Cancer eye (ocular squamous cell carcinoma) is the most common form of ocular neoplasia in sheep and cattle. The lesions typically occur on the eye (conjunctiva, nictitans membrane) and eyelids. The cause is likely multifactorial, but predisposing factors may include: age (more common in older animals); chronic UV exposure; breed (animals with nonpigmented skin and conjunctiva); and genetic factors (high incidence seen in certain lines or families of animals). Other factors, including viral infection, nutritional factors, have been implicated, but to our knowledge no viral agent has been consistently demonstrated.
This report describes a case in which about 1% of a mob of 1000 merino ewes developed ocular squamous cell carcinoma. Only 3-4 year old ewes were affected.
The owner of a property running around 1000 ewes in the Mallee district of South Australia and has been farming the property for 6 years noticed a few ewes (no rams) with eye lesions. Last year there were about 10 affected but since January 2016 he has noticed 8-9 ewes, some of which may have been early cases in October last year.
No neighbours have reported seeing any of the cases in their sheep, and they use rams from the same source stud. Sheep are normally vaccinated with Glanvac 3 in 1 and are drenched strategically on the basis of faecal egg counts.
On 11th Feb 2016 the owner reported that he was shearing, and had a number of affected sheep available, and was worried that the incidence of the condition was rising in his flock.
Lesions affected only one eye per sheep, with 5/7 in the left eye. The owner commented that lesions take about 2-3 months to develop to a noticeable stage, and do not appear to affect the sheep’s condition. They seldom get fly struck.
Only 3-4 year old sheep are affected, and all age groups are run together on the property. Only ewes were affected.
The lesions varied from 1cm to 10cm size and were invasive of soft tissue only on the ventral eyelid and appeared to be extending down the lacrimal duct. The lesions did not appear painful, all sheep were in good condition and comparable to others. There was minimal discharge from affected eyes.
On PM examination the lesions had the appearance of a solid tumour affecting soft tissue, and appeared to mainly affect lower eyelids and extending toward the nose following the lacrimal duct. There was no bony or eye involvement except for causing a keratitis from abrasion, this may have caused the small amount of discharge seen.
Two more extensive PMs were conducted (sheep 1 & 2) and 4 other sheep only the lesions were sampled.
The various tissues submitted from the eye lesions of these sheep, (haired skin from the eyelids or periorbital) and nictitans membrane, revealed extensive malignant neoplastic proliferation of keratinising squamous epithelium, i.e. ocular squamous cell carcinoma, associated with secondary ulceration, tumour necrosis, inflammation, haemorrhage and bacterial infection.
In cattle it is more common for only one eye to be affected with ocular squamous cell carcinomas, but occasionally the tumour may be bilateral, but not necessarily symmetrical. The disease typically inexorably progresses from the precursor stage to full-blown invasive cancer, which may then become secondarily ulcerated and infected. Bleeding, ocular discharge and self-trauma are common. Secondary myiasis may occur. Eventually, the tumour can invade the orbit and facial bones, with spread to regional draining lymph nodes and occasionally metastasis to distant sites, e.g. lungs and liver. Affected animals may have prolonged survival despite severe and extensive oculo-facial lesions; however, such animals often lose weight and become emaciated.
Despite the diagnosis of SCC these lesions did not have the characteristic appearance of bovine SCC where invasion of periorbital bony structures is common, and affected sheep do not appear to suffer pain from these lesions.
Despite being a rare condition in sheep the incidence on this property appears to be high. While it is possible that these ewes have a genetic predisposition to ocular squamous cell carcinomas, neighbouring flocks also use rams from the same stud, and have not seen these tumours and no ram ever been reported as affected.
On the basis that a genetic predisposition was possible it was recommended that the owner cull any affected ewes at the first sign of these lesions, and not keep progeny from these ewes for breeding purposes. Treatment of these cancers is unlikely to be successful or economic in affected sheep.
Although infectious tumours including enzootic nasal tumours in sheep caused by retrovirus and nasal adenocarcinomas have been reported overseas, it is assumed that these are exotic conditions in Australia (Windsor 2016).
This case was first reported to a livestock agent who advised veterinary involvement and demonstrates the benefit of a close working relationship with livestock agents. In SA livestock agents are often approached by producers as the first point of contact regarding disease or production issues on farm. Copies of this report were sent to the owner, reporting livestock agent and the source ram stud, for their information.