Hypomagnesemic Tetany in Cattle and Sheep

Core Definition & Pathophysiology:

Definition: An acute, complex metabolic disturbance and neurological condition caused by low dietary intake of magnesium (Mg).
Thresholds for Tetany:
- Plasma Total Mg (tMg): <1.5 mg/dL in general; clinical tetany usually occurs at <1.2 mg/dL in cattle and <0.5 mg/dL in sheep.
- CSF Concentration: Decreased tMg in the cerebrospinal fluid to <1.0 mg/dL.
The Magnesium Gap: Occurs when dietary absorption fails to meet the high requirements for maintenance (3 mg/kg body weight) and lactation (120 mg/kg milk).

Etiology & Risk Factors:

Primary Risk Group: Adult lactating animals are most susceptible due to the high loss of Mg in milk.
Dietary Triggers:
- Grazing on lush grass pastures or green cereal crops.
- Short-grass dominant pastures containing <0.2% Mg on a dry-matter basis.
Soil & Mineral Interactions:
- High potassium (K) and nitrogen (N) intake decreases Mg absorption from the rumen.
- Risk is higher in soils fertilized with potash and nitrogen (e.g., poultry manure).
- Low sodium (Na) and phosphorus (P) intake further inhibits absorption.
Environmental Stress: Inclement weather, transport, or reduced food intake.
Secondary Factor: Often involves concurrent hypocalcemia (blood calcium <8.0 mg/dL), which can be predisposed by metabolic alkalosis from lush pastures.

Clinical Findings

Acute Form:
- Sudden head throwing, bellowing, and "blind frenzy" galloping.
- Severe paddling seizures, chomping of jaws, and frothy salivation.
- Death usually occurs within a few hours.

Less Severe/Early Signs:
- Muscle twitching (face, flank, shoulder) and a stiff gait.
- Hypersensitivity to touch and sound.
Physiological Signs:
- Tachycardia: Heart rate approaching 150 beats/minute.
- Loud heart sounds audible without a stethoscope.
- Hyperthermia: Fever up to 105°F (40.5°C) due to excessive muscle activity.

Diagnosis

Antemortem:
- Presumptive: Based on history, clinical signs, and rapid response to treatment.
- Definitive: Low serum or urine Mg concentrations (Urine Mg is often undetectable).

Postmortem:
- CSF: Mg concentrations <1 mg/dL within 12 hours of death.
- Vitreous Humor: Mg concentrations measured within 24–48 hours of death.
Calves: Analysis of bone; a Calcium:Magnesium ratio ≥90:1 indicates deficiency (normal is 70:1).

Treatment & Management

Emergency Parenteral Therapy: Slow IV injection of combined calcium and magnesium solutions (e.g., 400 ml of 40% calcium borogluconate + 50 ml of 25% magnesium sulfate).
Important Precaution: The animal should not be stimulated during treatment to avoid triggering fatal seizures.
Subcutaneous (SC) Support: Additional Mg sulfate can be given SC.
Follow-up: Recovery is slower than hypocalcemia alone because it takes time to restore Mg levels in the CSF. Prevent recurrence by providing hay treated with 60 g of Mg oxide daily.

Prevention Strategies

Daily Supplementation: Because the body has no readily available Mg stores, supplements must be given daily (60 g for cattle; 10 g for sheep).
Pasture Management:
- Dusting herbage with powdered Mg oxide (500 g/cow).
- Spraying pastures with a 2% Mg sulfate solution.
Animal Husbandry: Protect out-wintered stock from wind/cold and ensure access to hay when grazing high-risk cereal crops or fertilized pastures.

Hypomagnesemic Tetany in Calves:

Target Population: 2- to 4-month-old calves fed milk only, or younger calves with chronic scours.
Mechanism: Mg absorption efficiency drops from 87% at 2 weeks old to 32% by 7–8 weeks old.
Differential Diagnosis: Must be distinguished from lead poisoning, tetanus, PEM, and enterotoxemia.

