HYPOMEGNESEMIC TETANY

( LACTATION TETANY , GRASS TETANY, WHEAT PASTURE POISONING )

ETIOLOGY :

• THE ETIOLOGY IS MULTIFACTORIAL.

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• LOW MAGNESIUM CONCENTRATION IN DIET . THE REQUIREMENT OF MAGNESIUM IN PREGNANT COWS IS ABOUT 1 – 1.3 G \ KG DM DEPENDING ON THE STAGE OF PREGNANCY , AND 1.8 – 2.2 G \ KG DRAY MATTER FOR LACTATING CATTLE .
• THERE IS NO FEEDBACK REGULATION MECHANISM TO CONTROL CONCENTRATION OF MAGNESIUM IN THE BODY OF RUMINANTS . THE CONCENTRATION OF MAGNESIUM IN BLOOD AND EXTRA CELLULAR FLUIDS ARE ESSENTIALLY DETERMINED BY THE BALANCE BETWEEN DIETARY INTAKE OF MAGNESIUM AND LOSS IN FECES AND MILK AND THE MODULATING EFFECT OF MAGNESIUM HOMEOSTASIS BY THE KIDNEY .
• DIETARY INTAKE :
• IN NORMAL CIRCUMSTANCES , MAGNESIUM ABSORBED FROM DIET IS SUFFICIENT TO MEET THE REQUIREMENTS OF THE BODY AND THE EXCESS AMOUNTS ARE EXCRETED IN THE URINE .

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* RENAL SECRETION :

• THE KIDNEY IS THE MAJOR ORGAN OF HOMEOSTASIS AND CAN ACT TO CONSERVE MAGNESIUM . MG IS FREELY FILTERED ACROSS THE RENAL GLOMERULUS AND IS REABSORBED WITHIN THE RENAL TUBULES .
• WHEN THE DIETARY INTAKE OF MG DECREASED , BLOOD AND INTERSTITIAL FLUID MG CONCENTRATION FALL ; EXCRETION OF MAGNESIUM IN URINE WILL CEASE WHEN SERUM CONCENTRATION FALL BELOW 1.8 MG \ DL . THE RENAL THRESHOLD OF MAGNESIUM EXCRETION IS PARTIALLY UNDER THE CONTROL OF PARATHYROID HORMONES , AND INCREASED LEVEL OF PARATHYROID HORMONES WILL ACT TO CONSERVE MAGNESIUM .
• MAGNESIUM RESERVE
• THERE ARE LARGE STORE OF MAGNESIUM IN THE BODY SPECIALLY IN THE BONES . THERE ARE AVAILABLE TO THE YOUNG CALVES , BUT MOBILIZATION DECREASES WITH AGE , AND IN THE ADULT RUMINANTS THERE IS LITTLE MOBILIZATION IN RESPONSE TO SHORT – TERM DEFICIT OF MAGNESIUM .
• IN RUMINANTS , THIS CONTROL MECHANISM FOR MG CAN MAINTAIN ADEQUATE CONCENTRATION OF MAGNESIUM IN BODY FLUIDS IN MOST PRODUCTION CIRCUMSTANCES , BUT IT CAN FAIL WHEN THERE IS HIGH REQUIREMENT FOR MAGNESIUM COUPLED WITH DECREASED INTAKE . THIS COMBINATION LEADS TO HYPOMAGNESEMIA AND HYPOMAGNESEMIC TETANY.
• INCREASED REQUIREMENT FOR MAGNESIUM IS ASSOCIATED WITH LOSS OF MAGNESIUM IN MILK DURING LACTATION . WHEREAS THE AMOUNT OF MAGNESIUM IN MILK IS NOT HIGH (12 MG \ KG).

FACTORS INFLUENCING ABSORPTION OF MG .

• IN THE ADULT RUMINANTS , MAGNESIUM ABSORPTION OCCURS IN THE FORESTOMACH WITH LITTLE ABSORPTION IN THE ABOMASUM AND SMALL INTESTINE . SOME ABSORPTION OCCURS IN THE LARGE INTESTINE, PARTICULARLY IN SHEEP , HOWEVER IT CAN NOT COMPENSATE FOR MALABSORPTION IN THE FORE STOMACH .
• A- NA : K RATIO :
• MAGNESIUM IS TRANSPORTED ACROSS THE EPITHELIUM OF FORE STOMACH BY AN ACTIVE SODIUM – LINKED ATPASE – DEPENDENT TRANSPORT SYSTEM . ABSORPTION AND SERUM MG CONCENTRATION IS INFLUENCED BY NA : K RATIO IN THE RUMEN , WHICH IS DETERMINED BY THE DIETARY AND SALIVARY CONCENTRATION OF NA & K . ABSORPTION OF MG INCREASE WITH INCREASING NA : K RATIO TO PLATEAU AT RATIO OF 5 : 1 . ABSORPTION IS SIGNIFICANTLY IMPAIRED IF THE NA : K RATIO LESS THAN 3 : 1 .

