Stephen J. Hernandez-Divers, BSc(Hons), BVetMed, Dipl RCVS Zoological Medicine
RCVS Specialist in Zoo & Wildlife Medicine (reptiles), Exotic Animal, Wildlife & Zoo Animal Medicine, Department of Small Animal Medicine & Surgery, College of Veterinary Medicine,
University of Georgia, GA, USA
This article provides an introduction to reptilian therapeutics. There are few drug preparations licensed for use in reptiles. Drugs authorized for use in other species or for humans may be administered under the responsibility of the veterinarian. It is important to note that some drugs may be very safe in certain species and fatal in another. For example, ivermectin while apparently safe in most snakes and lizards, causes death in most chelonia.
All reptiles should be accurately weighed before being medicated to avoid overdosage, and during treatment to monitor response.
All reptiles are ectothermic and a change in temperature may have profound influences on drug distribution, metabolism, excretion, and hence elimination half-life. Some therapeutic regimens state a fixed temperature at which the reptile should be held during treatment. The advantage of this approach is that where pharmacokinetic evidence exists the elimination of the drug will be known and constant. However, if this stated temperature is below or above the preferred optimum temperature zone for the species being treated then stress and debilitation may ensue. In addition, constant exposure to a fixed temperature is likely to cause stress and maladaption over a prolonged period of time.
Reptiles have well-developed renal and hepatic portal systems where blood from the caudal half of the body passes through the kidneys and liver before reaching the heart. However, studies have demonstrated that this effect does not always occur, and where it does occur it is unlikely to effect therapy. Another anatomical consideration is the large voluminous bladder of chelonians which may act as a drug reservoir and lead to a second therapeutic peak many hours after drug administration. The shell of tortoises, turtles, and terrapins is largely living tissue and therefore all chelonian medication should be based on total body-weight.
In general clinicians should always use pharmacokinetically derived drug dosages whenever possible. Second choice alternatives include published doses based on clinical experience and allometric scaling.
ROUTES OF ADMINISTRATION
Medications can be given by a variety of routes;
1. Topical applications for dermatological diseases
2. Oral administration by mouth, stomach tube or oesophagostomy tube (PO) for nutritional and fluid support, gastro-intestinal and absorbed systemic therapies
3. Intracloacal administration for fluid therapy
4. Subcutaneous injection (SC)-slow and unreliable
5. Intracoelomic injection (ICe)-slow but able to give large volumes therefore useful for fluid therapy
6. Intramuscular injection (IM)-rapid and reliable uptake, preferred route for many drugs
7. Intravenous and intracardiac injection (IV)-rapid and reliable, preferred route for most anaesthetic and emergency drugs
8. Intraosseous injection (IO)-rapid and reliable, easier to maintain than intravenous access, preferred for fluid therapy and emergency drugs
9. Intrasynovial injection (IS)-arthritic conditions
10. Intratracheal (IT) injection-respiratory diseases, especially when using systemically toxic drugs
11. Intrapneumonic injection (IP)-respiratory diseases, especially when using systemically toxic drugs
12. Nebulization (inhalation administration)-respiratory diseases, especially when using systemically toxic drugs
13. Intralesional or surgical administration-antibiotic impregnated polymethylmethacrylate beads, endoscopic injection, able to use systemically toxic drugs with greatly reduced systemic side-effects
Dermatological problems, particularly thermal burns and dysecdysis, are common presentations. Predisposing husbandry and environmental causes must always be considered if therapy is to be curative. In many cases of dysecdysis, bathing or the provision of high humidity for 6-12 hours will usually enable retained skin to be removed. Dilute povidone-iodine soaks can be used to provide antibacterial and anti-fungal effects when dealing with superficial infectious dermatitis. In aquatic turtles, it can often be necessary to reduce wound contamination by 'dry-docking' the animal until infection is under control. Silver sulphasalazine cream is a human preparation that is specifically designed for burns victims where the risk of Pseudomonas infection is high, and is therefore useful in reptiles.
Stomach tubing permits the delivery of fluids, foods and medications into the upper digestive tract. In cases of normal gastro-intestinal function the oral route represents a useful route for, especially for fluid and nutritional support;
Snakes. A relatively long rubber-feeding catheter is required to reach the stomach, which lies approx. 40-45% from snout to vent. In many cases available catheters are of insufficient length and oesophageal tubing would be a more accurate description. The catheter should be lubricated with a water-soluble lubricant before being introduced into the mouth via the labial notch. If a catheter of greater diameter than the glottis is used there can be no chance of inappropriate entry into the trachea. The catheter is slowly advanced as far as possible down the oesophagus before the contents of the syringe are deposited. After the catheter is withdrawn it is often useful to gently massage the ventral neck in a craniocaudal direction. The snake should then be returned to a vivarium and not disturbed if regurgitation is to be minimized.
