For more than a year, the world has closely followed the development, approval and deployment of various coronavirus vaccines that could bring an end to the global pandemic, debating every side effect and hurdle. But vaccines aren’t only used to spare humans from the ravages of disease; increasingly, they’re being used to conserve wild species threatened with extinction.
Humans vaccinate wildlife for a number of reasons, but most hinge on human health and the protection of livestock. Raccoons are vaccinated for rabies, white-tailed deer for tuberculosis, and wild boar for swine fever. But such immunization campaigns aren’t designed to save wildlife — they’re designed to save us.
That’s starting to change. Today, with environmental threats mounting, there is growing acceptance of the need to vaccinate wild animals to help save them from extinction. In 2015, the Wildlife Conservation Society along with several academic institutions convened the first “Vaccines for Conservation” international meeting in New York City to push for vaccinating threatened carnivores against canine distemper. But that’s just one among many initiatives under consideration by conservationists.
In the United States, scientists have developed an oral vaccine for prairie dogs, hidden in peanut-butter-flavored bait, to prevent plague, caused by the Yersinia pestis bacterium. Prairie dogs are the key prey species of endangered black-footed ferrets (Mustela nigripes).
Researchers are also racing to develop a vaccine for white-nose syndrome, caused by the Pseudogymnoascus destructans fungus, in hibernating bats. The fungus has killed millions of bats in North America and threatens some species with extinction. Scientists aspire to apply the vaccine by spraying it onto bats at roosting sites; when the bats groom themselves, they should ingest the vaccine.
The white-nose vaccine highlights just how difficult it can be to vaccinate wild animals. Entire bat or prairie dog populations can’t simply be captured and given a jab. And within some species, vaccines don’t seem to be very effective. For example, there’s little evidence that amphibians can acquire resistance to the devastating pathogenic chytrid fungus Batrachochytrium dendrobatidis through immunization. In recent studies, scientists at the University of South Florida were able to provoke an immune response to the fungus in Cuban tree frogs (Osteopilus septentrionalis), but they’ve yet to replicate the results with the critically endangered Panamanian golden frog (Atelopus zeteki).
Other vaccines, though, are advancing more quickly.
Chlamydia down under
For more than a decade, Peter Timms, a microbiologist at the University of the Sunshine Coast in Queensland, Australia, has been working on a chlamydia vaccine for koalas (Phascolarctos cinereus).
Chlamydia is a sexually transmitted disease that affects the koala’s reproductive system. It causes scarring and massive cysts in the reproductive tract, often leading to infertility. It can also lead to conjunctivitis in the animal’s eyes, causing blindness.
“In many areas, chlamydia is the single threat to koalas and can lead to a reduction of 50% in numbers,” Timms says.
Infected koalas are often treated with antibiotics, but at least one in three don’t survive treatment.
The marsupials, Timms says, are dependent on their gut microbiome. “Because koalas eat a diet — eucalyptus leaves — that is high in toxins, the koala liver is designed to detoxify those toxins. So, when you give them antibiotics, they also try to break it down and detoxify it.” This means veterinarians need to treat the animals for far longer — two weeks to a month — with daily antibiotics, but this much medicine also ultimately destroys their gut flora.
Instead of treating sick koalas, Timms says he hopes to prevent the disease from taking hold.
“The hardest [thing to develop] is a vaccine that prevents them from getting infected in the first place,” he says.
He and his colleagues have been conducting trials of an injectable vaccine on captive and wild koalas for years. These trials have shown that not only are infection levels reduced in vaccinated koalas, but that for already infected individuals, the progression to disease, when a koala would begin to exhibit symptoms, is significantly reduced. Moreover, they found that in at least 80% of koalas with eye disease, vaccination was able to reverse the disease’s effects.
With a safe and effective vaccine in hand, the next hurdle will be its deployment.
“Koalas are a challenge,” Timms says. “Unlike other species, such as big cats and bears that are radio-collared, most koalas are not closely monitored … it’s a more wild species from that perspective.”
