Biology and medicine of bearded dragons (Proceedings)

Bearded dragons (Pogona spp.) are omnivorous lizards that are native to Australia. Seven different species of bearded dragon are found in Australia, including the inland dragon (Pogona vitticeps), Northwest dragon (Pogona minor mitchelli.), Nullarbor dragon (Pogona nullarbor), western dragon (Pogona minor minor), Eastern dragon (Pogona barbata), Kimberley dragon (Pogona microlepidota), and Black-soil plains dragon (Pogona henrylawsoni). The inland bearded dragon is the species that is common to the pet reptile trade. This species is highly prolific in captivity, with (estimates of) over 250,000 animals being produced in captivity per year. As the popularity of these reptiles continues to rise, veterinarians can expect to encounter them more frequently in their practices. The purpose of this presentation is to provide attendees an overview of bearded dragon biology and common disease presentations.

Biology

Bearded dragons have a typical agamid-style, dorsoventrally compressed body. These animals were named for their dark, heavily-scaled gular beard. Dragons display their "beard" during both offensive and defensive behaviors. Dragons are acrodonts, so they have only one set of teeth. These lizards are geophagic, and can often be found "tasting" soil or other objects in their environment. Bearded dragons have long, narrow digits. These digits are adapted to climbing on rock and plant substrates. The tail of these animals is not autonomous. Males can be differentiated from females by the presence of large hemipenal bulges and large femoral/pre-vent scales.

Because of the vastness of the continent on which they originate, dragons may be found in both temperate and tropical climates. The humidity in these environments can vary dramatically, and the dragons may be found in arid to semi-arid areas. Bearded dragons are extremely adaptable to different habitats, and may be found in woodland, scrubland, and grasslands.

These animals are diurnal, sun-loving species. Dragons are primarily terrestrial, although they can also be semi-arboreal in those habitats that have appropriate climbing structures or plants. These "sun-loving" species prefer to bask in the morning and late afternoon hours in temperatures that approach and exceed 100o F. Because dragons are ectotherms, it is important to provide them an appropriate environmental temperature range. In general, a diurnal range from 85-100o F is appropriate; while a nighttime drop to 70-80o F will suffice. Dragons not provided an appropriate environmental temperature range may have a decreased metabolic rate and immune response, resulting in limited growth and chronic infections.

In Australia, bearded dragons are reproductively active during October. A clutch can range from 11-25 eggs. Eggs typically hatch in 55 days following incubation at 28o C. In captivity, female dragons may lay 3-4 clutches of eggs per year. It is important to monitor these highly productive animals closely, as they can expend significant amounts of energy on reproduction and die prematurely.

Nutrition

For years there has been very little research focused on identifying the specific nutritional requirements of reptiles. Omnivorous reptiles, such as the dragon, should be provided a diet comprised of both animal and plant materials. As juveniles, dragons tend to prefer animal proteins, while adult dragons tend to consume more plant protein in their diet. The author has found that romaine lettuce and other dark leafy lettuces are generally well received by the dragons. In the wild, these dragons have the opportunity to select their diet from a diverse number of insect prey items. These insects are nutritionally balanced because they can select from a variety of natural plant and animal food sources. In the United States, there are six invertebrates sold commercially, including the commercial cricket (Acheta domesticus), mealworm (Tenebrio molitor), superworm (Zoophobias morio), waxworm larva (Galleria mellonella), fruit fly (Drosophila spp.), and earthworm (Lumbricus terrestris). The primary advantage to using these invertebrates is that they are readily available through most pet distributors year round. Unfortunately, these prey items do not provide a complete and balanced diet for an insectivore. Most of these invertebrates are deficient in calcium, the exception being earthworms maintained in high calcium soils. Feeding or "gut-loading" commercial invertebrates prior to offering them to a dragon can help to increase the mineral content of the prey items. Dusting the prey item with a calcium carbonate powder may also help to increase the calcium content of the prey items.