Lizards and small crocodilians. Most species have powerful bites and therefore the use of metal, ball-tipped, feeding tubes are recommended. Mouth gags should be used to prevent biting through rubber feeding tubes. Feeding tubes usually pass unhindered down the spacious oesophagus into the stomach, which lies approx. halfway between the fore- and hindlimbs in most species.
Chelonia. The heads of small to medium land tortoises can generally be maintained in extension by placing the thumb and middle finger on either side of the head, caudal to the tympanic scales. Their mouths can be opened by applying a steady distractive force to the mandible and maxilla. The mouth can then be kept open by placing the index finger into the commissure of the mouth. Even small aquatic species have powerful bites, so that even though their heads can be manually extended a mouth gag is recommended in preference to the clinician's finger. The larger, stronger chelonians often necessitate some degree of chemical restraint to be able to pass a stomach tube. A rubber flexible stomach tube is preferred (with a gag if required) and passed to a left-central intracoelomic position.
Oesophagostomy tube. When repeated stomach tubing is required, especially in strong or shy chelonians, the placement of an oesophagostomy tube has both practical and welfare benefits. In debilitated animals manual restraint and local anaesthesia is usually sufficient but light anaesthesia can be used for short-term restraint.
Curved haemostats are introduced into the mouth and tented against the lateral aspect of the oesophagus, as far down the neck as possible. A stab incision is made through the aseptically prepared skin, over the haemostats, so that the jaws of the haemostats can be seen. A soft rubber feeding catheter (e.g., Cook Veterinary Products) can then be grasped and pulled through the incision and out of the mouth. The feeding tube is then re-directed caudally down the oesophagus and into the stomach. A tape butterfly is placed around the tube, close to its exit from the neck, and sutured to the neck. In chelonia, the tube should also be attached to the carapace. With proper catheter care (flushing after use and daily cleaning of tube entry site), these tubes can be safely maintained for many weeks and, in the author's experience, up to 4 months.
The intramuscular route is preferred because drug kinetics are more consistent and dependable. Snakes are generally injected in the epaxial muscles either side of the spine while chelonia and lizards are usually injected into muscle masses of the forelimbs.
It has been stated that due to the lack of a muscular diaphragm large volumes of intracoelomic fluid could theoretically compromise lung function. However, this seems extremely rare in practice. Snakes are injected in the caudal third of the coelom, cranial to the cloaca. Lizards are injected in the caudal coelom, infront of the pelvic limbs. Chelonia are injected in the ventral aspect of the prefemoral fossa.
Intravenous catheterisation is not easy in reptiles and cut-down procedures are often required under local or general anaesthesia. In large lizards, cephalic, abdominal and jugular vein catheterisation has been employed, while the jugular veins (right may be larger) are most accessible in snakes and chelonia. In large snakes, an emergency intracardiac catheter can be placed into the ventricle of the heart and maintained for up to 36 hours.
Intraosseous infusion is an easier critical care technique than intravenous catheterisation and can be employed in lizards, small crocodilians and chelonia. In lizards and small crocodilians a spinal needle (or 25g hypodermic needle for very small species) is inserted into the proximal tibia. The limb is flexed and the tibial tuberosity located and aseptically prepared. The needle is directed distally into the medullary cavity of the tibia. The aspiration of bone marrow, low resistance to flushing with heparinised saline, or radiography verify correct positioning. Great care must be exercised when dealing with osteodystrophic lizards, as limb fractures are a potential complication. In chelonians, the intraosseous needle can be inserted into the tibia or, in larger terrestrial tortoises, into the medullary cavity of the plastrocarapacial bridge. Syringe drivers are used to control the infusion rate.
The use of antibiotic impregnated polymethylmethacrylate beads has been used with success in a variety of reptiles. These beads can be used in cases of chronic cellulitis, osteomyelitis, and septic arthritis. Aminoglycoside beads are preferred and should be removed once infection has been controlled.
The increasing use of endoscopy and ultrasonography has permitted the guidance for the remote injection of drugs directly into lesions.
The advantage of these techniques is that potentially toxic drugs (e.g., aminoglycosides) can be used with reduced systemic side effects.
DRUGS AND DOSAGES
Space does not permit the inclusion of an appropriate formulary. Readers are referred to the references for more information on specific drugs and dosages.
1. Carpenter JW, Mashima TY, Rupiper DJ. 2001. Exotic Animal Formulary. Second edition. WB Saunders, Philadelphia. Pp39-105.
2. Mader DR. 1996. Reptile Medicine and Surgery. WB Saunders, Philadelphia.
3. Divers SJ, Lafortune M. 2000. Prescribing for reptiles. In Bishop Y (ed): The Veterinary Formulary. Fifth Edition. Royal Pharmaceutical Society of London. Pp 106-114.
4. Journal of Herpetological Medicine and Surgery. Published by the Association of Reptilian and Amphibian Veterinarians. Details available rafrom www.av.org