INTRODUCTION

Keeping small mammals, birds and reptiles as exotic pets has become increasingly popular over the last years. A great deal of information on the variety of species, on their behaviour, and on recent advances of animal management in combination with successful breeding in many species has caused this development. As a result, veterinarians are more often requested to provide prophylaxis and therapy to solve problems due to dietary deficiencies, mismanagement or physical disease. Today, an increasing amount of expert information about exotic pet medicine is provided in veterinary schools, at congresses and in professional journals.

Radiography has proven to be a revealing and essential diagnostic method in addition to clinical investigation of small mammals, birds and reptiles.

Radiographic technique

Although the small size of many exotic pets seems to contradict the necessity of a powerful X-ray machine, it is most helpful to have an equipment with high mA-capability and a short exposure time of 1/60 of a second or faster. The high mA-capacity allows the use of high-definition, slow-speed film-screen combinations or even non-screen envelope films that are necessary for identification of smaller details. The short exposure time minimizes degradation of radiographic detail caused by motion of the animal (motion blurring). Exposures are made at approximately 50 KvP, with mAs-settings adjusted to the film-screen combination, using a focal-film distance of at least 90 cm to minimize the influence of geometric unsharpness and distortion. A grid is not needed.

Especially with these small animals where low KvP--settings are used, collimation of the X-ray beam according to the size of the animal or part of the animal, and not to the size of the cassette, is essential. Furthermore, all general rules of radiation safety should be taken into account. Persons holding animals by hand should work with maximum distance from the primary beam, but even better is the procedure of fixating the animal directly upon the cassette using radiolucent adhesive tape and positioning devices. In both circumstances, adequate chemical restraint by using sedation or general anaesthesia is required.

Interpretation

To avoid incorrect radiographic diagnosis systematic analysis of the radiographs is essential. Interpretation includes evaluation of size, shape, position, delineation, density and homogeneity of all visible organs, evaluation of the comparative size of the organs, evaluation of patient nutrition, overall body condition and gastrointestinal contents, and evaluation of the results of contrast examinations.

The specific characteristics of the different species under examination should be taken into consideration.

1. Radiography of pet and racing birds

Radiographic techniques in birds have given a new dimension to avian medicine.

Most celomic organs are surrounded by air sacs providing excellent natural contrast.

A radiographic examination does not have to inflict any more stress upon the bird than a general physical examination. However, to prevent unwanted motion blurring of the radiograph or accidental induction of trauma to the restive animal, and to protect the radiographer, it is advisable to sedate the birds by i.v. injection of ketamine-HCL, or better, to anaesthetize the birds using inhalation anaesthesia with isoflurane.

Chemically immobilized birds are positioned directly on the cassette and fixated with radiolucent adhesive tape. For the lateral view, the bird is positioned on its right side. Both wings are hyperextended over the back. The legs can be stretched caudally or cranially, depending on whether the cranial or caudal celomic cavity has to be evaluated.

For the ventrodorsal view, the bird is positioned on its back, with the wings slightly extended and held down by adhesive tape or lead gloves.

In case of a suspected unilateral wing abnormality, always both wings are radiographed for reasons of comparison. For radiographic examination of the limbs, the hindlegs are stretched and pulled away from the body to avoid unwanted superposition of the pelvic region. For radiographic examination of the skull, lateral and ventrodorsal projections are used, supplemented by oblique ventrodorsal views when necessary.

Positive-contrast studies such as gastrointestinal barium studies, excretory urography and non-selective angiocardiography can be performed.

To perform a barium study, 25 cc/kg b.w. of a diluted barium w/v) is administered into the crop or directly into the proventriculus using a rigid feeding tube. Films are made at intervals: 0-½-1-2½-6-24 hours. During a normal barium study, at 1 hour the crop has emptied, and most of the barium will be present in the proventriculus, gizzard, and first small bowel loops. At 6 hours, the stomach will be empty, and the bowel loops will be filled with the contrast medium.

