Life at the Zoo

by Phillip T. Robinson

  Determining drug doses in zoo animals is an empirical task, since all drugs are potential poisons. If you read any drug pamphlet and the list of potential side effects (blindness, impotence, gastric ulcers, deafness, and so on) closely, you might begin to question whether the cure might inflict more damage than the disease itself. The challenge with all medications is to use them in a manner that maximizes their benefits while minimizing their toxicity. The use of new drugs in unfamiliar animals have caused many sleepless nights for zoo veterinarians. Pharmaceutical companies appreciate zoos because their veterinarians are inclined to experimental drugs and are usually willing to use them in trials on their patients. Zoo veterinarians are always on the lookout for newer, safer ways to medicate or sedate animals and try to acquire new drugs before they are available on the market.

  Since so many types of animals have to be treated, misgauging drug choices and dosages is always a hazard, as their metabolism and toxicities are often poorly understood in many species. Few comprehensive safety and efficacy studies have been done on medicines in zoo animals, and veterinarians have to make judicious, but often experimental, use of nearly every drug in the formulary. In reptiles, for example, the metabolism of many drugs is extremely slow, allowing the injection of certain antibiotics only every three days—compared to two to three times daily, as might be necessary in a mammal.

  Even the most cautious approaches to using medications in zoo animals can run amok. The rule of thumb with new drugs is to start conservatively—don’t medicate the whole herd at once! One very capable zoo veterinary colleague rigidly observed this principle, and from his cautious experience he had found a common worming medication to be safe and effective when used on his valuable collection of Sumatran tigers. This same product had also been used widely in many zoo species and was generally regarded as low-risk. None of his tigers had ever experienced any side effects, and this wormer was used annually in his preventive program of vaccination and parasite control. One year, however, all hell broke loose when all of the medicated animals began to show acute signs of nausea and retching, followed by convulsions and collapse. The signs progressed to respiratory failure, and, in only eight hours, every one of his beautiful tigers was dead or dying. The zoo staff frantically attempted to determine why this catastrophe was taking place. One of a veterinarian’s worst nightmares had become a reality.

  Complete necropsies were done on all of the tigers, but nothing conclusive was found at first. The immediate suspicion was that the medication may have contained some contaminants that caused the deaths, but toxicology tests of the product proved it to be free of any extraneous chemicals. Bewildered, perplexed, and in shock, the medical staff undertook a painstaking review of the care of these animals, and the cause was finally revealed. Employees had been spraying the tiger enclosures for several months with an organophosphate insecticide, and this prolonged chemical exposure had lead to subclinical organophosphate poisoning in the entire tiger group. The result was to render important enzymes ineffective in their systems that were critical in metabolizing the dewormer, turning it into a lethal poison. Safe and effective in normal circumstances, an inadvertent chemical exposure put the entire group into a fatal spiral.

  From infectious diseases and parasites to the ingestion of toxic substances and foreign bodies, the list of husbandry and environmentally induced problems is lengthy and evolving in zoos. In 1983, for example, to save on cost of materials after a rise in the price of copper, the US Treasury minted new pennies that contained 98 percent zinc and 2 percent copper by weight—earlier pennies were a 95 percent copper alloy. As a result, the numbers of cases of zinc poisoning in zoo animals rose noticeably, especially in marine animals that consumed coins tossed into pools by visitors. Exposed to the acidic gastric environment, zinc leached out and caused intoxication. Zinc poisoning in domestic dogs likewise increased for the same reasons.

  In the 1970s, zoo medicine articles started to appear in abundance in journals and veterinary magazines, reflecting the efforts of a critical mass of veterinarians who were now collaborating internationally. Pathologists began to summarize their findings, labs compiled the comparative results of various blood tests, and clinicians wrote about anesthetic drugs that worked, surgeries that were successful, and problems they had seen and treated. Many basic veterinary techniques had to be developed, some as simple as determining where to draw adequate blood specimens from various species. We poked a lot of holes in zoo animals simply to find the best places to obtain our diagnostic specimens. The animals with domestic counterparts advanced most quickly, and zoo veterinarians have always been grateful for that familiarity.

