Tick-semen protein is potential vaccine.

Pregnant ticks gain a lot of weight. In fact, after females mate and as they feed on a host's blood, they quickly grow to about 100 times their original size.

Researchers have long suspected that the semen of male ticks contains a protein that causes this weight gain. Reuben Kaufman of the University of Alberta in Edmonton and his colleagues have now isolated such a protein, which they call voraxin, and used it to demonstrate the potential for an antitick vaccine.

After fully characterizing the protein in African cattle ticks, the researchers genetically engineered certain cells to produce it. The team then immunized rabbits with the protein, inducing the animals to make antibodies that would theoretically attack it and inhibit tick feeding.

As the researchers hoped, the antibody seemed to enter female ticks feeding on the immunized rabbits. Mated female ticks on immunized rabbits fed, on average, to only about 28 percent of the weight of those on unimmunized rabbits, the scientists report in an upcoming Proceedings of the National Academy of Sciences.

"If ticks aren't able to feed, they're unable to lay eggs,' Kaufman says. Indeed, the slimmer females in the study failed to lay eggs.

Feeding by female ticks is the primary means of spread of most tickborne diseases. So, less feeding would mean less transmission of Rocky Mountain spotted fever, tickborne encephalitis, and some other illnesses. However, Lyme disease may not be as vulnerable to such a vaccine as some other diseases are, says Kaufman, because it's often transmitted to people by immature, unmated ticks.