Facial eczema is a disease of the liver caused by the toxin sporidesmin. Not all animals affected show physical signs or symptoms, yet unseen liver damage can limit performance long after facial eczema (FE) season.
Production losses caused by FE are often greater than they appear. Even if liver damage is insufficient to cause clinical signs, there will be sub-clinical (not obvious) effects on production of milk, meat, wool and reproductive performance. Some losses may not even be associated with FE as they happen later in the year especially around calving or lambing, when immune function is reduced and stress levels are high.
Zinc dosing in the run up to and during facial eczema seasons is probably the most widely but least understood form of FE protection. Zinc supplementation is believed to be protective against FE if it maintains cows’ blood serum zinc levels at 20 to 35 micromoles per litre.
Whilst no one knows exactly how zinc protects the liver, one hypothesis is that saturating the liver with zinc depletes it of copper. This competitive exclusion of copper is thought to prevent enzymes in the liver of turning sporidesmin from a non-toxic form into a toxic one.
An interesting case in the Waikato showed that despite adequate zinc treatment, cows that receive excessive copper in the run up to FE season (over 2 g of copper sulphate/day) still suffered severe liver damage. For this reason it is generally accepted that providing inorganic copper, e.g. copper sulphate during, and in the run up to FE season may increase susceptibility to FE.
There is also the possibility that high levels of free copper in the liver act as a catalyst in the formation of free radicals that cause liver and bile duct damage, so there may be a combined effect of high copper and toxic Sporedesmin.
Feeding high levels of zinc for an extended period depletes copper reserves, and could result in copper deficiency. Organic forms of copper are often fed in conjunction with zinc when used to combat FE. There do not appear to be issues regarding absorption and metabolism or organic with inorganic (e.g. zinc sulphate or oxide) minerals.
It is also believed that absorption and metabolism of organic minerals can be regulated according to requirements, unlike inorganic minerals, which is why they are considered safer to use. This may help explain why organic copper does not appear to increase the risk of FE when fed with zinc.
Whatever course you take, early intervention is critical – use prevailing weather conditions in combination with pasture spore counting to predict and identify periods of pasture toxicity, and take preventative action when local pasture spore counts trend upward of 20,000 spores per gram and weather conditions look favourable for sporulation.
As featured in NZ Dairy Farmer