Clinical Anatomy and Physiology of Exotic Species: Structure and function of mammals, birds, reptiles and amphibians, 1e

258 Pages·2005·56.87 MB·English

by  Bairbre O'Malley MvB MRCVS

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First published 2005
ISBN 0 7020 2782 0
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Knowledge and best practice in this field is are constantly changing. As new research
and experience broaden our knowledge, changes in practice, treatment and drug
therapy may become necessary or appropriate. Readers are advised to check the
most current information provided (i) on procedures featured or (ii) by the
manufacturer of each product to be administered, to verify the recommended dose
or formula, the method and duration of administration, and contraindications. It is the
responsibility of the practitioner, relying on their own experience and knowledge of
the patient, to make diagnoses, the determine dosages and the best treatment for
each individual patient, and to take all appropriate safety precautions. To the fullest
extent of the law, neither the publisher nor the author assumes any liability for any
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Contributors
Chapter 1 Amphibian anatomy and physiology
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Peter Helmer DVM
Avian Animal Hospital of Bardmoor, Largo, Florida, USA
and
Douglas P Whiteside DVM DVSc
Staff Veterinary, Calgary Zoo, Alberta, Canada
Chapter 12 Ferrets
John H Lewington BvetMed MRCVS
Member Australian Veterinary Association (AVA) and Australian Small
Animal Veterinary Association (ASDAVA), member of American Ferret
Association (AFA), World Ferret Union (WFU), South Australian Ferret
Association (SAFA), New South Wales Ferret Welfare Society (NSWFWS),
Ferrets Southern District Perth (FSDP)
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Preface
One of the main pleasures I have in working with exotic species is the fascinating diversity
among my patients. Daily in practice I see living evolution from frogs to snakes to birds and
small mammals. Each one presents a clinical challenge whether it is saving a tortoise found ix
drowning in a pond, treating a parrot with sinusitis or an anorexic rabbit. Yet we really need
to understand the basics – how reptiles breathe, the structure of the psittacine sinuses and
the complex gastro-intestinal physiology of the rabbit – before we can properly treat these
unique pets.
The internal structure and function of exotic species has always intrigued me, yet the
topic was traditionally not taught at Veterinary College. I wrote this book with the intention
of both redressing this balance and answering the many questions, which interest those who
work with exotics. Why, for example, don’t birds’ ears pop when they fly, why are rabbits
obligate nose breathers and how can a lizard drop its tail and grow a new one?
Over the last ten years veterinary knowledge of the medicine and surgery of exotic
animals has rapidly expanded yet the basic structure and function of these diverse species
have never been drawn together in a single text. With the increasing numbers of exotic pets,
veterinary surgeons are at a considerable disadvantage trying to treat sick reptile, avian and
rodent patients without having in-depth knowledge of the normal bare bonesbeneath.
This book, written by vets for vets, aims to merge the wealth of zoological research with
veterinary medicine – bringing the reader from the dissection table into the realms of
clinical practice and living patients. To this end, I have included clinical notes where
applicable and items of general interest about many species.
I hope this book will inspire vets in practice, veterinary students, nurses and technicians
to study this long neglected yet captivating subject and help them apply this knowledge
clinically to their patients.
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Acknowledgments
In writing this book I am grateful to veterinary surgeons Peter Helmer, Doug Whiteside and
John Lewington for contributing the excellent Amphibian and Ferret chapters.
x I would like to thank the Natural History Museum of Ireland who provided the sources
for the following illustrations: Fig 6.1, 6.2, 6.5, 6.12, 6.14, 6.15, 6.17, 6.25, 6.67, 9.6, and
11.7. Also Janet Saad for her exceptional snake photographs.
The Elsevier editorial team were wonderful with their belief in this project, their
constant support and endless patience. I would also like to thank Samantha Elmhurst for
her skilful and beautiful illustrations. And Tasha my poor dog who missed out on walks so
this book could be researched and written.
Lastly, I would like to dedicate this book to my beloved mother, the late Mary Pat
O’Malley, whose enthusiasm and encouragement kept me going as I endeavoured to juggle
the demands of lecturing and running my own exotic animal practice with writing this book.
