HYPOMAGNESEMIC TETANY (LACTATION TETANY, GRASS TETANY, GRASS STAGGERS, WHEAT PASTURE POISONING)

Tetany associated with depression of serum magnesium levels is a common occurrence in ruminants and divided into 2 type: 1-LACTATION TETANY((Grass Tetany))in which there may be a partial dietary deficiency of Mg ,but nutritional or metabolic factors reduce the availability of increase body loss of Mg. 2-HYPOMAGNESEMIC TETANY OF CALVES : In which there is a deficiency of Mg in the diet.

LACTATION TETANY ((Grass Tetany))

It is highly fatal disease of all classes of ruminant, and reaches it is highest incidence in lactating cows characterized by Hypomagnesaemia Hypocalcaemia Tonic –clonic muscular spasms Convulsions and death due to Respiratory failure .

Etiology

1-Short period of Starvation. 2-High level of K + in green cereal crops. 3- Young Green Grass has lower Mg content. 4-Heavy application of fertilizers rich in potassium and Nitrogen . 5-Pasture containing less than 0.4 Mg of the dry matter is likely to cause Hypomagnesaemia. 6-Diarrhea (decrease absorption). 7-There is a close association bet. Climatic condition and serum Mg levels in animals exposed to cold wet windy weather (cold increase urinary excretion of Mg).also there is a possible role of hyperthyroidism in the production of seasonal hypomagnesaemia.

Epidemiology Occur mostly in cattle grazing on lush grass in spring and animal grazing on wheat pasture .the incidence of the disease varies from year to year depending largely on climatic condition and management , the morbidity rate 12% in individual herds and up to 2% in a particular area, The mortality is high because of the short course and may reach 20%.

PATHOGENESIS Most clinical cases of the disease have serum magnesium levels below 1 mg/dL compared with the normal levels in cattle of 1.7-3 mg/dL seasonal hypomagnesemia Magnesium has many influences on Impulse transmission at the neuromuscular system, including effects on the release of acetylcholine, on the sensitivity of the motor end plate, on the threshold of the muscle membrane and on activation of the cholinesterase system. the increase excissive muscular contraction are duo to facilitation of transmission of impulses through the neuromuscular system the Mg levels in CSF are greatly decreased and suggested centeral effect on the brain.

Clinical Signs

Acute lactation tetany Sub acute lactation tetany chronic lactation tetany

Acute lactation tetany The animal may be grazing at the time and suddenly cease to graze, adopt a posture of unusual alertness and appear uncomfortable; twitching of the muscles and ears is also evident. There is severe hyperesthesia and slight disturbances precipitate attacks of continuous bellowing, frenzied galloping, and occasionally aggression. The gait becomes staggering and the animal falls with obvious tetany of the limbs, which is rapidly followed by clonic convulsions lasting for about a minute.

During the convulsive episodes there is: a- Opisthotonos b- Nystagmus c- Champing of the jaws d- Frothing at the mouth e- Pricking of the ears f- Retraction of the eyelids. The temperature rises to 40-40.5°C after severe muscle exertion; the pulse and respiratory rates are also high. Death usually occurs within 5-1 h and the mortality rate is high because many die before treatment can be provided. The response to treatment is generally good if the animal is treated early.

Subacute lactation tetany the onset is more gradual ,Over a period of 3-4 days, there is slight inappetence, exaggerated limb movements. The cow often resists being driven and throws her head about as though expecting a blow. Spasmodic urination and frequent defecation are characteristic. The appetite and milk yield are diminished and ruminal movements decrease. Muscle tremor and mild tetany of the hindlegs , straddling gait and trismus. Animals with this form of the disease may recover spontaneously within a few days or progress to a stage of recumbency with a similar but rather milder syndrome than in the acute form. Treatment is usually effective but there is a marked tendency to relapse .  

chronic lactation tetany Many animals in affected herds have low serum magnesium levels but do not show clinical signs. A few animals do evidence a rather vague syndrome including dullness, unthriftiness and indifferent appetite and may subsequently develop one of the more obvious syndromes.

Parturient paresis with hypomagnesemia This syndrome is described under parturient paresis and consists of paresis and circulatory collapse in an adult cow which has calved within the preceding 48 h but in which dullness and flaccidity are replaced by hyperesthesia and tetany.

CLINICAL PATHOLOGY Normal serum magnesium concentrations are 1.7-3 mg/dL These levels in cattle are often reduced in seasonal subclinical hypomagnesemia to between 1 and 2 mg/dL but risk for tetany is not present until the level falls to below 1.2 mg / dL. Total serum calcium levels are often reduced to 5-8 mg/dL and this may have an important bearing on the development of clinical signs. Serum inorganic phosphate levels may or may not be low. Estemation of Mg level in serum , urine , CSF,and also estimation of serum Ca level.

Differential Diagnosis . Cattle • Acute lead poisoning • Rabies • Nervous ketosis • Bovine spongiform encephalopathy. Sheep • Hypocalcemia • Phalaris poisoning • 'Stagger' syndromes.

TREATMENT 1-lV administration of preparations containing magnesium or magnesium and calcium are used. 2- . lV chloral hydrate may be administered to reduce the severity of Convulsions during treatment with magnesium. (e.g. 500 mL of a solution containing 25% calcium borogluconate and5%magnesium hypophosphite for cattle,50 mL for sheep) lV 3-followed by a S/C injection of a concentrated solution of a magnesium salt 50% (200 ml for Cattle and 20ml for sheep).

CONTROL 1-Daily Feeding of magnesium supplements 120 g is safe and effective but 180 g daily may cause diarrhea. The dose for sheep is 7 g daily or 14 g every second day. 2-The objective is to place a heavy 'bullet' of magnesium in the reticulum from which site it constantly liberates small amounts of magnesium - about 1 g/d. 3-Top dressing of pasture and Foliar dusting and spraying 4-Provision of shelter.