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.

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

Treatment

Outcomes