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Vaccination time for endangered wild bunnies

Do-or-die efforts aim to counter extinction-level threat posed by deadly disease

November 22, 2021 (published)
Screenshot from U.S. Fish and Wildlife Service video
The effort to bring Columbia Basin pygmy rabbits back from the brink — through breeding, reintroduction and now vaccinations against rabbit hemorrhagic disease virus 2 — requires a lot of people power and resources. The rabbits keep support high "by being so adorable," said Jon Gallie, a wildlife biologist with Washington Department of Fish and Wildlife.

Birders scouting for sandhill cranes, ibis and snowy egrets in the early morning hours at the San Joaquin River National Wildlife Reserve, 7,000 acres of protected wetland in Central California, might be surprised this fall by the appearance of a person here and another there, walking with purpose through mugwort, wild rose and willow, holding mysterious pillowcase-wrapped bundles against their stomachs.

If they look more closely, they might be able to make out the insignias and patches of wildlife agencies, conservation organizations and the Oakland Zoo on caps and coats. And if they linger on these human subjects, as each progresses toward an SUV loaded with medical supplies and parked under a picnic canopy, the patient observers could be rewarded with the sight of a pillowcase gingerly pulled back to reveal the ears and head of an unquestionably adorable riparian brush rabbit.

The endangered bunnies are being vaccinated against a lethal rabbit disease that has been killing wild lagomorphs, especially cottontails and jackrabbits, around the United States. The all-hands-on-deck initiative began a couple of months after reports surfaced in April 2020 of rabbit hemorrhagic disease virus type 2 (RHDV2) in Southwestern states. 

Easily spread and highly resilient, RHDV2 causes lesions throughout a rabbit's internal organs and tissues, particularly the liver, lungs and heart, resulting in bleeding. There is no cure, and the infection is often fatal.

If news of the virus outbreaks spooked rabbit-owner communities and wildlife management groups, it highly alarmed protectors of endangered rabbits.

In brief

"This is kind of like that plague-level of disease that can come in with little warning and wipe everything we have out with barely any time for us to figure out what happened," said Jon Gallie, a Washington Department of Fish and Wildlife (WDFW) biologist who runs the recovery effort for endangered Columbia Basin pygmy rabbits.

Veterinarians are helming interagency efforts in California and Washington built on the strategy that if an RHDV2 vaccine developed for domestic rabbits can be given to enough wild rabbits, it may help fend off this particularly perilous threat.

'Everybody just started pitching in'

The riparian brush rabbit (Sylvilagus bachmani riparius) is a small cottontail, one of eight subspecies of brush rabbits, sometimes called western brush rabbits, native to California. They weigh between 500 and 800 grams, are mostly light brown with a darker back, and can be distinguished from their more populous cousins, desert cottontails, by their inconspicuous tail and lack of black tips on their ears.

Listed as endangered federally and in the state, there are estimated to be about 1,500 riparian brush rabbits in the reserve and the nearby Dos Rios Ranch Preserve. Their numbers dwindled due to the loss of 90% of their riparian forest habitat to agriculture. Rabbits have generally been understudied, said Dr. Deana Clifford, senior wildlife veterinarian for nongame species at the California Department of Fish and Wildlife, so the historical prevalence of this subspecies is unknown.

The U.S. Fish and Wildlife Service has a decades-long history of managing the rabbits with captive breeding programs and translocations. Those programs got a jolt with news of RHDV2. Clifford said federal and state fish and wildlife agencies, Oakland Zoo and River Partners, a private riparian restoration group, and the California State University, Stanislaus, Endangered Species Recovery Program quickly banded together in June 2020.

"We call it sort of an emergency ‘coalition of the willing,' " she said. "Everybody just started pitching in."

While RHDV2 has not been identified in the refuge, it was found in a desert cottontail fewer than 50 miles away, as well as in domestic and wild rabbits elsewhere in the state. It has also been confirmed in other California brush rabbits, Clifford said, so she has no doubt riparian brush rabbits are susceptible.

By August, the coalition had mounted a safety trial to determine how the smaller riparian brush rabbits would respond to a vaccine developed for and, up to that point, used only in larger domestic rabbits. At the time, there were no vaccines produced in the U.S., so the team used Filavac, one of two RHDV2 vaccines approved in Europe, where the virus has been endemic for some time. Veterinarians, mainly in states with verified RHDV2 cases, have been permitted to import Filavac, from France, and a second vaccine, from Spain, since mid-2020.

Twenty rabbits were captured and transported to the Oakland Zoo, which provided its veterinary hospital and staff for the trial as part of its conservation mission. Here, the rabbits were vaccinated and monitored for 60 days. Clifford said a few of the rabbits were released early because they weren't doing well in captivity, which is very stressful for them, but there were no problems associated with the vaccine itself.

Oakland Zoo photo
Starting last year, hundreds of endangered riparian brush rabbits have been captured, vaccinated and released at the San Joaquin River National Wildlife Reserve to protect them from a deadly rabbit virus.

