Do horned rabbits really exist ? – Fibroma growths
Esther van Praag, Ph.D.
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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.
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.
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.
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,
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.
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