Describe the causes, clinical symptoms, diagnosis, prevention and treatment of hypomagnesemic tetany.

What is Hypomagnesemic Tetany?

Hypomagnesemic tetany is a metabolic disorder characterized by low serum magnesium (< 1.2 mg/dL). Magnesium is essential for proper nerve and muscle function; a deficiency disrupts neuromuscular transmission, leading to tetanic contractions.

Causes of Hypomagnesemic Tetany

The causes of HypoMag are multifactorial and often involve a combination of factors:

• Dietary Magnesium Deficiency: Lush, rapidly growing pastures, particularly in spring, are often low in magnesium. The rapid uptake of magnesium by the plant during growth dilutes its concentration.
• Impaired Magnesium Absorption: Conditions affecting the small intestine, such as parasitic infections or inflammation, can reduce magnesium absorption.
• Increased Magnesium Excretion: Stress, particularly around calving, increases magnesium excretion in urine and feces. Milk fever (parturition fever) can also exacerbate magnesium loss.
• Postpartum Stress: The physiological demands of lactation and parturition (calving) increase magnesium requirements and can lead to excretion.
• Potassium and Calcium Imbalances: High potassium intake can interfere with magnesium absorption. Calcium imbalances can also influence magnesium metabolism.
• Water Quality: Water sources low in magnesium can contribute to deficiency.

Clinical Symptoms

Clinical signs vary in severity:

• Early Signs: Restlessness, nervousness, muscle twitching (fasciculations), incoordination.
• Moderate Signs: Stiff gait, muscle tremors, hyper-flexion of joints, increased respiratory rate.
• Severe Signs: Collapse, convulsions, recumbency (lying down), and death. The sudden onset of severe signs is characteristic.

Diagnosis

Diagnosis is based on clinical signs and laboratory tests:

• Clinical Examination: A thorough physical exam and observation of clinical signs are essential.
• Serum Magnesium Measurement: A blood sample is taken and analyzed to determine serum magnesium levels. A level below 1.2 mg/dL is generally diagnostic.
• Electrolyte Panel: Assessing potassium and calcium levels helps identify contributing factors.
• Urine Magnesium Excretion: Can help determine if excessive magnesium loss is a factor.

Prevention

Prevention focuses on optimizing magnesium intake and minimizing losses:

• Magnesium Supplementation: Providing magnesium supplements in feed or water, particularly during periods of lush pasture growth. Magnesium oxide is a common supplement.
• Pasture Management: Rotational grazing and introducing mature grasses can improve magnesium content.
• Mineral Balancing: Ensuring proper calcium and potassium levels in the diet.
• Stress Management: Minimizing stress during calving through proper handling and nutrition.
• Water Quality Assessment: Testing water sources for magnesium content.

Treatment

Treatment aims to rapidly restore magnesium levels:

• Intravenous Magnesium Sulfate: The fastest and most effective method. Requires careful monitoring.
• Intramuscular Magnesium Sulfate: Slower absorption than intravenous administration.
• Oral Magnesium Supplements: Less effective for acute cases but can be used for maintenance.

Method	Onset of Action	Effectiveness	Requires	Limitations
Intravenous Magnesium Sulfate	Rapid (minutes)	Highly Effective	Venous access, trained personnel	Risk of side effects (slowed respiration)
Intramuscular Magnesium Sulfate	Slower (30-60 minutes)	Moderate	Proper injection technique	Slower onset, potential for tissue irritation
Oral Magnesium Supplements	Slow (hours)	Least Effective (for acute cases)	Consistent administration	Not suitable for emergency situations

Case Study: The Spring Tetany Outbreak in Wisconsin

In 2018, a dairy farm in Wisconsin experienced a significant outbreak of HypoMag during the spring flush of pasture growth. The cows, primarily Holstein, were grazing on lush ryegrass pastures with low magnesium content. Despite supplemental magnesium in the total mixed ration (TMR), several cows exhibited clinical signs of tetany, resulting in two fatalities. Post-mortem analysis revealed severely depleted magnesium stores in the liver. The farm implemented a rotational grazing system, supplemented with magnesium oxide in the TMR and drinking water, which significantly reduced the incidence of tetany in subsequent years. This demonstrated the importance of a multi-faceted preventative approach.

Recent Developments

Research is ongoing to identify genetic markers that predispose cattle to HypoMag, which could enable selective breeding for increased magnesium retention. Additionally, investigations into the role of gut microbiota in magnesium absorption are gaining traction.

Hypomagnesemic tetany remains a significant challenge for livestock producers, particularly in dairy farming. A proactive approach encompassing dietary management, stress reduction, and prompt treatment is crucial for preventing and managing this debilitating condition. The multi-faceted nature of the condition requires a holistic understanding of bovine nutrition and physiology. Continued research into genetic predispositions and gut health holds promise for further improving preventative strategies and minimizing the economic losses associated with HypoMag.