Trace element deficiencies in sheep
Deficiency of essential trace elements may lead to significant economic losses within affected sheep flocks; however diagnosis can be tricky as clinical signs are frequently mild and insidious in onset, such as poor growth rates, weak neonatal lambs, non-specific ill thrift or reduced feed intake. Problems tend to affect the whole group rather than individual animals and are frequently recurrent on a farm; organic units may be particularly susceptible due to increased reliance on home-grown feeds and restrictions on use of mineral supplementation. Diagnosis requires careful differentiation of trace element deficiency from other common causes of poor productivity such as chronic parasitic disease or inadequate nutrition; it is not uncommon for a combination of factors to be present, each of which will require appropriate intervention to correct. The most common trace element deficiencies in sheep include Copper, Cobalt, Selenium/Vitamin E and to a lesser extent Iodine and Manganese.
Copper is an essential component of several important enzymes which are required for normal cellular and metabolic function in the body. Deficiency occurs when there is insufficient copper available for the animal to use; either due to low copper intakes from grazing (relatively rare) or more commonly, excess intakes of copper antagonists (iron, molybdenum & sulphur) which ‘lock up’ dietary copper making it unavailable to the animal.
The most readily recognisable clinical sign of copper deficiency in sheep is the development of the non-treatable condition ‘Swayback’ in young lambs; a progressive hind limb weakness leading to paralysis due to damage to the spinal cord during foetal development mid-pregnancy in copper deficient ewes. Copper deficiency in older animals has been linked to poor fleece quality, reduced growth rates, anaemia and increased susceptibility to bacterial infections.
Swayback can be provisionally diagnosed by clinical presentation, however confirmation requires histological examination of the spinal cord following a post-mortem examination. Low blood plasma levels (<9.4µmol/L) suggest copper depletion, but do not reliably differentiate deficient diets from excessive antagonism. Liver tissue copper analysis is considered the gold standard assessment of the true copper status of an animal; liver biopsy can be conducted in live animals, although it is often simpler and less invasive to obtain samples from either post-mortem examination or the slaughterhouse.
Copper supplementation should only be carried out under veterinary advice and supervision following a diagnosis of deficiency, since sheep are also highly susceptible to copper accumulation and toxicity, with some breed predispositions e.g. Texels.
Cobalt is an essential component of vitamin B12 which is synthesised by rumen micro-organisms and is required for energy metabolism. The symptoms commonly associated with cobalt deficiency therefore actually manifest themselves as a result of a lack of vitamin B12 in the animal, rather than cobalt.
Weaned lambs in late summer/autumn are most at risk of cobalt deficiency (‘Pine’) which results in ill-thrift, lethargy, poor appetite, watery ocular discharge and poor fleece quality. As a consequence of deficiency lambs have poor immune function and are often more prone to infectious disease (e.g. clostridia, pasteurella); whilst concurrent worm burdens will exacerbate clinical signs by reducing gut vitamin B12 absorption, sometimes resulting in symptoms of deficiency even when nutrition and dietary cobalt levels appear to be adequate. In adults, reduced fertility and poor mothering ability have been reported, possibly as an indirect result of poor overall body condition.
Diagnosis is made by a combination of clinical signs supported by low blood or liver vitamin B12 levels. In addition, there is often a significant growth rate response to a supplementation trial.
Adequate short term supplementation can be achieved by regular monthly oral cobalt drenching, whilst longer term prevention can be provided by administration of intra-ruminal mineral boluses. Pasture dressing to increase cobalt levels in the soil can be effective, but is often not considered to be cost effective.
Selenium / Vitamin E
Selenium and Vitamin E act synergistically as protective anti-oxidants within the body and act to support immune function.
The most easily recognised clinical sign of selenium deficiency is ‘white muscle disease’, although disease prevalence is low. Typically, rapidly growing 2-6 week old lambs are affected, with sudden onset generalised stiffness, which may progress to an inability to stand within a few days if left untreated. In older growing animals selenium deficiency has been linked to poor daily live weight gain and in breeding ewes can cause embryonic death and poor fertility performance.
Diagnosis of selenium/vitamin E deficiency is made via a combination of history, clinical signs plus low blood glutathione peroxidase (GSHPx - a selenium containing enzyme) and vitamin E levels, or low liver tissue selenium levels. Animals with white muscle disease will have elevated muscle enzymes due to tissue damage (e.g. CPK, AST).
Treatment can be effected by oral selenium supplementation and/or selenium and vitamin E injection; whilst prevention can be achieved by regular oral supplementation, use of a sustained release injection preparation or use of intra-ruminal mineral boluses. Veterinary supervision of treatment or prevention programmes is advised, as although currently rarely diagnosed, selenium can be toxic to sheep if provided in excess.
Iodine & Manganese
Iodine is a component of the hormone thyroxine (T4) which controls energy metabolism. Deficiency is typically associated with an enlarged thyroid (goitre) and results in late abortions, stillbirth and/or increased neonatal mortality.
Manganese is essential to several enzyme systems. Deficiency is rare; however symptoms such as joint or bone abnormalities and a stiff gait are reported, along with potential negative effects on fertility due to poor oestrus expression and conception rates.
If your sheep flock is suffering from sub-optimal performance a veterinary investigation into possible trace element deficiency maybe worthy of consideration, however this should not be viewed as a ’golden bullet’ to solve all of your flocks health and disease issues. It is highly likely that a flock suffering from trace element issues will also have concurrent management or health problems such as chronic parasitism, which will be contributing to the overall flock health picture and should be investigated and addressed alongside any potential mineral imbalances.