• B - PROTEIN WHICH IS READILY FERMENTABLE AND LEAD TO INCREASED AMMONIA CONCENTRATION . A SUDDEN RISE OF RUMINAL CONCENTRATION OF AMMONIA IMPAIRS MAGNESIUM ABSORPTION IN THE RUMEN .
• C- THE UPTAKE OF MAGNESIUM IS INFLUENCED BY CARBOHYDRATE CONTENTS OF THE DIET . MG ABSORPTION IS IMPROVED WITH INCREASING AMOUNT OF READILY DEGRADABLE CARBOHYDRATE .
• D- VOLATILE FATTY ACIDS PROVIDE THE ENERGY FOR ACTIVE TRANSPORT OF THE MG ACROSS THE RUMEN WALL AND INCREASE MAGNESIUM ABSORPTION .
• E- OTHER DIETARY SUBSTANCES HAVE BEEN PROPOSED TO INFLUENCE THE ABSORPTION OF MG INCLUDING CALCIUM AND PHOSPHORUS , ORGANIC ACIDS SUCH AS CITRIC ACID , FATTY ACIDS AND ALUMINUM BUT THE SIGNIFICANCE OF THEIR ROLE IS CONTROVERSIAL .
• EPIDEMIOLOGY
• DISEASE OF ALL CLASSES OF RUMINANTS BUT REACHED ITS HIGH INCIDENCE IN OLDER LACTATING COWS EXPOSED TO BAD WEATHER OR GRAZING GREEN CEREAL CROPS OR LUSH GRASS – DOMINANT PASTURE .

PATHOGENESIS

• MOST CLINICAL CASES OF DISEASE HAVE SERUM MAGNESIUM LEVEL BELOW 1 MG \ DL COMPARED WITH NORMAL LEVEL IN CATTLE OF 1.7 – 3 MG \ DL . MAGNESIUM HAS MANY INFLUENCES ON IMPULSE TRANSMISSION AT THE NEUROMUSCULAR SYSTEM INCLUDING EFFECTS OF THE RELEASE OF ACETYLCHOLINE , ON THE SENSITIVITY OF THE MOTOR END PLATE , ON THE THRESHOLD OF THE MUSCLE MEMBRANE AND ON ACTIVATION OF CHOLINESTERASE SYSTEM . THESE OFFER AN ATTRACTIVE HYPOTHESIS FOR THE MUSCULAR IRRITABILITY SEEN WITH DISEASE .

CLINICAL FINDINGS

• LACTATING TETANY IS DESCRIBED IN ACUTE , SUB ACUTE AND CHRONIC FORMS .
• A- ACUTE LACTATING TETANY .
• THE ANIMAL MAY BE GRAZING AT THE TIME AND SUDDENLY CEASE TO GRAZE , ADOPT POSTURE OF UNUSUAL ALERTNESS AND APPEAR UNCOMFORTABLE ; TWITCHING OF THE MUSCLES AND EARS IS ALSO EVIDENT . THERE IS SEVERE HYPERESTHESIA AND SLIGHT DISTURBANCE PRECIPITATE ATTACK OF CONTINUOUS BELLOWING . THE GAIT BECOME STAGGERING AND THE ANIMAL FALL WITH OBVIOUS TETANY OF THE LIMBS WHICH IS RAPIDLY FOLLOWED BY CLONIC CONVULSION LASTING FOR ABOUT MINUTE .
• DURING THE CONVULSIVE EPISODES THERE IS :
• 1- OPSTHOTONOUS .
• 2- NYSTIGMUS .
• 3- CHAMPING OF THE JAWS .
• 4- FROTHING AT THE MOUTH .
• 5- PRICKING OF THE EARS .
• 6-RETRACTION OF THE EYELIDS .

