Abstract

Subcutaneously implanted microchips (Trovan, Electronic Identification Devices, Santa Barbara, CA, USA; AVID, Norco, CA, USA) are used as a convenient method to permanently and individually identify a variety of domestic, laboratory and exotic animal species.^4,6 Microchip implants are considered to be safe with implants eliciting only a mild tissue reaction in prospective studies.^1,4,5 Recently, soft tissue sarcomas associated with microchip implants were described in 36 of 4,279 (0.8%) laboratory mice used in a lifetime carcinogenesis study.⁸ This report describes the occurrence of soft tissue sarcomas associated with microchip implants in a degu (Octodon degus) and a feathertail gilder (Acrobates pygmaeus) housed at separate zoological institutions.

Case Reports

A 4-year-old female degu from the National Zoological Park had an identification microchip implanted in January of 1997. In September of 1997, the animal was presented for evaluation of a “hump-backed” appearance and an alopecic focus on the dorsal midline. On physical examination, a firm, irregularly shaped, 2.5-cm diameter mass was noted in the subcutis between the scapulae, just proximal to the alopecic focus and underlying the palpable microchip. A fine-needle aspirate from the mass demonstrated cells suggestive of a mesenchymal neoplasm, and the mass with the peripherally attached microchip was surgically excised. Histologic examination of the mass showed an invasive neoplasm composed of interlacing streams and bundles of pleomorphic spindle cells within a moderate collagenous stroma. The histologic diagnosis was fibrosarcoma. Two months postoperatively, the degu died due to sepsis caused by Pseudomonas aeruginosa. At necropsy, there was no gross or histologic evidence of the previously excised neoplasm.

The second case occurred in a 6-year-old female feathertail glider from the San Diego Zoo. An identification microchip was implanted in September of 1993. In July of 1998, the glider was presented for evaluation of a scabbed wound over the right dorsal midline. Physical examination revealed an underlying subcutaneous mass which was surgically excised. The mass measured 1.7x0.6x0.6 cm and surrounded the previously implanted microchip. Histologic examination showed osseous metaplasia and necrosis surrounding the microchip, which, in turn, was surrounded by a spindle cell neoplasm in which neoplastic cells appeared to produce a fibrillar to homogenous eosinophilic material interpreted as osteoid. The histologic diagnosis was of extraskeletal osteosarcoma. The glider progressively declined in condition after surgery and was euthanatized 5 days postoperatively. There was no evidence of distant metastasis at necropsy.

Discussion

Soft tissue sarcomas have been associated with foreign bodies composed of a variety of materials including metals, glass and plastics.² Foreign body sarcomas are likely best known to clinicians as rare complications of surgical implants.⁷ Recently, a liposarcoma occurring on the forelimb of a dog was associated with a traumatically acquired glass foreign body.³ Experimental studies in rats have shown that important factors associated with development of foreign body sarcomas include size and texture of the implanted material.² Larger implants and those with a smooth surface (perhaps like implanted microchips) are considered to be more tumorigenic. In general, foreign body sarcomas are thought to arise by malignant transformation of mesenchymal cells responding to the presence of the foreign body rather than by a direct effect of the foreign body on surrounding tissues.²

Subcutaneously implanted identification microchips are frequently used in small mammals at both zoological institutions involved in this report, and, with the exception of the described neoplasms, complications have only rarely been observed. The direct association between the described soft tissue sarcomas and the implanted microchips is compelling, and the possibility that identification microchips may rarely contribute to the development of soft tissue sarcomas in small mammals should be considered.

Literature Cited

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