Solar radiation appears to be a predisposing factor for the development of SCC (Hargis et al 1978). In humans, conjunctival scrapings are useful for in-house diagnosis (Spinak & Friedman 1977). This may also be the case for animals.

Melanoma is the most common primary intraocular tumour in dogs and cats. It arises from iris or ciliary body (anterior uveal tract), and is rarely choroidal. About 80% of intraocular melanocytomas and 95% of intraocular melanomas were found to arise in the anterior uveal tract. In dogs, intraocular melanoma tends to be benign, with a low rate of metastasis (5%), although they can still be locally invasive (Giuliano et al 1999). Mitotic index is suggested to be the best criterion for histological identification of ocular melanomas with high metastatic potential. Cell type or pattern of growth within the globe was not predictive of biological behaviour (Wilcock & Peiffer 1986).

In a series of 224 cases, tumour extension, tumour size and mitotic index were not found to be reliable predictors of survival. Dogs with MM had only a slightly decreased survival time compared to control dogs and dogs with melanocytomas. At the time of enucleation, most tumours invaded the sclera, but did not show extrascleral extension (Giuliano et al 1999). A worse prognosis was given in a study of 16 dogs with uveal melanomas, where 3 had metastasis within 3 months of enucleation. Benign melanomas tend to be darker and more heavily pigmented than MM (Wilcock & Peiffer 1986). Melanoma of the iris in dogs often appears as a discrete, brown nodule on the surface; however, it can be diffuse and can look like an iris cyst or benign iris melanosis.

Choroidal melanomas are rare (only 4% of canine uveal melanomas). Some remain static for many years; others invade the retina, sclera, optic nerve and orbit. One reported case of a 7-year-old Golden Retriever with a malignant choroid melanoma treated with orbital exenteration had no recurrence or metastasis on thoracic radiographs, and was clinically healthy for 23 months after surgery (Miwa et al 2005). Another Golden Retriever (3 years old) with a choroidal melanoma, with retinal detachment and exophthalmos, was treated with eye enucleation. Histology was benign, but 21 months later gross systemic metastases (lung and liver) were diagnosed as histologically identical to the primary choroidal melanoma (Hyman et al 2002).

In cats, anterior uveal melanomas are the most common primary intraocular tumour (Figure 25.5). Feline iris melanoma tends to present as a diffuse change rather than a discrete mass. A biopsy or aspirate may be helpful to differentiate benign iris melanosis, chronic uveitis and malignant melanoma in cats; however, obtaining a diagnostic sample may be difficult.

Malignant uveal melanoma in cats is very infiltrative and has a high rate of metastasis (Patnaik & Mooney 1988), frequently to liver and lungs. Iridial hyperpigmentation takes months to progress, and metastasis is late (1–3 years after eventual enucleation). Cats with melanomas that are larger and more invasive into the iris, ciliary body and scleral venous plexus (and with a higher mitotic index) are more likely to suffer metastasis. Benign anterior iridial melanoma may undergo late malignant transformation in a small number of cats to form larger, diffuse, malignant iris melanoma. In this subset of cats, early enucleation is important to avoid premature death from metastasis. Even with malignant transformation, cats with tumours confined histologically to the iris had good survival times, whereas cats with extensive ocular involvement or secondary glaucoma had shorter survival times (Kalishman et al 1998).

In humans with intraocular melanoma, the tissues around the eye are completely frozen to prevent flow of fluid and blood to or from the tumour, prior to enucleation. This is because manipulation of the tumour at enucleation increases intraocular pressure, significantly decreasing longevity by increasing metastatic potential. People are also advised not to rub or squeeze their eyelids (Fraunfelder et al 1977).

Histiocytic sarcoma (HS) may occur as an intraocular mass, especially in Rottweilers and Retrievers. In 15 of 26 dogs with ocular HS there were no concurrent systemic clinical signs at presentation, despite known systemic behaviour. Ocular HS has a poor prognosis, and may be differentiated from melanoma (usually a good prognosis) with immunohistochemistry (Naranjo et al 2007).

Sarcoma is the second most common intraocular tumour in cats, locally aggressive and highly metastatic. Previous ocular trauma which has occurred on an average of 5 years prior to sarcoma development is thought to be the cause (Dubielzig et al 1990). However, not all cases reported have a known previous history of trauma. Ocular trauma that triggers phthisis bulbi results in non-visual eyes that are at risk for late sarcoma development and should be enucleated (Ziess et al 2003). Feline leukaemia virus/feline sarcoma virus (FeLV/FeSV) does not seem to play a role in feline ocular sarcoma development (Cullen et al 1998). Enucleation prior to invasion of the optic nerve or sclera improves the prognosis (Dubielzig et al 1990). If complete removal is not possible due to tumour infiltration (even with orbital exenteration), recurrence is then expected and death will be from locally invasive tumour growth (into optic nerve, brain, periorbital tissues) within a few months of enucleation. The role of radiotherapy has not been evaluated in these patients. Metastasis is also reported.

Other primary tumours in dogs and cats are extremely rare. Blue-eyed dogs are at risk of developing spindle cell sarcoma in the anterior uvea.