• BETWEEN EPISODES THE ANIMAL LIE QUIETLY BUT SUDDEN NOISE OR TOUCH MAY PRECIPITATE ANOTHER ATTACK . THE TEMPERATURE RISE TO 40 – 40.5 ◦C AFTER SEVERE MUSCLE EXERTION . THE PULSE AND RESPIRATORY RATES ARE ALSO HIGH . THE ABSOLUTE INTENSITY OF THE HEART SOUNDS IS INCREASED SO THAT THEY CAN BE HEARD SOME DISTANCE AWAY FROM THE COW . DEATH USUALLY OCCURS WITHIN 1 – 5 HOURS 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 .
• B – SUB ACUTE LACTATING TETANY .
• IN THIS FORM OF THE DISEASE THE ONSET IS MORE GRADUAL OVER THE PERIOD OF 3 – 4 DAYS ; THERE IS SLIGHT INAPPETENCE ; WILDNESS OF FACIAL EXPRESSION AND EXAGGERATED LIMB MOVEMENT. SPASMODIC URINATION AND FREQUENT DEFECATION ARE CHARACTERISTIC . THE APPETITE AND MILK YIELD ARE DIMINISHED AND RUMINAL MOVEMENT DECREASED . MUSCLE TREMOR AND MILD TETANY OF HIND LEGS, STRADDLING GAIT MAY BE ACCOMPANIED BY RETRACTION OF THE HEAD AND TRISMUS . SUDDEN MOVEMENT , NOISE , THE APPLICATION OF RESTRAINT OR INSERTION OF THE NEEDLE MAY PRECIPITATE A VIOLENT CONVULSIONS .
• ANIMALS WITH THIS FORM OF THE DISEASE MAY RECOVER SPONTANEOUSLY WITHIN FEW DAYS OR PROGRESS TO STAGE OF RECOMBANCY WHICH SIMILAR BUT RATHER MILDER SYNDROME THAN IN THE ACUTE FORM . TREATMENT IS USUALLY EFFECTIVE BUT THERE IS MARKED TENDENCY TO RELAPSE.

C – CHRONIC HYPOMAGNESEMIA

• MANY ANIMALS IN AFFECTED HERDS HAVE LOW SERUM MAGNESIUM LEVELS BUT DO NOT SHOW CLINICAL SIGNS . THERE MAY BE SUDDEN DEATH . A FEW ANIMALS DO EVIDENCE A RATHER VAGUE SYNDROME INCLUDING DULLNESS , UNTHRIFTNESS AND INDIFFERENT APPETITE AND MAY SUBSEQUENTLY DEVELOP ONE OF THE MORE OBVIOUS SYNDROME .

CLINICAL PATHOLOGY

• SERUM MAGNESIUM LEVEL ABOUT 1-2 MG \ DL SUBCLINICAL BUT THE RISK OF TETANY IS NOT PRESENT UNTIL THE LEVEL FALL BELOW 1.2 MG \ DL .
• TOTAL SERUM CALCIUM LEVEL ARE OFTEN REDUCED TO 5 – 8 MG \DL.
• IN ACUTE TETANY SERUM POTASSIUM LEVEL ARE USUALLY DANGEROUSLY HIGH AND MAY CONTRIBUTE TO HIGH DEATH RATES .
• CSF MG LEVEL IS 1.25 MG \ DL WERE FOUND IN THE TETANIC COWS WITH HYPOMAGNESEMIA .
• URINE MAGNESIUM LEVEL IS LOW .
• DIFFERENTIAL DIAGNOSIS :
• A- IN CATTLE .
• 1- ACUTE LEAD POISONING.
• 2- RABIES .
• 3- NERVOUS KETOSIS .
• 4- BSE .

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• B- IN SHEEP
• 1- HYPOCALCEMIA .
• 2- STAGGER' SYNDROMES.

TREATMENT .

• IV ADMINISTRATION OF 500 ML OF SOLUTION CONTAINING 25 % CALCIUM BOROGLUCONATE AND 5 % MG HYPOPHOSPHATE FOR CATTLE AND 50 ML FOR SHEEP . FOLLOWED BY SC INJECTION OF CONCENTRATED SOLUTIONS OF MAGNESIUM SALTS .
• 200 – 300 ML OF 20 % SOLUTION OF MAGNESIUM SULFATE MAY BE INJECTED IV THIS IS FOLLOWED BY RAPID RISE OF SERUM LEVEL OF MAGNESIUM WHICH RETURNED TO NORMAL LEVEL WITHIN 3 – 6 HOURS .
• SC INJECTION OF 200 ML OF 50 % MAGNESIUM SULFATE SOLUTION HAS BEEN RECOMMENDED .
• * IV INJECTION OF MAGNESIUM SALTS IS NOT WITHOUT DANGER , IT MAY INDUCE CARDIAC DYSARRYTHEMIA OR MEDULLAR DEPRESSION MAY BE SEVERE ENOUGH TO CAUSE RESPIRATORY FAILURE . IF SIGNS OF RESPIRATORY DISTRESS OR EXCESSIVE SLOWING OR INCREASE IN HEART RATE ARE NOTICED , THE INJECTION SHOULD BE STOPPED IMMEDIATELY.

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• THE SUBSTITUTION OF MAGNESIUM LACTATE FOR MAGNESIUM SULFATE HAS BEEN RECOMMENDED TO PROVIDE A MORE PROLONGED ELEVATION OF SERUM MAGNESIUM LEVEL .

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• MAGNESIUM GLUCONATE 200 – 400 ML OF 15 % SOLUTION .