Every January, planes dump one million small plastic packets covered in fishmeal crumbles along the Texas-Mexico border. When the sharp teeth of feral dogs or coyotes pierce the plastic, a liquid rabies vaccine squirts into their mouths.
Not every animal that eats a packet becomes immune to rabies, but since the State of Texas started its oral rabies vaccination program in 1995, enough have become immune that the canine strain of rabies was eliminated from the U.S. in 2008. Continued flights keep canine rabies from re-entering the U.S. from Mexico. And though other animals, like foxes, skunks and raccoons, do still carry their own strains of the disease — and dogs and coyotes can still get these other strains from an animal bite — eliminating even one reservoir of rabies is an accomplishment.
Can this success be replicated to eliminate other zoonotic diseases — diseases that spread from animals to humans — such as plague, Lyme disease or even COVID-19? Some researchers are skeptical, like Melinda Rostal, a principal scientist in vector-borne diseases for the New York-based nonprofit EcoHealth Alliance. She says her organization prioritizes keeping people away from the animals that cause disease. The search for animal reservoirs can take decades, she says, and sometimes, when a disease is carried by mosquitos, ticks or other vectors, it’s more effective to target these disease-carriers instead.
Scott Williams, a Connecticut Agricultural Experiment Station (CAES) biologist and vaccine researcher says, “People are looking for that one silver bullet, but it doesn’t exist in a complex ecological system.”
But others are enthusiastic about the possibility of vaccinating wildlife to prevent the spread of zoonoses to humans. If the idea could succeed, it would be a boon to human health: Seventy-five percent of emerging infectious diseases are zoonotic. Across the U.S. and beyond, efforts are already underway.
Targeting Mice to Tackle Lyme
Other researchers have been inspired by the success of the oral rabies vaccine. The federal rabies vaccine program plans to build on previous success by eliminating raccoon rabies from the northeastern United States, says Richard Chipman, coordinator of the USDA Wildlife Services’ National Rabies Management Program. Similar to the Texas program, planes drop bait-wrapped vaccines in an area that runs north and south near the Appalachian Mountains to prevent raccoon rabies from spreading west.
A recently-tested, more-effective vaccine will allow the program to “march raccoon rabies back to where it came from, which is Florida,” over the next 30 years, Chipman believes, eventually eliminating the raccoon strain of rabies.
And this spring, US Biologic, a Memphis-based bio-tech company, and CAES announced a successful field test of a vaccine to prevent Lyme disease in white-footed mice, which are the main reservoir of the disease.
The human vaccine for Lyme disease was yanked off the market in 2002 because of slow sales. US Biologic saw an opportunity to blend its expertise in treating animal diseases with feed-based products with existing vaccine research to help the 300,000 people diagnosed with Lyme disease in the U.S. each year, says company CEO Mason Kauffman.
To test the vaccine in wild white-footed mice, Williams and his team set up feeding stations offering mouse kibble coated with tiny capsules of Lyme vaccine. They trapped and tested mice near the bait stations for four months, as well as at control locations. The mice trapped near the vaccine-dispensing stations showed fewer Lyme disease infections.
US Biologic Chief Science Officer Jolieke van Oosterwijk says the vaccine is safe for the chipmunks and gray squirrels that might eat the vaccine-laden bait, because vaccines target the infectious agent, not the host animal. “At least the way we’re doing it, there are no observed side effects.” The same is true of the oral rabies vaccine distributed by the USDA, Chipman says.
Baiting Prairie Dogs to Beat Plague
Some researchers are working to vaccinate wildlife not for humans’ sake, but for to protect their furry neighbors. Tonie Rocke, a research epidemiologist with the U.S. Geological Survey’s National Wildlife Health Center, has been working since 2003 to create a vaccine against plague in prairie dogs.
While the vaccine will benefit human health — there are as many as 15 human cases in the U.S. each year, says Rocke — she receives funding because the federally-endangered black-footed ferret depends on prairie dogs as its major food source. When plague wipes out a prairie dog colony, black-footed ferrets die too.
Rocke spent years adapting an oral rabies vaccine aimed at carnivores to deliver a plague vaccine to prairie dogs. Prairie dogs nibbled around the bait-encased vaccine packet, so Rocke hand-made a vaccine and bait matrix her lab, learning that the prairie dogs prefer peanut butter flavor. Today, Rocke is finalizing her research with field trials at a 1,000-acre prairie dog colony.
Treating Bats to Triumph over Coronaviruses
With some proof that wildlife vaccines work, the big question is whether they can prevent global pandemics such as COVID-19. Some researchers, such as US Biologic’s van Oosterwijk are optimistic. “Absolutely,” she says to the possibility. If science has identified an antigen that triggers an immune system response and knows the reservoir animal, US Biologic is ready to put its vaccine delivery methods to use, she says.
Sometimes the reservoir animal itself presents a hurdle. Bats are the reservoir for coronaviruses, such as the one causing the COVID-19 pandemic. In the existing oral vaccines for wildlife, animals eat a vaccine-laden bait off the ground, Rocke notes, but insect-eating bats feed in flight.
Rocke is developing two different vaccines for bats: one to prevent rabies in vampire bats in South America, and another to prevent white nose syndrome in insect-eating bats in North America. Her solution is to take advantage of bats’ meticulous grooming habits — think of a fussy house cat — to deliver a vaccine through a gel sprayed on the bats’ fur.
Keeping insect-eating bats alive in captivity is difficult, Rocke says, so she’s doing much of her research with field studies. “That takes much longer than doing the work in a model animal, like mice in a laboratory setting,” she says.
For Kaufmann, the effort is worth it. “Today we are so focused on the tail-end of treating COVID-19, the ventilators, the PPE,” says Kaufmann. “[van Oosterwijk] and her team focus on preventing the disease at the start, in the animal reservoir. I think both are important.”
Source : Discovermagazine