Hypomagnesaemia and new data on vitreous humour magnesium concentration as a post-mortem marker in ruminants

Magnesium deficit has been associated with many sub-clinical and clinical conditions in humans and animals. The incidence of hypomagnesaemia is high in lactating cows grazing spring pastures, occasionally resulting in the often fatal condition known as grass tetany. While plasma magnesium concentrations can be used to assess magnesium status in the live animal, post-mortem diagnosis of clinical grass tetany is difficult. Recent studies have investigated the potential of eye fluid magnesium concentration as a post-mortem marker of hypomagnesaemic tetany. In tetany induction studies carried out in adult ewes and lactating cows significant relationships were found to exist between the concentrations of magnesium in either cerebrospinal fluid or plasma and either aqueous or vitreous humour. In freshly dead animals aqueous humour magnesium concentrations of < 0.33 mmol/L in adult sheep and < 0.25 mmol/L in adult cattle were associated with severe hypomagnesaemia and tetany. However, aqueous humour was found to be unstable post-mortem. Vitreous humour was considerably more stable and a vitreous humour magnesium concentration in adult sheep of < 0.65 mmol/L for up to 24 hours post-mortem or < 0.55 mmol/L in adult cows for up to 48 hours was associated with severe hypomagnesaemia and tetany. Provided clear-fluid samples are taken from appropriate animals and processed correctly in the laboratory, the concentration of magnesium in vitreous humour is a useful and practical marker in the post-mortem diagnosis of hypomagnesaemic tetany in ruminants.