Chapter 1
Rabbits

1.1 Introduction

Domestic rabbits (Oryctolagus cuniculus) are descendants of wild European rabbits that were originally found on the Iberian Peninsula by the Phoenicians. Rabbits have been kept by humans for over 3,000 years, largely as a self-replicating source of food, but also for their pelts and later, as pets. Roman writings describe leporaria, walled yards, to keep rabbits contained for hunting. Rabbits were later kept in warrens, courtyards, and islands for breeding and food production, and breeding pairs were released on various islands and in newly discovered lands by naval circumnavigators to ensure a source of fresh meat was available on subsequent visits. In approximately 600 CE, a papal decree declared that unhaired newborn rabbits (laurices) were not meat but instead aquatic species and could be consumed during Lent, leading to a surge of rabbit breeding and domestication in monasteries throughout the Middle Ages. Selective breeding in monasteries gave rise to many of the coat color variants and breeds in existence today. Rabbit breeding for hunting and meat production was initially controlled by seigneurial privilege but with the rise of the middle classes in Europe, raising rabbits became very popular and increasing numbers of animals were kept as pets. Intensification of rabbit farming occurred in parts of Europe and Asia in the early part of the twentieth century, and later, in the early 1950s, in North America. Rabbit breeder associations and specialty hobbyist clubs have been in existence in Europe and North America for over 100 years. Rabbits make gentle and inquisitive pets and are popular with children and adults alike.

When considering rabbit disease, it is important to know the age of the animal and the typical life cycle, be it commercial or pet. The natural lifespan will vary depending upon the size of the rabbit, with larger breeds, such as Flemish giant and New Zealand white rabbits living between 7 and 9 years, while dwarf breeds will live longer; 10-14 years. Rabbits bred for commercial meat purposes in North America tend to be of New Zealand white or Californian background, both of which produce large, fast-growing animals with a high muscle to bone ratio. The white pelts harvested from these animals are used as a by-product for fur trim and felt production. Various spotted and colored hybrid animals may be seen on different farms; however, and other breeds are sporadically introduced from Europe to improve disease resistance and to provide hybrid vigor. In North America, breeding does and bucks kept in commercial meat operations are culled at 2-3 years of age (earlier in Europe) and young rabbits (fryers) are sent to market between 8-14 weeks of age, depending on the efficiency of the operation. Artificial insemination is used in some commercial operations to better manage disease, reproductive cycles, and parturition (i.e., "all in, all out" management systems). Many commercial rabbit breeders also breed or sell rabbits for the pet trade, either dealing directly with pet stores or distributors or by selling animals at the farm gate. While increasing numbers of commercial operations are more selective about barn entry and on-farm biosecurity practices are improving, significant gastrointestinal and respiratory diseases are still present on most of these farms in North America. Many pet rabbits, and there are over 48 breeds recognized by the American Rabbit Breeder Association, come from breeding operations that are not raising animals for meat. These breeding operations may or may not provide routine antimicrobial preventive medications to animals, and the patterns of disease seen in rabbits originating from these sources can be quite different than for commercial meat rabbit operations. Finally, fewer pet rabbits, usually New Zealand white or Dutch-belted breeds, may be adopted directly from research facilities. These rabbits usually originate in commercial laboratory animal breeding facilities and have a very high health status, often being free of gastrointestinal and respiratory infections, which are common in conventionally bred animals. Pet rabbits adopted from rabbit rescue agencies or animal shelters have unknown health backgrounds and may have come from situations of severe neglect. Thus, the types of diseases and conditions that are seen in rabbits will vary considerably, depending on the source.

Pet rabbits are often neutered at sexual maturity, which occurs between 3-6 months of age, with smaller breeds maturing more rapidly. Neutering is done to manage or prevent behavioral problems as well as to reduce the risk of certain age-related neoplastic conditions. Rabbits coming from research facilities as well as some commercial or show breeding operations may be identified by a tattoo inside one ear. Increasingly, pet rabbits receive micro-chip implants for identification purposes.

