Iodine deficiency in sheep and goats

Lucienne Downs - District Vet, Central Tablelands Local Land Services

A severe deficiency of iodine causes a lack of essential thyroid hormone production and the thyroid gland enlarges. The enlarged thyroid gland is called goitre. The swelling occurs in the throat area and can be as large as an orange. Goitre is mainly a disease of lambs and kids, it rarely occurs in calves. Goats have a higher requirement for iodine than other livestock.

Image: Lambs with goitre - Supplementing ewes with the trace element iodine can prevent goitre and potentially fatal deficiency complications, Photo courtesy of Bruce Watt 

Causes of iodine deficiency

• It mostly occurs due to insufficient intake of iodine from the pasture. The main iodine deficient areas in Australia are high rainfall areas in the Great Dividing Range and Tasmania. Many areas of the Central Tablelands have marginal iodine levels.
• Iodine deficiency may also be caused by goitrogens - substances within the feed which inhibit the utilisation of dietary iodine. Goitrogens have been detected in some legumes and forage crops, but are considered unlikely to be a significant cause of goitre.

Iodine deficiency is only a problem in newborn kids and lambs.

• Goitre may or may not be present in iodine deficient lambs and kids.
• Adults appear able to tolerate seasonal fluctuations in iodine supply by using reserves stored in the thyroid gland. This reserve is not available to the developing foetus.
• Thyroid hormones are essential for growth and development of the foetus, particularly the brain, lungs, heart and hair follicles, and contribute especially to survival of the newborn kid.
• Iodine deficiency makes newborn kids and lambs very susceptible to cold, wet weather and mortalities may be very high.

Intake of iodine from pasture

• Intake of iodine from pasture is lowest in late winter when the requirements of pregnant animals are at a peak.
• Seasonal conditions affect the iodine uptake of pregnant animals.
• Iodine deficiency is only likely to cause goitre in spring born lambs when there has been heavy autumn rains and lush pasture growth in autumn and winter.
• As goats have a higher requirement for iodine it has become a standard recommendation that pregnant does grazing in high rainfall areas receive a drench of supplementary iodine once or, in some cases twice, during the last two months of their pregnancy.
• Iodine supplementation of ewes depends on the region and seasonal conditions.
• Treatment of affected kids and lambs with iodine or thyroxine tablets is rarely useful and it is far more effective to ensure adequate iodine nutrition of the foetus.

Drenches

• The drench is made up by dissolving 28 gm of potassium iodide in one litre of water. Each doe should receive 10 ml.
• Iodine is highly volatile and it is important to mix the drench just before use.
• lodised salt licks and feed supplements containing iodine may not  prevent the problem because some animals avoid licks and it is impossible to accurately gauge intake with these systems.
• Drenching is cheap and accurate and can even be combined with some worm drenches, although it is wise to check this with the manufacturer.

Iodine deficiency also occurs in humans and having adequate quantities in pregnancy is critical. If you are interested in learning more there is a great interview between Professor Creswell Eastman and Richard Fidler that is well worth your time.