Do horned rabbits really exist ? – Fibroma growths



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Warning: this file contains pictures that may be distressing to some persons

Between the 16th and the 18th century, illustrations show the legendary horned rabbit, a hypothetical cross between the antelope and the hare. Several naturalists studied the horned hare and gave it the Latin scientific name of Lepus cornutus. It was popularly called “raurackl”, or "stag-hare”.

It is, nowadays, alleged that the "horns" around the head of the "Lepus cornatus" do not relate to imagination, but to the growth of papilloma or fibroma tumors. The latter develop after infection of a cell with the Shope papilloma virus, Shope fibroma virus or the leporipoxvirus.


Rarely observed horned cottontail,1607,7-153-10370_12150_12220-27256--,00.html

Typical flat Shope fibroma tumor on the ear of a cottontail

Shope fibroma virus

The Shope fibroma virus was discovered in 1931 by R.E. Shope. It is found mainly on the US continent, among the cottontail (Sylvilagus floridanus) population. It was soon realized that the virus is transmissible between cottontails and rabbits. A viral infection results in the development of gross and microscopic lesions called fibromas.

The virus is spread through bites of blood seeking insect (e.g. fleas, mosquitoes). Once a cutaneous cell is infested, it will undergo a transformation leading to the formation of a tumor.

Shope showed by different experimental techniques that the fibroma virus is related to the myxoma virus. This property is nowadays exploited by using the live Shope fibroma virus to vaccinate against myxomatosis.

European hares are known to carry a virus (Leporipoxvirus) responsible for fibromatous diseases. Rabbits are susceptible to this virus. Clinical signs include the growth of numerous skin nodules (up to 2.5 cm in size) on the face, eyelids and around the ears. The transmission mode of this virus is unknown.

Clinical signs and diagnosis

The symptoms of fibromatosis were accurately described by Shope in 1931, with tumors appearing essentially on the feet and limbs, and to a lesser extent on the face, the nose, the eyelids, and the back. In newborn rabbits and cottontails, it causes general and severe diseases; in adult rabbits, the tumors often regress naturally. This virus is, furthermore, known to cause a variety of tumors in cattle.

This disease not frequently found in pet rabbits. The first signs of infection are the thickening of subcutaneous tissues. A soft well-outlined swelling will grow slowly, and can develop into a large tumor, with a diameter of 7 cm, and a thickness of 2 cm. The large size leads to disturbances in the daily activities, such as movement and search for food.

Akira Yamanouchi

Rabbit suffering from Shope cystic fibroma

Akira Yamanouchi

Detail of the Shope cystic fibroma. The round lesion has been caused by a punch biopsy

The tumor consists mainly of connective tissue. Subcutaneous tumors can lead to compression and damage of the underlying tissues such muscles and tendons. In rare cases, it was observed that the tumor invades the muscle tissue.

Generally, the tumors regress spontaneously after 10 to 14 months.

Fibroma tumors must be differentiated from myxoma and papilloma tumors. The appearance of the fibroma is usually flat and it is located in the subcutaneous tissues, while the papilloma tumors have the aspect of a wart, with a well-keratinized surface.

The diagnosis is based on clinical signs and can be confirmed with a biopsy sample. Histopathological examination of the skin lesions shows intracytoplasmic inclusion bodies.

A virus called "malignant rabbit fibroma virus" has been isolated in rabbits. It can lead to fibrosarcoma. As it presents antigenic similarities with the fibroma and myxoma viruses, it is thought to be a recombinant of both viruses, but this is not yet well defined. The presence of this virus is accompanied by immunodepression, malignant tumors and infections.

Jeff Hymel

Sparky, a rabbit suffering from recurrent fibrosarcoma


Microscopic view of the fibrosarcoma tumor, removed from Sparky



Microscopic view of spindle cells and blood vessels with erythrocytes

Fibrosarcoma often start in the soft tissue and spread to a bone by direct invasion or transport of a metastatic cell via the blood circulation. Secondary fibrosarcoma may develop in lungs, heart, kidneys and lymph nodes.


Surgical removal is not recommended, because recurrence is quick and usually more extensive. If excision becomes necessary, it must be wide.

For more details, see: “Fibrosarcoma in rabbits”.


For detailed information on fibrosarcoma in rabbits,

Skin Diseases of Rabbits

by E. van Praag, A. Maurer and T. Saarony,

408 pages, 2010.



Thanks are due to Jeff Hymel and to Akira Yamanouchi (Veterinary Exotic Information Network), for the permission to use their pictures.

Further Reading

Hu J, Cladel NM, Pickel MD, Christensen ND. Amino acid residues in the carboxy-terminal region of cottontail rabbit papillomavirus E6 influence spontaneous regression of cutaneous papillomas. J Virol. 2002; 76(23):11801-8.

Singh SB, Smith JW, Rawls WE, Tevethia SS. Demonstration of cytotoxic antibodies in rabbits bearing tumors induced by Shope fibroma virus. Infect Immun. 1972; 5(3):352-8. 

Smith JW, Tevethia SS, Levy BM, Rawls WE. Comparative studies on host responses to Shope fibroma virus in adult and newborn rabbits. J Natl Cancer Inst. 1973; 50(6):1529-39. 

Friedman-Kien AE, Fondak AA, Klein RJ. Phosphonoacetic acid treatment of shope fibroma and vaccinia virus skin infections in rabbits. J Invest Dermatol. 1976; 66(02):99-102.

Block W, Upton C, McFadden G. Tumorigenic poxviruses: genomic organization of malignant rabbit virus, a recombinant between Shope fibroma virus and myxoma virus. Virology. 1985; 140(1):113-24.

Strayer DS, Cabirac G, Sell S, Leibowitz JL. Malignant rabbit fibroma virus: observations on the culture and histopathologic characteristics of a new virus-induced rabbit tumor. J Natl Cancer Inst. 1983; 71(1):91-104.

Strayer DS, Sell S. Immunohistology of malignant rabbit fibroma virus--a comparative study with rabbit myxoma virus. J Natl Cancer Inst. 1983; 71(1):105-16.

Strayer DS, Skaletsky E, Cabirac GF, Sharp PA, Corbeil LB, Sell S, Leibowitz JL. Malignant rabbit fibroma virus causes secondary immunosuppression in rabbits. J Immunol. 1983; 130(1):399-404.

Strayer DS, Skaletsky E, Leibowitz JL, Dombrowski J. Growth of malignant rabbit fibroma virus in lymphoid cells. Virology. 1987; 58(1):147-57.