Depending on where the rabbit originates, it may be euthanatized by the producer or a veterinary clinic for post-mortem evaluation. Currently accepted on-farm euthanasia practices include blunt force trauma applied to the back of the head, nonpenetrating captive bolt, cervical dislocation, and carbon dioxide inhalation. Animals killed by physical methods may demonstrate oronasal blood staining at post-mortem. Pen mates often cannibalize animals dying on farm and this type of trauma may be recognized by a lack of associated hemorrhage or inflammation. Sedation followed by intravenous or intracardiac barbiturate overdose or anesthetic inhalant gas overdose are more commonly used as euthanasia methods for pet rabbits by veterinary clinics. Because of very rapid tissue deterioration after death, when dealing with investigations of gastrointestinal disease in rabbits coming from commercial operations, it is highly recommended to have the client submit several live affected animals. Post-mortem examination and tissue sampling should occur as rapidly as possible after death.

1.2 Integument Conditions

Rabbit skin is thin and fragile compared with the skin of other species and the pelage consists of a dense undercoat of secondary hairs as well as larger, longer primary guard hairs. Rex rabbits have a defect in the hair growth cycle, resulting in a plush, short, velvety coat lacking the long guard hairs of other breeds. Rabbit hair normally undergoes periods of cyclic and seasonal growth and molting, which may influence hair regrowth patterns following clipping for surgical or other procedures. Regrowth does not occur evenly over the body with independent cycles noted for the dorsum and ventrum. Hair growth cycles also differ among breeds. For example, Angora rabbit hair follicles have been reported to have a growth phase of 14 weeks compared to a 5-week cycle in New Zealand white rabbits, as well as having a higher follicular growth rate. Both these factors likely contribute to the long hairs produced by the Angora breed.

Rabbits have numerous sebaceous glands associated with clusters of hair follicles on their chin (submental gland), feet, and prepuce, which are used for scent-marking (chinning) novel and familiar objects and spaces by sexually mature animals. Modified sebaceous glands are also found in the genital area, and within the rectum. Rabbits are not thought to possess sweat glands but do have modified apocrine glands within the ear (ceruminous apocrine gland), in the perineal area of both sexes, and in the submandibular area. Although mammary glands are not part of the integument, they are discussed in this section because of their cutaneous association.

The external ears of rabbits have only a vertical canal, i.e., they lack a horizontal canal, thus differing from those of other small companion animals, and the vasculature of the ears has an important function in thermoregulation, providing for counter-current heat exchange.

Healthy rabbits are fastidious in their habits and groom their pelage frequently, thus a poor quality, staring hair coat is a sign of poor health. Interestingly, rabbits with orodental disease have a 63 times greater risk of subsequently developing dermatologic disease. This is postulated to be due to decreased nutrition and poor body condition, matting from drooling or other discharge, and decreased grooming in unthrifty or painful animals secondary to chronic dental disease and abscesses.

Skin conditions represent almost 30% of pet rabbit primary care presentations. The most commonly reported conditions include pododermatitis, abscesses, alopecia, parasites, cutaneous masses, and moist dermatitis.

1.2.1 Alopecia

Focal or diffuse alopecia or hair loss is a common presenting condition and there are many potential causes for simple alopecia in rabbits, some of which may be physiologic. Rabbits typically undergo at least two cycles of hair growth and shedding annually and some breeds will undergo four or more cycles. Nutritional and disease status, hormone levels, age, and sex will all influence pelage quality, as well as growth and shedding patterns.

1.2.1.1 Presenting Clinical Signs

Uncomplicated alopecia may present as areas of focal to patchy to generalized hair loss. Owners may report overgrooming, hair pulling or chewing (barbering) or excessive shedding. The skin is intact in these cases unless there are lesions associated with trauma or fighting. Both long guard hairs and secondary hairs may be affected. Alopecia secondary to various endocrinopathies may be associated with vulvar swelling and thinning of the skin in does.

1.2.1.2 Pathology

An infectious etiology should be suspected if hair loss is accompanied by scaliness, pruritus, moist dermatitis or...