This article was originally published on The Conversation.
When the COVID-19 outbreak first emerged, many wildlife disease researchers like myself were not surprised. Some wondered if it wasn’t sooner; after all, it’s our job to observe, describe and study pandemic dynamics in animals.
For example, amphibians have been going through a global panzootic — animal version of a pandemic — for decades. In the late 1990s, researchers identified the amphibian kitride fungus, which causes the often fatal disease kitridomycosis, as the possible culprit behind frog and salamander declines and extinctions from Australia to Central America and elsewhere that began 10, 20, and even 30 years ago.
Scientists have found this pathogen on every continent inhabited by amphibians, and the extensive global amphibian trade has likely spread the highly deadly species around the world. The amphibian chytrid fungus is common in some geographic areas and, like the virus that causes COVID-19, can mutate rapidly and take new forms causing varying disease severity.
Conservation translocation is an increasingly popular way to save species that have experienced massive population declines. It involves the transport of organisms to recreate extinct populations, supplement existing ones, or create new ones in areas where the species has not previously been found. However, when the amphibian chytrid fungus is prevalent in the field, the frogs are likely to become ill again, hindering the success of the translocation.
To avoid the setbacks of the disease, researchers use a tool often used against human outbreaks: vaccinations similar to vaccines.
In our recent study, my research team and I vaccinated California red-legged frogs against chrysanthemum prior to translocation by exposing them to chyme in the laboratory. We wanted to see if we could activate their immune systems and give them an advantage against fungi when released. Our results were unexpected.
There’s nothing a cocktail can’t cure
Yosemite National Park has been actively transporting California red-legged toads to Yosemite Valley, where the kitrid fungus is already present, since 2017. We used a small subset of these displaced frogs in our study.
We collected wild frog eggs at a site where the species thrives, about 100 miles northwest of Yosemite Valley, then raised them in captivity at the San Francisco Zoo. When they turned into baby frogs, we bathed 20 of them in a “cocktail” of four live, active strains of the fungus. Three weeks later, an antifungal drug bath was given to stop the infection. The other 40 frogs that were not exposed to the fungus were also given an antifungal drug bath.
We then exposed 20 previously infected frogs to the fungus a second time, while 20 previously uninfected frogs were exposed to the fungus for the first time. We wanted to see how frogs with a second infection—that is, “vaccinated” frogs—compared to those that were infected only once.
What we found was surprising: 35% of frogs infected only once successfully cleared the infection without vaccination or an antifungal medication. This indicated that they had some degree of innate immunity, meaning that their immune system’s first line of defense was able to fight off the fungus. Additionally, frogs infected a second time had a 31% lower infection rate than those infected only once. This meant that the vaccine-like treatment also works by stimulating adaptive immunity, meaning their immune systems learn to recognize the fungus from their first exposure and fight it more efficiently. None of the frogs died from fungal infections.
We treated them with an antifungal before releasing them into the wild and observed them to make sure they were disease free. We wore tiny transmitters with beaded belts around their waists to track their infection and survival for three months.
Unexpectedly, we found no difference in disease burden between frogs that were never infected and frogs that were previously infected in the laboratory. This suggests that, at least in Yosemite, immunization of this species for kirid fungus may be unnecessary to ensure their survival after reintroduction.
In fact, California red-legged frogs released in Yosemite Valley are thriving three years after our experiment and six years after their first relocation. They hibernate successfully in cold winters and emerge in early spring to breed.
hope for the future
Our study provides a new approach to the emerging inoculation tool against the chytrid fungus. We put laboratory observations to the test in the real world, combining ex situ or laboratory experiments with on-site or field practice. Such work strengthens collaborations between wildlife managers and zoos, which are increasingly needed as the biodiversity crisis accelerates.
While it may seem like California red-legged frogs in Yosemite Valley don’t need vaccinations, that doesn’t mean other dangerous amphibian species don’t exist in the world. Research on chthridine vaccinations of other types has produced mixed results, ranging from improving survival to reducing the burden of infection associated with increased survival. One of the main challenges with this approach to protection is that even if vaccination improves survival after initial release, this immunity is not passed on to subsequent generations.
But there is hope. Researchers are working to identify genetic signatures associated with immunity to kitride fungus. If successful, breeding programs could artificially select protective traits and perhaps even use gene editing to give frogs a leg up against a pathogen that is devastating amphibian populations worldwide.
Andrea Adams is an Ecology Researcher at the University of California, Santa Barbara. Andrea Adams had previously received funding from the Yosemite Conservancy to conduct this research as a postdoctoral fellow in Yosemite National Park. Funding for his current academic assignment has been received from the U.S. Fish and Wildlife Service.