Fibrosarcoma in rabbits
Esther van Praag, Ph.D.
Warning: this file contains pictures that may be distressing to some persons
The word “sarcoma” comes from the Greek and means “fleshy growth”. Sarcoma is nowadays used to describe relatively rare a group of malignant tumors that involve the connective tissue. Although sarcomas are well-recognized tumors, their characteristics lead to confusion. Indeed, some type of sarcoma may present a combination of features of various different types of sarcoma. This lead to the widely accepted conclusion that the neoplastic development of a primitive mesenchymal cell can lead into different direction, thus different types of sarcoma.
The tumors are found is all parts of the body: forelimbs, hind limbs, chest, and abdomen; as well as in all types of tissues: muscle tissue, nerves, tendons, scar tissue, bones and their lining.
Four principles apply for sarcoma tumors:
• Location: a superficially located tumor is less likely to be malignant than a deeper located tumor;
• Size: bigger tumors are more likely to become malignant than small tumors;
• Growth: rapid growth tumors are more likely to be malignant than slow growing ones.
Fibrosarcoma seems more frequent in rabbits as generally assumed. These mesenchymal tumors are aggressive, have generally a rapid growth rate and are invasive into surrounding tissues, e.g. muscle or periosteum (tissue lining bones). Indirect invasion of transport of a metastatic cell via the blood stream is possible and metastasis can form in the lungs, heart, kidneys and lymph nodes.
A virus called "malignant rabbit fibroma virus" has been isolated in rabbits and can lead to fibrosarcoma. 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.
In rare cases, the fibrosarcoma can present myxoid features and will develop into a myxosarcoma.
Diagnosis and histology
The nature of the mass and the presence of metastases must be determined. This can be done by X-ray, ultrasound, and a biopsy.
Fibrosarcoma are heterogenous. The presence of “fishbone-shaped” fusiform cells within a collagenic stroma is characteristic and characteristic for a neoplasm of mesenchymal origin. Numerous cells are undergoing mitosis. Necrotic or hemorrhagic foci are commonly observed.
The complete excision of the fibrosarcoma lesions is necessary. If a limb is affected, amputation is the option of choice. Chemical treatments have been administered to rabbits suffering from fibrosarcoma, in order to reduce the size of the tumor in rabbits. Rabbits should be closely monitored after each treatment, as the drugs can affect the bacterial flora of the intestine and cause severe diarrhea.
• 1 mg/kg adriamycin IV q. 3 weeks, through peripheral catheter,
• Cytoxan: standard cat dose 4 days/week, PO
• L-asparaginase (Elspar): standard cat dose,
• 2mg/kg prednisone daily.
• Prednisone and L-asparaginase (Elspar) alone.
A characteristic of fibrosarcoma tumors is their high rate of recurrence (according to species, up 70% within year of surgical removal).
For detailed information on fibrosarcoma in rabbits,
see: “Skin Diseases of Rabbits”, by E. van Praag, A. Maurer and T. Saarony,
408 pages, 2010.
Thanks are due to C. Harvey, DVM (USA), Susan L. (USA), Jeff Hymel (USA), and Juliet Brown (Australia) for providing the illustrative material for this text. Thanks are also due to Sparky and Atticus, for his patience during picture-sessions.
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