In September, a vaccine produced by a biotech company in South Dakota, Medgene was given emergency-use authorization by the U.S. Department of Agriculture. The authorization means states will phase out permits to import vaccines. However, veterinarians currently vaccinating wild rabbits will likely need to continue using Filavac. 

They believe they'll be granted permission to import the French vaccine because it is the only RHDV2 vaccine with a track record of safety in smaller, wild rabbits and because it requires only one shot. Medgene's current recommended protocol is two-doses, with the second dose 21 days after the first.

‘We look for what we call rabbit runways'

So far, there have been three multiple-week vaccination campaigns at the refuge, first in the fall of 2020, then in spring 2021, and most recently, this fall.

Dr. Megan Moriarty, a wildlife veterinary resident at CDFW and the University of California, Davis, School of Veterinary Medicine, co-leads much of the fieldwork with Clifford; she described it to the VIN News Service.

On Monday evenings, six to ten people set out wire-mesh traps baited with a high-calorie mix of alfalfa meal, corn, oats, barley, and molasses. "We look for what we call rabbit runways," Moriarty said. "We try to place the trap right in the runway so that if a rabbit is hopping along their little path, they happen upon the trap."

Tuesday mornings, they check the traps. Sometimes, a rat, quail, sparrow or desert cottontail will have fallen for the bait. Most of these "opportunists," as Moriarty calls them, get a note in the record and their freedom. Cottontails are an exception. Also susceptible to RHDV2, they will be examined and have their blood drawn to add to the record. They do not receive a vaccine.

In a few cases, a riparian brush rabbit will be trapped repeatedly in a season. "A little bit trap-happy," Moriarty said, "these guys have figured out that there's yummy stuff in here." They are released without ado.

The other rabbits are transferred from traps and placed in pillowcase to minimize stimulation and stress.

Clifford found working with rabbits nerve-wracking at first. "Most of my experience is with carnivores … so to work with a prey species [was a change]," she said. "They're flighty. They're stressier. And it serves them in the wild." Everyone working on the project is trained in low-stress handling and the proper way to carry a rabbit so that it won't hurt itself kicking to get free. Rabbits have been known to break their backs that way.

All the rabbits are sexed, aged (based on weight), and have the length of their right ear and their right foot measured.

Oakland Zoo photo
Oakland Zoo staff Colleen Kinzley, vice president of animal care, conservation and research (left) and Dr. Alex Herman, vice president of veterinary services, attend a riparian brush rabbit wrapped in a pillowcase. A partner in the multi-agency vaccination effort, Oakland Zoo has run a safety trial at its veterinary hospital and sent staff into the field.

Unvaccinated riparian brush rabbits and those vaccinated last spring or fall — as indicated by their ear tags — are anesthetized, and a rectal swab and blood sample are collected before they receive either a primary vaccine or a booster shot between their shoulder blades.

Clifford said the team is collecting blood to determine whether the vaccine induces a detectable immune response in this particular subspecies and, if it does, to measure the duration of immunity.

The rabbits are anesthetized "because they're very small and somewhat challenging to collect blood from, and we don't want them to injure themselves," Moriarty said, explaining that the whole process takes only a few minutes. Afterward, they are re-swaddled in a pillowcase and given oxygen to bring them around.

The anesthesia station is set up in the back of an SUV equipped with drugs, oxygen tanks, blood collection supplies and ways to warm the rabbits if they get cold. "We do have a nice little mobile setup," Moriarty said.

After about 15 minutes, the rabbits are ready to be released at the same place where they were trapped.

"My favorite part, and I'm sure I'm not alone, is when you watch them hop away, and they don't look back," Moriarty said. "They don't know you're trying to help them, but they're happy to be out."

Clifford said she and Moriarty, who wrote a chapter on RHDV2 for a book on zoo and wild animal medicine, could not find any other examples in the literature of wild rabbits being vaccinated against RHDV2. However, vaccinating endangered species, in general, isn't unusual. Earlier in her career, Clifford helped inoculate endangered Channel Island foxes against distemper using a vaccine for ferrets.

After all the rabbits are processed, new traps are set in the evening, and the cycle repeats through Friday.

Moriarty said a typical day involves processing 20 to 30 rabbits, although they've had as few as five and as many as 50, adding up to around 240 rabbits per campaign. The group plans to continue the twice-yearly vaccination efforts, with a goal of keeping about 15% of the known population immunized.

"There's no way we're going to vaccinate every rabbit. We just can't," Clifford said. "But if we can keep enough vaccinated, then hopefully, with some natural survivors, if the population has a decline … there'd be enough brush rabbits in various areas of their habitat to recover."

'Our margin of error is nonexistent'

Screenshot from U.S. Fish and Wildlife Service video
Before Columbia Basin pygmy rabbits are vaccinated, biologists or veterinary staff use a tiny cookie-cutter like device to collect a small tissue sample for DNA analysis and population monitoring from the rabbits uniquely short, round ears.