  Some of the blood samples that we examined raised interesting questions, such as why camels and llamas have red blood cells shaped like footballs and why an animal the size of an elephant has red blood cells that are smaller than those of humans. After you obtain a blood sample from an animal, the next challenge is to obtain significant diagnostic interpretations of the findings—there is the chance that some of the resulting lab values are not meaningful. Few blood chemistry tests and other analytic procedures have been specifically developed to measure disease in zoo species. For example, even in domestic dogs, the human test used to measure the enzyme amylase (which may reflect pancreatic disease) is interfered with by the presence of several saccharides (sugars) in dog blood, invalidating the analysis. Fats present in the blood of some species may obscure the technology of some tests, and normal serum pigments may also create false readings for tests that measure color changes in serum samples.

  Physical exertion can markedly alter animals’ white blood cell counts, and excess muscle activity prior to and during the sample collection may raise the quantities of certain enzymes in the sample and eliminate their diagnostic value. One zoo pathologist suggested that the only way to truly determine the “normal” blood values of a wild animal is to cause its instantaneous death while it is in a blissful state at a feeding trough, and then immediately withdraw a cardiac blood sample for assay. Morbid imagery aside, he illustrated the problem fairly well. Overexertion of animals during forced exercise or physical restraint, particularly in hoofed animals, can produce muscle damage known as “overstraining disease” or “capture myopathy.” The damage to muscle (including heart muscle) can be severe enough to cause serious tissue injury, kidney failure, and fatalities. Parallel problems have been well known in overexerted race horses for centuries. Improved handling and chemical restraint techniques have significantly reduced injuries and made blood tests more useful by minimizing stress and exertion.

  Body parts that we might ordinarily expect to be present in an animal are absent in some of our zoo patients. For example, not all animals have gall bladders. Among the species in which this organ is absent are horses, zebras, elephants, rhinos, dolphins, camels, tapirs, and rats (mice have only a miniscule one). Carnivores have managed to avoid losing theirs, perhaps due to its usefulness in processing dietary fats. The gall bladder–“deficient” animals lack much in common, at least superficially, and I cannot explain why they are on a different evolutionary compass course, although their must be some underlying metabolic motives in play. Humans eagerly dispose of the gall bladder when it causes digestive pain and discomfort. Unfortunately for bears, their gall bladders are used widely in Asia for medicinal purposes, making this seemingly optional organ more of a handicap than an asset.

  Some traits are more evident than missing internal organs and a visitor to the zoo can observe the more obvious ones. Nearly all mammals have nasolacrymal ducts, the tear ducts that take the tear secretions from the eyes and direct them into the nasal passages. When humans’ eyes water excessively (from allergies, colds, crying, and the like), it causes our nose to run. But, ordinarily, the tear secretions that keep our eyes from drying out are not noticed because of their small volume. In the absence or obstruction of tear ducts the tears would spill onto our cheeks. When you next go to the zoo, observe the eyes of elephants and
seals and sea lions—none have nasolacrymal ducts. They can almost always be seen with a wet tear streak below each eye. In elephants, given the length of their trunks, long tear ducts would present “engineering” problems. With marine species, constant presence in and around water makes tear ducts unnecessary. Snakes have solved the problem by lacking tear glands entirely. Instead, they have a clear, dry spectacle covering the eye, which is shed periodically, along with the skin.

