AGING AND MISCELLANEOUS DISORDERS

Hair Chewing: Barbering

Hair loss due to hair chewing (barbering) occurs com­monly among young group-housed rabbits. Patchy alo­pecia may be present on the face and back, with no evidence of a concurrent dermatitis.

Wet Dewlap: Slobbers

Excess ptyalism occurs in heat-stressed rabbits and in rabbits with malocclusion. Secondary bacterial infec­tions, including necrobacillosis (Schmorl's disease), may arise, particularly in does with large dewlaps.

Tooth Abscesses

Tooth root infections are common in rabbits, and a number of opportunistic bacteria can be isolated from them. Abscesses often develop capsules with fistulous tracks, and osteomyelitis with retrobulbar involvement make them refractory to treatment.

Hutch Burn

Urine scalding of the perineal region may be a problem in young rabbits housed in filthy conditions. The skin may be hyperemic and excoriated, with serous exuda­tion. Lesions may be complicated by myiasis.

Pugilism

Fighting is common among group-housed male rabbits that have reached sexual maturity. Abrasions and hair loss are common in the combatants, including lacera­tions around the external genitalia. Rarely, aggressive males may mutilate both bucks and does. Injuries observed include skin abrasions and amputation of the tips of the ears.

Exfoliative Dermatosis and

Sebaceous Adenitis

This condition has been reported to occur in several different breeds of pet rabbits. Typically, there is a non­pruritic scaling dermatosis with patchy to coalescing areas of alopecia and scaling (Fig.

FIG. 6.82. Rabbit with chronic exfoliative dermatosis and sebaceous adenitis. Note the marked scaling lesions and alopecia over the dorsal aspect of the body (Source: M. Taylor and K.E. Linder.)

FIG. 6.83. Bilateral pododermatitis (sore hock) in an adult rabbit.

be associated with autoimmune hepatitis, thymoma, and cutaneous lymphoma in rabbits (see “Neoplasms”). Thus, it is important to pursue the possibility of the dermatosis having a paraneoplastic or neoplastic origin.

Pododermatitis: Sore Hocks

The plantar surface of rabbit feet is fully haired, but lesions consist of a circumscribed, ulcerated areas cov­ered by granulation tissue and necrotic debris (Fig. 6.83). Purulent exudate may be adherent to the lesions. The problem is most commonly seen in heavy, mature adults. Poor sanitation, trauma from poor-quality, wire-bottom cages, and hereditary predisposition are factors that may influence the incidence of the disease. Staphylococcus aureus is the most frequent bacterium isolated from lesions.

Prolapse of the Deep Gland of

the Third Eyelid

Swelling and protrusion of the third eyelid has been associated with prolapse of the deep bilobed gland of the third eyelid. Affected animals present with a uni­lateral or bilateral protrusion of the third eyelid from the medial canthus of the eye.

Vertebral Fracture and Degenerative Spinal Disease

Posterior paralysis due to vertebral fracture or dislocation occurs all too often in domestic rabbits. The axial and appendicular skeletons of domestic rabbits are relatively fragile in proportion to their muscle mass. An unsupported, sudden movement of the hind limbs

FIG. 6.84. Vertebral fracture in an adult rabbit due to improper restraint. Fractures typically occur in the lumbosacral region.

may exert sufficient leverage on the lumbosacral junc­tion to cause a vertebral fracture. Depending on the duration of the problem prior to euthanasia and necropsy, the hindquarters may be soiled with urine and fecal material consistent with incontinence. The site of the fracture (or luxation) is usually the lumbo­sacral region (L7) (Fig. 6.84). There may be extensive hemorrhage in the underlying psoas muscles. Changes vary from luxation to multiple fractures of the affected vertebra, with extensive damage to the lumbosacral spinal cord. Degenerative changes of the nucleus pul- posis, usually involving the distal thoracic spinal seg­ments, have been noted in rabbits as early as 3 months of age, with spondylosis among rabbits greater than 2 years of age.

Iatrogenic Nerve Damage

Following intramuscular injection of ketamine, xyla- zine, and acepromazine in the hind leg, rabbits were found to develop self-mutilation of digits. This syn­drome was associated with inflammatory change and degeneration of the sciatic nerve in the region of the injections. Intrathecal injection of preservative-free ketamine has been shown to cause severe damage to the spinal cord and nerve roots, but ketamine with preservative did not. Long-term restraint of rabbits with hind leg extension was found to result in degener­ation of sciatic nerves and skeletal muscle necrosis.

