Life at the Zoo

by Phillip T. Robinson

  It is a well-established rule that the veterinarian is ordinarily the first person to make contact with a dangerous zoo animal that has been drugged for handling. Everyone seems to agree that this incentive-based rule is a fair one, since the person giving the drugs should take responsibility for knowing how animals are supposed to react and risk their own neck first. And all of us who sedate zoo animals have had lingering thoughts that a malevolent bear or lion would fake its drugged state, await the touch of a zoo veterinarian, and savage him or her to a pulp.

  The keeper culture in zoos around the so-called magnum animals (dangerous cats, bears, and pachyderms) is a little different from that for animals such as kangaroos and birds. While many keepers have a pocketknife handy to take care of miscellaneous husbandry tasks, the big carnivore keepers often liked to carry a more formidable piece of cutlery in a belt scabbard. If I were stuck in a cage with a bear or a tiger, however, a CO2 fire extinguisher would probably be my choice of a defensive implement over a Bowie knife.

  Working on dangerous animals has been a touchy problem for generations of zoo veterinarians. Without chemical restraint to provide safety for both the animal and the handlers, accomplishing anything physical, however simple, was a daunting undertaking. With hoofed animals, it was common to rope and tie the more manageable species for treatments, hoof trimming, and other medical and husbandry tasks. Most zoos began by building steel “squeeze cage” contraptions that made it possible to force fractious animals into shipping or transfer crates and to restrain them by compressing them against a wall for as much time as it took to administer an injection. Oral tranquilizers are often unreliable and unpalatable and may have poor safety margins for patients in the high doses necessary for useful effect. On some occasions, it was possible to give an intravenous injection to a large animal that was mechanically compressed in a cage, but often difficult with all of the struggling, excitement, and limited access to veins amid the splashing urine, flying hair, growling, snarling, and confusion.

  Some of the early anesthetic procedures involved placing a rag soaked with ether or chloroform into a crate containing the patient. In the end, the combination of stress, along with the overheating and poorly suited drugs, caused an unacceptable number of injuries and deaths. Broken teeth, fractured legs, hyperthermia, and damaged muscles left another host of new problems to contend with. Neither were the risks to zoo personnel negligible in such close encounters, which had to be factored into all decisions to use manual restraint on larger creatures. Since the treatment could be more damaging than the original disease, attempts at physical interventions in larger animals were often avoided, except under the direst circumstances. I was fortunate to arrive in zoo practice when chemical restraint was well on the way to replacing the rodeo style of animal handling.

  Treating a sedated bear at the New York Zoological Park in 1906; Dr. W. Reid Blair (left) and director Dr. William Hornaday

  In his 1929 book In the Zoo, Dr. W. Reid Blair recounts several anesthetic procedures at the New York Zoological Park in the early 1900s: “In order to secure an animal properly and successfully, nothing is so important as that the person in charge of controlling the animal should possess good judgment, associated with plenty of courage and confidence. He must retain a level head in spite of any unusual incident which may chance to take place. He must guard against getting excited or unduly alarmed and provoked in case everything does not transpire as expected or desired.” These early anesthesia procedures involved generous quantities of inhaled chloroform and ether. The use of ether as an anesthetic was first described in the United States in the late 1840s, and, around the same time, chloroform anesthesia was first performed on humans in England. The public was slow to embrace general anesthesia with these two substances because of the occasional sudden death that occurred in humans due to the cardiac arrhythmias that they caused. In animals, the likelihood of anesthesia-related death was substantially increased as a consequence of the necessary use of physical restraint and the ensuing excitement and overstraining that ensued. To control pain, Blair often employed local anesthesia in his zoo cases by injections of cocaine and morphine. An eye cataract surgery on a partially grown Indian rhinoceros at the New York Zoological Park required eight ounces of chloroform and twelve ounces of ether, mixed together and applied through a large mask, while the rhino was tied and restrained on a floor covered with mattresses and straw. It took an hour before the rhino was sufficiently anesthetized to perform the surgery.

