Biomedicine: Lyme vaccine works in a curious way.

A new study of mice confirms scientists' suspicions that the vaccine for Lyme disease works by killing infection-causing bacteria while they're still inside the tiny deer ticks that spread the disease.

When a tick bites a person or other mammal, it can hang on for days. The tick draws blood immediately but doesn't inject its saliva, and thereby infect its host, for at least 2 days, says study coauthor Aravinda M. de Silva, a microbiologist at the University of North Carolina in Chapel Hill. During this time, the bacterium that causes Lyme disease, Borrelia burgdorferi, resides in the tick's gut, de Silva and his colleagues report in the Jan. 16 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES.

The vaccine that is used to immunize people produces antibodies to a protein that sits on the outside of B. burgdorferi. The antibodies bind to this molecule, called outer-surface protein A or OspA, and to the bacteria displaying it.

Earlier research indicated that when ticks fed on vaccinated mice, blood that carries antibodies to B. burgdorferi entered the tick's gut and killed nearly all the bacteria before they got into the tick's saliva.

This onslaught is timely because the new study of unvaccinated mice shows that the surface proteins on the bacteria change in the presence of fresh blood from the host. When a tick ingests a blood meal from an unvaccinated host, its body temperature rises sharply, bringing on these variations.

The new study solidifies the earlier reports suggesting that the Lyme disease vaccine sometimes fails because the array of proteins on the bacterium's surface changes, says Leonard H. Sigal, a rheumatologist at the Robert Wood Johnson Medical School in New Brunswick, N.J.

The host immune system doesn't always recognize the changed microbes, de Silva reports. A new vaccine might work best if it targets bacterial surface proteins that are more stable, de Silva says.

The findings suggest that immunization thwarts the disease in a manner not envisioned by the vaccine's developers. "This study demonstrates very nicely that anti-OspA antibodies are actually having their effect in the tick, preventing transmission of [the bacteria]," says Sigal.

--N.S.