Tracheal Injury Following Intubation

Erosive to ulcerative tracheitis has been reported to occur in rabbits following tracheal intubation and inhalation anesthesia. The subjects were New Zealand White rabbits that were kept under general anesthesia for a period of 4-5 hours. Tracheal lesions have been identified in rabbits acquired from several different research facilities that had been subjected to this procedure. Clinically, stridor and mild cyanosis are the usual presenting signs in affected animals. At necropsy, changes may be con­fined to congestion of the tracheal mucosa. In more advanced cases, necrotic debris and blood are present

FIG. 6.85. Reproductive tract of a Californian breed doe with a

history of intermittent vulvar bleeding due to rupture of endometrial venous aneurysms. Blood clots are present within the uterine lumina.

on the tracheal mucosa. On histopathology, mucosal lesions are usually most extensive in the sections imme­diately distal to the larynx. Changes vary from multi­focal mucosal ulceration to circumferential transmural ulceration of the tracheal mucosa.

Endometrial Venous Aneurysms

Multiple endometrial venous aneurysms have been asso­ciated with persistent urogenital bleeding. At necropsy, clotted blood is present in the uterine lumen (Fig. 6.85), and there are multiple blood-filled endometrial varices that consist of dilated, thin-walled veins (Fig. 6.86). These varices may rupture and bleed periodically into the uterine lumen, with subsequent hematuria. They have been observed in nonpregnant multiparous does. There is no evidence that predisposing factors, such as trauma or bleeding disorders, play a role in the disease.

Miscellaneous Genital Disorders

Cryptorchism may be occasionally encountered in bucks. In hot climates, bucks are prone to “seasonal infertility,” and seasonal involution occurs in the testes of some bucks during the nonbreeding season (see “Ana­tomic Features”).

Urinary Sludge and Urolithiasis

As noted previously, intestinal absorption of calcium in rabbits occurs in direct proportion to dietary intake, as intestinal uptake is not influenced by vitamin D. Renal

FIG. 6.86. Section of uterus from the previous figure. Note the marked dilation of the endometrial vessel and thrombosis.

calcium resorption and excretion are also in proportion to dietary intake, and regulated by parathyroid hor­mone, calcitonin, and vitamin D. When the resorptive capacity of the kidney is exceeded, calcium precipitates in the alkaline urine as calcium carbonate monohydrate, anhydrous calcium carbonate, and ammonium magne­sium phosphate, resulting in the cloudy nature of rabbit urine in postweanling rabbits. When metabolic demand for calcium rises, the urine becomes less cloudy. When rabbits are fed high-calcium diets, such as diets rich in alfalfa, become dehydrated, or are suffering from other systemic disease, the likelihood of excess urinary mineral excretion takes on the characteristics of mud-like

FIG. 6.88. Urinary bladder of a rabbit that died of urinary obstruction due to massive accumulation of intractable urinary sludge, which has the consistency of modeling clay. This material can accumulate rapidly.

“sludge.” This can happen very rapidly, resulting in acute urinary obstruction. The urinary bladder fills with intractable material that has the consistency of modeling clay (Fig. 6.88). Rabbits may also develop true uroliths in the renal pelves, ureters, urinary bladder (Fig 6.89), or urethra, resulting in hydronephrosis, hematuria, and obstructive uropathy.

Miscellaneous Kidney Disorders

Chronic renal disease, analogous to similar syndromes in aged rodents, is common in aged rabbits. Hydroneph­rosis may occur in association with urolithiasis. Fibrosis, with or without nephrocalcinosis, is common in rabbits over 10 months of age. Abscesses, pyelonephritis, and neoplasia (lymphosarcoma) may all involve the kidneys.

Amyloidosis

Amyloidosis occurs sporadically among rabbits, and is most likely to be observed in older rabbits that have been

FIG. 6.87. Uterine horn of a doe with mucometra. The uterine wall has been incised, and the uterus is surrounded by serosanguinous fluid from the lumen. Because the uterine horns and cervices in the rabbit are anatomically separate, mucometra may be unilateral or bilateral.

FIG. 6.89. Urolithiasis of the urinary bladder in a rabbit. (Source:

D. Imai, University of California, Davis, CA. Reproduced with permission from D. Imai.)

FIG. 6.90. Renal interstitial amyloidosis in a chronically immunized adult New Zealand White rabbit.

hyperimmunized or in rabbits with chronic infections, including pododermatitis and pyometra. Amyloidosis can also be readily induced experimentally. The kidney is the most commonly affected organ, in which amyloid deposits can be found in glomeruli, cortical interstitial tissue, or medulla (Fig 6.90). Mild early cases involve interstitial deposition in the medulla. Glomerular and cortical deposition tend to occur when medullary involvement is more severe. Medullary deposition may be associated with papillary necrosis or nephroli­thiasis. Stomach, intestine, spleen, myocardium, adre­nals, liver, lungs, and other organs may also be involved. Deposits in most organs are frequently perivascular, except spleen, in which deposits may be perifollicular or around the periphery of germinal follicles.

FIG. 6.91. Spleen of an adult rabbit with hemosiderin accumulation. This is a frequent incidental finding in aged rabbits.

Splenic Hemosiderosis

Aged rabbits tend to accumulate hemosiderin pigment in their spleens (Fig 6.91), which is probably due to excess iron in their diet.

