Rabies is becoming such a problem among wild animals that researchers in the United States and Canada are collaborating to find a way of vaccinating raccoons and foxes in the wild. They are experimenting with an oral vaccine that can be dropped from the air in baited pouches.

The Wistar Institute in Philadelphia, long a major rabies research center, has developed an experimental oral vaccine for raccoons. Scientists there are testing various baits to see what flavors and scents attract the omnivorous animals. The candidates are shellfish oil, turkey gravy, wild grapes, bananas and feta cheese.

The flavors are coated inside a perforated plastic bag that contains egg yolk, beef tallow and, in the flavor test versions, the antibiotic tetracycline. The bags have been dropped over suitable wild lands, and scientists are now asking hunters and trappers to give them carcasses or heads for study.

Because tetracycline produces fluorescent yellow bands in teeth, the scientists will be able to tell whether the animals have taken the bait. The pouches are harmless to animals and people.

Once the best bait is identified, similar pouches will be impregnated with rabies vaccine and scattered over rabies-infested parts of south-central Pennsylvania. The plastic pouch technique was developed by Canadian wildlife biologists to vaccinate foxes in Ontario.