Some pet owners elect to capture wild invertebrates to feed their dragons. It is important to only use invertebrates from areas where insecticides are not used. There are a number of invertebrates that produce toxins that can prove fatal to a reptile. Bearded dragons, and likely other reptiles and amphibians, are very susceptible to the toxins (lucibufagans) produced by lightning bugs (Photinus sp.). Dragons may die within minutes to hours of eating a single lightning bug. I strongly recommend against feeding wild caught invertebrates because of our limited understanding of the toxins produced by these organisms.

Diseases of dragons

Nutritional disease

Dragons not provided a balanced diet might develop secondary nutritional hyperparathyroidism (metabolic bone disease). Secondary nutritional hyperparathyroidism is most commonly diagnosed in animals offered a calcium deficient diet, phosphorus rich diet, and/or vitamin D deficient diet. Affected dragons are often depressed, anorectic, develop muscle tremors, have soft pliable bones and seizures. In severe cases, affected animals can die from secondary nutritional hyperparathyroidism. Fast-growing juveniles and reproductively active females are most commonly affected. Diagnosing this nutritional disorder can generally be done by taking a thorough history and physical examination. Inadequate nutrition is a common finding, and the physical examination findings are consistent with those described previously. Radiographs can be used to confirm the presence of pathologic fractures and a general loss of cortical bone density. Plasma biochemistries are not generally rewarding, and affected animals are normocalcemic. In terminal stages of the disease it may be possible to observe a hypocalcemia. Treatment should focus on stabilizing the patient. These animals should be considered an emergency, as a hypocalcemic episode could prove fatal. Calcium can be provided parenterally or orally. Oral calcium is considered more bioavailable. Vitamin D should also be given parenterally to ensure that the calcium is absorbed through the intestine. The provision of full-spectrum lighting should also be considered for these animals. Fluid therapy should be provided to correct any deficits. Caloric support should be provided to animals that are not eating on their own. The prognosis for these animals is generally poor to fair if the animals are diagnosed early and treated aggressively.

Commercially available larval insects have a very high fat content. Because these prey items routinely comprise a large portion of a captive dragon's diet, these lizards are susceptible to an over-accumulation of fat. Dragons, like other lizards, primarily store their fat in two large coelomic fat bodies. The fat bodies are anchored in the caudal coelomic cavity, and the cranial aspects of the fat bodies are unattached. In obese dragons, it is often possible to palpate the fat bodies. Obesity can lead to a number of health issues in captive dragons, including dystocia and hepatic disease. To limit the likelihood of complications associated with obesity, it is important that clients provide an appropriate diet for their dragons. The number of larval invertebrates should be limited in an adult dragon's diet, and ad lib greens provided. Dragons should also be removed from their enclosures regularly and provided exercise too. When exercising dragons it is important to consider the public health risks (e.g., Salmonella spp.) associated with these animals, and appropriate precautions taken to limit the spread of potential pathogens.

Infectious disease

Historically, veterinarians have attributed most reptile disease processes to bacterial infections. Although bacteria are frequently isolated from dragons with clinical disease, we often have difficulty in differentiating primary and opportunistic infections. It is possible that some of the diseases characterized as bacterial in origin were actually caused by viruses or other pathogens. The advent of new and improved diagnostic methods has enhanced our ability to characterize new pathogens in reptiles and amphibians. Many of these new and emerging pathogens are viruses and can only be isolated or identified using molecular diagnostic techniques.