  In defense of veterinarians, biologists in general are no smarter when it comes to deciphering anatomical variations. For some years it was thought that the beautiful eclectus parrot was two species. The males and females are totally different in their spectacular coloration—the males are a bright green and the females are a striking crimson color. This led field biologists to name them as two distinct species, until they finally came to the impossible conclusion that one species was all male and the other all female. In many parrot species, the external differences between males and females are more subtle and often inconspicuous altogether, and those species are referred to as lacking sexual dimorphism. In order to avoid ascertaining sex in parrots by a surgical endoscopic procedure to visualize internal testes or the ovary, the San Diego Zoo research department developed an innovative laboratory assay to measure the sex hormones that were excreted in their feces. Other tests are now available using a blood sample. Biologists also had it wrong again with a species of bat, Ametrida centurio; the females are so much larger than the males that each sex was classified as separate species for years.

  Historically, because of the nature of the animal trade, there has always been an element of uncertainty about the exact geographic location of origin of specimens caught for sale to zoos. While species may look very much alike, significant genetic differences may be present between different populations of the same species. These differences, especially when they involve something as fundamental as chromosome structure, can result in incompatibilities that cause infertility and unviable offspring. The San Diego Zoo’s spider monkeys were such a group until the zoo research director, Dr. Kurt Benirschke, decided to have a look at the chromosome profiles (karyotypes) within the zoo’s primate colonies. While outwardly similar in appearance, the spider monkeys were a mix of individual animals of closely related, but distinct, subspecies, geographically separated in their home ranges in the wild long enough to diverge genetically. As a result of this type of work, it has become possible to sort animals by their genetic profiles and to resolve genetic incompatibilities in spider monkey breeding programs. Chromosome analysis has been performed in a number of zoo animal species, thereby avoiding similar problems in other zoos. Cheetahs, on the opposite extreme, are extraordinarily homogeneous when it comes to their genetic makeup. Studies carried out in East and South Africa by the Smithsonian’s Dr. Mitch Bush and his colleagues have shown that the genetic differences between widely spaced populations of these cats are remarkably tiny. They are so similar, in fact, that skin can be taken from donor animals and easily grafted to animals more than a thousand miles away without any of the expected problems of rejection that would ordinarily be predicted to occur.

  The objective of collaborative breeding programs is to maintain enough ancestral gene diversity to preserve the potential of animals in captivity to be representative of wild types and to avoid inbreeding, which may lead to infertility, lower birth weights, and increased neonatal mortality. Only carefully managed breeding initiatives have the potential to assure this objective over time, through the creation of studbooks (genealogy documentation) and planned matings according to statistically calculated indices of “inbreeding coefficients.” In this manner, specific matings between individuals can be managed in order to avoid creating overrepresented lineages and overlooking the perpetuation of rare lineages in the captive population. Such evaluations can also provide objective information about the need to recruit additional founder animals from wild stocks.

  Zoo breeding programs generally strive to avoid interspecies hybridization or atypical variations. Occasionally, a “tiglon” or “liger” (tiger × lion), “lepjag” (leopard × jaguar), “zorse” (zebra × horse) or “zonkey” (zebra × donkey) is produced by private breeders for curiosity’s sake. The first portion of these names denotes the male and the second the female of these cross breedings. Sidewalk souvenir photographers in Tijuana, Mexico, create a zonkey by simply painting black stripes on a donkey. To appeal to the “believe it or not” sentiments of the visiting public, some zoos exhibit “white tigers,” “white lions,” albino animals, and even some bicephalic reptiles. The last two categories occur regularly in nature, but their survival is often short owing to their increased susceptibility to predation and other cumbersome issues. The purposely crossbred animals are throwbacks to man’s banal curiosity with the genetic manipulation of life and serve no real scientific purpose in zoos.

  Most zoos have a policy whereby every animal that dies on the zoo grounds will be subjected to a necropsy, helping us to understand what is normal for them and why they died. This information on disease is invaluable as a health management tool, as well as a great surgical anatomy preview. Contagious diseases, environmental intoxications, nutritional imbalances, parasitism, and numerous preventable conditions have been identified in deceased animals that can alter management practices to benefit the living. Important too, is the detection of diseases such as tuberculosis that may have a direct effect on employee and visitor health.

