Digestive problems can occur easily in house rabbits (5). Most of these problems in house rabbits are closely associated with their diet, with intestinal pathogens having less of an effect (25). Disorders, such as gastrointestinal stasis syndrome, acute gastrointestinal dilation or obstruction, diarrhea, enteritis complex, enterotoxemia, and neoplasia, could be included. The digestive problems that cause diarrhea and tenesmus could induce rectal prolapse (22).

If rectal prolapse occurs, it will appear as a pinkish rosette-like form projecting from the anus (21). Dyschezia and tenesmus associated with anorectal inflammatory disorders are common symptoms in many animals suffering from rectal prolapse (12). It can be followed secondarily to persistent tenesmus in animals with various diseases, such as severe parasite infections, diarrhea, intestinal neoplasia, and dystocia, including rectal polyps (18). Among them, rectal polyps in rabbits appear as a reddish proliferative mass around the anus that could be ulcerated or infected (3). Histologically, these polyps show well-differentiated figures and have proliferative squamous epithelium that covers inflamed fibrous stroma (9,13). If these polyps increase in size, they could cause continuous tenesmus and rectal prolapse in severe cases. This condition would likely develop into inflamed conditions like eosinophilic inflammation reported in several other species (3,10,14).

To the best of the author’s knowledge, there are no case reports of eosinophilic rectal polyps with rectal prolapse in house rabbits. Many cases of eosinophilic intestinal polyps have been reported in humans, dogs, and horses (8,10,11,15). The eosinophilic polyp of rabbits was reported only in experimental sinusitis with a nasal polyp model (20). Therefore, this case report will elaborate on rectal prolapse related to an eosinophilic rectal polyp in a house rabbit.

An 11-year-old male Rex rabbit weighing 2 kg was referred to Veterinary Medical Teaching Hospital of Seoul National University with a history of rectal prolapse caused by a mass. The owner reported clinical signs, such as hematuria and hematochezia with watery stool, which appeared after this polypoid mass had protruded from the anus. Physical examination of the rabbit showed low vitality, and the prolapsed rectum was coated with feces. The mass was highly vascularized and firm, with a cauliflower-like appearance (Fig. 1). The patient reacted to pain when the mass was palpated. Radiographic examination revealed a soft tissue opacity mass around the anus; although no significant evidence of metastatic lesions was identified, there was a limit due to the low serosal detail (Fig. 2A). Patency from the mass to the colorectal region was confirmed using barium sulfate contrast medium (Fig. 2B). Anorectal papilloma was the first to be suspected, given the visual shape, texture, aspect of vascularization, and lesion location. 50% dextrose solution and lidocaine gel (Lidocaine jelly 2%; Firson, Cheonan, Korea) were used to correct the edematous mass manually. Enrofloxacin (Baytril; Elanco, Indiana, USA) at 10 mg/kg PO SID and meloxicam (Metacam^®; Boehringer Ingelheim, Ingelheim am Rhein, Germany) at 0.3 mg/kg PO SID were administered for 10 days. Subsequently, the patient visited one more time with the same symptoms and received the same prescription. The prolapse was expected to relapse again, but the owner did not want further treatment like purse-string suture due to the anesthesia risk in the older rabbit.

Figure 1. 11-year-old house rabbit (Oryctolagus cuniculus) with rectal prolapse due to a protruding mass. The cauliflower-like mass was vascularized and had a firm touch.

Figure 2. Lateral view of the abdominal radiographic image from an 11-year-old Rex rabbit. (a) Rounded soft tissue opacity around the perianal region was observed. (b) Patency test using barium sulfate contrast agent showed the patency between the mass and colorectal region.

After 25 days, the rabbit made an emergency visit because of sudden prolapse and pain reaction. The rabbit was in a state of low vitality, as in the first visit, but the pain response was more severe than before. The mass was also hardened and enlarged compared to the last time, and the surface was necrotic (Fig. 3). Correcting the enlarged mass was impossible despite lavaging with normal saline and 0.05% chlorhexidine and attempts of manual reduction. Prednisolone (Solondo; Yuhan Foundation, Seoul, Korea) at 0.25 mg/kg PO SID and furosemide (Lasix; Handok, Seoul, Korea) at 1 mg/kg PO SID for a week were prescribed to prevent the inflammation and reduce the size of the mass, but there was no improvement. Two weeks later, the rabbit revisited the hospital for pre-operative assessment.

