A small Poll Dorset stud located just on the northern side of Gundagai reported unusually high lamb mortalities in a mob of stud ewes lambing down in May/June, 2010. The owner stated that the affected lambs looked pink and several had swollen throats. At this stage about 25% of his lambs had been born dead or died soon after birth. The property was visited at the next opportunity and a dead lamb with very obvious goitre was collected for detailed examination.
Goitre had been diagnosed in the past in the Gundagai district in goats but not to my knowledge, in lambs. Goats are the most susceptible livestock species, followed by sheep then cattle. The district had experienced above average summer rain and a good early start to autumn. Most properties had an excellent cover of good quality feed throughout late summer and autumn and little supplementary feeding was required before the onset of lambing. A second case was investigated on a property on the eastern edge of Gundagai soon after this where losses in a mob of maiden ewes were around 50%. The condition was also confirmed later that year on a property in the Illabo area and reported from a fourth property at Tarcutta.
The lamb weighing 3.6kg, had a severely enlarged thyroid gland and a sparse, fine cover of wool over the upper body. It had been alive at birth but had not suckled. The grossly enlarged thyroid weighed 50g (thyroid weight: body weight ratio of 13.9g/kg) and was nodulated and haemorrhagic. No other abnormalities were noticed.
Histopathology of the thyroid gland showed severe diffuse hypertrophy and hyperplasia of follicular cells. Follicles contained pale staining eosinophilic colloid.
Goitre in new born lambs is usually the result of iodine deficiency or the consumption of goitrogens contained in plants such as white clover and some brassica crops. These goitrogens block the uptake of inorganic iodide by the thyroid gland. The young ewes in the second case were the only sheep that had grazed forage brassica during pregnancy. There is also a rare inherited form of goitre that has been reported in poll dorset sheep but the condition had not been seen in this stud in previous years using sires from the same bloodlines.
The cases outlined in this paper are most likely to be the result of dietary iodine deficiency. Soils in this area are probably marginal for iodine and a combination of environmental factors could lead to goitre in certain years. The concentration of iodine in soil is much higher than the levels in pasture. Stocking rates throughout the district had declined due to several years of drought and above average rainfall during summer and autumn resulted in abundant pasture growth, excellent ground cover and little opportunity to ingest soil. High calcium intake can inhibit iodine uptake from the gut. Most producers in this area feed out calcium/magnesium supplements to pregnant ewes and water from bores or natural springs in this area are alkaline with a high calcium content. Selenium deficiency also predisposes animals to hypothyroidism. Selenium facilitates the conversion of thyroid hormones to their active form in the body. Selenium responsive illthrift is not uncommon in sheep in this district in the country east of the Hume Highway.
Goitre is associated with a significant increase in lamb mortality. Lambs with goitre are more likely to be stillborn, or have reduced vigour at birth. Iodine deficiency can result in reduced birth weight, impaired lung, brain, heart and wool follicle development. These factors when combined with impaired thermoregulation makes deficient lambs much more susceptible to cold stress. Enlarged thyroid glands may also increase the chance of dystocia.
Clinically obvious goitre only occurs sporadically even in areas with deficient soils so it can be difficult to determine when supplementation is needed. Thyroid weight to body weight ratios of new born lambs can be used to identify subclinical cases. A ratio of >0.8g/kg confirms goitre while thyroids between 0.4-0.8g/kg body weight indicate marginal iodine deficiency. A thyroid weight of greater than 2.8g has also been suggested as confirmation of goitre. Thyroid palpation can also be used to identify deficiencies. Normal thyroids should be just palpable or unable to be felt as you pass the thumb and forefinger over the gland in the neck.
In all of the cases outlined in this paper, the problem came to light very late in the lambing period and no attempt at treatment was made. Many of the changes in the foetus resulting from iodine deficiency such as impaired brain function and fewer secondary wool follicles, are irreversible after birth. Prevention is the best option.
Iodine can be provided to ewes during the third and fourth months of pregnancy by drenching at each time with 280 milligrams of potassium iodide dissolved in water and given directly or mixed with a drench. Alternatively 25g of potassium iodate can be mixed with 100kg of salt and provided as a lick. However, this less effective because intake is not controlled. Iodised oil products injected i.m. or s.c. provide sufficient iodine for around two years but are not available commercially in Australia.