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Diagnostic Challenge
Diagnostic Challenge
Figure 1. Lateral View of a 15-year-old, intact female French lop rabbit with an 8 ⫻ 6-cm mass in the right ventrolateral cervical region.
Figure 2. Same as Figure 1, cranial view.
History A 15-year-old, intact female French lop rabbit (Oryctolagus cuniculi) was presented to the Louisiana State University School of Veterinary Medicine for evaluation of a large mass in the right ventrolateral cervical region. The owner had first noticed the abnormality two days prior to presentation, when he found the rabbit in lateral recumbency with a decreased appetite and fecal output. On physical examination, the rabbit was depressed, laterally recumbent, and emaciated. An approximately 8 ⫻ 6-cm, firm, subcutaneous mass was visualized and palpated in the ventrolateral aspect of the right cervical region (Figs 1 and 2). A complete diagnostic workup consisting of a complete blood count, plasma biochemical analysis, radiographs, and biopsy was recommended to the owner, but the owner declined and opted for euthanasia and necropsy. Please evaluate Figs 1 and 2 and develop a diagnostic plan based on an owner’s permission and a primary differential diagnosis.
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Seminars in Avian and Exotic Pet Medicine, Vol 14, No 2 (April), 2005: pp 156 –158
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Diagnostic Challenge
Figure 3. Cut view during necropsy of the myxosarcoma.
Diagnosis Per the owner’s request, no presurgical diagnostic tests were performed. The rabbit was induced with isoflurane anesthesia, euthanized with sodium pentobarbital (Beuthanasia-D Special, Shering Plough Animal Health, Kenilworth, NJ USA), and the body submitted to the Louisiana Veterinary Medical Diagnostic Laboratory, Louisiana State University for necropsy. Differential diagnoses for the mass in this rabbit included abscess, foreign body reaction, and neoplasia. Gross necropsy revealed a 7.5 ⫻ 8 ⫻ 6-cm white, mucinous mass in the cervical region (Fig 3); multifocal white nodules in the lungs, ranging from pinpoint to 1-cm in diameter; hemorrhagic froth in the trachea. Histologic examination of a sample taken from the mass was consistent with myxosarcoma. Microscopically, the cervical mass was poorly demarcated, infiltrative, unencapsulated, and comprised of thin fibrous stroma with neoplastic mesenchymal cells arranged in interlacing bundles. The cells were stellate to spindle and surrounded by a homogenous, amorphous, basophilic, mucinous material (Fig 4a). Mitotic figures were rare. Necrosis
and fibrosis were prominent throughout the mass, and often infiltrated with granulomatous inflammation characterized by numerous foamy macrophages. Results of microscopic examination of the multifocal nodules in the lungs were similar to those of the mass described above (Fig 4b).
Comments Myxomatosis, caused by myxoma virus of the Leporipoxvirus genus, is a relatively minor disease in the tapeti (Sylvilagus brasiliensis) and brush rabbit (Sylvilagus bachmani), the virus’ natural hosts. In the European rabbit, however, myxomatosis often presents as acute fulminant disease.1 The virus is spread by arthropod vectors (eg, fleas and mosquitoes), direct spread by contact and inhalation, or from housing contaminated by fluid from infected rabbits.1 Clinical signs begin with development of a skin lesion, or myxoma, at the site of inoculation 4 to 5 days after exposure to the virus. After 9 to 10 days, secondary lesions develop on the nares, eyelids, lips, base of the ears, external genitalia, and anus.3 If the gonads are involved, sterility
may occur. Pneumonic signs associated with infection develop if exposure to the virus is via the aerosol pathway. Fatalities are often due to generalized exhaustion, secondary bacterial infection, or predation in wild rabbits.2 The large cervical myxosarcoma was presumably at the site of inoculation in this rabbit. The rabbit was kept in a cage located next to a door to the exterior of the house. The most likely scenario of inoculation was through the bite of an infected mosquito. The finding of a myxosarcoma, rather than a myxoma, appears to be somewhat unusual, as a literature search yielded no descriptions of such an occurrence due to myxomatosis. Myxomas and myxosarcomas can occur in all domestic species, though the incidence is rare. Both disease processes arise from fibroblast origins. Although rare, when metastasis does occur, the primary target organ is the lung.4 Local tissue involvement can be extensive, as evidenced by this case. Treatment for myxomatosis is nonspecific and involves supportive care, antibiotics, nonsteroidal analgesics, and a high environmental temperature. Increased ambient temperature appears to be correlated with recovery rate. Opioid analgesics appear to be ineffective.2 Exposure and subsequent clinical disease caused by the myxoma virus are prevented by controlling vectors, providing good husbandry, and vaccination. A related virus of the Leporipoxvirus genus, Shope fibroma virus, is relatively benign in it effects and has been shown to induce immunity in O. cuniculi. Vaccination programs involving shope fibroma virus and live attenuated myxoma virus have diminished
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Diagnostic Challenge
ment would have likely been of minimal value. This case was submitted by Trevor T. Zachariah, DVM, Orlando Diaz-Figueroa, DVM, and Michael Walden, DVM, Department of Veterinary Clinical Sciences, Louisiana State University School of Veterinary Medicine, Skip Bertman Drive, Baton Rouge, LA 70803 USA. © 2005 Elsevier Inc. All rights reserved. 1055-937X/05/1402-$30.00 doi:10.1053/j.saep.2005.04.013
References 1.
