Hypomagnesaemic tetany (grass tetany, grass staggers, lactation tetany)
Hypomagnesaemic tetany is a multifactorial complex metabolic disorder, which is characterized by reduction in magnesium concentration in blood and cerebrospinal fluid (CSF) with clinical ministrations of hyperesthesia, incoordination, muscular spasms, convulsions, respiratory distress and high mortality rate.
Aetiology: Low magnesium concentration in the diet and the presence of cations that interfere with the absorption of magnesium or its status in herbage, inclement whether, transport, increased requirement of magnesium during lactation and feeding of young rapidly growing grass contribute to onset of hypomagnesaemic tetany. High intake of potassium and nitrogen, and low intake of phosphorous reduce magnesium absorption from rumen. Grass grown over soil fertilized with potash and nitrogen tends to increase concentration of ammonia in rumen, which can impair absorption of magnesium. Insufficient concentration of magnesium in diet for pregnant and lactating cattle and pastures with magnesium level below 2 g/kg dry matter also increase the risk of hypomagnesaemia. Feeding of poor quality feeds and fodder prepared from pastures with marginal magnesium concentration and ingestion of large amount of lush grasses and green cereal crops (wheat) are important predisposing factors.
Clinical findings: Hypomagnesaemic tetany can occur in acute, sub-acute and chronic forms. In acute form, the normally grazing cows suddenly develop signs of anorexia, and abnormal behaviour. They appear uncomfortable, become hyperaesthetic, bellow and gallop in frenzy. Gait is staggering and animal falls and shows tetany of limbs followed by convulsions repeated at short intervals of about a minute. An increase in temperature (104- 1050 F) and respiratory rate is found. Death occurs within few hours. Some animals may be found dead without apparent illness. Response to treatment in the early stage is good.In sub acute form, the onset is more gradual over a period of 4-5 days. Animals show inappetance, change in facial expression, and exaggerated limb movements. There are spasmodic urination and frequent defaecation. Muscle tremors, mild tetany of hind limbs, unsteady-staggering gait and trismus are seen. Cows may recover spontaneously within few days or progress to recumbency.
Chronic hypomagnesaemia may be manifested by vague syndrome such as dullness, unthriftiness and variable appetite. There may not be any signs of disease and animal dies suddenly.
Diagnosis: The disease is diagnosed by clinical signs and response to treatment with magnesium or magnesium plus calcium preparations. Laboratory confirmation is based on low level of magnesium in plasma (<1.2 mg/dl) and CSF (<1.0 mg/dl). The magnesium level in urine is usually low in cows with hypomagnesaemic tetany.
Treatment and Prevention: Immediate treatment with intravenous infusion of 300-500 ml of combined calcium and magnesium solution (25% calcium borogluconate and 5% magnesium hypophosphite) for cattle, and 50 ml for sheep is effective. Animal should not be stimulated during treatment and heart rate should be closely monitored. For optimum results, additional magnesium sulphate (200 ml of 50% magnesium sulphate solution) can be given subcutaneously.Animals at high risk should be given daily doses of magnesium (60 g to cattle and 10 g to sheep) in form of oral supplement in combination with molasses, concentrate or jaggery. Magnesium content in herbage grown over a soil deficient in magnesium can be dusted with magnesium oxide powder (500 g/cattle) or sprayed with 2% magnesium sulphate solution at 1-2 week intervals.