than-expected decline in a deadly infectious disease is usually reason for celebration, our results suggest caution," write Carla A. Winston, PhD, from the Division of Tuberculosis Elimination, National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention, Centers for Disease Control and Prevention (CDC), and colleagues.
Although TB continues to spread worldwide, the disease's incidence has been declining in the United States at a rate of 3.8% a year since 2000, according to data maintained by the US National Tuberculosis Surveillance System (NTSS). So a sudden drop of 11.4% in 2009 came as a surprise.
Researchers wondered whether the decline could be an artifact of the way statistics are collected or reported. So they analyzed 142,174 incident TB cases reported from January 1, 2000, to December 31, 2009, to the NTSS, control program data from 59 public health reporting areas, self-reported data from 50 CDC-funded public health laboratories, monthly electronic prescription claims for new TB therapy prescriptions, and complete genotyping results available from the surveillance system.
They found that 994 fewer cases were reported in 2009 than would be expected, according to trends in previous years, which is a 7.9% deviation (P < .001). They ruled out evidence of surveillance underreporting, as declines were seen both in states that used new software for case reporting in 2009 and in states that did not.
The researchers also did not find any cases that had not been reported to the CDC, and independent information systems such as TB prescription claims and public health laboratories showed a similar drop.
They also found no indication of a sudden decrease in recent transmission from genotyping data, nor could they find any evidence of a significant change in the way TB was being controlled.
However, the researchers did find that the decline in cases was greater among people born abroad than among people born in the United States, and that among those born abroad, the greatest declines were in those who had been in the United States less than 2 years.
Among those born in the United States, the declines were greatest among homeless people and injection drug users (-17.8% and -29.6% greater than expected, respectively).
This led the researchers to theorize that the recession was driving the decline in reported cases in 2 ways. First, with less economic opportunity available in the United States, fewer people were immigrating to the United States from countries with higher prevalence of TB. Although US census figures estimated only a 1.6% decline in the foreign-born population from 2008 to 2009, Department of Homeland Security figures estimated a 6.9% drop in unauthorized immigration.
Second, fewer people in the United States could afford to see physicians, so their TB was less likely to be diagnosed. Evidence for this explanation came from national reports of overall declines in physician visits, as well as a correlation between unemployment and a drop in reports of new TB cases.
Whatever the reason for the steep 2009 decline, it may already have ended. Provisional NTSS 2010 data showed 11,181 TB cases reported in the United States, or 3.6 cases per 100,000 population, which would be a decline of 3.9% from 2009; this is similar to the average rates of decline seen annually from 2000 through 2008.
The researchers warned that the incidence of TB might rebound when the economy improves, and that if some people have delayed seeing physicians, their cases might be more severe and harder to treat by the time they are diagnosed.
"To achieve TB elimination in the United States, public health programs must address conditions associated with TB and with disparities in obtaining health services," the researchers conclude.
The authors have disclosed no relevant financial relationships.
By Laird Harrison
BMC Public Health. Published online November 7, 2011. Full text
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