  Dr. Werner Heuschele trimming antelope’s hooves at the San Diego Zoo, c. 1960

  For many years, however, keepers, veterinarians, and curators stood by helplessly as many animals experienced injuries and diseases that were not treatable with existing technology. As with yesteryear’s domestic horse, a major leg fracture in a large zoo animal, especially hoofed species, was often managed by euthanasia. Fortunately, with many problems, time is a great healer. An important element of practicing zoo animal medicine is simply knowing when to leave something alone, despite all of the well-intentioned enthusiasm and encouragement—actually, pressure—to try to fix it. Providing solitude, warmth, comfortable bedding and tempting foods often were the best prescriptions available. This still has great merit in the golden age of drugs in zoos.

  Beyond everything else, zoo veterinary medicine operates on the “precautionary principle”—a sort of Hippocratic credo which translates to: “The good achieved should not be exceeded by the harm inflicted,” or, in other words, if a proposed activity has significant consequences, and you don’t know what you are doing, don’t do it! After all, there are plenty of opportunities for catastrophe even when you know, or think you know, what you are doing.

  Overdosing with drugs is a prevailing veterinary concern because so little is known about the metabolism of many chemicals in zoo animal species. Zoo veterinarians have always sought out deceased animals to learn more of their anatomy and metabolism, and to pick up clues as to how they are likely to respond to medications. For example, if you needed to know how much of a giant tortoise’s body weight was made up of more metabolically active flesh, you simply waited for one to die and then dissected it into two piles and weighed them—muscles and other vascular tissues go in one pile, shell and bones into another.

  Later, reptile medicine took a giant slither forward with the embarrassing discovery that injecting certain drugs into the hind limbs of tortoises was more dangerous than giving the same injection in the front limbs. This may seem nonsensical, but the blood circulation of some reptiles, such as tortoises, is different from that of mammals. The veins that drain the rear limbs make a preferential circulatory pass through the kidneys (the renal portal system), where high concentrations of some drugs can cause tissue damage. If the drugs are injected instead into the forelimbs, this chemical toxicity is spared. Of course, as luck would have it, veterinarians typically give hind leg injections to most animals, as they learn to do in their training—an unfortunate custom for some earlier tortoise patients.

  Jenny the elephant at the London Zoo in 1874; she suffered from a paralyzed trunk and arthritis and died in 1875

  Determining safe and effective drug doses in zoo animals is an empirical task, since all drugs are potential poisons to some degree. We administer drugs to both animals and people with the objective of maximizing the therapeutic benefits while minimizing the toxic side effects. The use of new drugs in unfamiliar animals has caused many anxious moments in zoo practices. Pharmaceutical companies, as I have said, universally like zoos because zoo veterinarians are keen to try new experimental drugs. Due to simple economics, there are virtually no drugs developed and marketed specifically for zoological species. In fact, many drugs used in normal pet animal practice have not been created specifically for dogs and cats either. Part of the passion for newly emerging pharmaceuticals derives from the relentless attempts by zoo veterinarians to find safer, more effective ways to sedate and treat their patients. The combination of improved injecta
ble anesthetics and restraint agents with inhalation anesthetics—methoxyflurane, halothane, isoflurane, and sevoflurane (in the order of their introduction into medical practice)—made surgical interventions common and relatively safe for most animals. One of my patients was a Komodo dragon lizard that managed to fracture its arm; with the use of halothane inhalation anesthesia we were able to repair its fractured humerus with a metal bone plate and screws and achieve a total recovery. Hippos and giraffes, however, remain the top two anesthesia and restraint challenges in zoo practice today.

  Chemical restraint and anesthesia of zoo animals are used for a wide variety of purposes. In addition to physical examinations, imaging studies, treatments, and surgeries, there are myriad other tasks that are facilitated, such as genetic testing, semen collection, and artificial insemination. Zoo veterinarians were to find out by trial and error that some drugs worked well in some species but were entirely contraindicated in others. For example, a synthetic opioid drug called etorphine (M99), while highly effective in a rhinoceros, has disconcerting effects on felines. I tried it only once in an African lion and observed the animal apparently experiencing hallucinations, fear, and disorientation as it lunged around its cage and tried to scramble up a bare wall in terror.

