CHAPTER 6 CROCODILIANS
Modern-day crocodilians date back to the Mesozoic era over 65 million years ago, surviving to the present with relatively few evolutionary changes. Their biology, physiology, and anatomy are unlike those of any other reptiles. However, some species have suffered the effects of a changing environment and human encroachment; this situation has led to the formation of international programs dedicated to the preservation of threatened or near-extinct species. Of the 23 species of crocodilians, 13 can be found on the red list of endangered species.1 Taxonomic identification varies among all species, and there is debate over the proper classification of species and subspecies (Tables 6-1 and 6-2; Box 6-1). In addition to having wild crocodilian populations, countries such as Australia, India, Mexico, Papua New Guinea, South Africa, and the United States maintain intensive production operations for various species.
|Southeast United States
|Central and South America
|Cuvier’s dwarf caiman
|North, Central, and South America
|Fresh water crocodile/Johnston’s crocodile
|Africa and Madagascar
|New Guinea crocodile
|Papua New Guinea and Irian Java
|Mugger crocodile/Swamp crocodile
|Australia and Southeast Asia
|African dwarf crocodile
|Indian gharial/True gharial
In North America, the American alligator (Alligator mississippiensis) is the best-known crocodilian. Second to it is the American crocodile (Crocodylus acutus), a vulnerable species found in small numbers in Florida. The American alligator was considered a threatened species during the 1960s, but a captive rearing program in Louisiana has been successful at maintaining the estimated population at over 1 million animals.
The first type of captive rearing operations consisted of alligator farming. In farming operations, breeding pairs of alligators were kept in enclosed areas where they could mate and nest (Figure 6-1). The eggs were collected from the nests and incubated. The farming operations led to ranching operations in which eggs are harvested from the wild and incubated in private facilities (Figure 6-2). The alligators hatched on an “alligator ranch” are then raised for their hide and meat. To help maintain the wild populations, 14% of the hatched alligators are eventually returned to the wild. Louisiana is the primary producer of American alligators in the world. Captive rearing operations can also be found in Florida, Texas, Georgia, and other southern states within the United States where alligators naturally inhabit. In Louisiana the majority of the operations are ranches, whereas many farms still exist in Florida. Other crocodilian species, such as the Nile crocodile (Crocodylus niloticus), are primarily raised under farming operations in other countries.
In those states where alligator productions are present, there are opportunities for veterinarians to get involved with the industry. Some veterinarians, regardless of practice concentration, have alligator farmers or ranchers as part of their clientele and meet new challenges in their daily practice. A population management approach is necessary when working with these alligator production facilities. It is also advisable for veterinarians and farmers alike to be aware of the rules and regulations imposed by the local department of Wildlife and Fisheries.
In addition to being found in their natural environment, crocodilians are also found in zoos. Zoologic parks house different species of crocodilians for educational purposes, reproductive efforts, or both. As is the case with many other threatened species, zoologic institutions play a critical role in the captive reproduction of threatened species of crocodilians. Here the animals can be observed by specialists in a more controlled environment that mimics their natural habitat, although design and maintenance of the enclosures can be difficult. Disease prevention and control are other important considerations when it comes to oversight of threatened species.
Veterinarians may also encounter crocodilians as privately kept “pets.” In the past, the spectacled caiman (Caiman crocodilus) and even the American alligator (A. mississippiensis) have been sold to the general public. There are many reasons why crocodilian species should not be maintained as pets. In addition to the physical dangers and risks associated with keeping crocodilians, there is the misfortune of inadequate husbandry, adversely affecting the reptile’s health. Most crocodilians grow to be larger than other reptile species and therefore have significant space requirements. Like most animals requiring an aquatic environment, crocodilians need water that is clean and free of disease. Another husbandry issue is an incorrect diet, which can lead to metabolic abnormalities and overall poor health. Many private owners are unable or unwilling to provide these conditions for their crocodilians. In addition to these issues, most states will require special permits to own theseanimals. Most people lack the proper ownership permits, and veterinarians could be liable if they treat these illegal patients. The biggest challenge that veterinarians face when treating illegally owned animals is that they are often the last chance for humane treatment for these animals. It is the duty of veterinarians to educate these clients and persuade them to place these animals in appropriate zoologic institutions or other accredited facilities.