2.
3.
4. Figure 4. a) Photomicrograph of the cervical mass of the rabbit described in Figure 1. This section demonstrates the stellate and spindle mesencymal cells and surrounding mucinous material characteristic of myxosarcoma. b) Photomicrograph of a lung metastasis from the same rabbit, demonstrating the same features as the mass in Figure 4a.
clinical signs related to the disease and can reduce viral shedding of myxoma virus.5 In this case, the prognosis was guarded due to the malignant na-
ture of the neoplasm, its metastasis to the lung tissues, and the age of the patient. Due to the disseminated nature of the disease process affecting this rabbit, treat-
5.
Harcourt-Brown F: Textbook of Rabbit Medicine. Oxford, England, Alden Press, 2002, pp 377-380 DiGiacomo RF, Mare CJ: Viral Diseases, in Manning PJ, Ringler DH, Newcomer CE (eds): The Biology of the Laboratory Rabbit, 2nd ed. San Diego, CA, Academic Press, Inc., 1994, pp 172-178 Harkness JE, Wagner JE: The Biology and Medicine of Rabbits and Rodents, 4th ed. Philadelphia, PA, Williams & Wilkins, 1995, pp 249-251 Pulley LT, Stannard AA: Tumors of the Skin and Soft Tissues, in Moulton JE (ed): Tumors in Domestic Animals, 3rd ed. Berkeley, CA, University of California Press, 1990, pp 33-34 Marlier D, Mainil J, BoucrautBaralon C, et al: The efficacy of two vaccination schemes against experimental infection with a virulent amyxomatous or a virulent nodular myxoma virus strain. J Comp Path 122:115-122, 2000
Figure 1. Lateral View of a 15-year-old, intact female French lop rabbit with an 8 ⫻ 6-cm mass in the right ventrolateral cervical region.
Figure 2. Same as Figure 1, cranial view.
History A 15-year-old, intact female French lop rabbit (Oryctolagus cuniculi) was presented to the Louisiana State University School of Veterinary Medicine for evaluation of a large mass in the right ventrolateral cervical region. The owner had first noticed the abnormality two days prior to presentation, when he found the rabbit in lateral recumbency with a decreased appetite and fecal output. On physical examination, the rabbit was depressed, laterally recumbent, and emaciated. An approximately 8 ⫻ 6-cm, firm, subcutaneous mass was visualized and palpated in the ventrolateral aspect of the right cervical region (Figs 1 and 2). A complete diagnostic workup consisting of a complete blood count, plasma biochemical analysis, radiographs, and biopsy was recommended to the owner, but the owner declined and opted for euthanasia and necropsy. Please evaluate Figs 1 and 2 and develop a diagnostic plan based on an owner’s permission and a primary differential diagnosis.
156
Seminars in Avian and Exotic Pet Medicine, Vol 14, No 2 (April), 2005: pp 156 –158
157
Diagnostic Challenge
Figure 3. Cut view during necropsy of the myxosarcoma.
Diagnosis Per the owner’s request, no presurgical diagnostic tests were performed. The rabbit was induced with isoflurane anesthesia, euthanized with sodium pentobarbital (Beuthanasia-D Special, Shering Plough Animal Health, Kenilworth, NJ USA), and the body submitted to the Louisiana Veterinary Medical Diagnostic Laboratory, Louisiana State University for necropsy. Differential diagnoses for the mass in this rabbit included abscess, foreign body reaction, and neoplasia. Gross necropsy revealed a 7.5 ⫻ 8 ⫻ 6-cm white, mucinous mass in the cervical region (Fig 3); multifocal white nodules in the lungs, ranging from pinpoint to 1-cm in diameter; hemorrhagic froth in the trachea. Histologic examination of a sample taken from the mass was consistent with myxosarcoma. Microscopically, the cervical mass was poorly demarcated, infiltrative, unencapsulated, and comprised of thin fibrous stroma with neoplastic mesenchymal cells arranged in interlacing bundles. The cells were stellate to spindle and surrounded by a homogenous, amorphous, basophilic, mucinous material (Fig 4a). Mitotic figures were rare. Necrosis
and fibrosis were prominent throughout the mass, and often infiltrated with granulomatous inflammation characterized by numerous foamy macrophages. Results of microscopic examination of the multifocal nodules in the lungs were similar to those of the mass described above (Fig 4b).
