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.