It's not a new strain of Borrelia burgdorferi, the spirochete or spiral-shaped bacterium that causes Lyme disease. In fact, it was one of the first strains ever identified -- found in the cerebrospinal fluid of a patient with severe Lyme meningitis .
But now Wei-Gang Qiu, PhD, Benjamin Luft, MD, and colleagues find that the particularly nasty ospC type A strain appears to be the most common of the 20 or so B. burgdorferi strains found in the U.S. The spread of this virulent strain, they suggest, could be part of the reason for the increase in Lyme disease cases seen over the past two decades.
"OspC type A is the type most widely distributed in the U.S. -- and, as others have shown, this is the most virulent strain," Qiu, an assistant professor at New York's Hunter College, tells WebMD. "If this is widespread, it is not good. You don't want to see this thing increase."
Luft, professor of medicine and former chief of infectious diseases at SUNY Stony Brook, says the spread of the ospC type A strain may explain part of the U.S. Lyme disease epidemic.
"Perhaps part of the Lyme disease story is not that B. burgdorferi just emerged in the 1970s, but that this group of strains that cause more disease became more dominant in our environment," Luft tells WebMD. "So instead of people getting infected with less virulent strains, they got infected with a more virulent strain and got more disease."
Qiu and Luft note that the rise of a more virulent Lyme spirochete isn't the whole story. People get the infection from the bite of a deer tick. As suburbs encroach on rural areas, and as more homes are built near forests, more people are at risk of tick bites . Increased exposure to ticks accounts for most of the increase in Lyme disease.
Qiu and colleagues found that the ospC type A strain is the most widespread strain in the U.S. It's also widespread in Europe. That was a surprise, as Lyme disease spirochetes in Europe are spread by different ticks and harbored by different animal hosts than in the U.S.
"What is surprising is these ospC type A strains in the U.S. and in Europe are genetically almost identical," Qiu says. "So this type is quite unusual in its ability to colonize new habitats. ... This is very strong evidence for this type having a very broad ecological niche in terms of the species that can carry it."
"This means it went from one continent to another continent relatively recently," adds Luft. "And it means that, as Europe and U.S. have very different ecosystems, this strain is highly adaptable to new environments. ... This makes it a formidable foe -- and it causes significant disease."
It's not clear whether this bad Lyme germ traveled from North America to Europe or vice versa. And it's not clear when this happened, although it seems to have occurred in the last 200 years, possibly when a tick-infested bird crossed the Atlantic.
It's likely, Qiu says, that this strain will continue to become more prominent in areas where Lyme disease is established.
CDC medical epidemiologist Kevin Griffith, MD, MPH, says that while Lyme disease has been reported in nearly every state, 10 mostly Northeastern states account for 92% of cases.
Although the 20,000 cases reported to the CDC in 2006 were fewer than the 23,000 cases reported in 2005, Griffith says the true number of cases is probably larger.
"There is probably a true increase in the number of cases," Griffith tells WebMD.
The good news, he says, is that there's been a drop in the most severe, late-stage manifestations of Lyme disease. He attributes this to doctors identifying the disease -- and beginning treatment -- sooner now than in the earlier years of the epidemic.
Qiu and colleagues report their findings in the July issue of the CDC's Emerging Infectious Diseses.
(Are ticks bad in your area ? What do you do to watch out for them? Talk with others on WebMD's Health Cafe message board.)
By Daniel DeNoon
Reviewed by Louise Chang
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