Without a doubt, health problems arising from an inadequate diet are one of the major concerns in herpetological medicine. The key to managing nutritionally related diseases is to be familiar with the natural history of specific species. In ectothermic animals the metabolic processes governing digestion are dependent on environmental factors, mainly temperature. If these environmental factors are not optimal, even a perfectly balanced meal could go to waste because of inadequate digestion.
The class reptilia can roughly be divided into three nutritional groups: herbivores, omnivores and carnivores. Each of the groups has their own set of typical problems with different manifestations. Knowing to which nutritional category the affected species belongs, will help establish the differential diagnosis. Discussion of the following diseases will be organized according to the nutritional group of reptiles in which the disease is most likely to be seen. However, any disease may be found in nearly any species.
Generally, the diseases related to nutrition can be roughly divided into two groups: either caused by a deficiency or a toxic overdose of a certain nutrient.
Anorexia is probably one of the most common presentations of an affected reptile to the clinician. In assessing the patient's anorexia, it has to be determined whether the reason for an anorexic period is of physiological origin or related to improper management.
A thorough assessment of the captive conditions is essential in the clinical workup of an anorectic reptile to rule out improper management as the cause of the anorexia. A detailed description of the reptile's husbandry must be provided by the owner.
Common reasons for a reptile to become anorectic include both infectious and non-infectious diseases. Certain normal physiological states will make the animal refuse to eat (e.g. just before shedding, egg laying or hibernation, brumation, etc.).
As a general rule, a reptile going through a normal physiological phase of anorexia should not lose more than 10 percent body weight. To monitor this accurately, it is a good husbandry practice to weigh the individual animal on a routine basis (e.g. once a month).
Common Herbivore Diseases
Common species: most terrestrial tortoises, green iguana, prehensile-tailed skink and chuckwalla.
Calcium, phosphorus and vitamin D3 imbalance: Multiple different diseases affecting the bone are usually lumped into "metabolic bone disease." As more and more insight is gained into the specific pathophysiology of disease in reptiles, this term becomes too general to be useful and should be avoided. The following diseases are the most common:
Nutritional secondary hyperparathyroidism (NSHP): The causes for NSHP can be multiple, but it is mostly due to a severe imbalance of the Ca:P ration in the diet, no access to a full spectrum (UV-B) light source and a lack of activated vitamin D3 or a combination of the above. If not enough Ca is provided in the diet or if no vitamin D3 is available, demineralization of the skeleton (osteomalacia in adults and rickets in juveniles) occurs. Pathological fractures and chronic abscesses, especially around the jaw, are common findings.
If insectivorous reptiles are being fed invertebrates without Ca supplementation (e.g. gut loading the insects, dusting with powder) NSHP is triggered by the inverted ratio of the Ca:P ratio of the insect body's nutritional content.
Renal secondary hyperparathyriodism (RSHP): This disease is characterized by calcification of soft tissue and hypocalcemia. Chronic renal disease is the underlying pathology. This causes a hyperphosphatemia from decreased phosphate filtration in the diseased kidney. The elevated phosphate level in the blood decreases the hydroxylase activity due to negative feedback, leading to a decreased calcitriol (active form of vitamin D) level in the blood. The low calcitriol level fails to provide the negative feedback on PTH and a hyperphosphatemia is the result.
Common Carnivore and Insectivore Diseases
Common species: most chameleons (I), agamid lizards (I), all snakes, all adult amphibians, juvenile box turtles and aquatic turtles.
The primary energy sources for these reptiles is fat and protein. It appears that protein derived from animal sources is needed, and nonanimal product derived protein is inadequate for long-term nutrition.
Hypovitaminosis B1: This disease can be seen primarily in reptiles, which are being fed a large quantity of fish (e.g. garter snakes, aquatic turtles). The diagnosis, usually established by the clinical signs, will be primarily neurological in character (e.g. opisthotonos, blindness, torticollis), in combination with the feeding history (feeding fish species with high thiaminase activity).
This disease can be avoided by feeding fish that have been gutted, because most thiaminase is present in the gut, or by pre-cooking the fish, which will inactivate the thiaminase enzyme. Supplementation of B vitamins is also recommended; however, the deficiency can also occur even when fish are fed together with a vitamin B supplement because relatively small amounts of the enzyme thiaminase are able to deactivate very large amounts of thiamin.
Biotin deficiency: A biotin deficiency can only be induced if the reptile's diet consists 100 percent of raw eggs. Even though this seems very unlikely, cases have been reported in species like monitors (Varanus sp.) and gila monsters, which have received a diet consisting of only raw eggs. The raw egg contains the anti-nutritive factor avidin, which binds the biotin in the egg, making it unavailable to the body.
In the wild, most eggs consumed by reptiles are already fertile or have been incubated for a period of time, which in turn decreases the avidin content.
Clinical signs usually manifest themselves as cutaneous lesions and generalized weakness.
A change in the diet and vitamin supplementation is usually enough to reverse the damage.
Hypovitaminosis E: Reptiles that have been fed mainly obese rats can develop steatits because of the increased fat content of the prey animal. This disease should also be included in the differential diagnosis of aquatic reptilian species (e.g. crocodiles, turtles) that have been fed a diet consisting mainly of fish with a high percentage of polyunsaturated fatty acids.
If the food animals were not stored appropriately, the fatty acids in the carcass can become rancid, contributing significantly to the disease.
Clinical signs can be subtle, from anorexia to more specific signs, such as hardened fat pads. Supplementation with vitamin E and correct feeding techniques help prevent this disease.
Common Omnivore Diseases
Common species: Uromastyx lizards, box turtles, Asian turtles, bearded dragons, some aquatic turtles.
Hypovitaminosis A is a common presentation in box turtles and aquatic turtles. Vitamin A is essential in the health of the mucus membranes, which will show immediate pathological changes when faced with a shortage of vitamin A or beta carotene in the diet.
The mucous membranes will harden and thicken as a consequence of the deficiency. Swelling of the eyelids is a common presentation in turtles, and a "parrot beak" and/or aural abscesses in box turtles should make the clinician suspicious of a diet deficient in vitamin A or beta carotene.
Vitamin A/beta carotene supplementation can reverse the signs in most cases. I prefer the beta carotene supplementation because of the toxicity potential of concentrated vitamin A preparations.
Hypervitaminiosis A is a true toxicity and is usually caused iatrogenicly by an overdose injection of concentrated vitamin A preparation by the veterinarian. Clinical signs mimic a dermal burn and include sloughing of the skin. Treatment resembles burn wound therapy, including antibiotics, addressing the open wound and administering parenteral fluid.
Injections with highly concentrated vitamin A (e.g. 500,000 IU/ml) should be avoided and this preparation given only orally. Aquasol A (by Astra) is a good alternative for injection because it contains only 50,000 IU/ml.
Nutritional disorders should be considered in the differential diagnosis of every sick reptile. A thorough analysis of the husbandry protocols and physical examination will often reveal malnutrition as at least one of the underlying causes for the clinical presentation.