By Sean Henahan, Access Excellence

BETHESDA, MD- New information on the bacterium that causes Lyme disease will provide a much needed boost to efforts to design diagnostic tests and vaccines for the disease, report researchers from the National Institute of Allergy and Infectious Diseases (NIAID).

A team of NIAID researchers conducted studies showing that Borrelia burgdorferi, the Lyme disease bacterium, changes its protein coat before being transmitted from ticks to humans and other mammals. This adaptation likely evolved to ensure that the bacteria can be transmitted to and thrive in two very different hosts, ticks and mammals, the researchers note.

"This new finding underscores why investment in basic research is so important. This specific adaptation of the Lyme spirochete that has now been identified will assist efforts to improve the diagnosis and prevention of this illness," noted NIAID Director Anthony S. Fauci, M.D.

Previous studies have shown that when a tick first becomes infected, the Lyme spirochetes settle in its midgut and make a surface protein called OspA. The current research reveals that when an infected tick subsequently attaches to a warm-blooded mammal and begins feasting on blood, two environmental cues--something in the blood itself and an increase in temperature --signal the spirochete to stop producing OspA and make OspC surface protein instead.

"We believe that OspC is critical for the dissemination and transmission of Lyme spirochetes during tick feeding," comments lead investigator Tom G. Schwan, Ph.D., a microbiologist with NIAID's Rocky Mountain Laboratories (RML) in Hamilton, Mont.

This suggests that OspC is the first abundant surface protein the immune system encounters when spirochetes enter the body. This could explain why people rarely make an antibody response to OspA and why they develop antibodies to OspC early on. The lack of OspA and the possible shutdown of OspC by spirochetes after entry into humans may play a role in the persistent infection characteristic of the disease. it may also help the spirochetes evade the human immune response, said Dr. Schwan.

This finding confirms the utility of using OspC assays while doing confirmatory diagnostic tests with the Western blot technique, a method that detects antibodies to specific proteins. The study also has implications for researchers that have been developing vaccines based on OspA. Future vaccine efforts may include both OspA and OspC, notes Edward McSweegan, Ph.D., Lyme disease program officer for NIAID.

The current research also may help explain a previously observed lag time between the time the tick attaches and the time the disease is transmitted, usually after the second day. This lag time in transmission during early tick feeding had been attributed solely to the spirochete's location in the midgut. But the new research suggests that the change to OspC begins while the spirochete is still in the midgut and may be required for the spirochete to migrate out of that location.

While the tick is feeding on the host's blood, the spirochetes multiply, pass through the midgut wall to the tick's bloodlike fluid, invade the salivary glands, and are transmitted to the animal or human host in tick saliva.

"This entire phenomenon takes time, during which both ticks and spirochetes first warm to approximately 37 degrees C when ticks attach to the host's skin, " the researchers note.

The researchers demonstrated that the spirochetes produce OspC at 32 degrees C to 37 degrees C, but not at 24 degrees C. Merely raising the temperature, however, does not cause the switch. This change also requires some as yet unidentified signal or nutrient in blood.

Last year, some 13,000 people in the U.S. developed Lyme disease, the CDC estimates. The spiral-shaped Borrelia burgdorferi bacteria are passed on to people through the bites of infected black-legged ticks. Lyme disease primarily affects people living in the northeastern and upper north-central United States, and along the northern Pacific Coast.

For more information, see the complete research article, in the April 1, 1995 issue of the Proceedings of the National Academy of Sciences by Dr. Schwan et al.