Most of the material presented in this chapter specifically relates to the American alligator (A. mississippiensis). Although most of the alligator information will apply to other crocodilian species, there are variations among groups that will be mentioned. The natural environment and geographic distribution of a species will often determine disease exposure. Once in captivity, all species are susceptible to the same diseases within that environment. In addition, there are some inherent differences among the three families of crocodilians—Alligatoridae, Crocodylidae, and Gavialidae—that have emerged from studying them in captivity.
One of the most common questions a veterinarian may encounter regarding reptiles is “What is the difference between alligators and crocodiles?” The first difference is that they belong to two different families: the family Alligatoridae (see Table 6-1) includes the alligators and caimans, and the family Crocodylidae (see Table 6-2) includes all crocodiles. There is a third family, the Gavialidae (see Box 6-1), which contains the gharial, or gavial. Geographic location may help in the identification of some species. Alligators are thought to tolerate colder temperatures and live at higher latitudes, whereas crocodiles and caimans are less cold resistant and live in warmer areas.2 However, there are some anatomic features that will be most useful in differentiating alligators from crocodiles. The alligators and caimans have a broad, u-shaped snout, whereas crocodiles have a more narrow, v-shaped snout. This difference can be observed by looking at the dorsal aspect of the head (Figure 6-3). A more obvious distinction can be made when looking at their mouth from the side (Figure 6-4). Alligatorsand caimans have notches in the maxilla that fit the mandibular teeth. Therefore, they have no mandibular teeth visible if observed from the side with their mouth closed. On the other hand, crocodiles have the fourth mandibular tooth exposed when looking at them from the side with their mouth closed. Integumentary sensing organs, also known as dome pressure receptors (DPRs), are clear to gray pits present on the skin of crocodilians. Their function is not completely understood, but they may play a role as mechanoreceptors in prey detection or even as chemoreceptors aiding in detection of salinity levels.3,4 Alligators and caimans have DPRs only on the lateral aspect of the mandible (Figure 6-5), whereas crocodiles and gharials have DPRs all over the body, most noticeably on the ventral scales (Figure 6-6). The presence of DPRs can be used to differentiate the two main groups of crocodilian skins in the leather market. An additional feature that could be used for differentiation of alligators and crocodiles is the salt glands, which are absent from the tongue of alligators and caimans but well developed in crocodiles and gharials.
Figure 6-4 The lateral aspect of the head of a crocodile and an alligator. Notice that mandibular teeth are not visible in the alligator, whereas in the crocodile, the fourth mandibular tooth is clearly visible.
An interesting anatomic feature of crocodilians is the palatal valve, also known as the gular valve. There is some discrepancy as to the name of this structure and its two components, but I will attempt to describe them based on the anatomic location. Crocodilians have a true hard palate in the roof of the mouth that ends caudally in a soft palate. This soft palate has a ventral flap that is referred to as the velum palati. The velum palati is the dorsal component of the palatal valve, with its second and ventral component being the gular fold. This structure projects in a craniodorsal direction from the base of the tongue and has a cartilaginous base to it that is part of the larynx. Together, the velum palati and the gular fold form what is known as the palatal, or gular, valve (Figure 6-7). The function of this valve is to seal the pharyngeal cavity while under water to prevent aspiration. Crocodilians also have control of the nares and are able to open and close them as needed to prevent aspiration of water.
The respiratory system of crocodilians consists of well-developed lungs benefiting from a very effective inspiration aided by the intercostal muscles and the septum post hepaticum. The septum post hepaticum is a diaphragm-like muscle that creates a partial separation of the thoracic and abdominal viscera. A number of membranous connections separate the lungs and the liver, and an intricate mesentery system encompasses the gastrointestinal tract and viscera. All of these tissue structures may be necessary for allowing the changes in pressures that occur during diving.
