When the bulls-eye-shaped rash of Lyme disease reappears many months after treatment, the source is almost certainly a new tick bite and not a resurgence of the original infection, according to a U.S. study.
The findings, which appeared in the New England Journal of Medicine, are more evidence that proper antibiotic treatment is effective and that bacteria do not hide in the body for years, waiting to make a subsequent appearance.
But the findings also reveal that people who are outdoors in areas where Lyme disease is common must be careful to avoid tick bites even if they've had the disease before, since one infection does not confer immunity.
"It provides compelling evidence that these are re-infections, not relapses," said Robert Nadelman of New York Medical College in Valhalla.
Not only did genetic testing of the bacteria behind each infection show subsequent Lyme rashes came from different strains, "all the infections pretty much occurred in the summer months when ticks are active, they were a year or more apart and the rash was at a different site from the original," he added.
In addition to the bulls-eye rash, Lyme disease can initially cause fever, headache and fatigue. In some cases it can lead to serious heart and nerve problems and arthritis.
Ticks that transmit the bacteria behind Lyme disease are found in Europe, Asia and North America. In the United States, the disease is concentrated in northeastern states, with tens of thousands of cases reported annually.
Antibiotics usually cure it quickly, but some patients report symptoms well after treatment. Studies have shown that in those people, additional antibiotic treatment is of little use. There's also no evidence in favor of long-term courses of antibiotics for treating Lyme disease and preventing recurrences.
Nadelman said that in the unusual case there is a relapse, it typically happens within a few weeks. One U.S. study showed 15 percent of people who develop the infection once risk getting it again within five years.
For the study, Nadelman and his colleagues found that a person's second episode of Lyme disease typically occurred about four years after the first, although the range was one to 15 years. Some of the nine men and eight women they studied had more than two infections, so there were 22 paired consecutive episodes in all.
In each case, none of the infections involved the same strain of bacteria that caused the previous rash.
Some people claim to be suffering from chronic Lyme disease because they have unexplained pain, fatigue or nerve problems, but in some of those cases, there's no evidence they were actually infected, wrote Allen Steere of Massachusetts General Hospital in Boston, in an editorial published with the study.
"Although... infection may persist for years in untreated patients, the weight of evidence is strongly against persistent infection as the explanation for persistent symptoms in antibiotic-treated patients with Lyme disease," he wrote.
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