Figure 3. The mass became harder and larger than at the last visit, and necrotic changes of the surface were also confirmed.

In pre-operative assessment, blood tests, radiographic imaging, and fine-needle aspiration (FNA) cytology were conducted. Complete blood count and biochemistry panel revealed no remarkable findings except for the glucose level (172 mg/dL, Reference interval 75-150). Considering the duration of steroid use, it appeared to be stress-induced hyperglycemia rather than the steroid effects. In radiography, mild broncho-interstitial opacity caused by senile changes was observed at the overall lung field, but there was little evidence of metastasis about the mass. The FNA result showed no tumor cells other than blood cells, including red blood cells and eosinophils.

The rabbit was sedated with hydromorphone (Dilid inj.; Hana Pharm, Seoul, Korea) and midazolam (Bukwang Midazolam inj.; Bukwang Pharm, Seoul, Korea) with a dose rate of 0.1 mg/kg IM SID and 0.5 mg/kg IM SID, respectively. General anesthesia was induced with ketamine (Huons Medicare, Seongnam, Korea) at 25 mg/kg IV SID and maintained with isoflurane (Ifran liq.; Hana Pharm, Seoul, Korea) and oxygen on a circle system following V-gel^® (Docsinnovent, Hemel Hempstead, UK) intubation. The surgical site was clipped and disinfected with 70% ethanol and 5% povidone-iodine solution.

The patient was positioned in dorsal recumbency. The lesion was marked using a 3 cc syringe needle lid as a probe rod. Four stay sutures on the mucosal skin junction and three stay sutures on the rectal mucous membrane using a 3-0 polydioxanone (PDS^®II; Ethicon, New jersey, USA) were performed around the mass to evert the rectal wall and expose the entire mass (Fig. 4A). The base of the exposed mass was removed as a whole layer using electrocautery. The surface of loss caused by a whole-layer resection was sutured with simple interrupted sutures using 4-0 polydioxanone. All stay sutures were then removed, and the surgical site was put into the anus (Fig. 4B). Postoperative oral medications included Enrofloxacin (Baytril; Elanco, Indiana, USA) at 10 mg/kg SID and Tramadol (Tridol; Yuhan Foundation, Seoul, Korea) at 10 mg/kg SID for 10 days.

Figure 4. Surgical removal of the rectal polyp. (a) A 3 cc syringe needle lid was used as a probe, and several stay sutures were done to expose the polyp base. (b) Postoperative appearance of rectal polyp by electrocautery. Stay sutures around the surgical site were removed, and the prolapsed site returned into the anus.

Histopathological examination of the surgically excised mass was conducted. The mass was preserved in 10% neutral buffered formalin for 48 hours at room temperature. After complete fixation, the submitted samples were sectioned serially with 0.5 cm intervals. Four representative sections were processed and stained with hematoxylin and eosin (H&E) for the histopathological examination, followed by general procedures. With gross inspection, the mass was suspected to be chronically prolapsed anorectal tissue.

On gross examination, the mass was firm with a bright pink to red texture, probably due to the severe ulcer and erosion. The microscopy examination revealed a polypoid-like mass protruding from anorectal mucosa, with marked erosion and ulceration (Fig. 5A). With some discernable epithelial structures, there was an intestinal epithelial structure composed of mucinous cells, corresponding to large intestinal features. The mass was composed primarily of well-vascularized granulation tissue with numerous infiltrated inflammatory cells and edema. The majority of the inflammatory cells were eosinophils. Lymphocytes, plasma cells, heterophils, and macrophages were also observed (Fig. 5B). There were no remarkable etiologies that could induce chronic active eosinophilic inflammation, such as parasites or bacteria.

Figure 5. Microscopic examination of the surgically excised mass from the rabbit. H&E stain. (a) Low magnification of view, ×12.5, bar = 1 mm. The mass protruded from anorectal mucosa shows a polypoid-like appearance and severe surface ulceration. (b) High magnification of view, ×400, bar = 50 μm. The mass consisted of granulation tissue with numerous inflammatory cells. The majority of inflammatory cells were eosinophils.