The cardiovascular system of crocodilians also has special characteristics. Crocodilians have a four-chambered heart as opposed to the three-chambered heart found in other reptiles and amphibians. The circulation of blood through the crocodilian heart is like that in the heart of mammals, but the crocodilians possess a viable foramen of Panizza. This opening is located at the base of the heart between the left and right aortic arches and allows for venous admixture, which is essential during periods of diving to conserve oxygen.5 During diving, there is pulmonary hypertension. This in turn creates increased pressure in the pulmonary artery and the right ventricle, which forces deoxygenated blood through the foramen of Panizza into the left side of the heart and the aorta to be distributed through the body. This mechanism allows for conservation of oxygen and supplies oxygenated blood to those organs that require it the most, allowing some crocodilian species to stay submerged for up to 6 hours.6 A second anastomosis may be present in other crocodilian species as a vessel connecting the two aortic arches.2
Submandibular (Figure 6-8) and paracloacal glands and a gall bladder are present in crocodilians. The hard dorsal scales are known as osteoderms, bony plates lined by skin. Crocodilians have a smaller gastric compartment distal to the stomach that is a gizzard-like structure in which rocks and other materials may be found. It does not appear as evolved as the ventriculus in birds, however. Crocodilian intestines have a thick wall and can have well-developed diffuse aggregates of lymphoid tissues like Peyer’s patches. There is no urinary bladder, but the colon can hold large amounts of urine and water. Sexing can be performed by palpation of the cloaca; males have a phallus that can be palpated and extracted from the cloaca. Females have a well-developed clitoris that, depending on size, can be confused with a phallus. Internally paired gonads are found near the ventral surface of the kidneys.
Environmental considerations are variable depending on where a crocodilian species lives or is farmed. The aim of captive rearing operations is to produce a large number of animals in the most efficient way possible. In a zoologic or other educational institution, the goal is to exhibit the animals in an environmentally accurate artificial environment. The underlying policy of any aquatic enclosure should be clean water, appropriate diet, and enough space to accommodate the growth of the animals. As with most exotic animals, the challenge is to mimic a captive animal’s natural environment.
There are no specific references for the enclosure size of crocodilian species in captivity. The size of the enclosure will largely depend on the species of reptile and the purpose of their captivity. A general understanding of biology and natural behavior of the captive species is essential to designing appropriate enclosures. Although a zoologic institution housing the species might provide more specific advice, there are some general enclosure guidelines for the commercial production of American alligators: 1 square foot per alligator up to 24 inches in length (snout to tip of tail), 3 square feet per alligator for those between 25 and 48 inches in length, and an additional square foot of space for every 6 inches in body length beyond 48 inches.7 These are the recommendations for the maximum stocking rate for alligators in commercial operations.
The recommendations for a zoo or educational facility are to make the exhibit as large as possible, taking into consideration the species being housed. A consideration for larger species is territoriality requiring an expanded enclosure. Male crocodilians may become more aggressive during the reproductive season, and keeping them separated should be a consideration if space is a concern. Exhibits can be outdoor, indoor, or a combination of both. Outdoor exhibits can closely mimic the natural environment but also present more challenges for maintaining water and environmental quality as well as for controlling diseases. Geographic location will also play a role in the creation of outdoor exhibits, as not all species of crocodilians can tolerate cold weather. Finally, some species can dig considerably, and measures must be taken to prevent an escape.
The temperature and humidity requirements for crocodilians in captivity vary with the species. Once again, an understanding of crocodilian biology and natural history is needed to try and duplicate their natural environment. An important consideration is the allowance of circadian variations in light cycle and temperatures to mimic their natural environment. This is not the case in many commercial operations, where they are maintained at a fairly constant temperature and humidity to achieve faster growth. From a health standpoint, this may allow for cross exposure of reptiles in commercial operations to infectious organisms that typically affect mammals. As the commercial reptiles are maintained at higher temperatures, new diseases commonly associated with mammals may adapt to living inside a reptile host and lead to clinical disease. In an enclosure, the temperature can be maintained via heating elements contained within the concrete slab, in line water heaters, or both. The water temperature must also be maintained during the refilling of the pen or enclosure to avoid significant temperature variations.