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1
Amphibian anatomy and physiology
Peter J. Helmer and Douglas P. Whiteside
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INTRODUCTION The larval stages rely on fins to move through their aquatic
environment, in a manner similar to fish. Metamorphosis
With over 4000 species described, the class Amphibia includes the development of legs for terrestrial locomotion
represents a significant contribution to the diversity of (Figs. 1.1–1.6). The dual life cycle remains evident as the
vertebrate life on earth. Amphibians occupy an important limbs of many amphibians remain adapted, for instance
ecological niche in which energy is transferred from their with webbing between the toes, for aquatic locomotion.
major prey item, invertebrates, to their predators,
primarily reptiles and fish (Stebbins & Cohen 1995). TAXONOMY
The first amphibian fossils date back approximately 350
million years. Current evidence indicates that they
Amphibians are classified into three orders (Table 1.1):
descended from a group of fish similar to the coelacanth
(Latimeria chalumnae) (Boutilier et al. 1992; Wallace et al. 1. Anura (Salientia) – the frogs and toads
1991). These fish had functional lungs and bony, lobed fins 2. Caudata (Urodela) – the salamanders, newts, and
that supported the body. Further refinements of these fea- sirens
tures allowed amphibians to be the first group of verte- 3. Gymnophiona (Apoda) – the caecilians
brates to take on a terrestrial existence. The class name
Amphibia (derived from the Greek roots amphi, meaning Anura
“both,” and bios, translated as “life”), refers to the dual By far, the Anura represent the greatest diversity of
stages of life: aquatic and terrestrial. amphibians, with over 3500 living species divided among
Multiple features support the role of amphibians as 21 families. Anura comes from the Greek, meaning “with-
an evolutionary step between fish and reptiles. The 3- out a tail,” and with the exception of the tailed frogs
chambered heart represents an intermediary between the (Leiopelmatidae), the remainder of anurans have either a
2-chambered piscine model and the more advanced 3- very poorly developed tail or lack one (Fig. 1.7). The larvae
chambered heart of the reptiles. are unlike the adults, and lack teeth. Neoteny, the condition
The trend toward terrestrial life is also evident in the in which animals become able to reproduce while arrested
respiratory system. Most species have aquatic larval forms developmentally in the larval stage (Wallace et al. 1991), is
where gas exchange occurs in external gills. Metamorphosis not present. The anuran families are listed in Table 1.2
to the adult, usually a terrestrial form, results in the develop- (Frank & Ramus 1995; Goin et al. 1978; Mitchell et al.
ment of lungs. These primitive lungs are relatively ineffi- 1988; Wright 1996, 2001b).
cient compared to those of other terrestrial vertebrates,
Caudata
and respiration is supplemented by gas exchange across the
skin. Secretions of the highly glandular skin help to main- The order Caudata comprises nine families, with around
tain a moist exchange surface; however, amphibians are 375 species described (Table 1.3). Urodeles have a long
restricted to damp habitats. tail, with the toothed larval forms often being similar in
Most amphibians are oviparous, similar to fish and most appearance to the adults. Neoteny is common among the
reptiles. Though their eggs must not be laid in completely salamander families, with the axolotl (Ambystoma
aquatic environments, the ova lack the water-resistant mexicanum) (Fig. 1.8) being the most common example
membranes or shell of reptiles and birds, thus they must be (Frank & Ramus 1995; Goin et al. 1978; Mitchell et al.
deposited in very damp places to avoid desiccation. 1988; Wright 1996, 2001b).
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Clinical Anatomy and Physiology of Exotic Species
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Figure 1.1 • Egg mass of Dyeing poison frog Dendrobates tinctorius.
(Photo by Helmer.)
Gymnophiona
Although there are approximately 160 known species of
caecilians, which are classified into six families (Table 1.4),
clinicians will likely see them only on a sporadic basis. They
are limbless, with elongate worm-like bodies, and short or
absent tails (Frank & Ramus 1995; Goin et al. 1978;
Mitchell et al. 1988; Wright 1996, 2001b).