This type of investigation can be used to indirectly determine position and size of the liver or urogenital organs, to delineate suspected masses in the celomic cavity, and to investigate the esophageal and gastric walls, and to evaluate passage through the gastrointestinal tract.

The other positive-contrast examinations by i.v. injection of iodine contrast media are seldom performed. Excretory urography can be of value for delineation of the ureters prior to surgery of the cloaca. Non-selective angiography can be performed for evaluation of the size of the right and left ventricles in birds that are suspected of cardiomyopathy.

For both investigations, a dosage of 2cc/kg b.w. iodine contrast medium (Hexabrix®) is injected in the brachial vein. The sequence of filming must be very rapid due to the short circulation time in birds: 0-30 sec-60 sec-5 min.

Many abnormalities can be diagnosed by radiographic studies. Abnormalities of the musculoskeletal system include fractures and luxations, osteomyelitis, arthritis, and nutritional disorders. Concerning the respiratory tract, inflammation (Aspergillosis), tympany and rupture of air sacs, and changes in the parabronchial structures produce characteristic radiographic patterns. Also changes of the digestive tract as dilatation of the crop and proventriculus, abnormalities of the gizzard and intestines, and the presence of foreign bodies can easily be recognized. Egg binding and neoplastic enlargement of the gonads and kidneys are abnormalities of the urogenital tract. Celomic spilling of abnormal eggs or contents of collapsed eggs result in generalised serositis, with increased density of the celomic cavity. Cardiac enlargement and fluid accumulation are signs of circulatory problems. Abnormal enlargement of the liver and spleen are visible with a variety of diseases. Abnormal fat deposits and ventral herniations are common abnormalities of the celomic cavity.

2. Radiography of small mammals: guinea pigs and rabbits

Because small mammals are shy and panic easily, radiographs should be made with chemical restraint of the animal. For this, a combination of ketamine and xylazine can be used: ketamine 20 mg/kg b.w. + xylazine 4 mg/kg b.w., injected intramuscularly (for rabbits add 0.5 cc 1% atropine).

Newer drugs include the use of medetomidine (Domitor®), 400 microgram/kg b.w., and its antagonist Antesedan®).

Inhalation anaesthesia with isoflurane using a small cap can also be used safely.

To prevent unwanted superposition of appendicular structures on the abdominal or thoracic organs, the front limbs and hindlegs are pulled away from the body using a small forceps or string.

For radiographic examination, the same routine projections that are used in general small animal radiography can be applied: DS-lateral for thorax, abdomen and skull, DV for thorax and skull, VD for abdomen, and ML, AP or PA for the limbs.

Contrast examinations that can be performed include positive-contrast retrograde urethro-cystography for evaluation of bladder rupture and other bladder wall abnormalities, and negative air-contrast or positive barium-contrast gastrography for evaluation of gastric foreign bodies and large-sized gastric wall abnormalities, incl. pyloric outflow obstruction.

Most of the abnormalities of the musculoskeletal, circulatory, respiratory, digestive, and urogenital tract that are routinely seen in dogs and cats can also be found in guinea pigs and rabbits. In guinea pigs, special emphasis is placed on urological problems, and in rabbits on gastric trichobezoars, osteomyelitis of the mandibular jaws and vertebral fractures.

3. Radiography of reptiles: chelonians (tortoises, turtles), lizards and snakes

Reptiles that may be presented for radiographic examination include chelonians (tortoises and turtles), lizards and snakes. Especially with tortoises, turtles and snakes, due to their special anatomy radiographic examination is difficult. In addition to this, unfamiliarity with normal radiographic anatomy of reptiles makes interpretation of radiographs of diseased animals a questionable experience.

