Grass Tetany by André Voisin

CHAPTER 4

Lower content of magnesium in the blood serum accompanies grass tetany


Levels of magnesium and calcium in the blood serum of cows suffering from grass tetany

Hypomagnesaemic grass tetany, by its very definition,is accompanied by a diminution in the magnesium content of the blood serum. Depending on the individual case, there may or may not be a simultaneous diminution in the calcium level of the blood serum. In general, there is no appreciable variation in the content of mineral phosphorus.
With milk fever there is first and foremost a very considerable drop in the calcium and mineral phosphorus levels whereas the magnesium level exhibits a very slight tendency to rise. This is demonstrated by Table 2, which shows the means established by SJOLLEMA more than thirty years ago over a large number of cows.

Table 2: Magnesium, calcium and phosphorus contents in the blood serum of cows suffering from grass tetany or milk fever

Means of this nature do not illustrate many of the individual cases that arise. For example, SJOLLEMA observed very low magnesium contents in association with normal calcium contents 1 in certain cows suffering from grass tetany. He found magnesium contents of 0-45 - 0-22 and 0-82 mg./100 c.c. in cows showing normal calcium contents, viz. 9-45 - 9-5 and 9-1 mg./100 c.c. respectively.
W. M. ALLCROFT studied 406 cows that were clinical cases of hypomagnesaemia. He found no hypocalcaemia in the few heifers attacked. Half of the cows that had become hypomagnesaemic in the course of their first lactation were hypocalcaemic. From the fourth lactation onwards three out of four hypomagnesaemic cows were also hypocalcaemic, and this proportion increased with every lactation. SEEKLES found a higher proportion and is of the opinion that 90 - 100% of the cases of hypomagnesaemia are accompanied by hypocalcaemia.
The older the cow, moreover, the more frequent the occurrence of hypocalcaemia in hypomagnesaemic tetany. In young cows 2 attacked by tetany the calcium content of the blood serum is generally normal. 3
Underfeeding, which is an important factor in winter grass tetany, gives rise to a hypomagnesaemia which is always accompanied by hypocalcaemia. 4 The result is that, when underfeeding is a contributory factor of any importance in hypomagnesaemia, hypocalcaemia will tend to be more marked.
The calcium content of the blood serum, therefore, is a very uncertain criterion to adopt in distinguishing between grass tetany and milk fever. The magnesium and mineral phosphorus levels are much more reliable indicators. 5

Seasonal variations in the magnesium content of the blood serum

Unfortunately the magnesium content of the blood serum of the cow at grass 6 is subject to seasonal variations which, as shown in Figure 2 concerning cows at grass throughout the year, are as much as 100%. 7 It will be seen below, moreover, that an old ewe has a lower content of magnesium in its blood serum on the average than a young animal.
The different causes of this variation make it difficult to fix for serum a minimum magnesium content below which the animal can be said to be suffering from hypomagnesaemia.

Figure 2: Variations in the magnesium content of the blood serum of cows at grass throughout the year.

Difficulties of analysing the blood of an animal suffering from, or that has died from, tetany

For various reasons it is a difficult business to sample blood in order to ascertain whether an animal is suffering from hypomagnesaemia. In effect, it takes easily two days for the sample to be sent in to the laboratory and analysed, and by the time the result is available, giving the veterinary surgeon the information he requires, the chances are that the animal will be already dead.
The actual sampling operation presents difficulties, moreover. If it is done a short time after death or even just before death the information it yields is unfortunately unreliable, because at this time magnesium is released into the serum by the tissues.
Worse still, if the blood sample is to provide accurate information it should be taken BEFORE the tetany attack, because in an animal suffering from convulsions there is an immediate rise in the magnesium level of the serum due to the release of magnesium by the muscles (see Figure 5). This has been observed in rats as well as cows.

Lower calcium and magnesium limits

In the light of these reservations, what indications can the analysis of blood serum provide?
If the magnesium content of the blood serum before convulsions is lower than 1-20 mg./100 c.c., then hypomagnesaemia is the probable cause: if it is lower than 0-90 mg./100 c.c. hypomagnesaemia is the definite 8 cause of the convulsions.
So far as calcium is concerned, when the content of the serum is less than 6 mg./100 c.c. hypocalcaemia may be contributing to the occurrence of convulsions, when it is less than 5 mg/100 c.c. hypocalcaemia is definitely contributing to the convulsions. 9 




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Notes



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  1.  Cf. the simultaneous and slow drop in the 
magnesium and calcium levels in the blood serum of suckling calves 
(Figure 6) and the hypomagnesaemic tetany that ensues. *


 

    


  2.  Noting that grass tetany is less frequent in 
young than in old cows (see Part VII). *


    
       

  3.  Note the opinion of certain research workers 
(MERSHON) that 
in adult animals there was no tetany unless the
hypomagnesaemia was accompanied by hypocalcaemia.*


    
       

  4.  Some workers have found very low copper 
levels in the blood serum of cows or ewes suffering from grass tetany. 
They considered that this serious hypocupraemia must play 
an important part in the tetany. *


    
       

  5.  Bearing in mind that observations made on the 
relationship between the magnesium content of the blood serum and the 
appearance of nervous disorders are often contradictory. 
This may be explained by the fact, noted by GREENBERG 
as far back as 1938, that there is not necessarily a 
correlation between the extra-cellular magnesium of the 
blood serum and the magnesium in the cells, particularly 
those of the neuro-muscular system.
  In the case of delirium tremens in humans a better 
correlation has been found of nervous disorders with 
the magnesium content of the red corpuscles than with the
magnesium content of the blood serum. *


    
       

  6.  BROCHART likewise found a seasonal variation 
in the magnesium of the serum of cows kept in the stall. *


    
       

  7.  For the influence of temperature and humidity 
on grass tetany see Chapter 34. *



    


  8.  It should be remembered that whereas grass 
tetany is always accompanied by hypomagnesaemia, hypomagnesaemia 
can be present without convulsions (see Figure 16, Chapter 26). *



    


  9.  Remembering that in the case of milk fever 
the reduction in the level of blood-serum calcium is accompanied 
by a reduction in its level of mineral phosphorus (see Table 2). *




 



  
















 

       
         TABLE 2 
  
       
Magnesium, calcium and phosphorus contents in the blood serum
 
of cows suffering from grass tetany or milk fever



	

	 
	Content in the blood serum (mg. in 100 c.c.)
	

	

	Mineral
	  Calcium   
	  Magnesium   
	  Phosphorus  
	  Ponderat Ratio   
	

	

	 
	(Ca)
	(Mg)
	(P)
	Ca/Mg
	P/Mg
	

	

	Normal
	9 · 35
	1 · 66
	4 · 57
	5 · 62
	2 · 75
	

	

	With grass tetany
	6 · 65
	0 · 45
	4 · 33
	14 · 80
	9 · 62
	

	

	With milk fever
	4 · 35
	2 · 19
	2 · 16
	1 · 99
	0 · 99 



       
N.B. 1. The figures represent the mean for a large number of cows.

       2. The samples were taken at the time of 
tetany or milk fever attacks.

After SJOLLEMA *