Light requirements for reptiles are still a controversial subject. In general, a source of ultraviolet B (UVB) light for herbivorous and omnivorous reptiles is recommended. Ultraviolet light is essential for the synthesis of vitamin D3, specifically its active form 1, 25 dihydroxyvitamin D, which is essential for the metabolism of calcium and phosphorus. A lack of vitamin D3 can lead to inappropriate calcium absorption, which in turn creates a metabolic imbalance resulting in metabolic bone disease. Metabolic bone disease, specifically secondary nutritional hyperparathyroidism, is recognized in many reptile species housed with an inappropriate source of UVB light, fed a diet deficient in calcium, or both. Carnivorous reptiles may also benefit from UVB light but are thought to obtain enough vitamin D3 and calcium from their prey. The UVB light requirement of crocodilians is unknown, but as true carnivores they may thrive with minimal exposure to UVB light. It is a common practice on alligator ranches to raise animals in darkness with no source of UVB light or a normal light cycle. Most animals will grow well under these conditions, and some have reached adulthood without signs of metabolic diseases. However, I have also observed evidence of metabolic bone disease in a subset of captive American alligators being fed a commercial diet with no exposure to UVB light. In these cases veterinarians must also consider the possibility that the commercial diet may be deficient in calcium. Anecdotal stories from alligator ranches claim that weak, anorectic animals appear to improve after being exposed to sunlight over a period of time. Further research is needed to determine the UVB light requirements of crocodilians and the potential benefits of exposure to UVB light. Natural unfiltered sunlight is the best source of UVB light, but various artificial sources are available (e.g., fluorescent UVB light bulbs, mercury vapor light bulbs).
The two main substrates in crocodilian exhibits are water and soil/sand. The species, age, and feeding habits must be taken into account; avoid substrates that may be ingested by accident and may lead to impactions. It is also important to prevent the public from throwing coins and trash into exhibits, as this may represent a source of toxicity and a cause of impactions. In commercial operations, a smooth covering is applied to the concrete to preserve the quality of the hide. Either an epoxy coating or plastic liners are routinely used as substrate.
Crocodilians are true carnivores, as evidenced by their short gut and oral cavity. As such, they require a high protein diet, low in fiber. Their feeding habits in the wild will vary with age and food availability. Early on, their diet will con-sist of small invertebrates, amphibians, and reptiles. As they grow, they will eat larger prey of the type described earlier and will incorporate fish and birds into their dietary regime. With time, and depending on the species, size, and food availability, crocodilians will start eating mammals. There have been few studies investigating the nutritional requirements and feeding protocols of alligators.8–10 Various commercial feeds are available for alligators maintained in captivity. The commercial rations consist of dry pelleted diets that try to provide full nutritional requirements. These diets can be found with a 45%, 47%, or 56% protein content, less than 11% fat, and approximately 3% fiber content.11 Refined commercial alligator diets are widely used in production operations but may prove too expensive and/or inappropriate for the long term. A variety of whole prey feeds, such as chicken, nutria, and fish, are also recommended. If using nutria, be sure that it has not been killed using lead shot; if the lead is ingested, toxicosis can occur.12 When feeding frozen fish to crocodilians, provide a vitamin B supplement or another meat source to prevent thiamine deficiency. To prevent dietary associated problems, purchase meat from a reputable source.
All animals should be quarantined before their introduction to a production or zoologic facility. A detailed history should be obtained from the source facility, including information regarding diseases to which the animals in question may have been exposed. One should always purchase animals from a reputable individual or institution. Little information will be known about wild-caught animals. A minimum quarantine period of 60 to 90 days is recommended in a building that is separate from the main facility. During the quarantine period, the animals can be examined for any sign of illness, and diagnostic tests (complete blood count [CBC], plasma or serum chemistry, West Nile virus antibodies, etc.) can be performed to assess their overall health status. If the alligators originated from an area where WNV is endemic, it is advisable to test for previous exposure to this virus.
Unfortunately, a true quarantine process does not often occur. Quarantine is limited by the availability of appropriate facilities, as well as by the time and production budget. Quarantine is also a challenge in commercial operations where there may be a large population of animals. Nonethe-less, zoologic institutions and production facilities should try to establish an adequate quarantine program for their crocodilians.
