Biology

Leporidae (rabbits and hares) and Ochotonidae (pikas) are the two families in the order Lagomorpha.²⁷ With a worldwide distribution, this order contains 12 genera and 81 species (Box 41-1). Unlike rodents, lagomorphs have a second set of upper incisors known as “peg teeth.” Lagomorphs are born with three pairs of upper incisors, but the most lateral on each side is lost soon after birth. Jaw movements of rabbits, hares, and pikas are vertical or transverse. Lagomorphs are hindgut fermenters, herbivorous, and practice cecotrophy. Although lagomorphs were once classified as rodents, they only illustrate convergent evolution on the basis of their gnawing teeth.¹⁷

Although rabbits and hares have a short tail, pikas lack a tail.²⁷ Most lagomorph males have testes located in a scrotum in front of the penis, similar to marsupials, and lack an os penis. Two to five pairs of mammary glands are found in females. Hearing, smell, and touch are well developed, and vocalizations in most species are minimal. Eyes are laterally positioned, providing a circular field of vision. Sensory hairs are positioned around the nose and above the eyes.¹⁷

The Leporidae family contains 54 species found within 11 genera.²⁷ Although the term “hare” should probably be reserved for those animals in the Lepus genera, the terms rabbit and hare are often used interchangeably and applied incorrectly. Hares are born fully furred and precocial, are generally larger, and have ears larger than those of rabbits, with dark ear tips. Rabbits are born naked and blind and usually associate in groups compared with the more solitary hare. Hares and jackrabbits comprise 29 species in the genus Lepus, and rabbits have 25 species, 14 of which are Sylvilagus, or cottontails, in the remaining 10 genera. Unlike many other mammals, female leporids are usually larger than males. The long hindlimbs are well adapted for running, with thick hair, instead of footpads, on the soles of all the feet. The dental formula of leporids is usually (incisor [I] 2/1, canine [C] 0/0, premolar [P] 3/2, molar [M] 3/3) ×2 = 28.

The European wild rabbit Oryctolagus cuniculus, which likely originated from the Iberian Peninsula and southern France following the end of the Pleistocene, is the origin of the domestic rabbit.²⁷ Domestic rabbits range in size from 1 kilogram (kg) to over 7 kg and are divided into over 60 fancy and fur breeds and over 500 varieties.³³ Box 41-2 lists biologic and physical data of domestic rabbits. Table 41-1 lists hematologic and serum biochemical values of the domestic rabbit and hare.

The family Ochotonidae (pikas, mouse hares, or conies), found in Eurasia and western North America (Figure 41-1), contains one living genus and one recently extinct genus and up to 37 subspecies.^18,27 Pikas have long, fine, dense fur and short legs with five digits on each front foot and four digits on each rear foot. Weight ranges from 125 to 400 grams (g), and both genders are of the same size. The dental formula of pikas is (I 1/1, C 0/0, P 3/2, M 2/3) ×2 = 26. Females have four to six mammae. Males have abdominal testes that descend into the skinfolds at the base of the penis during breeding season.

Pikas are generally social, but the social structure varies. Steppe-dwelling (burrowing) pikas may live in large colonies organized into family units, whereas the species found in talus and boulder-fields defend individual territories (Chris Ray, personal communication, 2012). Pikas gather food in late summer and stack it in heaps called “hay piles”; the weight of food in each pile can be up to 20 kg, and they can survive suboptimal conditions by feeding on these stores. They are primarily diurnal, most active in the early morning and evening, and do not hibernate.²⁷ The steppe-dwelling pikas may be threatened by human encroachment, but all pikas may be most threatened by climate change.³²

Feeding

Domestic rabbits have specific recommended dietary requirements including 13% to 18% dry matter (DM) dietary crude protein; 12% to 16% dietary crude fiber; 7,000 international units (IU) vitamin A per kilogram of food; 40 milligrams (mg) vitamin E per kilogram of food; 2 mg vitamin K per kilogram of food; and 0.5% to 1% DM calcium.⁵ Of these requirements, fiber is especially important. Because fiber is essential for the production of short-chain fatty acids and gastrointestinal (GI) motility, diets containing less than 10% crude fiber often result in enteritis.