Figures 1.3–1.5 • Progression of metamorphosis of Dyeing poison frog
Figure 1.2 • Developing embryos of Dyeing poison frog Dendrobates Dendrobates tinctorius. The process from egg to adult takes approximately
tinctorius. (Photo by Helmer.) 3 months. (Photo by Helmer.)
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Amphibian anatomy and physiology
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Figure 1.7 • Adult Red-eyed tree frog (Agalychnis callidryas). h
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Figure 1.6 • Young adult Dyeing poison frog Dendrobates tinctorius. n
(Photo by Helmer.) s
body temperature required for optimal digestion is likely
different from that required for gametogenesis (Goin et al.
5
1978; Whitaker et al. 1999; Wright 1996, 2001d).
METABOLISM A number of physiological and behavioral adaptations
have developed in amphibians that allow them to control
Based on the theory of metabolic scaling, larger amphibians,
in general, will require proportionately fewer calories than
smaller animals. Metabolic requirements also vary with
Table 1.2 Composition of the order Anura
environmental temperature and activity level. Active, food-
seeking species, such as Dendrobatid frogs, have a higher Family Representative species
energy requirement than those species that ambush prey,
Brachycephalidae Saddleback toads
such as the horned frogs (Ceratophrysspp.). Metabolic rate
Bufonidae True toads
will increase by up to 1.5 to 2 times with illness or surgical
recovery, and by up to 9 times with strenuous activity Centrolenidae Glass frogs
(Wright & Whitaker 2001). Formulae for the determina-
Dendrobatidae Poison frogs
tion of metabolic requirements of various amphibians are
Discoglossidae Painted frogs
presented in Table 1.5.
Heleophrynidae Ghost frogs
Thermoregulatory and hydrational homeostasis
Hylidae Treefrogs
Hyperoliidae African reed frogs
Amphibians are poikilotherms (ectothermic), relying on a
combination of environmental heat and adaptive behavior Leiopelmatidae Tailed frogs
to maintain a preferred body temperature. This preferential
Leptodactylidae Tropical frogs
temperature is dependent on a number of factors, includ-
Microhylidae Narrowmouth frogs
ing species, age, and season, and is essential for optimal
metabolism. However, the ideal body temperature is also Myobatrachidae Australian froglets
dictated by specific metabolic processes; for example, the
Pelobatidae Spadefoot toads
Pelodytidae Parsley frogs
Pipidae Clawed frogs
Table 1.1 The class Amphibia is composed of three orders
Pseudidae Harlequin frogs
Order Representative species
Ranidae True frogs
Anura Red-eyed treefrog
Rhacophoridae Flying frogs
(Agalychnis callidryas)
Rhinodermatidae Darwin’s frogs
Gymnophionia Caecilians
Rhinophrynidae Mexican burrowing toads
Caudata Tiger salamander
(Ambystoma tigrinum) Sooglossidae Seychelles frogs
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Clinical Anatomy and Physiology of Exotic Species
Table 1.3 Composition of the order Caudata Table 1.4 Composition of the order Gymnophiona
Family Representative species Family Representative species
Ambystomatidae Mole salamanders Caeciliidae Common caecilians
Amphiumidae Amphiumas Ichthyophiidae Fish caecilians
Cryptobranchidae Giant salamanders Rhinatrematidae Beaked caecilians
Dicamptodontidae American giant salamanders Scolecomorphidae Tropical caecilians
Hynobiidae Asian salamanders Typhlonectidae Aquatic caecilians
Plethodontidae Lungless salamanders Uraeotyphlidae Indian caecilians
ns Proteidae Neotenic salamanders
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bi Salamandridae True salamanders
hi Table 1.5 Formulae for determination of caloric needs
Sirenidae Sirens
p of resting amphibians at 25ºC
m
A Order Caloric requirement per 24hours in kcala
their body temperatures to a limited degree. The most Anuran 0.02 (BM)0.84
6
obvious of these are postural and locomotory controls that Salamander 0.01(BM)0.80
allow the amphibian to actively seek or move away from
Caecilian 0.01(BM)1.06
heat sources. Another important method of thermoregu-
lation is peripheral vasodilation and constriction to regulate
a Value should be increased by a minimum of 50% during periods of injury or illness.
body core temperature, often in conjunction with glandular BMrepresents the animal’s body mass in grams.
secretions to regulate evaporative cooling in some species (Adapted from Tables 7.1-7.4 in Wright KM and Whitaker BR, 2001).