Routine physical exams of crocodilians may be incorporated into a zoologic institution’s preventive health program. As animals get larger in size, performing physical exams becomes more hazardous. Chemical immobilization can be used, if needed, to perform necessary examinations, particularly on large crocodilian species. In commercial operations, the skins of a subset of the production animals will be examined at intervals before the anticipated time of slaughter. The time of hide examination presents an opportunity for veterinarians to examine the animals in more detail. Other than hide examinations, routine physical exams are not commonly performed in crocodilians from commercial operations.
Biosecurity is an essential part of disease prevention in any animal facility. The creation of a sanitation station at the entrance of each building is recommended to prevent introduction of disease organisms. These stations should contain a foot bath and a hand-washing station to decrease the opportunity of disease transfer between exhibits or buildings. A brush should be provided at each foot bath to thoroughly clean boots and shoes. The solution for the bath can be made of bleach or other commercial disinfectants, preferably with virucidal activity, and should be changed daily. Organic material will contaminate a foot bath, rendering it ineffective. The foot bath should be used before and after entering the building. A hand-washing station should consist of a water source, a sink, hand soap, and disposable hand towels. A waterless hand sanitizer product or exam gloves will suffice in lieu of a hand-washing station. In addition to the sanitation station, separate tools for working in each building are required. Separate working tools also prevent the transfer of diseases via nets, brooms, rakes, and so on. The buildings themselves must also be maintained free of pests and thoroughly cleaned whenever possible. In commercial operations, it is common practice to empty and disinfect the buildings after the slaughter period, before the introduction of new animals. Water quality is one of the main issues of concern when keeping crocodilians in captivity, and many health problems can be prevented if attention is paid to acceptable water quality.
Manual restraint of crocodilians is essential for physical examination, administration of medications, administration of anesthetics, and relocations. The size and species of crocodilian will determine the best and safest restraint methods to be used. An experienced crocodilian handler must be available to help restrain the animal. Although alligators and caimans are usually thought of as being less aggressive than crocodiles, this may not always be true. All sizes and species should be handled with the safety of the people as well as the animal in mind. Crocodilians less than 1 m in length (snout to tip of tail) may be handled by one or two individuals. Those between 1 and 2 m in length should be handled by at least two or three individuals. Those longer than 2 m in length will require at least four to five individuals. Various tools (e.g., pole snares, nets, squeeze cages, traps) also can be used to restrain crocodilians. The head, tail, and limbs must be immobilized and controlled. Once the animal is under control, the mouth is secured with strong tape or a rope. Albino and leucistic animals can have increased skin sensitivity compared to the normal pigmented individual; therefore, additional care should be employed to avoid irritation of the skin. Restraint is stressful for the animal, and contact must be limited to the time it takes for the procedures being performed. (See Anesthesia for more discussion on chemical restraint.)
Signs of illness in captive crocodilians are usually nonspecific. Anorexia, lethargy, a change in behavior, or death may be the first indication that something is wrong in a collection of animals or commercial operation. Adequate observations of the animals made by the personnel in the facility should be taken seriously. A visit to the facility is best undertaken during feeding time to avoid additional stress to the animals. At this time, the feeding and water quality policies can be evaluated. A thorough history should include information about the number of animals, source, age, most recent introduction, quarantine practices, feed, frequency of feeding, water quality parameters, clinical signs, time since first signs were observed, recent changes in management techniques, and any treatments such as salt, bleach, or antibiotics. Within a commercial operation, a subset of animals should be collected for disease diagnostics and necropsy. In addition to the obtaining of routine samples, tissues should be frozen for possible bacterial, fungal, or viral cultures. Within a zoologic institution, sacrificing live animals may not be possible, but veterinarians should obtain diagnostic samples from those with and without clinical signs. Necropsies should be performed in all dead animals. Live animals should undergo physical examination.