Timms says it makes sense to start vaccinating individuals brought into wildlife hospitals; around 700 or 800 koalas arrive every year just at the Australia Zoo Wildlife Hospital in Queensland. It might also be feasible to vaccinate populations of koalas impacted by road development.
“In those cases, the animals are being closely monitored and it might be possible to mount a vaccination program,” he says.
The koala chlamydia vaccine is currently in the middle of the regulatory approval phase, and Timms says he’s hopeful that within one to two years they can begin wide-scale immunization of koalas.
The Australian government, he adds, has traditionally focused on habitat protection to conserve koalas, “but we’re getting to a stage that disease management should be an integral part of broader conservation management.”
Not just for dogs
Vaccination of wild animals for conservation has not always been without controversy.
In the early 1990s, efforts to inoculate African wild dogs (Lycaon pictus) against rabies were falsely implicated in their extinction from the Serengeti in Tanzania. Researchers theorized that the darting of wild dogs and invasive handling techniques may have increased their stress hormones, compromising their immunity and reactivating latent forms of the rabies virus in their bodies. Another study around the same time suggested that a modified-live canine distemper vaccine may have been responsible for killing four African wild dog puppies.
Despite the fact that research since then has disproved those theories, “many authorities over the world will not consider vaccinating threatened wild carnivores,” says Martin Gilbert, a veterinary scientist at Cornell University in the U.S. Rather, in cases where a wild animal is threatened by a disease, environmental authorities often elect to vaccinate the perceived sources of those viruses, such as stray dogs or livestock, to reduce transmission, instead of the wild animal itself.
This was the idea first proposed to protect the Siberian or Amur tiger (Panthera tigris) from canine distemper virus, which showed up in the early 2000s. Though the disease, which plagues the respiratory system and, later, the brain, appeared to affect only two tiger populations in Siberia, wildlife managers grew greatly concerned; in 1994, an outbreak of CDV killed roughly one-third of the Serengeti’s lion population.
Gilbert found that if small tiger populations, like the one in southwest Primorski, are exposed to even modest levels of CDV, it’s 65% more likely the population will go extinct within 50 years.
“Initially, we thought this was going to be a dog issue — tigers eat dogs regularly,” Gilbert says of the origin of the virus. Veterinarians, therefore, assumed that vaccinating domestic dogs in Siberian villages and controlling their movement would mitigate the threat to wild tigers. But Gilbert’s latest research reveals that, surprisingly, dogs aren’t the reservoir population of canine distemper in Siberia. Instead, when Gilbert and his colleagues sampled wildlife carcasses found in fur traps and along roadsides, they found high levels of CDV antibodies in the brain tissue of everything from raccoon dogs (Nyctereutes procyonoides) to badgers (Meles leucurus) to Siberian weasels (Mustela sibirica).
Canine distemper is likely transmitted to tigers from their prey, Gilbert says. “In late summers, tigers will sit along badger runs and kind of pop them like candy as they come running along their trails through the forest.” Such findings indicate that “the only feasible approach to mitigating the impact of distemper on the tigers would be to vaccinate the tigers themselves.”
The logistics are daunting. Many people living in Siberia have never even seen a tiger. And no wild tigers have ever been vaccinated for any disease.
“We’re never going to be able to vaccinate the whole population of tigers,” Gilbert says. But he adds it’s possible inoculation could be done on a passive basis: conservationists could vaccinate tigers captured due to conflicts, or orphaned cubs in rehabilitation. He’s found that if veterinarians were able to vaccinate just two tigers per year, they would reduce the extinction risk by 75% in Russia’s Primorye population.
The other challenge is that there is no dedicated CDV vaccine for big cats. Today, they only exist for ferrets and dogs. To repurpose a vaccine for a different species, clinical trials may need to be conducted anew.
