Hypomagnesaemia

Hi, this post describes the pathogenesis, clinical signs and treatment of hypomagnesaemia. Enjoy :)

Hypomagnesaemia is a condition resulting from abnormally low levels of magnesium in the blood plasma. It is also known as grass tetany, grass staggers and lactation tetany in ruminants. It is common among ruminants and is seen more frequently in lactating animals as well as young calves fed on whole milk. 

Pathogenesis

Hypomagnesaemia is the result of an imbalance between magnesium inputs and outputs. The condition often occurs along with hypocalcaemia. This is because when plasma magnesium concentrations fall below 0.5mM, calcium homeostasis fails and hypocalcaemia results. Also, hypocalcaemia triggers the drainage of magnesium from the brain which causes hypomagnesaemia. 

There are several factors which contribute to the pathogenesis of hypomagnesaemia. 

Dietary Factors

In terms of the diet, high potassium intake may significantly reduce magnesium absorption from the GIT. High potassium concentrations in the rumen may result from grazing pastures on soils that are naturally high in potassium or fertilized with potash, salt deficiency, or from a change in diet from dry feed to lush pasture. 

Low sodium concentrations in lush green grass fail to meet the dietary requirements of lactating cows, which leads to high potassium levels in the rumen and decreased magnesium absorption. 

High crude protein intake and rumen ammonia in the diet may also decrease magnesium availability. This can be found in young lush grass which has been treated with nitrogen fertilizers. 

In addition, low amounts of fermentable carbohydrates as well as high amounts of lipid or fatty acid concentrations in the diet may decrease magnesium availability. Lipids and fatty acids from fertilized grasses may for magnesium soaps in the rumen which aren’t able to be absorbed. A high rumen pH may also reduce the solubility of magnesium in the rumen. 

Seasonal Changes

Winter-calving cows and winter-lambing ewes are at greater risk of hypomagnesaemia. This is because they are exposed to cold, lush pastures which are low in soluble carbohydrates and higher in nitrogen and fats. These animals also have a higher demand for calcium and magnesium for their milk. 

Phosphorus and Calcium

Low levels of phosphorus in the rumen have been shown to impair the absorption of magnesium. 

Animal Factors

The rate of absorption of magnesium decreases with age. Older cows also have more fat and so have less available magnesium in their body fluids. Angus and Angus-cross breeds are more susceptible to hypomagnesaemia than Hereford, Jersey, Holstein and Brahman breeds. High milk production can also cause this condition because much of the dietary intake of magnesium is lost in the milk. Colostrum also has two to three times more magnesium than regular milk. 

Environmental Factors

Factors in the animal’s environment which may cause stress may also trigger grass tetany in lactating cows and ewes. These factors include: exposure to bad weather, rain and wind; a sudden change in feed and feed quality; low temperatures; mustering; transport; and shearing.

Clinical Signs

Hypomagnesaemia occurs in three forms: acute, sub-acute and chronic. 

Acute

This form requires urgent treatments. Signs include:

• The animal suddenly stops grazing and becomes extremely alert 
• Twitching of ears and muzzle 
•  Animals may gallop in a wild frenzy 
• Staggering, then the animal goes down into lateral recumbency with frantic paddling and tonic convulsions which may last up to a minute. 
• During the convulsions nystagmus, champing of jaws, clicking of eyelids, the head arches back and frothing at the mouth occurs. 
• Pulse and respiration rate are elevated. 
• Death can occur within 30-60 minutes and mortality rate is high if not treated.   

Sub-Acute

The onset of this form is more gradual. 

•  Animals may have abnormally low levels of magnesium in the blood for some time without showing symptoms. Symptoms only occur when triggered by an environmental stressor.
•  Slight in appetence 
• Wildness of facial expression and exaggerated gate 
• Difficult to drive 
•  Spasmodic and frequent urination and defecation 
•  Milk yield and ruminal movements in crease 
•  Blinking and clicking of eyelids when approached 
• Fine muscle tremors develop 
• Unsteady and straddling gate 
•  Convulsions may occur is the animal becomes excited 
•  Cows frequently go down when milking 
• Animals with this form of the disease may recover spontaneously or progress to recumbency. 

 

Chronic Form

This is a vague syndrome that may affect herds with low levels of plasma magnesium. They may not exhibit any clinical signs but some animals may show: 

• Vague dullness 
• Ill-thrift 
• Indifferent appetite.

The chronic form of hypomagnesaemia may progress to the acute form. It may also occur in animals that have recovered from the subacute form. 

Treatment and Prevention

The first step in treating hypomagnesaemia is to correct the magnesium deficiency. This involves administering magnesium intravenously while monitoring the heart rate. This is because magnesium salts are toxic and may result in respiratory failure. Combinations of magnesium and calcium borogluconate may be given as hypocalcaemia and hypomagnesaemia often occur together. The animal should improve within 3 to 5 hours of treatment and should be left undisturbed during this period. Follow up doses of magnesium may be administered orally or subcutaneously. If the animal is throwing convulsions, sedation is required if possible. Relapses are common but the provision of legume hay is helpful in preventing this.  

Prevention of hypomagnesaemia is dependent on the continuous intake of adequate amounts of magnesium. Supplements of magnesium oxide or magnesium sulphate (Epsom salts) may provide this. Supplementation may be via loose lick, magnesium treated hay, magnesium in the grain or water, magnesium bullets and magnesium blocks. 

Well, that's what we need to know about hypomagnesaemia for now. Let me know if you have any questions :)