  The dose range for different species of hoofed animals is significantly different between the San Diego Zoo and its sister facility, the San Diego Wild Animal Park, only thirty-five miles away. I attribute the reasons for these to the degree of confinement and differences in the physical condition (cardiovascular fitness and muscle mass) of animals that reside in larger areas where they received much more exercise. Compared to animals in the zoo, most of these animals also are accustomed to greater freedom of movement and personal space, which undoubtedly affect the level of arousal and excitement during the apprehension process. Similar parallels occur in free-ranging animals in the wild, which ordinarily require even higher doses for successful immobilization in the field.

  The new age of chemical restraint in zoos was exciting, but sometimes disconcerting, involving daily experimentation with unfamiliar drugs in novel patients. The word about new drugs, both good and bad, traveled quickly among zoo veterinarians in widely separated institutions long before the first results were published in veterinary journals. If there was an oddball case to sedate, we often telephoned around to find a trusted colleague with the same or similar species in the collection to see if he or she had any useful experiences to share. On one such occasion I called Dr. Clint Gray, the head veterinarian at the National Zoo in Washington, about a pygmy hippopotamus in San Diego that had a dental problem. I needed to do an oral examination on him and possibly cut back his tusks to correct a malocclusion. Chemical immobilization procedures in the larger common hippo in zoos have had a dismal track record—about half of the hippos under sedation died of anesthetic complications, mostly from respiratory collapse from their inability to breathe properly under their massive weight when unconscious out of water. I could find no published drug doses for the pygmy hippo, its diminutive relative, which weighed in at about six hundred pounds. Dr. Gray, a former cattle and horse veterinarian (and life of the party at annual zoo veterinarian conventions), immediately offered me an exact dose of a sedative on the phone. Given his extensive experience, I felt greatly relieved. As soon as I hung up the phone I made plans for dental surgery the next day. The dentistry went well, and our little female hippo recovered fine from the anesthesia. Grateful for such capable advice, I called Clint back to thank him for sharing his drug doses with me. He said, “Great! I’m glad it worked out on your pygmy because, well, I’ve never actually had to sedate one of them before—and I always wondered if that drug would work on them. . . . And since it worked so well for you, I’ll be sure to try it the next time I have to handle one of those critters. Thanks for the information, Phil.”

  The fundamental strategy of modern zoo medical practice is the prevention and early diagnosis of problems. In the late 1950s new drug-delivery equipment came on the scene and began to allow veterinarians to intervene diagnostically and surgically with increasing degrees of success. The invention of the Cap-Chur dart gun system by a Georgian named “Red” Palmer made it possible to inject drugs remotely into animals with aluminum projectile darts. In the 1960s, the Marlin Perkins Wild Kingdom television series showed this equipment at work in numerous episodes involving the chemical restraint of wildlife. Unlike real life, however, the show conveniently left out the tedious and risky segments of film where animals run off, stumble, bash into trees, or experience other, sometimes fatal, complications. In Marlin’s world, darted animals safely fell under a reliably sublethal chemical spell while hardly blinking an eye. Jim Fowler, his trusty assistant on the program, was always there to explain how marvelously well things were going.

