The Sunday News
Farming Issues
Introduction
CANCER Eye (Ocular squamouscell carcinoma) is the most common cancer in cattle. It is a skin cancer occurring on the eye and the eyelids. Estimates of its prevalence vary from one percent to 20 percent in some herds. Some of the predisposing factors are heritable and in certain line of families, nearly all families of cattle may be affected in their lifetime. Older cattle are the most vulnerable in all the herds, and indications are that 22 percent of total condemnation in mature cattle 7,3 percent of condemnation in young cattle (less that eight teeth), or younger than approximately four years are due to cancer eye. Overall cancer eye represents 47 percent of all cancer that result in condemnation.
Cancer lesions or development
Cancer eye growth can occur on the eyeball, eyelid or on the third eyelid. All cancers develop from precursor lesion, but less than half of this develop into cancer. The precursor lesion can be:
— a plaque (a slightly elevated opaque area on the eyeball).
— a papiloma (a wart-like growth protruding from the eyeball).
— a keratoma (a horn-like projection attached to the eyelid).
— a small ulcer on the eyelid.
As the disease progresses from the precursor stage to the cancer stage, the tumours become ulcerated. Bleeding and weeping are common. Bacteria invade the lesion which usually develops especially into a festering foul smell growth. The cancerous tissue also grows inwards, invading the deeper tissue behind the eye. It can progress to the lymph nodes of the head and to body organs such as the lungs and the liver.
Cancers that begins in the third eyelid or outer eyelid usually invade the deeper tissue quickly than do those that start on the eyeball. Untreated cattle may live for two to five years after the first appearance of a lesion although such cattle may become weak and emaciated within six months, indicating the involvement of internal organs. It is more common for only one eye to be affected, but occasionally the condition may occur in both eyes.
Factors affecting incidence of cancer eye
Breed
The disease is very synonymous with cattle that have areas of un-pigmented skin or conjunctiva. Breeds such as Hereford, Polled Hereford and White faced Friesian are most susceptible, with Hereford crosses being less susceptible than the pure breds. Cancer eye is rare in breeds with fully pigmented skin such as Angus and Santa Gertrudis.
Strain and sire
Susceptibility to the disease varies within strains and sires. The heritability of cancer eye has been moderate at 0.4.
Pigmentation
Cattle with unpigmented skin around the eye are more susceptible to cancer eye. Cancers usually start on unpigmented skin, but can then spread to pigmented areas. Eyelids pigment is present at birth and is easily accessed. Eyeballs pigment usually takes five years to fully develop. Pigmentation of the eyeballs are highly heritable and appear to be generally related.
Conformation
It is likely that protruding eyeballs are more susceptible than “hooded” eyes, which are more protected from the sun. One such example of indigenous breeds with protruding eyes is the Tuli breed which is susceptible to opthalemia.
Age
Older cattle are more prone to cancer eye. The disease is uncommon in cows under five years. Males and females are equally susceptible but the cancer is seen less frequently in male because steers are sent for slaughter at an early age, and bulls tend to be culled at an earlier age than cows.
Nutrition
It has been reported that high level of nutrition and growth rate increases the risk of cancer eye developing. Cancer eye is more common during and after drought due to longer hours spent grazing in the sun, more sunlight, dust and flies and because of stress lowers immunity.
Pregnancy
Cancer eye often seems to develop more rapidly in the latter half of the pregnancy. This may be due to increased stress or the immune suppression associated with pregnancy. It may simply be that farmers become more aware of cancer eye when faced with the decision of whether its humane to keep an affected cow long enough to calve and raise the calf.
Virus
Papiloma virus has sometimes been isolated from animals with cancer eye, but advanced virological techniques have so far failed to reveal any definite association between papiloma virus and the development of these tumours.
Ultraviolet solar radiation
Increased ultraviolet is thought to trigger the predisposed cattle to cancer eye. Therefore cattle living at low latitudes and high altitudes may be at greater risk because latitude and the daily hours of sunlight contribute to the amount of ultraviolet radiation.
Prevention
Cancer eye can be controlled by selective breeding. Selection for pigmented eyelids is most effective and can result in rapid improvement in the herd. This selection is recommended for all Hereford herds. Eyeball pigmentation develops slowly, but apparently is genetically linked to eyelid pigmentation so selection for one, in effect select for both. Treated cows and their progeny should be culled as soon as practicable to select against this form of cancer.
Breeds not committed to producing pure Hereford should consider cross breeding if cancer eye is a problem. There are strain and sire differences. Breeders who identify families that are more susceptible, or sires whose progeny are more susceptible should move away from lines in their breeding programmes.
Treatment
Surgical treatment of cancer eye is possible particularly in the early stages, and is warranted if for example a cow has a young calf. Small lesion can be removed by electrocautery (burning) or cry-surgery (freezing). A veterinarian may also surgically remove lesion from the eyelid or third eye, scrap a lesion from the eyeball, or remove the whole eye. In some cases, removal of very early precursor lesions may result in permanent cure.
However, more than 60 percent of cases recur and it is not good to continue to breed from treated cattle or from their progeny. Since it is undesirable to keep animals that are predisposed to cancer eye, vaccination is not recommended as a management tool, as it will make selection against the disease more difficult. Never breed from affected or treated animals, control by culling animals with early signs of cancer eye.
Mkumbuzi Siziba is a specialist in livestock rearing. He can be contacted on [email protected]>