Though scientists have struggled to develop a vaccine for amphibians plagued by chytrid fungus, the jab’s usefulness extends beyond protecting threatened mammals. Yellow-eyed penguins (Megadyptes antipodes), native to New Zealand and considered the world’s most endangered penguin species, have been in sharp decline for the last 20 years.
The birds are dying from avian diphtheria, a bacterial infection that affects the penguins’ upper respiratory tract. Diphtheria primarily impacts chicks, with up to 93% of young yellow-eyed penguins contracting avian diphtheria every year in some northern populations. Nearly three-quarters of them die.
The bacterium infects the tongue and mouth of young chicks, blocking the oral cavity with pus and ulcers.
“They can’t feed properly so they die of malnutrition,” says Vartul Sangal, a molecular scientist at Northumbria University in the U.K. Though avian diphtheria also affects adult penguins, the mortality rate is much lower. The bacterium is likely more effective at killing chicks because they have a less developed immune system, Sangal says.
Eventually, the diphtheria infection can spread to the rest of the penguin’s body, resulting in sepsis.
Sangal and his colleagues recently identified the bacteria strain causing these ravaging infections for the first time. Researchers collected swabs from the mouths of infected penguin chicks and sent these samples to Sangal, who isolated the bacterium in the laboratory and sequenced its genome.
“It was very exciting,” he says. “We also now know the mechanism of how it is causing the infection.” One of the genes, he found, produces a protein that helps the bacterium survive inside the host.
This information, he says, is important for developing a vaccine against the bacterium. If scientists can modify the protein to protect the penguins from infection, they’ll be able to develop a defense.
“We definitely need some intervention,” Sangal says. “The penguins are treated with antibiotics but it doesn’t work very well.”
It’s still not known where the chicks are picking up the disease from in their environment, or how exactly it moves from penguin to penguin.
“It is being spread in the nest, but it could be from the mother to the chicks, or from chick to chick,” Sangal says.
Producing and distributing a vaccine for yellow-eyed penguins remains an uphill battle. Sangal estimates an effective vaccine will take between five and 10 years to develop. First, scientists will need to test it on other birds, such as chickens, to ensure it’s safe for penguins. Then they will need to inject the vaccine into wild penguins, preferably adults. In studies of other avian diseases in chickens, researchers have found it’s possible to vaccinate a mother bird and immunity will transfer her to egg yolks.
Ultimately, there is no silver bullet in wildlife conservation. Effective vaccines, however, can serve as critical weapons in the fight against extinction.
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Mourya, D. T., Yadav, P. D., Mohandas, S., Kadiwar, R., Vala, M., Saxena, A. K., … Bhargava, B. (2019). Canine distemper virus in Asiatic lions of Gujarat state, India. Emerging Infectious Diseases, 25(11), 2128-2130. doi:10.3201/eid2511.190120
Olagoke, O., Quigley, B. L., & Timms, P. (2020). Koalas vaccinated against koala retrovirus respond by producing increased levels of interferon-gamma. Virology Journal, 17(1). doi:10.1186/s12985-020-01442-7
Rocke, T. E., Kingstad-Bakke, B., Wüthrich, M., Stading, B., Abbott, R. C., Isidoro-Ayza, M., … Osorio, J. E. (2019). Virally-vectored vaccine candidates against white-nose syndrome induce anti-fungal immune response in little Brown bats (Myotis lucifugus). Scientific Reports, 9(1). doi:10.1038/s41598-019-43210-w
Saunderson, S. C., Nouioui, I., Midwinter, A. C., Wilkinson, D. A., Young, M. J., McInnes, K. M., … Sangal, V. (2021). Phylogenomic characterization of a novel corynebacterium species associated with fatal diphtheritic stomatitis in endangered yellow-eyed penguins. mSystems, 6(3). doi:10.1128/msystems.00320-21
This article by Gloria Dickie was first published by Mongabay.com on 20 August 2021. Lead image of a koala bear by Nathan Rupert via Flickr (CC BY-NC-ND 2.0).
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