(Goin et al. 1978; Whitaker et al. 1999; Wright 1996, 2001d).
A change in skin color to modulate absorption of solar energy adaptations (increased fibrinogen, shock proteins, and
is another significant adaptation that has been studied in glucose transporter proteins, and the appearance of ice
terrestrial anurans. Melanophores (melanin-rich pigment nucleating proteins in blood that guide ice formation), the
cells)in the skin of amphibians can regulate internal melanin accumulation of low molecular weight carbohydrates
aggregation or dispersal, thus changing the skin to a lighter (glycerol or glucose) in blood and tissues, and increasing
coloration to enhance reflectivity, and thus decrease heat plasma osmolarity through dehydration. These adaptations
absorption in periods of light. In addition, some anurans serve to lower the freezing point of tissues (super-cooling)
have extraordinarily high skin reflectivity for near infra-red and promote ice growth in extracellular compartments.
light (700–900 nm), owing to their iridophores (color pig- Amphibians that are freeze tolerant have also good tissue
ment cells), which significantly reduces solar heat load anoxia tolerance during freeze-induced ischemia (Lee &
(Kobelt & Linsenmair 1992, 1995; Schwalm et al. 1977). Costanzo 1998; Storey & Storey 1986).
Finally, a number of crucial physiological adaptations are Physiology, behavior, pathology, and therapies are all
found in wild temperate anuran and caudate species that influenced by temperature; therefore it is important for the
are necessary for winter survival. These include protein clinician to realize that amphibians must be kept within
environments that allow for them to stay within their pre-
ferred optimal temperature zone (POTZ) for normal meta-
bolic homeostasis (Whitaker et al. 1999; Wright 2001d). It
is equally important that amphibians not be subjected to
rapid temperature fluctuations because thermal shock may
ensue (Crawshaw 1998; Whitaker et al. 1999).
CLINICAL NOTE
Amphibians that are kept above their POTZ may show signs
of inappetence, weight loss, agitation, changes in skin color,
and immunosuppression. Those kept below the POTZ may
become inappetent, lethargic, develop abdominal bloating
associated with bacterial overgrowth from poor digestion,
have poor growth rates, or become immunocompromised.
Figure 1.8 • Axolotl (Ambystoma mexicanum). (Photo by Whiteside.)
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Amphibian anatomy and physiology
Thus enclosures that contain a mosaic of thermal zones physiological mechanisms to excrete excess water while
are ideal to allow the amphibian to thermoregulate normally conserving plasma solutes (Goin et al. 1978; Mitchell et al.
(Whitaker et al. 1999; Wright 2001d). 1998; Wright 2001d).
Due to the permeability of most amphibians’ skin, desic-
cation is always a threat to survival, necessitating the devel- GENERAL EXTERNAL ANATOMY
opment of physiological adaptations and behaviors to ensure
hydrational homeostasis in aquatic or terrestrial environ- The three orders of amphibians are quite different in their
ments. Amphibians are limited in their activities and ranges external appearance. Salamanders are lizard-like in form,
as their evaporative water loss is greater than that of other covered in glandular skin, have four legs (except the sirens,
terrestrial vertebrates. Some species of amphibian, such which are lacking the pelvic limbs), and lack claws on their
as axolotls and mud puppies, are totally dependent on an digits. External feather-like gills may or may not be present.
aquatic environment, and even most terrestrial amphibians The tail is usually laterally flattened. The salamanders range
A
must remain moist in order for gas exchange to be effective in total length from 1.5 inches (4 cm) to over 60 inches m
(Boutilier et al. 1992; Shoemaker et al. 1992; Wright 2001d). (1.5m). The anurans, or frogs and toads, are tail-less as p
h
For most captive amphibian species, a relative environmen- adults. External gills are absent. Anurans generally have i
b
tal humidity of greater than 70% is appropriate as it provides longer hind legs than fore, and commonly have webbed, i
a
a humidity gradient and the animals can then select a level unclawed toes. Depending on the species, the glandular n
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that is suitable for them. Clinicians should always remain skin may be smooth or bosselated. The snout-to-vent length
aware of the need for the amphibian patient to remain in of anurans ranges from 3/8 inch to 12 inches (1–30 cm).
