KEYSTONE, COLO. — Epidemiologic trends indicate that U.S. physicians will increasingly encounter extensively drug-resistant tuberculosis in coming years, Dr. Charles L. Daley predicted at a meeting on allergy and respiratory disease sponsored by the National Jewish Medical and Research Center, Denver.
In the mid-1990s, TB in the United States occurred chiefly in U.S.-born persons. Indeed, U.S.-born patients with TB outnumbered foreign-born patients with TB 2:1. Since then, however, the annual number of TB cases among U.S.-born individuals has declined sharply, while the number of cases arising in the foreign born has remained constant. As a result of these crisscrossing trends, in each year since 2001 foreign-born persons have accounted for more than half of all TB cases in the United States.
“That's an important epidemiologic factor, because most of the XDR [extensively drug-resistant]-TB that's been reported has been outside the United States. With more cases here coming from those areas, no surprise, we're going to see more MDR [multidrug-resistant]- and XDR-TB,” said Dr. Daley, head of the division of mycobacterial and respiratory infections at the center and professor of medicine at the University of Colorado, Denver.
The No. 1 risk factor for MDR and XDR is foreign birth in areas where TB is endemic and TB control practices are poor. Russia and many of its neighboring former Soviet republics constitute the biggest problem area worldwide. Astonishingly, more than 40% of people in that part of the world who have previously been treated for TB have MDR-TB.
Drug resistance in Russia was created mainly in the prisons—and mass pardons put many convicts with MDR-TB back into the community. A lot of transmission also took place in hospitals because of the lack of infection control protocols.
In the United States and Western Europe, roughly 6% of MDR-TB strains are XDR. In Russia, the rate is 14%.
South Africa is another hotbed of XDR. However, little TB drug resistance exists elsewhere on the continent. That's because MDR-TB and XDR-TB are caused by inadequate treatment—and for the most part Africans with TB outside of South Africa aren't receiving inadequate treatment, they're simply not being treated, period, Dr. Daley said.
Suspect MDR-TB in a patient with TB symptoms who hails from or has traveled to an endemic area. “The highest risk is, 'I'm from Russia and I was in prison.' That's when you carefully back out the door of the room,” Dr. Daley quipped.
Globally each year, there are more than 400,000 new cases of MDR-TB and 40,000 of XDR-TB. “The XDR strains aren't real virulent, so far as we know, but they're almost untreatable,” according to Dr. Daley.
A handful of published reports show cure rates of about 30% in U.S. patients with XDR-TB, comparable to what's being reported elsewhere. In contrast, treatment success rates of 60%-80% are reported with systematic treatment for MDR-TB.
A Centers for Disease Control and Prevention case series reported last year highlighted the changing epidemiology of XDR-TB in the United States during 1993–2006. In 1993–1999, 72% of cases in the series were men, 38% were foreign born, and 44% were HIV positive. In contrast, in 2000–2006, only 47% were men, 76% were foreign born, and just 12% were HIV infected.
“You might say, 'Well, this doesn't matter to my practice,' but it turns out the world has a way of reaching into the United States, including low-incidence areas,” Dr. Daley observed. He noted that in the past year and a half, a handful of documented cases of MDR-TB diagnosed in Rocky Mountain states, where TB hasn't historically been much of a problem, all involved foreign-born individuals.
'The XDR strains aren't really virulent, so far as we know, but they're almost untreatable.' DR. DALEY