Hypomagnesaemic tetany in calves is common and is often accompanied by hypocalcaemia. It is caused by a dietary magnesium deficiency exacerbated by a high intake of calcium, which causes depletion of magnesium stores and lower serum and bone levels of magnesium. Milk is an adequate source of magnesium in very young calves. However, the efficiency of magnesium absorption decreases up to about three months of age, when maximum susceptibility to the disease occurs.
Other factors may predispose hypomagnesaemic tetany. Scouring reduces magnesium absorption. Chewing fibrous material, such as floor bedding, increases salivation and thus causes greater losses of endogenous magnesium. Hypomagnesaemic tetany in calves often coexists with other diseases, especially enzootic muscular dystrophy.
The disease is most common in housed, inadequately fed animals. Calves aged 2-4 months or over are most at risk when fed solely on a diet of whole milk, as with veal calves. However, the disease also occurs in calves running at pasture with their dams. Calves receiving the greatest quantity of milk and growing most rapidly are more likely to be affected, as they have a greater need for magnesium.
Tetany does not occur until serum magnesium levels fall below 0.8 mg/dl (0.33 mmol/l), and is most severe at levels below 0.6 mg/dl (0.25 mmol/l), although calves may have levels even lower than this and show few clinical signs. It is probable that hypocalcaemia precipitates tetany, in animals rendered tetany-prone by low serum magnesium levels.