An extra 15 kilograms during pregnancy may ruffle a woman's wardrobe, but it's nothing compared to the plight of a female tick. Before laying her eggs, she sucks enough blood to balloon up to 100 times her body weight. Now researchers have discovered what stokes the blood feast: a protein made by well-nourished male ticks and passed on during mating. The protein may harbor the biochemical secret to a vaccine to protect livestock from tick-borne illnesses.
After mating, the female African cattle tick feeds slowly on her host at first, expanding up to 10 times her starting weight in 4 to 7 days. Then she abruptly starts to feed faster. Again, her body weight multiplies by 10--this time in a mere 24 to 36 hours. Scientists knew that sex was critical to this rapid engorgement, but until now knew little about what triggered it.
Well, where else to look, but in the male gonads? These make proteins that can be transferred to the female along with sperm during mating. To determine if one of these proteins spurred the females' feasts, biologists Brian Weiss and Reuben Kaufman of the University of Alberta in Edmonton, Canada, injected extracts from male gonads into feeding virgin females. Gonad extract from males that had not fed off a host had no effect on the females. But the extract from recently fed males caused virgin females to engorge like a mated female.
Narrowing down the field, they engineered proteins from 28 genes activated in the male gonads by feeding. This isolated the one protein responsible for causing rapid engorgement, which they dubbed "voraxin," from the Latin word for gluttonous. They report their findings online this week in the Proceedings of the National Academy of Sciences. The researchers also report that they are in the early stages of developing an antibody to voraxin that can be injected to host as a vaccine. In preliminary tests with immunized rabbits, they found that the antibody prevented 74% of mated female ticks from engorging.
Such a vaccine would not only reduce the ticks' effects on livestock, but it would also greatly reduce the females' egg-laying capacity, and thus the overall tick populations and risks of tick-borne diseases, says Anthony Kiszewski, an instructor in the Harvard School of Public Health in Boston. "The discovery is certainly novel, with potential practical applications in agriculture," he says.