Zinc is used as a feed supplement in deficient diets and also in New Zealand and Australia in recent years it has become a prophylactic method for the control of facial eczema.
Zinc is available from many feed sources and deficiency in animals is rare. However high dietary levels of phytates, copper, magnesium, cadmium and calcium increase the dietary requirement for zinc. Dairy cattle, pigs and poultry have high daily requirements for zinc.
If supplementation is required zinc sulphate at appropriate dose rates can be used on a daily basis. Also zinc gluconate and zinc methionine are both available as tablets for zinc supplementation of small animals.
The mechanism of sporidesmin toxicity and the role of zinc in prophylaxis:
Sporidesmin is a mycotoxin contained in spores produced by the fungus Pithomyces chartarum during warm humid conditions. The major target site is the biliary epithelium, where the toxin causes necrosis and inflammation ultimately ending in cholestasis and liver dysfunction.
The mechanism of cytotoxic action of the mycotoxin, has been extensively investigated. It was demonstrated that active oxygen radicals could be generated by sporidesmin both in vitro and in vivo. If the toxic changes due to sporidesmin poisoning do involve the production of oxygen free radicals, then any treatment to reduce the rate of radical formation by this mycotoxin would reduce the toxicity.
A number of metals were found to inhibit the radical formation by combining with certain groups on the sporidesmin toxin (thiol groups) and forming stable complexes called mercaptides. These metals in decreasing order of formed mercaptide stability are:
Cd > Hg > B, > Zn > Ag > Sb > Cu > Co > N: > As(III) > Pb > Mn
Zinc is one of the most potent inhibitors of active oxygen generation both in vitro and in the cell model used. If this is the mechanism by which zinc protects against sporidesmin toxicity in vivo, then it is unlikely that another metal of this type would supersede zinc as a practical means of controlling facial eczema. Only the Group IIb metals, mercury and cadmium were found to be significantly more active; however these metals have an exceptional toxicity to living tissues.
Active oxygen radical formation from sporidesmin is also inhibited by compounds which chelate copper, (copper may act as a catalyst for the intracellular reaction). This suggests that any superoxide radical anion production from sporidesmin in vivo, would be mediated by the intracellular pool of copper. Therefore, the mechanism whereby zinc salts provide protection against the harmful effects of sporidesmin toxin in living animals may be related to the ability of zinc to inhibit the intestinal absorption of copper.
The following are general recommendations for the use of zinc in the prevention of facial eczema in cattle and sheep. There are now reported cases of some poisoning. In general these have resulted from the erroneous use of zinc sulphate. Both cattle and sheep have been affected.
The dose rate of zinc oxide for cattle is 30 mg/kg body weight daily
i.e. for a 340 Kg Jersey cow 10g ZnO/day
for a 390 Kg Friesian cow 12g ZnO/day
When to dose
Zinc dosing is only required while the animals are grazing toxic pasture but should be commenced before they graze such pasture. Because of the high risk of toxicity, it is important that dosing with zinc salts should be confined to periods of known or suspected danger. Zinc dosing should not be undertaken routinely throughout the facial eczema season.
Note that dosing with zinc salts does not give total protection. However in conditions found in most field outbreaks it will reduce both the number of animals affected by the FE toxin, and the amount of damage done to the livers of affected animals.
Dosing frequency
Research has shown that daily dosing is not essential, provided that the dose rate is increased to supply the same total weekly intake of zinc as would occur with daily dosing.
Oral dosing
This is the only way to guarantee the correct intake for the animal. Drenching with zinc sulphate to prevent facial eczema is not recommended.
Zinc oxide
This is the cheapest form of zinc. However, as an insoluble powder it is more difficult to mix and administer. Zinc oxide (ZnO) is less hazardous than zinc sulphate because it is less of an irritant and far less soluble.
Drench recipe: Mix 1kg zinc oxide powder with 2.5 litres of water.
Stir until a smooth lump free cream is formed.
The addition of Nutrimol, Maxicrop or Sea Magic (Yates) will assist in the suspension of the zinc oxide.
It may be necessary to adjust water content slightly, to get a suspension that will readily flow through an automatic drenching gun. If too stiff, add more water. If the oxide settles out too quickly add more dry powder. Keep stirred during drenching and wash the gun out after use. Ensure that a finely ground source of zinc oxide (free of contaminants) is used.
Dose rate: 10ml/100kg live weight. Sufficient for 85-100 cows.
Zinc salts have a role only as a short term protection on properties with occasional eczema problems, when no other methods are suitable.
Long term dosing of dairy cattle with zinc oxide may interfere with copper and selenium metabolism, although it has not yet been shown to induce either selenium or copper deficiency. Another possibility of long term dosing is pancreatic injury. However if this occurs recovery does occur once zinc dosing ceases.
As well as zinc oxide and dosing in cattle, zinc sulphate has been used for facial eczema control through water dispensing units.
The daily dosing of sheep is largely impractical and even unnecessary. Provided the dose rate is increased to supply the same total weekly intake of zinc as would occur with daily dosing, adequate protection is given with dosing at 6 7 day intervals. The high dose rate given to sheep may produce temporary inappetance and salmonellosis has been recorded in sheep following such dosing.
The mixture for sheep can either be the standard cattle mix as above or:
1kg of ZnO + lLH2O + 0.2 L "x" = 1.45L mixture
x = suspending agent (see above).
The dose of this mixture is:
21ml per week per ewe (55Kg)
11ml per week per lamb (30Kg).
Dose before and during the danger period.
Intraruminal zinc boluses
Intraruminal zinc boluses are also available for use in sheep and cattle. These have led to zinc toxicity in some cases where calves have been under the minimum recommended weight for dosing.
Residues
Zinc does not accumulate in the body, however, tissue residues may be present for several weeks. Animals overdosed with zinc should be held from slaughter until the zinc levels fall below the acceptable limits which may be 2 3 weeks or more.