Grass Tetany by André Voisin

CHAPTER 3

External symptoms of tetany


Present limited subject

Before examining the symptoms of grass tetany it is well to bear in mind that the various forms of convulsions or paralysis known as tetanies can be triggered off by a diversity of causes. 1 Tetany, therefore, is not a disease but a symptom that may be the result of several causes, one of the best known being disturbed calcium metabolism. 2 In the present instance it is the symptoms of grass tetany caused by a disturbance of the magnesium metabolism that are being studied: that is, tetany of animals at grass, tetany accompanied by a low magnesium content in the blood serum 3 or, in other words, hypomagnesaemic tetany.

Early symptoms of grass tetany

An experienced and informed observer can diagnose whether tetany is a menace to an animal or herd, and this may help the "protective" measures outlined in Part IX to be put into operation. In effect, one or more animals in the herd threatened will walk very stiffly, their hind legs having lost flexibility. It is this swaying "hackney gait" which gives the impression that the animal is staggering: hence the English name "grass staggers". The animal has no appetite and looks mournful. Its eyes are glazed and bulging to a certain extent. 4
Any excitement, the least noise or shock can cause the muscles of the shoulders and face to quiver and the eyelids to tremble. Often it is sufficient to clap one's hands in front of the cow's eyes to trigger off these, what might be described as "preliminary" convulsions. Apart from noise and excitement, a low temperature 5 or a high wind can help to unleash the latent attack. Again, if the animal is to receive an injection the contact between the needle and the skin may be sufficient to produce convulsions with perhaps a fatal attack of tetany.
If this stiffness in walking becomes manifest in several animals in a herd, particularly during the dangerous period when they have just gone out to grass, the effect of "triggering factors" must be avoided at all costs, whether making the animals run or giving the least shout. The "protective" measures, which will be discussed in Part IX, must be put into operation immediately.

Onset of an attack of convulsions

The onset of an attack of convulsions is rarely observed, the animal generally being found on the ground already affected. When circumstances permit the beginnings of such an attack to be watched the legs of the animal, who, as has just been said, had a very stiff gait, will become completely rigid for a few seconds, whereupon the animal, unable to maintain its balance any longer, falls to the ground. In less severe cases this fall may be preceded by a fairly long period during which the animal is subject to muscular tremors and lateral jaw movements. It will arch its back and throw back its head. Finally, in some mild cases the animal returns to normal after a period of these nervous symptoms.

Appearance of the convulsed animal on the ground

After the onset of an attack of convulsions the animal will be lying on the ground, its hooves stiff, its head thrown back and its eyes convulsed. Periods of rest alternate with the attacks, during which the animal rolls its eyes and grinds its teeth with a lateral movement of the jaw, which generally remains closed, emitting a large quantity of saliva. From time to time the animal may be able to move its stiff hooves as if it were running. The heart beat is fast and irregular. Respiration may stop for a period of up to 30 seconds. Despite its efforts, the animal cannot raise itself from the ground, for it does not seem able to bend its legs. Sometimes it is able, either alone or with help, to get into a sitting position and will rise up suddenly and run a few yards, only to fall down again stricken with a renewed attack of convulsions.
The temperature to begin with is normal, with a slight tendency to drop, but as the animal struggles its temperature rises, sometimes markedly, and may reach 104° F. (40° C.).
Attacks that are not fatal can last for several minutes: some last 4-5 hours.
Where the animal does not react well to magnesium treatment, it remains limp, either standing or lying down, and the fever persists or increases. It refuses all food and dies within two or three days.

Difference between the external symptoms of grass tetany and milk fever (or vitular fever)

An attempt has been made to distinguish the two types of convulsions common among ruminants, namely tetany and milk fever, on the basis of the external symptoms. This is not always easy, the symptoms of the two conditions being very similar. 6 AS LARVOR, BROCHART and THERET state, "it is a case of a whole combination of symptoms that are clearly different, although liable to present intermediate forms".
Already in 1931 WARRINGSHOLZ, in Holstein, had tried to distinguish milk fever from grass tetany on the grounds of the external symptoms. Not much progress would appear to have been made in the intervening thirty years, and the distinctions made by the German veterinary surgeon continue to afford valuable aid.
According to WARRINGSHOLZ, the following symptoms can be distinguished on the whole, although the distinction is not absolute:
  1. Muscles
    • Milk fever: prostration of the muscular system.
    • Grass tetany: muscle stiffness, contractions of the tail.
  2. Muzzle
    • Milk fever, mouth hardly open.
    • Grass tetany: mouth closed and difficult to open; grinding of teeth; in general, frothing of the mouth.
  3. Eyes
    • Milk fever: sleepy eyes.
    • Grass tetany: wild, blood-shot eyes, frequently rolling.
  4. Head
    • Milk fever: thrown to the side.
    • Grass tetany: thrown back.
  5. Sensitivity
    • Milk fever: diminished.
    • Grass tetany: much greater
  6. Pulse
    • Milk fever: vigorous and slightly increased.
    • Grass tetany: feeble and much more rapid.
  7. Udder
    • Milk fever: soft.
    • Grass tetany: more or less normal, no excessive softness.
  8. Temperature
    • Milk fever: low.
    • Grass tetany: normal or high.
Some of these characteristics distinguishing the two conditions will now be discussed briefly.

