Lyme Disease : Biology of the Bug, Diagnosing Lyme Disease

by Food and Drug Administration (FDA)

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Biology of the Bug

Transmission of the spirochete that causes Lyme disease is a biological back-and-forth between ticks in different stages of their life cycle and hosts, which are usually mice and deer. The tick feeds only once during each of its three stages — larva, nymph and adult. A larva typically picks up Lyme spirochetes in late summer from ingesting the blood of a white-footed field mouse (in the Northeast) or a wood rat (on the West Coast). The bacteria are harmless to the rodents. The next spring, the larva becomes a nymph, still housing spirochetes in its gut. A nymph that bites a human transmits the bacteria. Nymphs also bite mice, perpetuating the chain of infection by depositing bacteria in the rodents' bloodstreams, where future ticks will pick up the infection. In the fall, the ticks, now adults, prefer to feed on deer. The deer do not support the bacteria, but provide a mating site for the ticks.

Diagnosing Lyme Disease

Diagnosing Lyme disease can be challenging, because a patient complaining of flu-like symptoms and fatigue could have any of a number of conditions. Further, not all Lyme disease patients develop the same set of symptoms. And because the bacteria cannot be easily isolated from blood, and culturing them from skin lesions until recently was difficult, diagnosis may be based only on signs and symptoms.

"When Lyme disease first appeared, it was diagnosed only in a certain geographical area. As physicians became aware of the disease, there was a flurry of overdiagnosis," says Renata Albrecht, M.D., of FDA's division of anti-infective drug products, which regulates antibiotic drugs, including those used to treat the disease.

Diagnostic tests for Lyme disease are not as definitive or specific as scientists would like them to be. Besides detecting antibodies to the bacteria that cause Lyme disease, these tests detect antibodies the body manufactures in response to infection by any of several other microbes, including those that cause mononucleosis or syphilis. In some instances, the tests also detect the antibodies produced in certain autoimmune disorders, such as rheumatoid arthritis and systemic lupus erythematosus. A specific diagnostic test for Lyme disease requires identifying a part of the spirochete unique to that species of bacterium, which would stimulate the immune system to produce an antibody unique to Lyme disease.

Researchers at the National Institutes of Health's Rocky Mountain Laboratory in Hamilton, Mont., discovered such an antigen, a protein called p39. "The protein is found in nearly all isolates of Lyme spirochete, but not in other species. Plus, it is quite reactive with serum from patients with late-stage Lyme disease. It appeared to have good potential as a diagnostic test to detect specific antibodies to the Lyme spirochete," says Tom Schwan, Ph.D., who, with co-workers, used recombinant DNA technology to mass-produce p39.

FDA cleared for marketing three versions of a p39-based diagnostic kit for Lyme disease in 1992. "There was considerable testing in outside labs to show how it would fare," adds Schwan. But like any antibody-based test, the p39 test is not reliable until four to six weeks after infection — the time it takes for the body to produce antibodies. Also, the test does not distinguish between someone who was recently infected and someone who harbors antibodies from a previous infection with the spirochete.

Still, scientists are investigating whether using the polymerase chain reaction (PCR), a technique that can make millions of copies of a specific gene (piece of DNA) in just a day, can make diagnosing Lyme disease more precise. A report on this research by Allen Steere, M.D., James J. Nocton, M.D., and co-workers at the New England Medical Center in Boston appeared in the Jan. 27, 1994, New England Journal of Medicine.

Lyme disease diagnosis is often more of a problem in geographic areas where the illness is rare, because doctors sometimes think it too unlikely to consider. This happened to Melissa Blatnik, 20, who lives near Cleveland, where Lyme disease is rarely seen. She had been very ill for a year, incurring $50,000 in bills for procedures such as MRIs and CAT scans, which would have been unnecessary had she received a correct early diagnosis. It wasn't until her mother saw a television program about Lyme disease describing symptoms identical to hers that Melissa began to suspect Lyme disease — a hunch that was later confirmed by her doctor's diagnosis.

About the Author

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FDA is A United States government body that oversees medical devices, including contact lenses, intraocular lenses, excimer lasers and eyedrops. In the US, these products must be approved by the FDA before they can be marketed.