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Hypomagnesemia
Synonym(s): hypomag, hereford disease lactation tetany hypomagnesaemia
Introduction
- Cause: low blood magnesium (Mg) levels, as a result of inadequate dietary magnesium intake or poor absorption from the gut.
- Signs: hyperesthesia, ataxia, seizures, sudden death.
- Diagnosis: blood magnesium levels.
- Definition: serum ionized magnesium <1.5mg/dL or <0.65mmol/L.
- Treatment: intravenous and subcutaneous magnesium/calcium preparation.
- This is a true emergency and treatment must be initiated ASAP.
- Treatment goes beyond stabilising the individual cow.
- These cases are often the tip of the iceberg and a herd approach will be necessary.
- Less experienced clinicians would be encouraged to seek the assistance of their senior colleagues when advising farmers as to herd management of these cases.
- Prognosis: variable. Dependent upon early intervention and treatment.
Age predisposition
- Lactating cattle and fast growing calves predisposed due to increased Mg demand.
- Older beef cattle.
- Young calves being fed milk only.
- Scouring calves on milk/milk replacement diet.
Cost considerations
- Acutely affected animals usually represent an underlying herd problem.
Special risks
- Animals with severe hypomagnesemia are at risk of cardiac dysrhythmias Dysrhythmias.
Pathogenesis
Etiology
- Insufficient dietary Mg Magnesium: an overview - fails to meet maintenance requirements, growth requirements or lactational requirements.
Predisposing factors
General
- Lush pasture: especially spring growth - fast growing.
- Lactating cattle: due to Mg loss in milk.
- Fast growing calves: due to increased Mg demand.
- Exposure to extreme weather (reduced food intake and increased stresses).
- Beef cattle and dry cows predisposed if left to overwinter out at pasture with no supplementation.
Specific
- Most commonly seen around the time of calving.
- High levels of Potassium Sodium and potassium - an overview (in soil) and Nitrogen (from fertilizer)- reduces Mg available for uptake by the cow.
Pathophysiology
- Magnesium Magnesium homeostasis is an essential cation involved in many intracellular enzymatic reactions, including regulating the calcium channel function, neurotransmission, vasomotor tone, muscular contraction and cardiac excitability.
- Magnesium is primarily an intracellular cation, with only 1% being found in the serum.
- Intracellular magnesium is primarily in the bone (67%), muscle (2%) and other soft tissues (11%).
- Extracellular magnesium exists in three forms:
- Protein-bound.
- Complexed with anions (such as phosphate).
- Ionized.
- Ionized magnesium is the physiologically active form.
- Total serum levels therefore may not reflect either the active moiety or the total body concentration.
- Magnesium is antagonistic to the actions of calcium Calcium and phosphorous - an overview.
- Cows with hypomagnesemia usually become symptomatic when hypocalcemia Hypocalcemia and hypophosphatemia - overview (< 2mmol/L) presents concurrently.
- A significant role for intracellular magnesium is as a co-factor for the calcium-ATP pump that moves intracellular calcium into the sacroplasmic reticulum.
- It performs a similar role with the sodium-potassium-ATP pump that moves these electrolytes across cell membranes.
- Reduction of the activity of these ion pumps is likely to be the major cause of the clinical signs of hypomagnesemia.
- Magnesium’s role in active transport of Ca and K ions across cell membranes plays an important role in the process of nerve impulse conduction. This in turn affects muscle contraction and cardiac muscle activity.
- This altered skeletal and cardiac muscle function leads to the symptoms of ataxia, muscle weakness and dysrhythmias Dysrhythmias.
- If not corrected can progress to death.
Timecourse
- Acute form: sudden death.
- Sub-acute form: may persist for days.
Diagnosis
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Treatment
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Prevention
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Outcomes
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Further Reading
Publications
Refereed Papers
- Recent references from PubMed and VetMedResource.
- Allcroft R & Burns K (2011) Hypomagnesaemia in Cattle. NZ Vet J 16 (7).