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This study documents an outbreak of congenital goitre in a flock of sheep under intensive observation for behaviour during lambing on pasture containing a high proportion of white clover (Trifolium repens). Thyroids were palpably enlarged in half of the lambs and much enlarged in 12% of lambs, reaching 42 g/kg bodyweight. The incidence of goitre increased as lambing progressed and with increasing ewe age, but tended to decrease with increasing litter size. Gestation period increased with increasing thyroid size. Lamb mortality was 3-fold higher in lambs with moderately or grossly enlarged glands than in lambs with little or no gland enlargement. Major factors contributing to death were poor vigour of lambs at birth, possibly due in part to impaired thermoregulation, and difficult or prolonged birth. Lambs with severe goitre were susceptible to heat stroke apparently through tracheal constriction. Activity of lambs was depressed according to the degree of thyroid enlargement. The correlations between various lamb behaviours and plasma thyroxine were no better than with the thyroid enlargement score. Twins had a higher mortality and less vigorous behaviour than singles with similar thyroid enlargement. Twins with much enlarged glands were particularly at risk. It is suggested that slight to moderate thyroid enlargement might not be detected in practical farming and could be more common than usually supposed. In the presence of other factors that prejudice survival, such as cold weather, prolonged and difficult birth and multiple birth, this 'subclinical' goitre could contribute to death of lambs. |
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