Hypomagnesemic Tetany in Cattle and Sheep

Core Definition & Pathophysiology:

Definition: An acute, complex metabolic disturbance and neurological condition caused by low dietary intake of magnesium (Mg).

Thresholds for Tetany:

Plasma Total Mg (tMg): <1.5 mg/dL in general; clinical tetany usually occurs at <1.2 mg/dL in cattle and <0.5 mg/dL in sheep.

CSF Concentration: Decreased tMg in the cerebrospinal fluid to <1.0 mg/dL.

The Magnesium Gap: Occurs when dietary absorption fails to meet the high requirements for maintenance (3 mg/kg body weight) and lactation (120 mg/kg milk).

Etiology & Risk Factors:

Primary Risk Group: Adult lactating animals are most susceptible due to the high loss of Mg in milk.

Dietary Triggers:

Grazing on lush grass pastures or green cereal crops.

Short-grass dominant pastures containing <0.2% Mg on a dry-matter basis.

Soil & Mineral Interactions:

High potassium (K) and nitrogen (N) intake decreases Mg absorption from the rumen.

Risk is higher in soils fertilized with potash and nitrogen (e.g., poultry manure).

Low sodium (Na) and phosphorus (P) intake further inhibits absorption.

Environmental Stress: Inclement weather, transport, or reduced food intake.

Secondary Factor: Often involves concurrent hypocalcemia (blood calcium <8.0 mg/dL), which can be predisposed by metabolic alkalosis from lush pastures.

Clinical Findings

Acute Form:

Sudden head throwing, bellowing, and "blind frenzy" galloping.

Severe paddling seizures, chomping of jaws, and frothy salivation.

Death usually occurs within a few hours.

Less Severe/Early Signs:

Muscle twitching (face, flank, shoulder) and a stiff gait.

Hypersensitivity to touch and sound.

Physiological Signs:

Tachycardia: Heart rate approaching 150 beats/minute.

Loud heart sounds audible without a stethoscope.

Hyperthermia: Fever up to 105°F (40.5°C) due to excessive muscle activity.

Diagnosis

Antemortem:

Presumptive: Based on history, clinical signs, and rapid response to treatment.

Definitive: Low serum or urine Mg concentrations (Urine Mg is often undetectable).

Postmortem:

CSF: Mg concentrations <1 mg/dL within 12 hours of death.

Vitreous Humor: Mg concentrations measured within 24–48 hours of death.

Calves: Analysis of bone; a Calcium:Magnesium ratio ≥90:1 indicates deficiency (normal is 70:1).

Treatment & Management

Emergency Parenteral Therapy: Slow IV injection of combined calcium and magnesium solutions (e.g., 400 ml of 40% calcium borogluconate + 50 ml of 25% magnesium sulfate).

Important Precaution: The animal should not be stimulated during treatment to avoid triggering fatal seizures.

Subcutaneous (SC) Support: Additional Mg sulfate can be given SC.

Follow-up: Recovery is slower than hypocalcemia alone because it takes time to restore Mg levels in the CSF. Prevent recurrence by providing hay treated with 60 g of Mg oxide daily.

Prevention Strategies

Daily Supplementation: Because the body has no readily available Mg stores, supplements must be given daily (60 g for cattle; 10 g for sheep).

Pasture Management:

Dusting herbage with powdered Mg oxide (500 g/cow).

Spraying pastures with a 2% Mg sulfate solution.

Animal Husbandry: Protect out-wintered stock from wind/cold and ensure access to hay when grazing high-risk cereal crops or fertilized pastures.

Hypomagnesemic Tetany in Calves:

Target Population: 2- to 4-month-old calves fed milk only, or younger calves with chronic scours.

Mechanism: Mg absorption efficiency drops from 87% at 2 weeks old to 32% by 7–8 weeks old.

Differential Diagnosis: Must be distinguished from lead poisoning, tetanus, PEM, and enterotoxemia.