Within the past few years there has appeared in Western Australia, and more recently in other States to a smaller degree, a hitherto undescribed disease of sheep characterised by breeding failure, dystokia, and prolapse of the uterus of the ewe and curious metaplastic changes in wethers, which also may show milk production. These conditions were studied by Bennetts and his co-workers in Western Australia and they came to the conclusion that the cause was to be found in the pasturing of the animals on subterranean clover. Dwalganup strain, an early strain widely used in that State. It appears that attention first was drawn to the condition by the frequent occurrence of prolapse of the uterus, and that the other manifestations of the disease came to light as the investigations proceeded. However, there is considerable variation in the incidence of infertility, dystokia and uterine prolapse on the different properties and Bennetts states that the development of the problem on a property may be heralded by the dramatic appearance of dystokia. Whatever the onset, the final result is that the infertility progressively becomes more serious until breeding is discontinued as unprofitable. Western Australian experience indicates that the age of the pasture does not appear to influence the appearance of the disease, since it has appeared suddenly on recently and long established pastures in the same season. As an indication of the serious nature of the problem the following figures are quoted.
|1943||21%||400 lambs and 140 ewes died|
In an article published in 1946 Bennetts and others stated that this breeding problem has become general during the past few years, and that it appears to be confined to pastures of Dwalganup clover. The figures quoted show what serious losses are caused by the condition, since many of the farmers in the affected districts have been forced to discontinue breeding.
The effects of the clover are described as follows:—Infertility. There are no characterisic features. There is no failure of oestrum. The ewes come more or less readily to service but fail to conceive. Histological examination shows that the endometrlum is cystic.
Dystokia. This is due to short-lived, feeble labour; the ewe showing no concern or discomfort until the advent of complications due to the death of the foetus. The labour is of very short duration, being only sufficient to rupture the foetal membranes and permit the escape of the fluids. Commonly there is no external evidence of the lamb at this stage, but delivery is carried out easily. Most often, however, the condition of the ewe is not noticed until the limbs of the lamb are noticed protruding from the vagina. At this stage humped attitude, occasional abdominal straining and udder distended with milk may attract attention. Ewes that have suffered from dystokia may not be noticed for weeks or months later when the flock to brought into the yards. There is usually then putrid vaginal discharge, unthriftiness and sometimes abdominal straining.
Uterine Prolapse. This is prolapse of the inverted uterus leading to more or less complete eversion. It is seen rarely in the earlier stages and generally is not observed until the prolapse is visible externally. With such cases the exposed mucous membrane at first is congested deeply, but later it becomes greyish and necrotic.
Mammary Development in Unbred Ewes. A high incidence of cases of udder development and milk secretion in unbred and virgin ewes frequently occurs in sheep pastured on subterranean clover. This has been noted in Spring and early Summer months, particularly in favourable seasons.
Lactation in Wethers. This has occurred on a number of properties. There is marked development of the teats with secretion into an enclosed space bounded by the teat wall and a capsular membrane embedded in the subcutaneous tissue at the base of the teat.
The actual cause is believed to be a hormone-like substance which is postulated to be present in the clover whilst active growth is proceeding, but which is found mainly in clover burrs when these are formed. This hormone or hormone precursor is stable and hay made from the green clover and dry burrs contain it in considerable quantities. Similar results to those found in the field have been produced experimentally by oestrogenic hormones.
The area involved in Western Australia is in the south-west Division where the light soils of low fertility responded well to treatment with superphosphate and where there is now an abundant pasture of subterranean clover. These pastures have been established from two to fourteen years.
The disease in South Australia has been investigated by Schinkel. He found that two-tooth ewes transferred from sub-clover pasture to pasture free of clover were highly infertile for two subsequent lambings. At the first lambing after removal the degree of dystokia was high but it was low at the second lambing.
Ewes transferred as maiden two-tooths, in their first lambing ten months later had 40 per cent. to 80 per cent. dystokia, although their conception rate was high. In a number of lower age groups the breeding capacity of some sheep already was affected after 12 to 14 months' grazing on Sub. clover.
The indications are that when the infertility becomes established it is permanent.
Control. There is some evidence from grazing experiments that balancing the clover pasture by grasses or oat crops gives some measure of success, and this is recommended at present by the Western Australian Dept. of Agriculture. It is obviously important that the sheep must not have a sole diet, of subterranean clover and that they must have access to other foods.
In Western Australia it has been found that if grass or green oats are available to sheep little clover is eaten because during the growing stage the Dwalganup strain is not very palatable.
The Disease in N.S.W. The first suspicion that subterranean clover might be causing breeding troubles in sheep was raised in the Tumbarumba area, although before this a farmer at Capertee had reported prolapse of the uterus in sheep grazing on sub-clover. The condition at Holbrook was complicated by the coincidental occurrence of Toxaemic Jaundice. In a few flocks at Holbrook abortions occurred. Examination of the material revealed no bacteria, and the fact that the abortions caused no appreciable ill health did not support a hypothesis that they were due to the known bacterial causes of ovine abortion. Abortions were reported only in sheep on sub-clover pastures. Infertility and dystokia were not reported, but in view of the circumstances and the work being done in Western Australia, guinea pigs were sent to the District and the Stock Inspector, Mr. McBarron, arranged for them to be fed a sole diet of sub-clover on one of the properties. After three weeks feeding they were returned to Glenfield and the uteri examined. Sixty per cent. of the guinea pigs were found to be affected with cystic endometrium. It seems, therefore, that the mid-season clover at Tumbarumba contains the hormone-like substance.
The definite diagnosis of the presence of infertility and other manifestations of the condition reported in Western Australia was made as a result of a visit to a property at Grenfell by the D.V.O. West. This property contains 1750 acres, of which 1100 are almost pure sub-clover, and 200 to 300 acres are about 40 per cent. sub-clover. Lambing percentage dropped from 80 to 40 per cent. over four or five years. No dystokia, prolapse or abortion has been noticed, but udder development and milk secretion in maiden ewes and wethers occurs. Also, metaplastic changes the genitalia of wethers has been seen; about 60 per cent. of the wethers suffer from balanitis.
Other localities where the disease is suspected to be present are:—
Cumnock. Stud sheep on sub-clover pastures have shown lowered lambing percentage during the past two or three years; whereas flock sheep on natural pastures have maintained a high percentage.
Wombat. Milk secretion has been reported in wethers.
Brocklesby. Sheep grazing on sub-clover have a lambing percentage of 50; sheep on natural pasture, 80.
Waugoola. Milk secretion has been seen in wethers. Uterine prolapse and abortions occur in ewes.
Woodstock. Lambing percentage of sheep on sub-clover has fallen over the past four years, but no drop has occurred in sheep on natural pasture.
Molong. Mammary development occurs in lambs and wethers.
Oberon. Lambs from B.L. Stud have fallen from 102 per cent. to 69 per cent, in the past five years. Seventeen per cent. of stud sheep require aid at lambing. Sheep on sub-clover pasture. Cross-bred sheep on mixed pasture have maintained their level of lamb production over the same period.
Rylstone. Balanitis and enlarged bulbo-urethral glands have been seen in wethers on well established sub-clover pasture.