Ocular Neoplasia in Cattle
The most frequent ophthalmic neoplasms in cattle are the squamous cell carcinoma complex and the orbital infiltration associated with lymphosarcoma (see Bovine Leukosis). The latter, with extensive invasion of the orbital structures, results in progressive bilateral exophthalmia, reduced ocular mobility, exposure keratitis, and corneal ulcerations that can lead to perforation.
Ocular squamous cell carcinoma (cancer eye) is the most common neoplasm of cattle. It results in significant economic loss due to condemnation at slaughter and a shortened productive life. It occurs more frequently in the Bos taurus than the Bos indicus breeds, and it is seen most often in Herefords, less often in Simmentals and Holstein-Friesians, and rarely in other breeds. The peak age of incidence is 8 yr; actual incidence varies from 0.8% to 5.0% among herds. The cause is multifactorial, with heritability, sunlight, nutrition, eyelid pigmentation, and perhaps viral involvement playing roles. The medial and lateral limbal regions (corneoscleral junction) are affected most frequently, but the eyelids, conjunctivae, and nictitating membrane may be affected. Bilateral involvement varies but can be as high as 35%. Eyelid and conjunctival pigmentation are highly heritable and can reduce the frequency of lid squamous cell carcinomas, but they have limited effect on the development of tumors of the conjunctiva and nictitating membrane. The cancerous or precancerous lesions are bilateral or multiple in the same eye in ~28% of cases. Ultraviolet radiation and a high plane of nutrition are contributing factors. The viruses of infectious bovine rhinotracheitis and papilloma have been isolated from the neoplasms, but their significance is unknown.
The lesions usually begin as benign, smooth, white plaques on the conjunctival surfaces; they may progress to a papilloma and then to a squamous cell carcinoma or go directly to the malignant stage. Lid lesions usually begin as either an ulcerative or a hyperkeratotic lesion (cutaneous horn). While in this benign stage, ~30% may spontaneously regress. The tumor may become quite large without invading the globe, but invasion into the eye and orbit and metastasis to parotid and submandibular lymph nodes occur in late stages of the disease. Diagnosis usually is made by the typical clinical appearance but can be confirmed rapidly by cytologic examination of impression smears. The intraocular tumor invasion must be differentiated from severely damaged and disorganized eyes after trauma or advanced infectious keratoconjunctivitis (see Infectious Keratoconjunctivitis).
Squamous cell carcinomas may respond to excision, cryotherapy, hyperthermia, radiation therapy, local chemotherapy using 5-fluorouracil, immunotherapy, or often a combination of these therapies. Surgical excision is indicated for small lesions or for debulking the larger lesions before cryotherapy or hyperthermia. Superficial keratectomy can be used to excise the limbal plaques, papillomas, and squamous cell carcinomas. After superficial keratectomy and tumor removal, cryotherapy, hyperthermia, or a permanent bulbar conjunctival graft have yielded excellent short-term results, but recurrence at the same or a different site is ~25%.
For advanced lesions confined to the globe, enucleation is recommended. When adjacent tissues are affected, removal of the globe and all orbital contents (exenteration) should be performed. Immunotherapy is still experimental, and the resulting tumor regression may be temporary. Radiation therapy is not practical in the field but may be an option for valuable animals.
Owners of problem herds should be advised of the heritability factor, with affected animals and their offspring culled to decrease the incidence of tumors. Active breeding bulls with ocular squamous cell carcinoma should be culled.