Hypomagnesaemia (grass tetany or grass staggers) is the most common killer of mature lactating cows on the Central Tablelands of New South Wales, especially so in the autumn and winter of 2021, but it is surprisingly rare in lactating ewes. Hypomagnesaemic tetany has been reported in lactating ewes grazing grass-based pastures in Southern NSW, Victoria, New Zealand and the United Kingdom (Sykes AR 2007, Whittaker and Pitman 2012, Constable et al., 2017, West et al., 2017), but this is the first case the first author has diagnosed in sheep.
In early cases, the other two important metabolic diseases of pregnant and lactating ewes, hypocalcaemia and ketosis (pregnancy toxaemia) can be differentiated from hypomagnesaemic tetany with a clinical examination and biochemistry. Hypocalcaemic ewes collapse into sternal recumbency but are initially bright, alert and responsive. Ketotic ewes initially remain standing but are dull, only partially responsive and may appear blind. Ewes with hypomagnesaemic tetany collapse in convulsions in lateral recumbency and die soon after. Pregnancy toxaemia occurs before lambing while hypocalcaemia can occur both pre and post lambing. In contrast, hypomagnesaemic tetany is usually seen in lactating ewes. While the presentation of hypomagnesaemic tetany is acute, ewes with ketosis or hypocalcaemia collapse and become comatose before death hours to days later. Ewes with clinical hypocalcaemia have serum calcium concentrations less than 1.2-1.4 mmol/L. Ewes with primary ketosis have beta-hydroxy butyrate concentrations greater than 2.5-4 mmol/L. Ewes with hypomagnesaemic tetany have serum magnesium levels less than 0.4mmol/L.
Terminal ewes usually present as dull and recumbent and may be either primarily or secondarily hypomagnesaemic, hypocalcaemic and/or ketotic .
On 6 July 2021, the owner of 310 aged lambing first cross ewes running in two mobs, concerned that the pasture was insufficient, mustered and moved one mob of 140 to an adjoining paddock. The ewes, joined to Poll Dorset rams, had been lambing for a month. Shortly after movement to the new paddock the owner observed two ewes that collapsed in lateral recumbency, shaking and convulsing, with opisthotonos and salivation. Both ewes died within 10-15 minutes of the onset of these signs. The owner estimated that over a period of one month he lost 25 ewes in total. He speculated that at least some of these additional deaths occurred under similar circumstances.
The pastures were short and green and dominated by annual grass species such as silver grass (Vulpia bromoides) and barley grass (Hordeum leporinum) with less than 10% subterranean clover (Trifolium subterraneum). The ewes were supplemented with approximately 3 kg of light grain oats and 3 kg of oaten hay per head per week. They also had access to blocks (Furneys Ewe and Lamb + 30% Lupins, containing 30-40% salt, 8% calcium and 1% magnesium).
Two ewes were necropsied on 6 July 2021, within 2-3 hours of death. Both ewes were aged with either a broken mouth or small spread incisors. Both were in body condition score 2 with normal lactating udders. No other gross abnormalities were noted. Aqueous humour samples were collected from both ewes.
|EWE 1||EWE 2|
|BHB||<0.80||mmol/L||1.20 H||1.26 H|
|Aq. Hu Ca||1.13-2.03||mmol/L||1.09 L||0.74 L|
|Aq. Hu Mg||0.65-1.55||mmol/L||0.33 L||0.34 L|
Aqueous humour samples collected from both ewes showed significantly lowered magnesium, lowered calcium, and minor elevation in beta-hydroxybutyrate.
Hypomagnesaemia in sheep is rarely identified in ovine submissions to Elizabeth Macarthur Agricultural Institute. Approximately 12,200 diagnostic ovine submissions were submitted to the laboratory between July 2006 and July 2021. The magnesium serum and aqueous humour reference ranges employed during this period at Regional Laboratory Services, Benalla, were 0.74-1.44 mmol/L for serum and 0.48-1.50 mmol/L for aqueous humour (changed to 0.65-1.55 mmol/L in 2017). Of these 12,200 submissions only 16 cases included sheep with serum magnesium concentrations of less than 0.50 mmol/L, and only 7 cases included sheep with aqueous humour magnesium concentrations of less than 0.48 mmol/L.
As noted by Herd and Peebles (1962), ewes may be hypomagnesaemic, but it is often a stressor, such as mustering or even dogs that triggers hypomagnesaemic tetany. If our case definition of grass tetany or hypomagnesaemic tetany is that it occurs in lactating, hypomagnesaemic ewes and is characterised by a sudden onset of excitability, convulsions and death soon after, then only 8 of the 23 cases meet these criteria over this 15-year period.
Of the 23 cases with hypomagnesaemia identified above, six do not meet the criteria of hypomagnesaemic tetany because other diseases including polioencephalomalacia, pyrrolidine alkaloid poisoning, haemonchosis, malnutrition, primary hypocalcaemia, ketosis and listeriosis were the primary problem. Of the three cases that occurred in young sheep (lambs or hoggets), one was consistent with transit tetany, one was associated with urea poisoning and another with hypocalcaemia.
It has been suggested that concurrent hypocalcaemia may contribute to, or be the dominant factor in, hypomagnesaemic tetany (Constable et al., 2017). West et al., (2017) reported that the New Zealand experience was that hypomagnesaemia can occur without hypocalcaemia. In this case both ewes sampled had mildly to moderately low aqueous humour calcium levels.
In live animals, blood analysis can be used confirm hypomagnesaemia. Most commonly, the presentation is of an acute death. post-mortem, magnesium in blood shows rapid increases in concentration as intracellular leakage contributes to the extracellular pool. In contrast, eye fluid concentrations do not show these rapid post-mortem changes and are the preferred sample for diagnosis.
While both calcium and magnesium levels are more stable in the aqueous humor than in the blood, they do rise after death making interpretation more difficult in the more mature carcase especially if the time of death is not known. Other metabolites such as ammonia, potassium and phosphate also rise with time post-mortem and in specific cases may be used as an indicator of the post-mortem time interval prior to sampling the eye fluid.
In this case, potassium, ammonia and phosphate concentrations are consistent with samples obtained within three hours of death on a cold Central Tablelands morning. The very low concentrations of magnesium observed are consistent with a diagnosis of primary hypomagnesaemia as the cause of death of these ewes.