Tortoises and Turtles

Radiographic examination of chelonians is complicated by the presence of the shell that necessitates special positioning. Routine projections include a dorsoventral projection, using a vertical X-ray beam with the tortoise in prone position, a laterolateral projection, using a horizontal X-ray beam with the tortoise in prone position, and a craniocaudal projection, using a horizontal X-ray beam with the tortoise in prone position. The laterolateral and craniocaudal projections can also be made with a vertical X-ray beam. For those projections the animal is fixed with tape on a rack or wooden block, and positioned on its side for the laterolateral view or positioned upright for the cranio-caudal view. Under normal circumstances, these routine radiographs of chelonians can be made without sedation. When waiting for the radiographic examination the animals are turned on their back. Just before the radiographic exposure, the chelonians are turned back in normal position. When reorientating, the animals extend head and limbs from underneath the shell. At this moment the radiographs must be made. When necessary, sedation can be applied using ketamine hydrochloride, 25-80 mg/kg bw.

Barium sulfate contrast medium (20 cc/kg bw.) can be used for outlining gastrointestinal tract. The contrast medium is administered using a stomach tube. Passage through the intestinal tract can be very prolonged (1-20 days) depending upon species and environmental circumstances.

Abnormalities that can be investigated radiographically include changes due to metabolic bone disease (shell, skeleton), fractures of shell or limbs, osteomyelitis, arthritis, pneumonia and pleuritis, presence of foreign body in the gastrointestinal tract, enteritis, constipation, and urogenital problems with egg binding, articular gout or urological calculi.

Lizards

Radiographic examination of lizards can be difficult depending on species and size of the animal. Small lizards as monitors and geckos and aggressive lizards as crocodiles must be sedated (ketamine hydrochloride, 20-50 cc/kg bw.). Sometimes the snout has to be tied and the tail fixed. Larger skinks and green iguanas are calm, and may be radiographed without sedation. Normal radiographic projections include a laterolateral and dorsoventral projection, using a vertical X-ray beam with the animal either in lateral or in prone position. The animals are placed directly upon the cassette. Also in these animals, it is possible to study the gastrointestinal tract with a barium sulfate contrast study (25-50 cc contrast suspension/kg bw.). As with chelonians, in the early hours of passage only the stomach and first intestinal loops will be visualized, while a complete passage may take days depending upon species and circumstances. The danger of barium constipation due to the long passage time necessitates oral hydration of the lizards.

Radiographic interpretation of radiographs of lizards may be complicated by the presence of calcified scales in the skin. Abnormalities that can be radiographed are listed with the chelonians. Differences include the low radiographic opacity of the egg shells, and the possible presence of developed embryos within the body cavity with certain species.

Snakes

Radiographic examination of snakes is hampered by their special anatomy. Localization of internal organs is difficult, and necessitates the presence of radiographic/ anatomic descriptions (articles, textbook, etc.). For routine laterolateral and dorsoventral radiographs, snakes can be placed directly upon the cassette, or can be radiographed when placed in a X-ray permeable Perspex box (dorsoventral projection, in coiled up position) or in a Perspex tube (dorsoventral or lateral projection, in more or less extended position). With longer snakes, it is necessary to radiograph the animal in segments.

Ketamine hydrochloride (25-75 mg/kg bw.) is used for sedation of restless and poisonous snakes. Passage of a barium sulfate contrast suspension may be enhanced by massaging the belly of the snake. In this way, passage time may be shortened to several hours.

Radiographic abnormalities include fractures and deformations of the vertebral column, foreign body and constipation of the gastrointestinal tract, visceral mass lesions that obstruct normal barium passage (e.g., kidney tumors), and problems with the urogenital tract like egg binding or pyometra.

In all cases, radiographic examination of reptiles can be a valuable additive to the details of the difficult clinical examination of these animals. However, its full potential is only achieved in the hands of an experienced veterinarian.

References

1.  For further information and pictorial displays, the reader is referred to the "Atlas of Diagnostic Radiology of Exotic Pets" by Rübel, Isenbügel and Wolvekamp, Wolfe Publ. Ltd., London and W.B. Saunders, Philadelphia , 1991, ISBN 0 7234 1642 7.