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This article was published in 1975
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Goitre in Lambs
John W. Plant, B.V.Sc., Special Veterinary Research Officer Veterinary Research Station, Glenfield.
Goitre, an enlargement of the thyroid gland, may result from either hypothyroidism or hyperthyroidism. In some cases, there may be histological changes in the thyroid gland indicative of goitre, not accompanied by any enlargement of the gland.
Extent of the Losses.
Most outbreaks in New South Wales have occurred on the Tablelands and Slopes and appear to be more frequent in the southern areas. In Tasmania, losses are commonly associated with river valleys.
In most flocks, the disease does not occur every year. Outbreaks are usually sporadic and on some properties, only odd cases occur whilst on other properties, up to 75% of the lambs born have been affected. Goitre occurs more frequently in lambs born to young ewes. Goitre is more common in wet years.
Breeds affected.
In New South Wales, goitre has been observed in lambs born to Border Leicester-merino cross ewes (sired by Dorset Horn and Poll Dorset rams) and in Polwarth, Corriedale, Zenith, Merino, Suffolk, Poll Dorset and Dorset Horn Lambs.
Symptoms of Goitre.
Many affected lambs are born dead or die within 3-4 days of birth. Some premature lambs may be seen often with a poor wool covering. Not all affected lambs die and there appears to be no subsequent effect on growth rate and reproduction.
Not all affected lambs will show clinical signs of goitre. In some lambs, severe changes may be observed histologically in an apparently normal thyroid. These lambs are often weak and lethargic and susceptible to cold.
Causes of Goitre.
An adequate supply of thyroid hormone is necessary for the normal development of the foetus and newborn and its subsequent survival. There are 2 main causes of goitre: —
1). A deficiency of dietary iodine essential for the synthesis of thyroxine.
2). The presence of goitrogens in the diet which interfere with the synthesis of thyroxine, even in the presence of adequate dietary iodine.
3). A genetic predisposition to goitre.
4). Excess amounts of iodine in the diet resulting in thyrotoxicosis.
The iodine requirement of the foetus increases rapidly from about 100 days of gestation until near parturition and in this period, a deficiency of iodine can exert an effect. There is also a possibility of competition between the dam and the foetus, especially in cold, wet weather, when the iodine requirement of the ewe will be increased in order to maintain body temperature.
1. Iodine deficiency
Animals obtain most of their iodine requirements whilst grazing. Water does not make a major contribution to the iodine intake of animals. Pregnant and lactating animals require 0.8 ppm iodine in the dry matter intake, and other animals require 0.12 ppm to provide all their daily iodine requirement.
a). Iodine in soils
The iodine content of the rocks from which most soils originated is low, but in many cases there has been an accumulation of iodine in the soil, so that the iodine content of the soil is higher than that of the original rocks.
Soil iodine is replenished by oceanic iodine. This does not occur in the interior of large land masses, nor when the prevailing winds are offshore. High rainfall can result in leaching, so that the iodine level is often low in soils in valleys and areas subject to flooding. The iodine level is higher in acid soils than in alkaline soils and it is higher in clay soils than in sandy soils in the same region.
In most soils there is a continual alteration in the iodine level with losses by leaching and by grazing and replenishment from oceanic sources via the atmosphere and from grazing animals via faeces. The iodine levels in soil are highest in the top 2 cm., and there is continual recycling through grazing animals. Some fertilisers are rich in iodine, eg. Chilean nitrate of soda.
Ingestion of soil is an important source of iodine to the grazing animal. In South Australia, the iodine level in the thyroid gland in drought situations has been twice that observed in sheep in the same localities in good seasons (Dawbarn and Farr, 1932). In a stocking rate trial in New Zealand (Healy et al. 1972), Losses from goitre were observed in the low stocking rate group. Pasture iodine levels were identical, but the soil content of the faeces was higher in the high stocking rate group and the iodine level of the faeces was correlated with the soil content. The topsoil contained more iodine than the herbage. They suggested that the goitre in the low stocking rate group was the result of a mild winter, with better pasture growth resulting in less ingestion of soil.
b). Iodine in Pastures
In New Zealand, the soil type and soil iodine level is not correlated with herbage iodine levels, but there are marked species and varietal variations seen in iodine levels. In the same soil, perennial ryegrass (Lolium perenne) had a higher iodine level than Italian ryegrass (Lolium multiflora) and both had a higher iodine level than short rotation ryegrass, a hybrid of the two (Butler et al. 1956). The iodine level in perennial ryegrass is a strongly inherited character (Butler and Glenday, 1962).
