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You asked. Ars answers. Here’s how to give an electric eel an MRI

Enlarge / Veterinarians at Chicago’s Shedd Aquarium have pioneered a lot of unusual procedures to diagnosis and treat the animals in their care—including figuring out to give MRIs to electric eels.

Right before Thanksgiving, we reported on how Chicago’s Shedd Aquarium solved the Curious Case of the Missing Chloroquine. The antiparasitic drug is typically added to the water for new animals in quarantine, but it was mysteriously disappearing. The culprit: hungry, hungry microbes. The post included a throwaway line about how the aquarium vets also had the lowdown on how to give an electric eel an MRI.

That bit seemed to resonate with readers, and we received several queries about how, exactly, this feat might be accomplished. You asked. We wanted answers. So we turned to Bill Van Bonn, the clinical veterinarian in charge of the aquarium’s Center for Animal Health and Welfare, which boasts a state-of-the-art animal hospital for monitoring the health of all the animals in the exhibits and treating them as necessary. Dr. Van Bonn and his colleague, Dr. Karisa Tang, were happy to oblige.

Van Bonn describes the veterinary team at the aquarium as “family practitioners” rather than specialists, although they are able to draw on world-class expertise as needed from the greater Chicago area. And since there isn’t a lot of diagnostic and treatment precedent in the literature for many of the animals in their care, they practice comparative medicine by necessity.

As we’ve reported previously, electric eels are technically knifefish. The eel produces its signature electric discharges—both low and high voltages, depending on the purpose for discharging—via three pairs of abdominal organs composed of electrocytes, located symmetrically along both sides of the eel. The brain sends a signal to the electrocytes, opening ion channels and briefly reversing the polarity. The difference in electric potential then generates a current, much like a battery with stacked plates.

It’s that ability that makes giving the creatures an MRI tricky. According to Vanderbilt University biologist and neuroscientist Kenneth Catania, the creatures can vary the degree of voltage in their electrical discharges, using lower voltages (around 10 or 12 volts, akin to a car battery) for sensing their environment, navigating, and hunting. “They live in murky waters where there’s not much light or visibility,” Van Bonn told Ars, which is why they have such tiny eyes. The eels use higher voltages (100 volts per foot of animal, typically amounting to between 400 and 500 volts) to stun and kill prey.

Van Bonn initially wondered if perhaps he and his staff wouldn’t need to worry about those higher voltage discharges, given that the aquarium eels are in captivity. Poison dart frogs in captivity, for instance, don’t produce their trademark poison because they don’t need the defense mechanism while in managed care. He tested this by placing a voltmeter in the eel’s watery habitat. It registered a low, 10-volt potential. But when he poked the animal, the voltmeter’s reading shot up dramatically. Conclusion: the eels need to be handled with great care.

Enlarge / Electric eels are capable of generating voltages as high as 500 volts to stun and kill prey.
Clay Harrison/Getty Images

While an electric eel discharge might not damage the MRI equipment, it could produce an artifact on the resulting image that renders the image useless. And nobody wants the handlers to be electrocuted. So the first step is figuring out a way to anesthetize the eel. The handlers must don latex gloves and nonconductive footwear for the process to interrupt any conductive circuit that might form.

According to Tang, the aquarium vets usually put a powder in the water to anesthetize fish, unlike humans, who breathe it in via a gas. Eels breathe air, but they also breathe through the water with gills, so the team uses the same method as they do for other fish. Correct dosages must be determined through trial and error, and that trusty voltmeter is in the water to alert the team to any discharges, should the animal not be completely asleep. Once anesthetized, the MRI can proceed as it would for any nonelectric fish or tubular animal.

Also, since an anesthetized eel can theoretically drown, the vets must administer little puffs of oxygen over the oral cavity. Humans have a trachea, and the air travels through that to the lungs so we can breathe, but electric eels “have this cool wrinkled tissue on the inside of their mouths,” Tang told Ars. “So all you have to do is puff oxygen just over the tissue in there.”

The “unhappy triad of O’Donoghue”

"Perform knee surgery on a bullfrog? Sure, we can do that."
Enlarge / “Perform knee surgery on a bullfrog? Sure, we can do that.”
Maryann Flick/Getty Images

When you specialize in treating weird, exotic animals, you’re going to rack up a few good stories. “Every time we work with one of the animals, we learn something new,” said Van Bonn—like the time he had to figure out how to perform knee surgery on an American bullfrog. Whenever the Shedd Aquarium acquires new animals, the creatures are first placed in the quarantine habitat to prevent them introducing any outside pathogens into the aquarium’s carefully controlled environment.

One day, an aquarist brought Van Bonn a bullfrog that had hurt its leg while wrestling with another male of the species. Van Bonn recognized the injury as the froggy equivalent of a blown knee. This particular injury, common in human athletes, occurs when there are tears in the medial collateral ligament, the medial meniscus, and the anterior cruciate. “That means that the leg is just kind of flopping out to the side and dangling,” he said. It’s a serious injury, known in the literature as “the unhappy triad of O’Donoghue.”

Human knee surgery is a common practice, and the anatomy is well documented in the medical literature. However, “I didn’t even know if frogs had a cruciate ligament,” Van Bonn recalled. “Nobody writes about the surgical anatomy of a bullfrog’s leg.”

He eventually found the necessary anatomical diagram in a 19th-century German treatise. Van Bonn also had to construct his own artificial froggy ligaments out of suture material to repair the knee. As for anesthetizing the animal, it turns out that—for some strange reason and despite being inordinately sensitive to environmental contaminants—bullfrogs require six times more anesthetic to knock them out than fish.

Beluga whales can't be transported to the second-floor hospital via elevator. Their habitat has a secret pool with a platform that can be raised as needed.
Enlarge / Beluga whales can’t be transported to the second-floor hospital via elevator. Their habitat has a secret pool with a platform that can be raised as needed.

What if the animal needing treatment is larger—say, the size of a beluga whale? The animal hospital is located on the second floor of the Shedd building, and a beluga can’t be transported in the elevator. According to Van Bonn, every habitat in the aquarium also has a behind-the-scenes care area. In the case of the Oceanarium, where the belugas live, there is a special pool under an edifice designed to look like a Pacific Northwest mountainside, with a special platform that can be raised as needed.

But much of the care, such as taking blood samples, can be done in situ with the help of the husbandry staff. And while there aren’t instruments designed specifically for beluga whales, commercial horse endoscopes, for instance, work just fine.

“There are years and years of experience and training behind all these one-off stories about how do you reconstruct a bullfrog’s knee, or how do you build a nebulizer for an anaconda, or what level of pain medication will help a shark,” said Tang. Because taking care of the Shedd Aquarium’s aquatic menagerie involves so many unusual cases, the team members are dedicated to publishing scientific studies about their work to help build up the literature, disseminating what they’ve learned to other zoological and aquatic veterinarians around the world.

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