Definition and Etiology
Hypomagnesemic tetany in cow (lactation tetany/ grass tetany/grass staggers) is a complex metabolic disorder characterized by hypomagnesemia (<1.5 mg/dl in serum), hyperexcitability, muscular spasm, seizures, respiratory distress, collapse, and death. It occurs in calves aged between 2 and 4 months which are fed exclusively on milk because of the fall in the magnesium absorption level from 87 percent at 2-3 weeks to 32 percent at 7-8 weeks of age.
Predisposing Factors
Hypomagnesemia arises during inclement weather due to starvation, transport, grazing on immature grass pasture containing <0.2% Mg on a DM basis. Absorption of magnesium increases with an increasing Na:K ratio (5:1), and it is impaired if ratio is <3:1.
Clinical Signs
Acute lactation tetany – twitching of the muscles and ear, severe hyperesthesia, continuous bellowing, frenzied galloping, staggering gait. During convulsive episodes, there are nystagmus, champing of jaws, frothing at the mouth, pricking of ears, and retraction of eyelids. Hyperthermia (40° to 40.5°C), and increased pulse and respiratory rates. Subacute lactation tetany – gradual onset of clinical signs which usually takes 3 to 4 days inappetence, wildness of facial expression and exaggerated limb movements, spasmodic urination and frequent defecation, decreased ruminal contractions, muscle tremor, mild tetany of hind legs and tail with an unsteady, straddling gait, and retraction of head and trismus. Chronic hypomagnesemia – sudden death with no premonitory clinical signs. Few animals show vague syndrome like dullness, unthriftiness and reduced milk yield. Clinically, in calves, the condition is characterized by hyperesthesia, shaking of the head, opisthotonos posture, retraction of the eyelids, muscle tremors of limbs and frothing at the mouth. Clinical signs occur when serum magnesium concentration falls below 0.3-0.7 mg/dl.
Diagnosis
Diagnosis is based on clinical signs and is confirmed by response to treatment. Laboratory test for measurement of blood magnesium levels (normal range in cattle: 1.8-2.4 mg/dl and sheep: 2.2-2.8 mg/dl) and low CSF (<1 mg/dl) level. Herd diagnosis can be accomplished by estimating the urinary magnesium concentration (normal level: 1-20 mg/dl). Low urine Mg is good presumptive evidence of hypomagnesemia. Urinary magnesium concentration below 1 mg/dl indicates danger of tetany. Some affected animals may have concurrent hypocalcaemia and hypophosphatemia.
Differential Diagnosis
Rabies (History of dog bite, abnormal vocalization, aggressive behaviour), lead poisoning (anaemia, blindness, hyperesthesia), nervous ketosis (sudden drop in the milk yield, anorexia), hypovitaminosis A (blindness, absence of ocular reflexes) and nitrate poisoning (chocolate brown colour mucous membrane).
Treatment
Treat with calcium magnesium borogluconate (1.86 percent calcium borogluconate, 5 percent magnesium hypophosphite and 20 percent dextrose anhydrous, 400-500 mL, slow IV in large animals and 50-100 ml in small animals), depending upon the severity of the condition. Alternatively, administer magnesium sulphate (@ 200 ml of 50 percent solution, SC). Provide magnesium sulphate (100-200 g) daily in the diet at least for 3 to 5 days after initial intravenous infusions. Administer xylazine (@0.05 mg/kg IM) to reduce convulsions. In calves, treat with magnesium sulphate injection (100 ml of 10 percent solution, slow IV) followed by oral supplementation of MgO (10 g/day). Prevention and Control Animal at high risk should be moved to low-risk pastures during the grass tetany season. Daily feeding of at least 60 g of magnesium oxide (MgO) or magnesium chloride per day to lactating dairy cows
STANDARD VETERINARY TREATMENT GUIDELINES FOR LIVESTOCK AND POULTRY