The reports on incidence of spontaneously occurring tumours in rabbits are few probably because the rabbit population is not permitted to survive long enough to collect data on tumour incidences¹,²,³-⁴⁷,⁸,⁹-¹⁰. Most rabbits are killed before 2 years of age. The normal life span of rabbit is about 7 to 8 years. Nephroblastoma, (also called as embryonal nephroma) in domestic rabbits has been found to be the third most commonly occurring neoplasm. The exact cause of nephroblastoma is unknown, but probably it develops in foetal tissue due to an underlying genetic factor³,⁵. In view of the paucity of information¹,²,⁴,⁷, a spontaneously occurring nephroblastoma in a rabbit has been reported in the present communication.

An enlarged tumorous mass was recorded in a male German Angora rabbit aged about one year (born on 15^th October, 2001 and died on 5^th October, 2002). The rabbit was fed ad libitum on green fodder and about 100 to 150 g of feed daily (Composition: Mustard cake 20%; maize 20%; rice bran 58.5%; mineral mixture 0.5%; common salt 0.95% and vitamin mixture 0.005%). He served does two times (1^st service on 24^th April, 2002 and 2^nd service on 1^st June, 2002) without any difficulty. A litter size of 5 was obtained after each service. There was a gradual increase in body weight except in the last one month before death. The animal was apparently healthy without showing any abnormal clinical signs of significance. The rabbit was found dead suddenly. A thorough necropsy was conducted on the same day and the representative tissue samples were processed routinely for histopathological examination. The tissue sections were stained with HE stain and examined under a microscope.

Grossly, the tumourous mass was found in the left caudal aspect of body cavity which was enlarged and replaced left kidney. The mass was about 10 cm in length and 7 cm in width weighing 235g. The growth was circumscribed, encapsulated (fibrous, gelatinous and translucent) and lobulated with irregular lumpy surface areas showing discrete projections of nodules. The colour of the tumour mass was brownish to tan with irregular patchy areas of yellowish cream colour (Fig. 1). On palpation, it was firm and meaty in consistency. The vascular structure and collecting tubules/ducts appear to be not involved. Cut section revealed yellowish cream coloured firm, fleshy surface with small irregular patchy areas of tan colour. Few round to oval circumscribed encapsulated areas were also observed over cut surface. The cortex and medulla could not be differentiated over cut surface. No cysts or haemorrhages could be observed. The other non tumorous, apparently normal right kidney was about 4 cm in length and 2.5 cm in width weighing only 18 g. No gross lesion of significance was observed in other vital organs at necropsy.

Microscopically, the tumourous mass was comprised mainly of neoplastic cells (blastema like cells) and epithelioid components resembling primitive glomeruli and tubules with varied differentiation. The blastemal cells were arranged in dense clusters or sheets with high cell density in a lobular pattern. In multifocal areas, the sheets of blastemal cells were supported by a delicate connective tissue stroma. Neoplastic cells contained hyperchromatic nuclei and inconspicuous cytoplasm. They were pleopmorphic with variably distinct cell borders. The nucleus was irregular to oval in shape and contained finely stippled basophilic chromatin and one or more acidophilic nucleoli. Few structures suggestive of foetal, primitive and nonvascularized glomeruli were observed within the tumorous mass. They were frequently of a complex nature with a papillary configuration and entrapped in connective tissue stroma. At some places in the tumorous mass, almost fully differentiated glomeruli along with few differentiated tubules were observed. These apparently normal glomeruli/tubules were separated by a thick band of connective tissue from the tumourous tissue like a capsule. The separating connective tissue was highly vascular with engorged blood vessels. The tumourous mass predominantly consisted of structures suggestive of primitive renal tubules. These renal structures were enclosed in a delicate connective tissue stroma. They were of cuboidal to columnar type of cells arranged in clusters of cells or tubules or rosettes. Sometimes, these cells appeared as tortuous tubules enclosed in a delicate connective tissue stroma (Fig. 2). These structures of probable renal tubular origin were randomly distributed throughout connective tissue bands/mesenchyma. Sometimes, glomerulus like invaginations of epithelial cells were noticed within these tubular structures. It was difficult to differentiate the neoplastic structures from glomeruli to tubules. Some times, the neoplastic structures of glomeruli/tubular appearance were observed around a blood vessel. The connective tissue stroma was loose and fine. Fibrous reaction was only as a simple ramifying stroma within and around the tumour. Increasing collagen formation appeared to be associated with ischaemic necrosis of tumour tissue. No recognisable areas of fibrosarcomatous elements or specialized connective tissue such as smooth muscle cells or striated muscle, adipose tissue, cartilage or osteoid tissue were noticed. There was no definite evidence of malignancy in mesenchyme. However, in some areas, the pattern of collagen fibres was irregular. At some other places, necrotic areas (coagulative to liquefactive) of both neoplastic tissue and connective tissue were also observed. Majority of the blood vessels were engorged with erythrocytes. There was no presence of neoplastic cells within the lumen of blood vessels. The histological examination of major vital organs particularly lungs, liver, intestine and heart did not reveal the presence of metastatic neoplastic tissue.

Fig. 1:Tumourous left kidney and apparently normal right kidney.

Fig. 2:Tortuous renal tubules confined in a connective tissue stroma in tumourous kidney. Dilated blood vessels were in abundance. HE × 100.

The gross and histological lesions of tumorous mass was suggestive of benign nephroblastoma or embryonal nephroma. Flir¹ reported embryonal nephroma in rabbits in 1954. Huang⁴ detected nephroblastoma by abdominal palpation in 25% of Angora rabbits at a rabbit breeding farm. He also observed that the incidence of nephroblastoma was considerably higher in Angora breed of rabbits than in broiler rabbits. In a 4 years study (2001 - 2004) at Mukteswar, a single case of nephroblastoma was recorded in a German Angora rabbit out of 188 necropsies of different breeds (German Angora - 55. Soviet Chinchilla - 84, Russian Grey Giant - 3 and New Zealand While - 16) conducted. This observation of spontaneous nephroblastoma in Angora breed of rabbits supported the findings of Huang⁴. The present tumour appears to be of benign in nature without any metastasis in major vital organs or presence of any malignancy in mesenchyme. Specialized connective tissue such as smooth muscle cells or striated muscle, adipose tissue, cartilage or osteoid was not noticed in mesenchyma of neoplastic tissue contrary to the observations of Wilm's tumors reported in human beings. This is in agreement with the earlier observations⁶,⁷.

Nephroblastoma in a rabbit