Affected ewes may just be found dead. Blood calcium concentrations do not usually fall until signs develop, so monitoring using blood tests is not useful. Veterinarians can arrange for measurement of the dietary anion-cation difference in feed samples for dairy cows, and this can warn when cows are at risk. It also occurs in cattle of any age that are undernourished or on a falling plane of nutrition. Basically the cause is a low magnesium intake, and changeable bad weather, yarding and transportation all predispose.
Wind chill is known to be a particularly important contributing factor.
Hypomagnesaemia in cows usually causes sudden death but sometimes obvious signs develop before death such as increased nervousness, body tremor, walking with stiff legs, collapse with paddling and the head held back. It's not unusual for affected cows to be extremely aggressive. Affected cows soon develop fits and die. Hypomagnesaemia in ewes sometimes causes dramatic signs such as body tremor, walking with stiff legs, collapse with paddling and the head held back. Sometimes they are just found dead, but more often the signs are rather subtle, with an increase in nervousness and anxiety that can be easily overlooked or misinterpreted.