  After the necropsy, some rarer animals are reserved for shipment to museum collections while others are reduced to ashes in the hospital crematorium. An animal incinerator was installed in the original San Diego Zoo hospital building, and Dr. Charles Schroeder, the zoo’s veterinarian, nearly burned the hospital down when a lanolin-rich wild sheep exploded in flames, shooting a fiery plume out the top of the building. This led to the construction of a succession of freestanding crematoria that saw every imaginable species enter their heavy steel doors over the years. The later zoo crematories had their own episodes of incendiary indigestion. In one, a large dromedary camel was being burned in sections when the custodian pushed in a bushel-sized load of fat from the camel’s hump. For the next twenty minutes a grease fire engulfed the firebox and got so hot that the fire department was put on alert. The coup de grace for our crematory, however, was a large seizure of hashish that we destroyed as a special favor to the federal Bureau of Narcotics and Dangerous Drugs. Seized in a drug bust on the Mexican border, the hashish arrived in several vanloads under the watchful eyes of drug agents, who meticulously inventoried each parcel as it was stacked into the fire chamber. The burners were lit and the inferno got underway, ejecting a huge cloud of cannabis smoke from the smokestack. Fueled by plant resins, the flames rendered the contents into ashes. Shortly after this incendiary bravado, an inspection of the firebrick liner revealed severe cracking, along with warping of the support steel, requiring an expensive overhaul. Future requests for cremating drug bust seizures were politely declined, and the greasy animals were sent to a rendering contractor.

  Pathology work brings up a lot of “What’s this thing?” questions. If I had it to do over again I might be a pathologist—if only for the reason that pathologists’ mistakes are seldom fatal with animals, and they get the coveted last word.

  9. HOLDING THE TIGER

  Zoos Say Yes to Drugs

  The air was heavy with musky scent as we dragged the tiger through the steel-barred door and into the unlit animal bedroom behind the big cat grotto. Several keepers pressed closer to look when the tiger suddenly reared his head in a wild-eyed stupor. Dr. Sedgwick admonished the keepers blocking the entrance: “Just remember folks, if he tries to get on his feet, I won’t be the last one out the door!” Everyone stumbled backward, ducking their heads in a hasty retreat through the narrow opening and into the sunlight. Two locks were snapped in place on the metal door as a third secured the chain-link safety cage. The tiger was left in peace to reorganize his brain cells, wh
ich had been temporarily derailed by a substance that went by the street names “Angel Dust,” “Monkey Juice,” “Rocket Fuel,” and “PCP.” The drug’s proper chemical name was phencyclidine hydrochloride.

  A breakthrough in zoo medicine, phencyclidine hydrochloride was eventually replaced by a shorter-acting compound called ketamine, now known widely by its contemporary street name, “Special K.” After observing hundreds of animals sedated with this drug, it is difficult to imagine the appeal that drives the illicit manufacturing and abuse that continue today. Starting in the 1960s, phencyclidine had become the new miracle drug for sedating nearly all nonhoofed mammals in zoos, from monkeys and gorillas to lions and grizzly bears. From a terminology standpoint, zoo animals can be “chemically immobilized,” which means that they are restrained so they can be safely handled, or “anesthetized,” which indicates that they are not experiencing pain while immobilized.

  Dr. Charles Sedgwick sizes up leopard anesthesia candidate

  Zoo veterinarians are expected to forgo the basic preliminary processes of competent anesthesia that are mandatory in virtually every other type of medical practice—the comprehensive pre-anesthetic physical examination. In human and veterinary practice, the lack of this critical evaluation would be considered malpractice if there were complications. With zoo animals, however, it is simply part of the normal territory and places great reliance on prior observations to assess risk. An animal may have serious preexisting problems that are not evident, even to experienced eyes; yet zoo veterinarians lack the benefit of comprehensive screening for cardiac malfunction or compromising liver and kidney diseases—the very organs essential to detoxifying the drugs they must use on their patients.