BIBLIOGRAPHY FOR AGING AND MISCELLANEOUS DISORDERS

Tooth Abscesses

Tyrrell, K.L., Citron, D.M., Jenkins, J.R., Goldstein, E.J.C., & the Veterinary Study Group (2002) Periodontal bacteria in rabbit mandibular and maxillary abscesses. Journal of Clinical Micro­biology 40:1044-1047.

Exfoliative Dermatosis and Sebaceous Adenitis

Florizoone, K. (2005) Thymoma-associated exfoliative dermatitis in a rabbit. Veterinary Dermatology 16:281-284.

Florizoone, K., van der Luer, R., & van den Ingh, T. (2007) Sym­metrical alopecia, scaling and hepatitis in a rabbit. Journal of Veterinary Dermatology 18:161-164.

Prelaud, A.R., Jassies-van der Lee, A., Mueller, R.S., van Zeeland, Y.R.A., Bettenay, S., Majzoub, M., Zenker, I., & Hein, J. (2012) Presumptive paraneoplastic exfoliative dermatitis in four domestic rabbits. Veterinary Record 172:155.

White, S.D., Campbell, T., Logan, A., Meredith, A., Schultheiss, P., Van Winkle, T., Moore, P.F., Naydan, D.K., & Mallon, F. (2000) Lymphoma with cutaneous involvement in three domestic rabbits (Oryctolagus cuniculus). Veterinary Dermatology 11:61-67.

White, S.D., Linder, K.D., Schultheiss, P., Scott, K.V., Garnett, P., Taylor, M., Best, S.J., Walder, E.J., Rosenkrantz, W., & Yaeger, J.A. (2000) Sebaceous adenitis in four domestic rabbits (Orycto- lagus cuniculus). Veterinary Dermatology 11:53-60.

Prolapse of the Deep Gland of the Third Eyelid

Janssens, G., Simoens, P., Muylle, S., & Lauwers, H. (1999) Bilateral prolapse of the deep gland of the third eyelid in the rabbit: diagnosis and treatment. Laboratory Animal Science 49:105-109.

Vertebral Fracture and Degenerative Spinal Disease

Green, P.W., Fox, R.R., & Sokoloff, L. (1984) Spontaneous degener­ative spinal disease in the laboratory rabbit. Journal of Orthopedic Research 2:161-168.

Jones, T., Lu, Y.S., Rehg, J., & Eckels, R. (1982) Diagnostic exercise: fracture of the lumbar vertebrae. Laboratory Animal Science 32:489-490.

Tracheal Injury Following Intratracheal Intubation

Nordin, U. & Lindholm, C.E. (1977) The vessels of the rabbit trachea and ischemia caused by cuff pressure. Archives of Oto­rhinolaryngology 215:11-24.

Nordin, U., Lindhom, C.E., & Wolgast, M. (1977) Blood flow in the rabbit tracheal mucosa under normal conditions and under the influence of tracheal intubation. Acta Anesthesiologica Scandi- navica 21:81-94.

Phaneuf, L.R., Barker, S., Groleau, M.A., & Turner, P.V. (2006) Tracheal injury after intratracheal intubation and anesthesia in rabbit. Journal of the American Association of Laboratory Animal Science 45:67-72.

Iatrogenic Nerve Damage

Mendlowski, B. (1975) Neuromuscular lesions in restrained rabbits. Veterinary Pathology 12:378-386.

Vachon, P. (1999) Self-mutilation in rabbits following intra­muscular ketamine-xylazine-acepromazine injections. Cana­dian Veterinary Journal 40:581-582.

Vranken, J.H., Troost, D., de Haan, P., Pennings, F.A., van der Vegt, M.H., Dijkgraaf, M.G., & Hollman, M.W. (2006) Severe toxic damage to the rabbit spinal cord after intrathecal administration of preservative-free S(+)-ketamine. Anesthesiology 105:813-818.

Endometrial Venous Aneurysms

Bray, M.V., Weir, E.C., Brownstein, D.G., & Delano, M.L. (1992) Endometrial venous aneurysms in three New Zealand White rabbits. Laboratory Animal Science 42:360-362.

Urinary Sludge and Urolithiasis

Eckermann-Ross, C. (2008) Hormonal regulation of calcium metab­olism in the rabbit. Veterinary Clinics of North America Exotic Animal Practice 11:139-152.

Amyloidosis

Hinton, M. & Lucke, V.M. (1982) Histological findings in amyloi­dosis or rabbits. Journal of Comparative Pathology 92:285-294.

Hinton, M. & Lucke, V.M. (1982) Ultrastructure of the kidney in amyloidosis of rabbits. Journal of Comparative Pathology 92:295-300.

Hofmann, J.R. Jr. & Hixson, C.J. (1986) Amyloid A protein deposits in a rabbit with pyometra. Journal of the American Veterinary Medical Association 189:1155-1156.

Miscellaneous Kidney Disorders

Hinton, M. (1981) Kidney disease in the rabbit: a histological survey. Laboratory Animals 15:263-265.