Two weeks after surgery, the feeding response of the patient was good since the day after the surgery, and cecotrophy existed normally. Physical examination showed that the surgical site was well ingested into the anus, and there were no other inflammatory or pain reactions (Fig. 6). Any recurrence of clinical signs 6 months after initial presentation was not detected from the rabbit.

Figure 6. Re-examination performed after two weeks from surgery showed no significant complications.

Basic physical examination information, including body temperature, heart rate, and respiration rate, is omitted from the case summary. This is because there were evaluation restrictions based on the patient’s condition. During rectal temperature examination, bleeding and pain reactions were too severe to proceed at the visit because the thermometer can unintentionally tear the thin and vulnerable mucosa of the rectum (26). The blood test was also not carried out before the final decision to undergo surgery. The same treatment was repeated without proceeding with surgery despite the deterioration of the patient's condition because of the owner's anxiety about anesthesia depending on the patient's senility.

The rectal polyp, which was considered to induce rectal prolapse in this case, is often called anorectal papilloma because of its cauliflower and fungated shape and appearance at the anorectal junction (13,16,23). One study reported that these polyps had been misnamed as anal papilloma (2). Anorectal papilloma is a common benign polyp in rabbits, but its pathogenesis is unclear. The condition was a similar appearance to viral-induced polyps of other animals (3). A virus transmission study was conducted to prove that the origin is viral, as is the case with oral papilloma in rabbits, but the result was unsuccessful (28).

In many cases, papillomas do not cause problems and resolve with time (27). Given the patient's old age, manual reduction with 50% dextrose solution for preventing prolapse was done first. On the other hand, it did not resolve itself and continued to protrude, so surgical removal was finally performed. Anorectal papilloma should be removed carefully because it is highly vascular with the potential for recurrence (13).

Only the rectal mucosa was resected during surgery because the mass was derived from the epithelium, and complications could occur if the entire layer were resected. The side effects of intestinal surgery involving wall incision include leakage, which can be caused by anastomosis failure and bacterial peritonitis after leakage (6). Near the root of the mass, however, there was a full-layer adhesion caused by inflammatory material, which led to the removal of the entire layer. An analysis of rectal full-through surgeries in dogs and cats revealed several complications, such as tenesmus, bleeding, and incontinence (1). Nevertheless, there were no complications at the surgical site as of the last recheck in this case.

Interestingly, histopathological examination of the removed mass revealed eosinophilic proctitis that was not accompanied by an increase in white blood cells (WBC). This is due to the fact that the total WBC count of rabbits barely changes even in infectious diseases, so the redistribution of WBC should be checked through WBC differential count (24). Unfortunately, the WBC differential count was not performed in this case, which was the limitation of this report. Some of the causes of eosinophilia in mammals include parasites, hypersensitivity reactions, or chemotactic agents that stimulate eosinophils (4). Although the cause of eosinophilic enteritis is not clear, few studies in which eosinophilic enteritis was observed in rabbits have been conducted due to infectious diseases (17,19). On the other hand, no infection was detected by a histopathological examination in this case. In addition, before and after surgery, prescription has not changed significantly, but clinical symptoms have improved significantly. Therefore, the probability of infection would be low, even though it cannot be completely ruled out.

Another cause of eosinophilic proctitis is an inflammatory response from polyps. The rectal polyp with eosinophilic inflammation might have led the rabbit into chronic tenesmus, rectal prolapse, and finally, the painful condition. Among the studies currently reported, there are several case reports of some mammals undergoing eosinophilic rectal polyps or rectal prolapse but not for rabbits. One research reported a rectal prolapse case in a rabbit caused by an inflammatory polyp, but it was not related to eosinophilia in histopathological examination (7). Also, Therefore, further research will be needed to identify the relevance between these conditions.

In conclusion, in cases where manual reduction is not considered practical, surgical removal of the inflammatory lesion may be curative or at least successful in rabbits in the long-term suppression of clinical signs and prognosis.

This report was supported by Seoul National University Teaching Hospital and the Research Institute for Veterinary Science, Seoul National University. This subject is supported by the National Institute of Wildlife Disease Control and Prevention as “Specialized Graduate School Support Project for Wildlife Disease Specialists”.

The authors have no conflicting interests.