Increased irritability of the animal is characteristic of hypomagnesaemic tetany

Sensitivity is quite contrary 7 in hypocalcaemic milk fever and hypomagnesaemic grass tetany, and can be considered as clearly differentiating the two syndromes. Some research workers have even claimed that hyper-irritability is characteristic of the hypomagnesaemic animal. 8 It is because of this increased irritability that hypomagnesaemic animals fall down in a convulsion at the least noise or if they experience any emotion, such as fear, or any excitement such as is caused by the prick of a syringe needle. It is also the reason for cows in herds where animals have been or are about to be attacked by hypomagnesaemic tetany going mad and attacking humans.
The cow that is only suffering from hypocalcaemia, on the other hand, is quite apathetic.
Naturally this distinction is valid only in clear cases of hypomagnesaemia and hypocalcaemia. In intermediate cases, or where there is a combination of hypomagnesaemia and hypocalcaemia, symptoms of irritability may just as easily be found as those of apathy. As a rule, this is the case with the hypomagnesaemia, generally accompanied by hypocalcaemia, that is brought about by under-nourishment.

High muscle tonus in grass tetany, low tonus in milk fever

So far as the muscles are concerned there are grounds for stressing with SJOLLEMA that:
In grass tetany the muscular tonus 9 is high, that is, the muscles are stiff. In milk fever the muscular tonus is low, that is, the muscles sag. 10
LARVOR, BROCHART and THERET likewise stress that grass tetany is characterized by contractions, hard muscles, muscle tremors and constriction of the jaws. Here again, however, there are intermediate and combined forms.

Interval between calving and the appearance of grass tetany or milk fever

The delay in appearance after calving can be a help in distinguishing grass tetany from milk fever. In general, tetany appears between the 1st and 12th week 11 following calving (Table 1), whereas hypocalcaemic milk fever most frequently occurs in the first three days after calving.

Table 1: Incidence of grass tetany in the weeks following calving

In this connexion there is no absolute rule in either case, but merely probabilities. Nevertheless, if a grazing animal is attacked by convulsions more than 10 days after calving the chances that it is suffering from hypomagnesaemic grass tetany are great. If, on the other hand, the convulsive manifestations appear less than 5 days after calving it is probably a case of milk fever. These are merely probabilities, and during the intermediate period from the 5th to the 10th day in particular there is occasion for every possible doubt.




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Notes



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  1.  Such as: magnesium deficiency; calcium 
deficiency; Vitamin D 
deficiency; potassium deficiency or excess; removal of the 
parathyroids or simply hypoparathyroidism; absorption of large 
quantities of sodium phosphate, sodium citrate, etc.;
hyperventilation of the lungs; acidosis or alkalosis; disturbance 
of the suprarenal cortex; chronic nephritis causing uraemia; 
various toxic elements, etc. etc. *


    
       

  2.  Most frequently the result of a defect or of the removal of the 
parathyroid glands. *


    
       

  3.  There are three points to be noted:

    1.  This diminution in the magnesium content of the blood serum 
   may or may not be accompanied by a variation in the serum's 
   content of other mineral elements, e.g. reduced calcium content.

     
    2.  A low magnesium content in the blood serum is not 
   necessarily accompanied by tetany.
     
    3.  Tetany is due to a disturbance of the 
   magnesium metabolism, the deficiency of the element being but 
   one of the causes.

    Finally, it will be seen below that the magnesium content 
of the blood serum may return to almost normal, following an 
attack of tetany, but the animals remain just as sensitive to 
the factors that trigger off the convulsions of the condition. *
 

    


  4.  Compare the symptoms of hypomagnesaemic tetany 
in milk calves and sheep. *



    


  5.  For the influence of temperature on the 
frequency of tetany see Part VI *



    


  6.  The distinction is even more difficult in ewes 
than in cows. *



    


  7.  This difference in effect on irritability between 
hypocalcaemia and hypomagnesaemia will be better 
understood when the mechanism of the neuro-muscular 
junction has been studied (see Part IV). *


    
       

  8.  For example, MERSHON. *


    


  9.  Tonus is the slight degree of contraction 
usually present in muscles when they are not in active movement. *

 
    
      

  10.  The differences are more marked in cows than in 
ewes. *


    
       

  11.  When the animals go out to grass in the 
spring SJOLLEMA noted that 55% of the cases of tetany took place 
in cows that had been calved for more than 6 weeks. With regard to the 
interval elapsing after the animals go out to grass, SJOLLEMA 
established that 46% of the cows were attacked during their 
first week at grass, 28% during the second week and 26% after 
more than two weeks. *
    



 
















       
     

       
               TABLE 1 

       
  Incidence of grass tetany in the weeks following calving


       
  
Weeks elapsed
 since calving 
      		   
      % of the total number of
 cows attacked by grass tetany 
  

  
 1 - 4
          
       38 - 66
  

  
 5 - 12   
          
       37 - 24
  

  
 13 and above
          
       13 - 33
    

  
 Not established
          
       10 - 77 
  


 

      
N.B. These findings were calculated on 195 cows.

From SELLERS.   *