Hypocalcaemia in sheep is a non-seasonal, feed related condition that occurs predominantly in late pregnant and lactating ewes and weaner lambs. There is an increased incidence in older ewes and in multiparous ewes. I have seen an increase in the number of hypocalcaemia cases in my area over the past 8 years due to an increased reliance on feed wheat/ oat crops particularly in pregnant ewes and for weaned lambs. Some proprietary supplements sold in the area have been associated with hypocalcaemia losses primarily due to a low calcium to phosphorus ratio in them. Apart from a link to cereal based feeds, hypocalcaemia can also occur on oxalate containing pastures. Common plants associated with this in our area include oxalis/ soursob (Oxalis pes caprae) and pigweed (Portulaca spp). The acute form of oxalate toxicity presents as hypocalcaemia. Chronic cases are marked more by nephrosis, ill thrift and anaemia (Radostits et al, 2007).
Calcium: phosphorus imbalance in feed is the most likely cause. Recommended calcium: phosphorus ratios should be 2:1 (Dickson et al, 2011). An analysis of a ration provided to lambs in a feedlot trial conducted by Geoff Duddy at Yanco Agricultural Institute in 2006, showed that the ration containing the commercially available pellets had a ratio of calcium to phosphorus ratio (Ca: P) of 0.84:1 (Geoff Duddy, pers. comm.). Winter wheat has a Ca: P of 0.8:1 and cereal grains have a Ca: P of 1:6 - 1:17. In lamb feedlot systems, addition of roughage and CaCO3 to the diet has been found to reduce the plasma and urinary concentration of phosphorus and has an effect on the development of hypocalcaemia as well as urolithiasis (Godwin et al, 1982).
Oxalate containing plants can also precipitate hypocalcaemic events. Sheep can adapt to oxalates if exposed to low concentration over time. All sheep are susceptible to oxalate poisoning but the risk is considered higher in late pregnant and lactating ewes, as well as hungry sheep place on large amounts of the toxic plants (Radostits et al, 2007).
In ewes, the disease is most likely to occur in the last trimester of pregnancy and in the first 6 weeks of lactation, with most cases occurring in the 2-4 weeks before lambing (Nosdil G et al, 1981). In late pregnant ewes, hypocalcaemia can occur concurrently with hyperketonaemia (pregnancy toxaemia). It can also occur at the same time as hypomagnesaemia. Our area has infrequent occurrences of grass tetany, and I have only had a small number of samples indicating hypomagnesaemia and hypocalcaemia in sheep, but would expect this to differ in the areas to the east of us.
Hypocalcaemia is most likely to follow a stressful event, which can include (but is not limited to) movement of sheep; yarding of sheep off feed or, less commonly, on feed which has low calcium: phosphorus ratio; husbandry procedures; inclement weather; pregnancy toxaemia.
Include ataxia, muscle tremors, recumbency (sometimes in a frog-leg position), salivation, ruminal stasis and tachypnoea. Tachycardia is often present and heart sounds are often muffled although I have heard them to be very loud particularly in hot weather conditions. Body temperature is often increased as a result of muscle tremors. Hypocalcaemia is associated with a lowered uterine activity during parturition (Robalo Silva et al, 1984) and can be linked to an increase in dystocias due to uterine inertia. In one study, plasma calcium was higher in ewes with hypocalcaemia which were still able to stand than in ataxic or recumbent cases, and magnesium levels were lower in ewes with tetany (Nosdil G et al, 1981). This correlates with an observed enhanced outcome for both ewes and lambs that are still standing when treated.
Very little is evident other than marked congestion in most cases.
In the field, this is usually based on clinical signs, history and response to treatment. In live sheep, take blood in plain tubes prior to treatment to confirm your diagnosis. In dead sheep, aqueous humor works reasonably well if they haven't been dead for too long. Pregnancy toxaemia should also be considered in late pregnant ewes (usually with cerebral signs as well). Grass tetany in ewes is uncommon in the Riverina, but consider it a possibility in ewes with lambs at foot that have been yarded overnight or longer.
Intravenous calcium borogluconate is the preferred treatment given slowly to effect (monitor heart rate and intensity). The recommended dose is generally no more than 20 ml I/V. There is a high incidence of cardiac arrhythmia during intravenous treatment with calcium borogluconate, and slowly injecting these solutions is less likely to cause complications. A subcutaneous injection of approximately 20 ml is usually given concurrently as a 'depot'. In treatment of large numbers of affected sheep, farmers generally rely on subcutaneous (SC) injections. Intraperitoneal treatment is equally quick and appears to have a better outcome than SC.
One study found that the mortality rate for ewes treated post-parturition was higher than that for ewes treated in late pregnancy (Nosdil G et al, 1981).
I have not found retreatment successful in cases caused by a nutritional imbalance and especially in ewes. Cases appear to either recover or not following the initial treatment. Often larger/ overweight ewes that have been recumbent for more than 2-3 hours appear to have a poorer prognosis for recovery in my experience. Retreatment in cases of acute oxalate poisoning has appeared to provide advantage in sheep that stay down or recover and then go down again.
Although low calcium diets are recommended in cattle in the prepartum period, in sheep these appear to precipitate hypocalcaemic events. As such, it is recommended that high risk groups, such as late pregnant and lactating ewes and weaned lambs are supplemented with calcium carbonate when grazing crops or other diets with low Ca: P.
I prefer loose licks made with a mixture of super-fine limestone and coarse salt. I recommend that lambs are imprinted onto this method of supplementation whilst still with the ewes as it will make prevention easier at the high-stress period following weaning if the lambs are going onto cereal based feeds. Calcium molasses blocks don't always have a good uptake by all sheep and the blocks can be eaten by a few dominant sheep. Calcium and salt added into grain feeds in a mix-all also works effectively as long as the limestone doesn't settle out. The rate of supplementation is 1.5 - 2%. Some producers don't like this method as saliva in the bottom of the feeder can cause a 'cement' of lime to build up leading to issues with feed delivery. It is also abrasive on augers and feeders. I have observed that producers using dolomite instead of limestone in the lick tend to have a quicker return to risk if the lick runs out and is not replaced quickly enough.