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This article was published in 1958
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Cancer in Sheep

P. D. CARTER, B.V.Sc., Veterinary Inspector, Coonabarabran

During the late summer and autumn of 1958, instances of unusually high Cancer incidence among sheep was observed on the north-west black soil plains in the Burren Junction area. The usual incidence of this disease, in this locality, could be put approximately at 1% among Merino sheep of mixed ages in an average year.

One flock concerned consisted of 1,600 Merino ewes, two years of age and upwards; with no sheep in the mob over six years. These lived through a severe drought period between July, 1957, and January, 1958; during which time they were hand-fed continuously. Summer rains in January, 1958, brought a fush of feed which included mainly grasses of a wide variety of types and various weedy species. There was little growth of clovers, medics or other soft herbage species.

The owner drafted out 98 from this mob which showed obvious signs of ear cancer; at least 70 of these having developed between January and May, 1958.

Cancerous growths affected mainly the ears and were the usual horny epitheliomatous types commonly found in sheep. They did not appear to develop specifically from ear marking scars, but were located mostly in the centre parts of the top side of the ears; which normally in the Merino receive the full light of the sun. In numerous sheep it was apparent that lesions were developing from several foci simultaneously; and on both ears. A few cases, four or five, showed cancerous growths on other parts, such as near the eyes or nose. The incidence appeared higher in older sheep, though the flock as a whole could not be regarded as old. Particular note was taken of the hair covering of the ears. This was well developed in the flock, which appeared to be average or better than average in this regard.

No particular photosensitisation was noticed by the owner during the summer of '57-58, though careful examination showed that some scalings had taken place; which strongly suggested photosensitisation with skin damage had occurred some time previously. Light and heat from the sun was apparently only one aspect of the high cancer incidence, as hot summers in previous years had not produced similar outbreaks. The incidence of cancer in other flocks in the area appeared to be much higher than usual generally, and on other properties up to 8% of cases occurred. The condition was not recognised by owners until autumn, but the initial damage almost certainly was done during the summer. Some hot weather was experienced in January, February and March, 1958, and the exposure to the summer sun of newly-healed skin following photosensitisation was the possible explanation of this high incidence, Photosensitisation also occurred in many sheep in the spring of 1957; where a brief growth of burr medic appeared after some light rains.

It is difficult to incriminate any particular plant species as a cause of photosensitation, as the variety of growth was so great. A species of panic, P. coloratum, or Coolah grass, is prevalent in the area; while Tribulus terrestris also grew vigorously after the January rains.

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