Airborne rabies vaccine would reach most raccoons, Cornell test shows

VETERINARY MEDICINE
December 1994
Contact: Roger Segelken (607) 255-9736, hrs2@cornell.edu

ITHACA, N.Y. - If hungry raccoons thought their dreams came true last summer when food fell from the sky, they were partly right: Tests by Cornell University veterinarians and biologists proved that oral vaccine - concealed in flavored baits and dropped from aircraft - can immunize most raccoons against rabies.

Some 84 percent of Ithaca-area raccoons that were subsequently live-trapped and tested for a biological marker had eaten the bait and would have been immunized against rabies - if the bait actually contained vaccine. Instead, the test by the Cornell College of Veterinary Medicine's Diagnostic Laboratory and Canada's Ontario Ministry of Natural Resources used placebo baits because a new, genetically recombined virus vaccine for raccoon rabies awaits licensing by the U.S. Department of Agriculture.

Both the commercially manufactured bait used in the Summer 1994 test and similar bait developed at the Cornell Department of Food Science are good candidates for vaccine delivery, Laura L. Bigler reported Nov. 16 at the Fifth Annual International Rabies in the Americas Meeting in Niagara Falls, Ontario. Bigler is a wildlife biologist in the veterinary college's Diagnostic Laboratory. Cornell began bait development because existing baits were not readily accepted by raccoons in urban and suburban environments, she told the international meeting.

Controlling rabies is an international concern because various forms of the disease spread to new areas as easily as an animal walks, flies, swims or rides across borders, Bigler noted. In southern Ontario, where fox rabies is being controlled by aerial distribution of oral vaccine, raccoon rabies is the next threat. Raccoon rabies has progressed northward through the eastern United States at approximately 25 miles a year, drastically reducing populations of raccoons and posing a serious health threat to humans and domestic animals. The closely related forms of rabies, including those specific to foxes, raccoons and bats, can also infect humans, and post-exposure treatments in New York alone have cost millions.

Scientists in Europe, Canada, Mexico and the United States have shown that it is more cost-effective to vaccinate wild animals against rabies than to live with the disease indefinitely, Bigler noted. Aerial distribution of oral vaccine is also seen as a less labor-intensive way of immunizing wild animals, compared to the trap-vaccine-release method, which was tried in 1992 by Cornell researchers in the Ithaca area. In that successful experiment, about 1,000 raccoons were live-trapped, vaccinated against rabies with hypodermic needles, then returned where they had been captured.

The recombinant vaccine, which cannot cause rabies in animals, would be encapsulated in cookie-sized baits made of rancid tallow, food grade wax and marshmallow flavored sugar - the recipe found most enticing to wild raccoons. During the 1994 aerial tests, capsules of edible iodine took the place of vaccine. Less than a month after 3,600 placebo baits were air-dropped over an 18.5- square-mile area, live-trapping examinations of raccoons found the biological marker in the blood of more than four-fifths of the animals.

"If the baits carried vaccine, those raccoons would be immunized against rabies by now," Bigler said.