Bearded dragon adenovirus was first reported in Australia in the early 1980's. The virus was not characterized in the United States until more than a decade later. Since that time, the virus has spread through the bearded dragon population in the USA and should be considered endemic. Transmission of the virus is primarily by the direct route (fecal-oral), although vertical transmission may also be possible. Affected animals may present with anorexia, weight loss, limb paresis, diarrhea and opisthotonous. Concurrent dependovirus and coccidial infections have also been observed in neonatal bearded dragons. Biopsies of the liver, stomach, esophagus, and kidney may be collected to confirm diagnosis (ante-mortem). On histopathology, basophilic intranuclear inclusion bodies are strongly suggestive of adenoviral infection. Currently, there is no non-invasive ante mortem diagnostic test to confirm adenovirus in the reptile; however, the author is currently working on a polymerase chain reaction (PCR) assay to detect adenovirus in the feces of affected animals. There is no effective treatment for adenoviral infections, although supportive care (e.g., fluids, enterals, and antibiotics) may be useful in stemming the secondary effects of the disease. Again, very little is known regarding the epidemiology of this virus; therefore, special precautions should be taken when working with affected animals. Because there is no effective treatment, affected bearded dragons should be culled from breeding populations.

Coccidiosis is a major cause of morbidity and mortality in bearded dragons. The species that has been identified in bearded dragons is Isospora amphiboluri. These endoparasites are especially problematic in neonatal dragons, often resulting in stunting, diarrhea, and death. Whereas most coccidial infections in higher vertebrates are self-limiting, these infections often persist in bearded dragon colonies. Eliminating coccidia from bearded dragons is difficult. Historically, most of the therapeutics used to eliminate coccidia have been coccidiostatic and produced inconsistent results. Ponazuril, a coccidiocide, is different. Dosing dragons with ponazuril at 30 mg/kg by mouth twice (48 hours apart) is sufficient to clear dragons.

Microsporidians are obligate intracellular parasites. The life cycle of these parasites includes both merogenic and sporogenic phases. These parasites are common in lower vertebrates (e.g., fish), but have also been implicated as a concern in humans with acquired immunodeficiency virus. Bearded dragons infected with these parasites can present with a similar clinical picture as adenovirus or coccidiosis. Affected dragons are anorectic, unthrifty, cachectic, and may die acutely. Diagnosis is generally made at post-mortem. Hepatic and renal necrosis is common, although other organ systems (e.g., intestine and gonads) may also be affected. There is no effective treatment. To limit the likelihood of introducing this parasite into a collection, herpetoculturists should only acquire animals from reputable breeders and quarantine any new arrivals for a minimum of 60-90 days.

Oxyurids, or pinworms, are a common finding in captive bearded dragons. These parasites are generally found in the large intestine, although occasionally they are found more cranial in the small intestine. These parasites are considered as commensals by some veterinarians. In the author's experience, dragons with oxyurid infestations are generally asymptomatic. In these cases, the author does not recommend treatment. If an animal was found to have clinical disease (e.g., diarrhea) associated with the presence of the parasites, than the author recommends treating the dragon with an appropriate anti-parasitic (e.g., fenbendazole, 25-50 mg/kg per os once a day for 5-7 days).

Anesthesia and surgery

There are many different occasions when a bearded dragon may require surgery (e.g., foreign body removal, organ biopsy, abscess removal). There are a number of different compounds that can be used to anesthetize a dragon. The author recommends ketamine (3-5 mg/kg intramuscularly) or propofol (5-10 mg/kg) as induction agents, although propofol can also be used to provide complete anesthesia. Once induced, the dragon is intubated and placed on isoflurane. The author recommends positive-pressure ventilating (4-6 times per minute) the animals during a procedure to limit the likelihood of respiratory arrest.

The approach to surgery in dragons should follow standard techniques for other reptiles. The author recommends a paramedian incision for intracoelomic procedures. This should reduce the likelihood that the large ventral abdominal vein will be accidentally incised; however, if the vessel is lacerated, it can be ligated without problem. Care should be taken when performing surgery on a dragon as the tissues are more delicate than higher vertebrates (e.g., dogs and cats). Incisions generally require 4-6 weeks to heal, so the author recommends using non-absorbable sutures for skin closure to reduce the potential for dehiscence.