In a trial in New Zealand, the iodine level of perennial ryegrass seedlings ranged from 0.10 - 0.28 ppm dry matter (Alderman and Jones, 1967). Similar Variations were observed in white clover seedlings, ranging from 0.14 ppm (New Zealand Strain) to 0.4 ppm (Polish strain) dry matter.
In some cases, application of nitrogenous fertilisers has reduced the iodine level of pastures (Alderman and Jones, 1967), but in others there has been no effect (Horn et al. 1974).
c). Effect of season on soil and pasture
In Tasmania, a relationship has been shown between the incidence of goitre in sheep and the occurrence of high rainfall in the premating period and during pregnancy(Mason, 1974). This could be due to several causes: —
i). leaching of iodine from the soil by the high rainfall;
ii). washing of the pasture, reducing the soil intake;
iii). increasing pasture availability, thus reducing the need for the sheep to graze close to the ground and reducing the intake of soil.
iv). dilution of concentration of available iodine through the greater bulk of pasture produced.
The incidence of goitre on individual properties may vary from season to season, despite similar iodine levels in the pasture.
Goitrogens as a Predisposing Cause.
There are two common goitrogens: —
i). cyanogenetic glycosides - the effect of these can be reversed by an adequate dietary intake of iodine. There is some doubt on their role in the development of goitre.
ii). goitrins - these are found in the Cruciferae family and the effect is not commonly reversed by additional iodine.
3. Genetic Predisposition to Goitre.
A genetic predisposition to goitre has been reported in merino sheep (Mayo and Mulhearn, 1969). They believed that the goitre was due to an autosomal recessive gene with incomplete penetrance, and that the effect was influenced by phenotype and environment. Many outbreaks of goitre have been observed in Dorset Horn sheep and there is evidence that there is a genetic factor involved (Watson et al. 1962).
On some properties in New South Wales, goitre occurs regularly and these properties there may be a genetic predisposition to goitre.
4. Thyrotoxicosis
Excess intakes of iodine can lead to an enlargement of the thyroid gland. An increased incidence of thyrotoxicosis was observed in Tasmania in the 1960's following iodation of bread in an attempt to control goitre in children (Connolly, 1971).
Enlarged thyroid glands have been observed in lambs born to ewes which received excess amounts of iodine in late pregnancy.
In summary, there are three main factors which may contribute to outbreaks of goitre. These are a simple iodine deficiency influenced by soil, pasture type and rainfall, the effects of pasture goitrogens and genetic predisposition.
CONTROL AND PREVENTION OF GOITRE.
There are several recommendations which can be made on properties where goitre or iodine deficiency is a problem:
i). When there is a high rainfall in the premating and early pregnancy period the risk is higher. Heavy stocking in selected paddocks may assist by increasing the intake of soil iodine.
ii). grazing pregnant ewes on pasture containing members of the family Cruciferae should be avoided.
iii). iodised salt blocks may be of value, but many animals may not be protected. Iodine will be leached from the block and not all the stock will eat from the block.
iv). organic iodine preparations given intramuscularly may exert an effect for one or more years.
v) an iodine drench given to ewes during pregnancy will protect the lamb. Ewes should be dosed at the beginning of the 4th and again at the beginning of the 5th month of pregnancy using either:
a). 280 mg of potassium iodine
b). 360 mg of potassium iodate.
vi). an iodine capsule is being developed by the C.S.I.R.O. which will slowly release iodine into the runen.
vii). consideration could be given to changing the lambing period to avoid cold, wet conditions, when the effect of iodine deficiency would be more marked.
References
ALDERMAN, C. and JONES, G.I.H. (1967) J. Sci. Pd. Agric. 18:197
BUTLER, G.W. and GLENDAY, A.C. (1962) Aust. J. Biol. Sc. 15:183
BUTLER, G.W. JOHNSON, June M., FLUX, D.S. and PETERSEN, G.B. (1956) Proc. NZ. Inst. Agric. Sci. :52
CONNOLLY, R.J. (1971) - Med. J. Aust. 1:1268
DAWBARD, Mary C. and FARR, F.C. (1932) - Aust. J. Exp. Biol. Med. Sci 10:119
HEALY, W.B., CROUCHLEY, G., GILLETT, R.L., RANKIN, P.C. and WATTS, H.M. (1972) - N.Z. J. Agric. Res. 15:778
HORN, F.P., REID, R.L. and JUNG, G.A. (1974) - J. Anim. Sci. 38:968
MASON, R.W.(1974) Tasm. J. Agric. :130
MAYO, G.M.E. and MULHEARN, C.J. (1969) - Aust. J. Agric. Res. 20:533
WATSON, W.A., BROADHEAD, G.D. and KILPATRICK, R. (1962) - Vet. Rec. 74:506