The Rhino with Glue-On Shoes (10 page)

Anesthesia for a Frog

by Mark Stetter, DVM

O
ne of the coolest things about frogs is that they can breathe through their skin. Of all the animals I work with, I think they are my favorite—and for a long time I was frustrated with the existing methods of frog anesthesia. A common technique involved using a powdered fish anesthetic, MS-222, dissolved in a water bath. In fish, it is a very safe and effective method. As the fish swims, the drug becomes absorbed across the gills and into the bloodstream; the patient falls asleep in a couple of minutes and rapidly wakes up when removed from the medicated water. In frogs, this drug is much more slowly absorbed through the skin, requiring as many as thirty minutes to induce anesthesia and a very long time (sometimes hours) for recovery. There is also the fact that many terrestrial frogs dislike being forced to take a long bath in the anesthetic water.

It seemed to me that frogs deserved a better anesthetic protocol, and that the key to this lay in their amazing skin. I wondered if a liquid anesthetic, applied directly, might possibly work.

I continued to ponder the concept until the right opportunity finally arose. At the time (1996), I was working as a veterinarian at the Wildlife Conservation Society's Bronx Zoo in New York City. One day, I was on rounds at our Central Park facility when our curator of reptiles and amphibians approached me and said, “Mark, I need you to look at a frog while you're here this morning. I think there was a big frog rumble in the tank last night, and when I came in this morning one of the frogs looked pretty beat up.”

We had our first patient! Frances was an ideal candidate—a beautiful poison dart frog from our new South American rain forest exhibit. She was brightly colored, with alternating markings of vivid blue and black. But although these little frogs seem cute and harmless to us, they can be quite savage toward each other. They often exhibit aggressive behavior in the effort to establish dominance, territory, and breeding rights. Frogs are no different from other animals in this respect.

Poison dart frogs get their name from hunters in South America. Indigenous peoples have long used the toxic excretions from the skin of this species as a fast-acting poison. When the tip of a dart is rubbed on the skin of the frog, the dart becomes a lethal projectile to bring down animals in the forest. Even more interesting, rain forest poison dart frogs do not produce this toxin when housed in zoos and aquariums. Nothing else about the animal changes; a captive frog looks and behaves exactly like a wild one. Scientists think the rain forest frogs ingest the primary components of the toxins via certain wild insects and other foods found only in the jungle—hence their deadly skin.

Somehow, Frances had been injured in the group frog fight, and her left eye appeared to have been punctured. Although she was a fully grown adult, Frances was only the size of a dime, and her eye was about the size of a blunt pencil tip. I would need our surgical microscope to see the extent of the damage and, if possible, repair it—a very delicate procedure. But how could I safely anesthetize her and ensure that she didn't move, not even slightly, while I performed this critical surgery under the microscope?

It was time to try my new frog anesthesia program.

The anesthetic gas called isoflurane is used in people, dogs, cats, horses, and a variety of wildlife species. Purchased as a liquid in a small glass bottle, isoflurane is poured into a metal compartment inside the anesthetic machine. A tank of oxygen is also connected to the machine. When the machine is turned on, it mixes isoflurane with oxygen and forms an anesthetic gas. The usual method of delivering the gas is to place a mask over the patient's face or insert an endotracheal tube in his windpipe (trachea). With frogs, both of these options are difficult, if not impossible. Holding a tiny face mask over the nose of a small, slippery frog is not an easy task. (I know, I've tried it.)

If frogs can breathe through their skin, why can't we just apply the isoflurane directly on their skin?

I knew from my previous work that you can place the entire frog in a mask, creating a little frog anesthetic chamber, and the frog will eventually fall asleep. The problem with the frog chamber is that it takes a very long time for the anesthesia to be effective.

Frogs can also be directly injected with anesthetic, like any mammal, but the effects are variable and the dose difficult to measure. If too little is given, the patient will move, making delicate surgery impossible. If too much is given, it's lethal. In a patient this small, even with the world's smallest syringe and a lot of drug dilution, injectable anesthetic was neither safe nor practical.

If I could succeed in applying the liquid isoflurane directly to the frog's skin, I'd not only be able to avoid the prolonged anesthetic gas chamber method, I'd also be developing a technique that could be used anywhere. Researchers could utilize this method for sedating amphibians in the field; it could also be used by zoo and aquarium vets in countries that might not have access to expensive anesthetic machines.

Liquid isoflurane is very volatile, so much so that a single drop evaporates in a matter of seconds. I knew that I'd need to mix the liquid anesthesia with something that would slow the evaporation rate and allow greater time for the anesthetic drug to be absorbed through the frog's skin.

I decided to combine the liquid isoflurane with water and a type of skin lubricant, K-Y Jelly, to create an elixir. I mixed the ingredients into a thick, syrupy solution that could be gently applied to our frog, calculating that the jelly would slow the rate of evaporation.

I applied a couple of drops to Frances's back, then placed her in a small clear plastic container. I pulled my chair up to the surgery table and waited. Within a couple of minutes, I could see the drug starting to take effect. Frances was getting a little woozy and was swaying from side to side. It was working! But would she become anesthetized enough so that the surgery could be performed? And if so, for how long would the two drops work?

After about five minutes, Frances could not sit upright; she rolled onto her side, sound asleep. I waited a couple of minutes longer and then removed her from the container for a full anesthetic check. In a frog, a “full anesthetic check” isn't performed with fancy EKGs, stethoscopes, or pulse oximeters. In even the tiniest frogs like Frances, you can actually see the heart beating beneath the skin. She had a good heart-beat, there was no movement, and when I pinched her tiny little toe there was no response. Perfect!

