Selenium is an essential trace element required for optimal animal health; however, large parts of New Zealand’s farming country are marginally to markedly deficient in this element. In higher dosages selenium is highly toxic to stock and the margin of safety between health and poisoning is very small.
There are currently over twenty products containing selenium as sodium selenate licensed for use in livestock. These can be administered as a ‘pour-on’, injected, orally drenched or top-dressed on pasture. Many of the injectable products available for sheep contain vaccines or vitamins as well as sodium selenate. This makes them especially attractive to farmers who may not realise that the selenium in them has the potential to be four times more toxic than the same concentration of selenium administered orally.
Three to four week old lambs are by far the most commonly affected age group for selenium toxicity. These lambs are usually given selenium at tailing but, at this age, should not be given any more than 1-2 mg of sodium selenate at any one time.
• Do not use injectable products containing selenium on very young lambs if you can avoid it. Drenches containing selenium are safer;
• Do not use more than one selenium containing product on the same lamb;
• Check the manufacturer’s instructions on the product label;
• Make sure you do not exceed a total dose of 1-2 mg/lamb by any method.
Lambs poisoned by selenium will typically die within the first 24-48 hours after dosing and are usually either discovered dead or frothing at the mouth with breathing difficulties. A post mortem will reveal marked lung oedema with fluid in the airways.
A diagnosis of selenium poisoning is confirmed by measuring the concentration of selenium in the liver. Toxic concentrations are usually taken as >30,000 nmol/Kg.