Baiting Ebola

Chimps testing the Ebola vaccine bait

Courtesy of IDT Biologika. Credit: Andrea Schaenzler

At the Leipzig zoo's Wolfgang Köhler Primate Research Center last summer, a 3-year-old female gorilla named Kibara was going berserk. She had just been given a new type of food, deep-red colored candies with a rich mango scent. Kibara had never smelled mango before, and she couldn't get enough of the aromatic treat. "Kibara was crazy, running from one point to another, cracking open [the candies], and eating them up," recalls Martina Neumann, a behavioral biologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig. "She was just like, 'this is sweet; I love it.'"

Kibara was the subject of a pilot study on delivering oral vaccines to great apes. Now that a handful of experimental Ebola vaccines have proven effective in laboratory monkeys, researchers need to find a way of delivering a vaccine to apes in the wild. But the baits used for decades to deliver a rabies vaccine would melt and dissolve in the hot, wet tropical rainforests of Africa. Apes also have nimble hands, meaning they handle baits differently than foxes and raccoons, and their palates lean more to sweet fruits than the rabies baits' fishmeal flavor. A completely new design was needed.

In 2007, Peter Walsh led a group of Max Planck primatologists who teamed up with experts from the German vaccine manufacturing company, IDT Biologika, for a preliminary feasibility study into designing an oral Ebola vaccine. "I know all the science, but I don't know anything about the vaccine development business," says Walsh, a quantitative ecologist who had shown that a single Ebola epidemic killed approximately 5,000 gorillas in a 2,700-km² region of central Africa (Science, 314:1564, 2006).

The design they narrowed in on was a 6-centimeter, puck-shaped lozenge, containing agar, fructose and eventually, they hoped, the viral vaccine. The bait was coated in paraffin wax to withstand the harsh African climate, but what color and scent would best attract the apes? From August 2007 to April 2008, the researchers ran near-monthly trials testing different color-smell combinations with the captive gorillas, chimps, and orangutans at the Leipzig zoo.

The researchers tried three different fruit colors (red, orange, and yellow) with three different scents (banana, fig, and mango). On the first choice trial, however, the 27-year-old silverback gorilla named Gorgo simply ate all four baits he was given. Across all the trials, the color of the bait didn't seem to matter much, but there was a clear preference for mango scent, especially among the chimpanzees. Because they were studying only a limited number of captive animals, though, Adrian Vos, a wildlife biologist with IDT Biologika who was involved with the trials, is quick to note that the trials don't qualify as a scientific food-preference study, and none of the work has been published in any peer-reviewed journals. "We only wanted to see if the animals would accept the baits, and how they would handle them," he says.

Walsh is now planning a trip to Africa this summer to test the baits in the field. First, he plans to investigate bait placement strategies. The best bet, says Vos, is probably placing baits low in the tree canopy to avoid both ground-scrounging competitors like rodents, as well as monkeys and birds higher up in the tree tops.

For now, the baits will just contain sugar, but ultimately Walsh and Vos plan to incorporate a live, attenuated Ebola virus. One concern is "spillage," says Vos: The apes either drop much of the bait on the floor or swallow it whole, so only a small amount of virus is usually absorbed in the cheek epithelial cells. Thus, the baits should contain a live virus that can replicate in the blood system and build up a large enough viral titer.

There is another concern: A live virus can revert to a virulent Ebola strain, which might harm other animals that are drawn to the candy-like bait. So Walsh also plans to test hypodermic darts containing freeze-dried vaccine - he calls them biobullets - though finding the apes and getting close enough to vaccinate can be difficult. "The gorillas and the chimpanzees will decide which approach is more suitable," says Vos.