  Cap-Chur pistol and tranquilizer dart

  These new darting systems were based on three formats: a Crossman CO2 pistol, a CO2- charged rifle, and a .28-gauge rifle powered by .22-caliber powder charges. An earlier crossbow model proved too brutal for general use. Their practical darting ranges are from ten to one hundred feet, and the hollow aluminum darts typically hold between three to ten milliliters of liquid drug. All immobilizing darts are capable of inflicting serious injury to animals—bruising muscles, breaking legs, puncturing tendons, and penetrating body cavities. The aluminum ones are much more hazardous than later, lightweight varieties. The Cap-Chur darts were made up of three segments: the body, the tail, and the needle. These latter two parts screw onto each end of the threaded aluminum tube. A rubber plunger within the body of the dart separates the liquid drug on the forward side from the small impact-activated powder charge that fits into the rear side of the plunger. When the dart strikes an animal, a spring-loaded internal weight strikes and ignites a powder charge, driving the plunger forward and expelling the contents through a large 14-gauge needle. To reduce the tendency of the dart to bounce off of an animal, metal collars or barbs adorning the needle shaft catch in the skin, retaining the dart long enough for injection of the drug. In its earlier applications, its intended markets were for “non-lethal chemical restraint of criminal suspects,” for medicating domestic livestock and in urban animal control work. It was tested on so-called volunteer prisoners in the beginning, who probably sought to perform good deeds in exchange for future consideration at parole time. However, it found its most versatile market among zoo and wildlife veterinarians, who used it for delivering both immobilizing drugs and antibiotics. While it never caught on in law enforcement, they have been put into wide use by municipal animal control agencies and wildlife biologists. Several other, more recent, systems, most notably the “Telinject” brand of dart rifles and pistols, are now available. These are superior, lighter plastic or nylon darts, which are significantly less traumatic and more accurate. The immobilizing drugs used in these darts must be readily absorbed by intramuscular injection, which eliminates many sedative drugs in human and veterinary medicine to begin with. The margins of safety for some chemicals were narrow between an immobilizing dose and a lethal dose, especially with nicotine alkaloids (“Cap-Chur-Sol”) and muscle-immobilizing drugs. The green Cap-Chur-Sol label appropriately displayed—in bold print—a large skull and crossbones and the word “poison.”

  Darting a patient at the bear grotto, San Diego Zoo

  Telinject rifle and darts

  The drug curare was originally obtained from the leaves of the tropical plant, Chondrodendron tomentosums, by indigenous South American native hunters, who applied it to hunting darts projected through long blowpipes. It causes neuromuscular paralysis, but not relief from pain; alone, is inappropriate for surgery. A drug that similarly causes muscle paralysis without analgesia, called succinyl choline, was used by equine practitioners for years in the absence of other, safer drugs for castrating horses in the field. Overdoses of both of these compounds can cause severe temperature regulation disturbances, paralysis of the respiratory muscles, and asphyxiation, unless artif
icial ventilation is employed. Their unpredictability, especially when given intramuscularly, resulted in highly variable and risky procedures in zoo animals.

  Many animals, particularly free-ranging wildlife, died of complications from earlier capture drugs in the field because the compounds lacked reasonable safety margins and were often administered by personnel who were untrained in emergency treatment measures required to resuscitate animals. Neuromuscular paralyzing drugs quickly fell out of favor as soon as the first glimmers came from newer pharmaceuticals. In the 1960s the growing availability of a new class of drugs called dissociative anesthetics (cyclohexamines) opened a new vista for safely restraining cats, bears, apes, monkeys, and a host of other zoo animals. These have been subject to widespread human drug abuse and require close oversight to prevent diversion for “recreational” purposes.

  Other delivery systems for remotely administering drugs to exotic animals include a variety of commercial and homemade blowpipes. Used for centuries by South American forest peoples for capturing monkeys, birds, and other small game with drugladen dart projectiles, the blowpipe has proven to be an effective device at shorter range, delivering liquid drugs through lightweight nylon and plastic darts with a minimum of noise and traumatic impact. The ideal anesthetic/immobilizing agent would have certain key properties—none have them all. Some drugs are meant simply to provide physical control of an animal so that it may be handled safely and without injury to itself. Others have analgesic (pain-relieving) properties essential to performing surgery. The ideal immobilizing drug is (1) rapid-acting; (2) suitable for intramuscular injection; (3) effective in volumes usable in a projectile dart; (4) safe over a broad dose range; (5) chemically stable without refrigeration and when mixed with other drugs; (6) not highly toxic to humans with accidental exposure; (7) reversible with a specific antagonist (antidote); (8) analgesic in surgical procedures. No existing drugs qualify for all of these criteria. In many cases, inhalation anesthetics are used as a supplementary adjunct to injectables for longer procedures involving surgery.