Even more dire are the circumstances of the Columbia Basin pygmy rabbits, which live in the sagebrush steppe of Eastern Washington. The genetically distinct subpopulation of pygmy rabbit (Brachylagus idahoensis) was extirpated in the wild in 2000, according to WDFW's Gallie. "At that point, they were so few of them that the last dozen or so were captured from the wild and placed in a captive breeding program."

Since then, they've been bred with closely related Great Basin pygmy rabbits. The kits are reintroduced into three wild areas. While it's unclear whether conservationists will be able to preserve the genetic characteristics of the Columbia Basin group, there is no doubt an RHDV2 outbreak would extinguish all hope.

"Our margin of error is nonexistent; we couldn't handle a 50% mortality event just because we don't have enough to rebound from that," Gallie said.

Brownish to dark gray, the pygmy rabbits are unlike other rabbits and hares in that they have short, round ears, small hind legs, and lack white on their tails. They are the smallest rabbit in North America. Weighing less than a pound apiece, two can fit in a pair of cupped hands. They are unique among North American rabbits in subsisting almost entirely on sagebrush.

Dogged by extreme weather, increasingly catastrophic wildfires and disease, the population of Columbia Basin pygmy rabbits regularly spikes and crashes, Gallie said.

His best estimate of the current number — and he cautions it is just an estimate — is about 150 rabbits total, including about 35 that live within sprawling breeding enclosures on the steppe. He and his team monitor populations in the winter by counting burrows and collecting fecal pellets for genetic testing and tracing. Since tissue samples are taken from all kits before they are released, testing allows the biologists to identify wild-born rabbits. The results for last year have been delayed because the labs were busy with COVID-19-related work.

WDFW and the U.S. Fish and Wildlife Service held off inoculating pygmy rabbits until after the Oakland Zoo safety trial on the riparian brush rabbit. "We wanted to make sure that there was no immediate response, like anaphylaxis or anything," said Dr. Katie Haman, a wildlife veterinarian with WDFW.

Once they felt they could safely vaccinate, they began with the rabbits in breeding enclosures. "Unfortunately, of the 22 that we vaccinated the first go-round, we did end up losing 10 of them," Haman said.

"It was certainly a big blow because we have so few rabbits to begin with, especially after the fires that happened prior to our vaccination efforts," she added, referring to the Pearl Hill Fire in 2020, which wiped out nearly half of the population. "It did make us take pause and really consider the pros/cons and a risk analysis for continuing the vaccination effort." 

As part of that analysis, three carcasses were sent to the Washington Animal Disease Diagnostic Laboratory for testing. They revealed no evidence of an adverse vaccine reaction. Instead, the animals were found to be undernourished and likely died because they couldn't tolerate the stress of capture and handling.

Produced by the U.S. Fish and Wildlife Service Washington Ecological Services Office

Washington Department of Fish and Wildlife biologists talk about the Columbia Basin pygmy rabbit recovery program.

In an attempt to avoid further losses, Gallie and Angel Hastings, a WDFW veterinary technician working on the project, dove into their database to look up at pygmy body weights over time, to identify when the rabbits are typically well-nourished so they could target those periods. They also established a cutoff body weight below which a rabbit won't be vaccinated. 

"We have vaccinated probably 100 rabbits since then ... and have not seen any negative side effects from the vaccination effort," Gallie said.

Once RHDV2 was identified in Oregon, Montana and Idaho in early 2021, the team looked beyond the breeding stock.

"When it was still far away, down in the Southwest, it was like, 'Well, let's just start,' "Gallie said. "But then it showed up in our neighborhood, so to speak, and that ramped up the urgency."

They began holding kits longer than usual in the breeding enclosures so they could be vaccinated at 10 weeks, as recommended for domestic rabbits, before being released. They also captured some rabbits from the wild — about 30 so far — and vaccinated them.

Unlike the California effort, the pygmy capture doesn't include anesthesia or blood draws. "We collect a little bit of biometric data — the weights, age, sex — and then we take a little DNA sample off its ear," Gallie said.

He added that Hastings, the veterinary technician administering the shots, is "incredibly thorough on checking her injection site and giving a slow injection" to minimize the animal's discomfort.

Gallie, who has headed up the pygmy rabbit program since 2016, is often asked why so much effort is put into so few rabbits. He said the easiest answer is that preventing extinction for all native plants and animal species is required under the Endangered Species Act.

"When the active reintroduction efforts began a decade ago, no one was sure it would actually work," Gallie said. "And so it was like, 'Let's just throw it on the table, we're going to try everything. If it works, great. If it doesn't, then you know, we did everything we could.' "


VIN News Service commentaries are opinion pieces presenting insights, personal experiences and/or perspectives on topical issues by members of the veterinary community. To submit a commentary for consideration, email news@vin.com.



Information and opinions expressed in letters to the editor are those of the author and are independent of the VIN News Service. Letters may be edited for style. We do not verify their content for accuracy.



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