Comments Myxomatosis, caused by myxoma virus of the Leporipoxvirus genus, is a relatively minor disease in the tapeti (Sylvilagus brasiliensis) and brush rabbit (Sylvilagus bachmani), the virus’ natural hosts. In the European rabbit, however, myxomatosis often presents as acute fulminant disease.1 The virus is spread by arthropod vectors (eg, fleas and mosquitoes), direct spread by contact and inhalation, or from housing contaminated by fluid from infected rabbits.1 Clinical signs begin with development of a skin lesion, or myxoma, at the site of inoculation 4 to 5 days after exposure to the virus. After 9 to 10 days, secondary lesions develop on the nares, eyelids, lips, base of the ears, external genitalia, and anus.3 If the gonads are involved, sterility
may occur. Pneumonic signs associated with infection develop if exposure to the virus is via the aerosol pathway. Fatalities are often due to generalized exhaustion, secondary bacterial infection, or predation in wild rabbits.2 The large cervical myxosarcoma was presumably at the site of inoculation in this rabbit. The rabbit was kept in a cage located next to a door to the exterior of the house. The most likely scenario of inoculation was through the bite of an infected mosquito. The finding of a myxosarcoma, rather than a myxoma, appears to be somewhat unusual, as a literature search yielded no descriptions of such an occurrence due to myxomatosis. Myxomas and myxosarcomas can occur in all domestic species, though the incidence is rare. Both disease processes arise from fibroblast origins. Although rare, when metastasis does occur, the primary target organ is the lung.4 Local tissue involvement can be extensive, as evidenced by this case. Treatment for myxomatosis is nonspecific and involves supportive care, antibiotics, nonsteroidal analgesics, and a high environmental temperature. Increased ambient temperature appears to be correlated with recovery rate. Opioid analgesics appear to be ineffective.2 Exposure and subsequent clinical disease caused by the myxoma virus are prevented by controlling vectors, providing good husbandry, and vaccination. A related virus of the Leporipoxvirus genus, Shope fibroma virus, is relatively benign in it effects and has been shown to induce immunity in O. cuniculi. Vaccination programs involving shope fibroma virus and live attenuated myxoma virus have diminished
158
Diagnostic Challenge
ment would have likely been of minimal value. This case was submitted by Trevor T. Zachariah, DVM, Orlando Diaz-Figueroa, DVM, and Michael Walden, DVM, Department of Veterinary Clinical Sciences, Louisiana State University School of Veterinary Medicine, Skip Bertman Drive, Baton Rouge, LA 70803 USA. © 2005 Elsevier Inc. All rights reserved. 1055-937X/05/1402-$30.00 doi:10.1053/j.saep.2005.04.013
References 1.
2.
3.
4. Figure 4. a) Photomicrograph of the cervical mass of the rabbit described in Figure 1. This section demonstrates the stellate and spindle mesencymal cells and surrounding mucinous material characteristic of myxosarcoma. b) Photomicrograph of a lung metastasis from the same rabbit, demonstrating the same features as the mass in Figure 4a.
clinical signs related to the disease and can reduce viral shedding of myxoma virus.5 In this case, the prognosis was guarded due to the malignant na-
ture of the neoplasm, its metastasis to the lung tissues, and the age of the patient. Due to the disseminated nature of the disease process affecting this rabbit, treat-
5.
Harcourt-Brown F: Textbook of Rabbit Medicine. Oxford, England, Alden Press, 2002, pp 377-380 DiGiacomo RF, Mare CJ: Viral Diseases, in Manning PJ, Ringler DH, Newcomer CE (eds): The Biology of the Laboratory Rabbit, 2nd ed. San Diego, CA, Academic Press, Inc., 1994, pp 172-178 Harkness JE, Wagner JE: The Biology and Medicine of Rabbits and Rodents, 4th ed. Philadelphia, PA, Williams & Wilkins, 1995, pp 249-251 Pulley LT, Stannard AA: Tumors of the Skin and Soft Tissues, in Moulton JE (ed): Tumors in Domestic Animals, 3rd ed. Berkeley, CA, University of California Press, 1990, pp 33-34 Marlier D, Mainil J, BoucrautBaralon C, et al: The efficacy of two vaccination schemes against experimental infection with a virulent amyxomatous or a virulent nodular myxoma virus strain. J Comp Path 122:115-122, 2000