The preferred diet for the pet rabbit is a high-quality, high-fiber (15% to 16% crude fiber) pelleted diet containing 13% to 18% (ideally 16%) crude protein, at a rate of cup pellets per 2.3 kg body mass divided into two meals per day.⁷ A fiber content below 15% may increase the potential for anorexia and diarrhea, and one greater than 16% reduces feed palatability. However, a fiber content of 18% to 22% helps prevent obesity in pet rabbits and often is used in mature laboratory animals. Some rabbits do well when pellets are offered ad libitum (freely), unless overeating and obesity become problems or an inadequate amount of loose hay is consumed. The pellets are supplemented with loose hay (mixed grass hay, timothy hay, or high-quality dried grass clippings) ad libitum. Alfalfa hay may be offered throughout the growth stages and then discontinued because of its high protein and calcium content. The diet can be supplemented with a small amount of dark fibrous, leafy greens and fresh vegetables (1 cup per 2.3 kg body mass) and small amounts (up to 1 tablespoon per 2.3 kg body mass) of fresh fruit, daily or several times per week. It should be noted that growing rabbits and females in late gestation may require twice as much food, and lactating females may consume three times as much food as an adult in maintenance.

The skeleton in rabbits represents only 7% to 8% of the body weight (as opposed to 12% to 13% in cats).¹⁶ Rabbits are prone to fractures of the back and hindlimbs because of hindlimb musculature and the delicate nature of the skeleton. Support of the hindquarters is essential when transporting rabbits to avoid causing injury; rabbits may be carried with one hand under the thorax or holding the scruff, with the second hand supporting the hindquarters. The rabbit should be placed in a cage with its rear facing the back of the cage and the hindquarters supported to reduce chances of injury from the rabbit kicking. A nonslip mat should be used when examining the rabbit or placing the animal in the cage. Alternatively, the rabbit can be wrapped in a towel with the head covered to prevent struggling. A hand may be placed over the eyes to calm the rabbit while using care not to obstruct the nostrils, since rabbits are obligate nasal breathers. Hyperthermia should be avoided when using a towel during handling or examination. Noises or smells from predator animals may stress the sensitive rabbit.

Pasteurella multocida is a non–spore-forming, bipolar, gram-negative rod. Although P. multocida is a common cause of mortality in brown hares (Lepus europaeus) in Europe, the bacterium is associated most commonly with disease in domestic rabbits.⁷ P. multocida is transmitted from acutely infected rabbits, through direct contact, via fomites, or via inhalation or wounds.^7,25 The incubation period of P. multocida is about 2 weeks. In a survey of rabbits with signs of upper respiratory disease, P. multocida was the most common isolated bacterium in greater than 50% of rabbits.³⁰ The bacteria colonize the nares or cause production of nasal exudate if the host is unable to resist infection. Snuffles, a common upper respiratory tract disease in domestic rabbits, is caused by a local overgrowth of P. multocida in the nasal epithelium. However, cultures of the nasal epithelium are frequently positive for P. multocida in clinically normal adult rabbits housed in institutional colonies (40% to 72%)¹⁰ and in conventional rabbitries (28% to 31%).⁹ P. multocida may proliferate and spread via the trachea (to the lungs), the nasolacrimal ducts (to the conjunctiva), the eustachian tube (to the middle ear, inner ear, and then the brain), or the bloodstream (septicemia) to the lungs, heart (endocarditis), reproductive organs (orchitis and pyometra-endometritis), skin, and subcutis (subcutaneous abscesses).^7,25 Pasteurellosis infection in the lungs may result in fibrinopurulent pneumonia, pleuritis, and cranioventral pulmonary abscesses. Ophthalmic involvement (conjunctivitis, hypopyon, or retrobulbar abscesses) also may be seen with pasteurellosis. Serologic tests such as enzyme-linked immunosorbent assay (ELISA), and polymerase chain reaction (PCR) tests are available to detect pasteurellosis in rabbits. Culture and sensitivity testing are recommended because organisms other than Pasteurella species may be isolated.

Torticollis (head tilt or wry neck) in domestic rabbits usually is caused by the extension of P. multocida infection from the nasal cavity to the inner ear via the eustachian tube and middle ear or may arise centrally in the medulla or cerebellum (Figure 41-2).⁷ Other causes to consider, although less common, are otitis externa, cranial trauma, listeriosis, encephalitozoonosis, ascarid migration, or extension of ear mite infection. If infection is unilateral, the head of the rabbit tilts down on the affected side. Occasionally, nystagmus may be present. Affected middle ears are characterized by tympanic bullae filled with thick, yellow exudate, which may be seen on radiographic or computed tomography (CT) examination of the skull. The head tilt may or may not respond to antibiotic therapy. Additional supportive care measures, including syringe feeding and protection of the down eye, which may be prone to ulceration from rolling, may be warranted. Prognosis is favorable if a positive response is exhibited within the first week after therapy, and therapy is continued for 1 week after the resolution of clinical signs.