Off to surgery she went. Even with the surgical microscope, this was going to be a challenge. I had our veterinary intern scrub in with me to maximize the chance of success. When performing microsurgery, it's important to keep your eyes focused on the image in the microscope. You don't want to look up for any reason, and your elbows need to stay planted on the table. Your assistant is in charge of exchanging the surgical instruments in your hand.

Operating microscopes are commonly used for eye surgery in both humans and animals, particularly for cataract removal. This type of delicate procedure requires magnification for the surgeon and a complete lack of eye movement from the patient. Even a partial eyelid blink can be disastrous. In Frances's case, I was not only concerned about movement, I also knew we'd be pushing the limits of the microscope, designed for work on human-size eyes. Our entire patient was smaller than a human eye!

Examination under the microscope confirmed the damage: Frances's eye had been ruptured, and she was not going to be able to see out of it again. At this point, our only hope was to save her eye and minimize the chance for infection. Bacteria or fungal organisms could infect the eye, spread throughout her body, and jeopardize her life. We used the smallest type of suture material available to place a single stitch through her cornea. Usually a corneal laceration would require several sutures to repair, but in this small patient, only one was needed.

It took Frances about thirty minutes to wake up from the anesthesia. This was a good indicator that I had used the correct amount of anesthetic to begin with. After the surgery, she spent the night in the hospital's frog ICU—a warm, moist, quiet, well-monitored plastic container. The next day, she went back to the rain forest building for close observation.

I'd been concerned that Frances might need additional surgeries and that we would be forced to remove the eye altogether. But when I made a house call to check the frog in her exhibit a couple of days later, it was obvious she was going to heal well. The tissue of the injured eye looked healthy and she did not appear overly bothered by either the injury or the surgery. The keepers had helped her find her food and given her some extra attention after she returned from the hospital.

In the wild, frogs need both of their eyes to feed. They have exceptional vision, which they use in their aggressive capture of flying insects. Now Frances would need help catching her food on a long-term basis. To give her a competitive edge, the zookeepers found a way to slow down the live insects: they chilled them. By placing the food in the refrigerator prior to mealtime, they could slow the insects' flight, allowing Frances to get her necessary food requirements.

Since that day with Frances, I have used the topical isoflurane jelly protocol with many frogs and toads. By presenting this method of amphibian anesthesia at various scientific meetings and writing about it for veterinary medical books, I have also shared this technique widely with my colleagues. But it was Frances, our tiny poison dart frog, that made it all happen.

III

G
ETTING
P
HYSICAL

Zoo vets face unique challenges because of the vast number of species in their care, the potentially endless list of medical problems, and the great variety of conditions in which wild animals live. They must be prepared to be emergency doctors, primary caregivers, anesthesiologists, surgeons, even gerontologists. Every day is different, and many cases are one of a kind. The work can be exhausting, uncomfortable, and dangerous.

We begin our examination of every patient with what sounds like a simple process: observation. Since most wild animals have an innate fear of humans, even this first step can be hard on the doctor. In field situations, we may track the patient for hours or even days before we catch a glimpse of it. If the animal has learned to recognize the vet, we may end up hiding in vehicles, climbing trees, or sitting motionless at a distance until the animal accepts and ignores our presence. Sometimes the only option may be to join the animal in its environment: plenty of zoo vets have donned a wet suit and scuba gear and swum with their patients, even with sharks.

Once we decide it's necessary to restrain, anesthetize, or move a wild animal, we plan our approach. These procedures may carry a certain degree of risk, depending on the animal and the circumstances. Whereas an inch-long Texas dung beetle can be held gently in a pair of forceps, a 250-pound tiger requires a carefully placed anesthetic dart. Human safety is always a top priority. And even with the best planning, anything can happen.

If we don't have personal experience with the particular animal or situation—always a possibility—we review what we've done with closely related animals, read the literature, and call for advice.

Many smaller birds, reptiles, amphibians, and invertebrates can be carefully held in a net or in gloved hands. To minimize stress, it's best to lightly anesthetize them for an exam. But larger species that can bite, kick, scratch, and expel venom must be immobilized from the start. In such cases, we use a variety of potent injectable anesthetics in addition to gas anesthesia. Sometimes these drugs can be injected by hand, with the animal in a net or restraint box. More often, we inject them from a distance by a flying dart, using a light-weight CO2-powered pistol or rifle.

Zoo vets tend to work long hours in difficult settings, whether it's kneeling on a concrete floor performing surgery or crawling through brambles to see a patient. Even a state-of-the-art veterinary hospital can feel like the bush when you've been working for sixteen hours every day for a week. When we work in the field, whether easily accessible or remote, all of the necessary equipment and staff must be brought to the patients. Checklists are essential, given that something unpredictable always happens. Most of us subscribe to the rule that says plan, plan, plan . . . and then be flexible.

Zoo vets are known for their stamina, strong constitutions, steady hands, good aim, and healthy knees—with a bit of ego added to the mix. In this next selection of stories, these vets get physical. One works nonstop for a solid month to fly a group of dolphins to a new home. Another takes a risk while examining one of the world's largest saltwater crocodiles. Others brave logistical challenges and harsh conditions before they even have a chance to get their hands on their patients—a Bactrian camel, a herd of escaped bison, a mountain gorilla, and a forest elephant.

Lucy H. Spelman, DVM