7
moist settings when being examined (Whitaker et al.1999). Caecilians are limbless and resemble a snake or worm. They
Behavioral responses to minimize water losses include have a very short tail, if one is present at all. Small olfactory
postural changes and limitation of activities to periods of and sensory tentacles are present in the nasolabial groove just
elevated humidity. One well-documented physiological rostral to the eye. Total length varies from 3 to 30 inches
adaptation to prevent water loss that has been described in (7.5–75cm) (Stebbins & Cohen 1995; Wright 2001b).
South American treefrogs (Phyllomedusa spp.), and likely
exists in other treefrog species, is the secretion of a water- SKELETAL SYSTEM
proofing substance from lipid glands in their skin (Heatwole
& Barthalamus 1994; Wright 2001d). This waxy exudate is There is significant diversity of skeletal elements among
smeared over the surface of the frog with stereotyped move- amphibians. Caecilians lack pectoral and pelvic girdles, as
ments of the feet and imparts a surface resistance to evapo- well as the sacrum. Locomotion in this group is primarily
rative losses comparable to many reptiles. Other described achieved through worm-like regional contraction of the
physiological mechanisms in terrestrial amphibians include body (vermiform motion), or lateral, eel-like undulations
stacked iridophores in the dermis, and dried mucus on the (Stebbins & Cohen 1995; Wright 2001c).
epidermis (McClanahan et al. 1978; Wright 1996, 2001c). Salamanders (Fig. 1.9) typically have four limbs, though
It is important to realize that these protective mechanisms the hindlimbs are greatly reduced in the mud eels (Amphiuma
are often lacking on the ventral surface of amphibians; the spp.) and missing in sirens (Siren spp. and Pseudobranchus
ventrum serves as an important route for water uptake spp.) (Stebbins & Cohen 1995; Wright 2001c). Generally,
from the environment, with some anurans even having a four toes are present on the forefoot and five on the hind,
modified area on their ventral pelvis, known as a “drinking although this is variable between species. Salamanders are
patch,” that is responsible for up to 80% of water uptake capable of regenerating lost toes and limbs. Cleavage planes,
(Parsons 1994). or predetermined zones of breakage, are present in the tails
of many species so that when the animal is threatened
or injured the tail breaks free of the body. This is known
CLINICAL NOTE asautotomy; the lost tail will regenerate (Stebbins & Cohen
Absorption of water from the gastrointestinal tract is 1995).
negligible in most species, thus oral fluids are of little benefit Anurans have several adaptations for saltatory locomotion
in rehydrating an amphibian. For most terrestrial species, or jumping. They have four limbs, and the hind legs are
shallow water soaks and subcutaneous or intracelomic dilute elongated (Fig. 1.10). There are generally four toes on the
fluid administration are most effective in combating forefoot and five on the hind foot. The vertebrae are fused
dehydration (Whitaker et al. 1999; Wright 2001d). and the vertebral column is divided into the presacral,
sacral, and postsacral regions. The sacrum itself is not
present, and the pelvic girdle is fused. The forelimb is com-
Aquatic amphibians face a different problem in that they posed of the humerus, a fused radio-ulna, carpals,
are constantly immersed in a hypo-osmotic environment. metacarpals, and phalanges, and the hind limb is formed by
Overhydration is a constant threat, with plasma expansion the femur, fused tibiofibula, tarsals, metatarsals, and
resulting in cardiac stress. To combat this, they have developed phalanges. Caudal vertebrae are replaced by a fused