Once the animal is properly restrained, a physical examination can be performed. For safety purposes, veterinarians must be aware of the location of the head and tail at all times when performing the examination. A protocol should be followed for the examination process in crocodilians as in any other species. The oral examination can be performed if a speculum (e.g., PVC pipe, piece of wood) is inserted in the mouth before securing it with tape or a rope (Figures 6-9 and 6-10). Examine the eyes for evidence of discharge and assess their function (Figure 6-11). Examine the skin for any evidence of trauma or dermatitis, and then palpate the extremities, joints, musculature of neck, pelvic region, and tail. Joint swelling is often noted with infectious disease such as mycoplasmosis or trauma. Poor body condition may be reflected in atrophy of the muscles. In commercial operations, it is important to examine the skin on the animal’s ventral aspect because this is the area where many disease manifestations will be noted. It is also common to find tooth marks, scratches, and lacerations on the ventral epithelial surface. Finally, examine the vent and cloaca for abnormalities. If working in a commercial operation, a veterinarian must examine multiple animals to determine if an observation is associated with disease. If working in a zoologic institution and any findings are suspected to be infectious in origin, other animals in the exhibit should be examined. As with any species, an examiner must know what a normal presentation is in order to recognize clinical signs of disease.
Diagnostic tests used to determine crocodilian health status are no different than those used with other animal groups. CBCs, chemistry panels, and bacterial cultures can all be performed in crocodilians. However, there are limitations when it comes to the interpretation of test results because of the lack of reference ranges available for the multiple crocodilian species. In addition, other diagnostic tests, such as those based on polymerase chain reaction (PCR) technology, enzyme-linked immunosorbent assay (ELISA), and other antibody or antigen serologic tests, are usually not validated for crocodilians. A clinician must inquire about the specifics of the tests being performed to make an accurate interpretation of the results. Published literature and the experience of other colleagues are invaluable for interpreting diagnostic test results of crocodilian species. Also there may be generally accepted diagnostic tests for diseases that have been recognized and studied in crocodilians, such as mycoplasmosis and WNV.
There are various sites for venipuncture in crocodilians. The ventral coccygeal vein can be accessed from either the ventral or the lateral aspect of the tail (Figure 6-12). This vessel lies ventral to the vertebral processes and on midline with the vertebrae. A second alternative is the supravertebral sinus, located on midline at the junction of the head and the neck (Figure 6-13). The examiner must be careful not to go too deep at this site, or physical damage may occur. A third site is an unnamed vessel located on both the left and right sides of the neck. This vessel can be approached from the dorsal aspect of the neck and is surrounded by muscles, decreasing the risk of coming in contact with nervous tissue (Figure 6-14).13 A 3-ml syringe and a 22-gauge needle are recommended for collecting blood from crocodilians. Lithium heparin and EDTA tubes are used for plasma chemistry analysis and CBCs, respectively. If collecting serum, the tubes may have to sit for at least 45 to 60 minutes before centrifugation to allow proper separation of the serum from the blood cells.
As with other species, CBCs and chemistry panels are a fundamental part of diagnostic testing of crocodilians. These tests can help assess the overall health status of the animals. A CBC can show evidence of acute or chronic inflammation that may prompt further investigation or lend direction to a definitive diagnosis. Chemistry panels can give insight into the hydration status and the health of the liver and kidneys. Also of importance are electrolyte values, which can show abnormalities related to poor diet and husbandry. A measurement of packed cell volume and total solids and/or total proteins is also an essential health indicator for crocodilian species. Fine needle aspirates, impression smears, and fluid analysis are diagnostic tools that can be useful in determining a definitive diagnosis. Urinalysis is not a practical test in crocodilians due to the absence of a urinary bladder and the fact that urine will be highly contaminated in a voided sample.
The hemocytometer with the Unopette Eosinophil Determination for Manual Methods stain (Becton Dickinson and Company, Franklin Lakes, NJ 07417-1885) is often used to obtain a CBC of crocodilian species and other reptile and avian species. A blood smear stained with Diff-Quick (Quik-dip stain, Mercedes Medical Physician and Laboratory Products, 7490 Commerce Ct., Sarasota, FL 34342) is also needed to complete the total estimated white blood cell count and obtain the cell differential count. Interpretation of CBCs from crocodilians can be challenging for veterinarians unfamiliar with the morphology of their white blood cells, which can vary from that of other reptile species. This variation found in crocodilian species is typical and more pronounced among reptiles. To become familiarized with the different cell types and their appearance, veterinarians should spend time scanning blood smears. This will often help veterinarians differentiate heterophils from eosinophils before the initiation of the differential count. Alternatively the samples can be submitted to a commercial laboratory that runs CBCs on blood collected from exotic animal species.