Grass Tetany by André Voisin

CHAPTER 35

Are the characteristics of spring grass tetany different from those of autumn and winter tetany?


Can spring, autumn and winter tetanies be distinguished?

Having examined the influence of external conditions, particularly cold and humidity, on grass tetany, it might be useful to discuss autumn and winter tetany, these forms sometimes being differentiated from spring grass tetany.1 A clear distinction, however, appears to be very difficult.2
It was stated at the beginning of this book that the disturbed magnesium metabolism that gives rise to grass tetany has two main causes:
1. an imbalance in the composition (mineral and organic) of the herbage, leading to direct or indirect magnesium deficiency;
2. under-nourishment.

When hypomagnesaemia caused by under-nourishment was being studied (in which case it is accompanied by hypocalcaemia) it was stated that it had two main forms:
(a) Primary under-nourishment, the result, under grazing conditions, of too little grass being made available to the animal.
(b) Secondary under-nourishment, resulting from digestive upsets based on many causes: too much moisture in the herbage, excess of crude protein leading to the production of excessive amounts of ammonia in the rumen, lack of palatability of herbage low in sodium, etc.

Unbalanced herbage composition and inadequate quantities harvested are implicated in all tetanies

On the basis of these few, brief considerations it can be said that:
1. Spring grass tetany is caused mainly by the imbalances in the composition of the herbage.
The hypomagnesaemia develops rapidly in a week or two, as if from the effect of a shock, probably from the combined effect of a diminution in the resorption of magnesium in the digestive tract and of disturbance of the neurohormonal system which is no longer able to maintain a constant level of magnesium in the blood serum and call upon the few mobilizable reserves of magnesium. AS BLAXTER states, the fall in the magnesium level of the blood serum in these circumstances takes place too rapidly to be attributable to a "nutritional deficiency", in the proper and classical sense of the term.
2. Autumn and winter tetanies are caused mainly by under-feeding. The amount of herbage present in the poor season is frequently very low; if the sward is covered in snow no grass at all will be harvested, and frequently all that is available to the animal is supplementary feeding (straw, maize cobs, lowquality forage, etc.) with a very low magnesium content. This under-feeding slowly robs the organism of its few mobilizable reserves of magnesium, which accounts for the description: "slow developing seasonal hypomagnesaemia" (generally accompanied by hypocalcaemia).
In actual, everyday circumstances, however, the causes of the two types of tetany are not so clearly distinct. In fact:
Secondary under-feeding can play a part in spring grass tetany despite the abundance of herbage at this time. It has been seen that digestive upsets can prevent the animal from harvesting large quantities of herbage, particularly as the latter is frequently very low in dry matter, much too rich in protein and too low in carbohydrates (and crude fibre): not to mention the low palatability of herbage deficient in sodium.
Autumn and winter tetany is caused both by the meagre quantity of herbage harvested by the cow and by the unbalanced composition of that herbage, which favours hypomagnesaemia.3

The difference in the nutritional causes of the two types of tetany is therefore one of degree and not an absolute difference.

Role of temperature in the two types of tetany

Temperature plays a part in both types of tetany, but here again there are mere shades of difference rather than clear differentiation. It might be said that:
Spring grass tetany appears 4 when, following a cold spell of weather, there is a sudden rise in temperature triggering the growth of young grass; this being all the more dangerous, as ill-advised dressings of fertilizer unbalance the latter's composition.
Autumn and winter tetany is triggered particularly by a very low absolute temperature,5 which accentuates the effects of under-nourishment.

But absolute temperature and rise in temperature enter into play in both types of tetany:
(a) The lower the absolute temperature, the more marked the effect of the sudden rise in temperature in the spring.
(b) A rise in temperature, such as takes place in autumn 6 and winter, can trigger a slight, sudden growth of grass,7 which will accentuate dangerously the hypomagnesaemia already in existence as a result of under-nourishment. It may be seen, for example, in Fig. 20 that a rise in temperature at the end of October has triggered an attack of tetany.

Reaction of the two types of tetany to magnesium therapy

Hypomagnesaemic grass tetany caused by under-nourishment appears to be the outcome of the slow exhaustion of the very small, mobile reserves of magnesium in the organism, which are as slow to build up again as they were to become exhausted. As a result, the animal attacked by autumn and winter tetany, which is caused mainly by under-feeding, reacts badly 8 to a parenteral injection of magnesium salt, which is excreted very rapidly in the urine, and has no time to reconstitute the meagre reserves of mobile magnesium. After 24 hours the magnesium content reverts to the low level present before the injection.
With spring grass tetany, which results from hypomagnesaemia of a rapid and transitory nature, injection of magnesium appears to cause shock, re-establishing the mechanisms that control the level of magnesium in the blood serum, which quickly returns to more or less normal. This is true only if various organs of the animal's body have not deteriorated to any great extent.
This difference in the effectiveness of treatment is obviously smaller when the causes are less clearly differentiated: that is to say, when under-nourishment is a contributory factor in spring tetany or when the mineral imbalances of the herbage contribute towards autumn or winter tetany.

Types of animal affected by the two types of tetany

So far in this chapter a distinction has been made between:
spring grass tetany;
autumn and winter grass tetany;

on the basis of their causes and the speed with which hypomagnesaemia appears. But, if the types of animal affected are taken into consideration it appears that grass tetany should be divided up into:
spring and autumn tetany;
winter tetany.

Cows of the dairy breeds, that is, cows that are milked, are particularly affected by spring and autumn tetany. In winter they are kept indoors and cannot therefore be affected. The cows of the beef breeds,9 on the other hand, who nurse their calves and, in many countries, remain at grass for a large part of the winter, are particularly susceptible to winter tetany, although this does not mean that they are not affected by spring or autumn tetany. The same rule applies to ewes with their lambs that remain at grass all the year round, but this problem will be dealt with in the next chapter.

Hypomagnesaemic, transport tetany

To conclude this chapter on the different types of tetany, a few words must be said about a particular kind of hypomagnesaemic tetany: transport tetany, also referred to as travel tetany or railway disease. All these names stem from the fact that it appears following a long journey by train, truck or simply on foot.
It has been reported in the gestating or newly calved cow, in ewes and in goats, both during gestation and lactation. In general, however, it mainly affects the lactating animal in the first three weeks following parturition. This tetany occurs for preference in May-October, particularly in animals that have been at grass and are then subjected to a long journey. In other words, grazing appears to prepare the animal for this particular form of tetany, which therefore links it with grass 10 tetany.11
Unfortunately, there is little information available on the characteristics of the blood serum of animals suffering from transport tetany. Hypocalcaemia and hypomagnesaemia appear to be present simultaneously, however.12
From the point of view of protection against transport tetany, the animals should be removed, gradually, if possible, from the pastures a few days before they are to be transported and gently accustomed to dry feeding (forage, etc.), which is what they will generally receive on a journey by rail or road. During the journey itself it must be seen that the animals have sufficient food and water. If possible, a magnesium supplement should naturally be offered. The only practical method appears to be to feed a concentrate containing magnesium. The normal therapeutic treatment consists in injecting magnesium and calcium salt simultaneously, as in the case of grass tetany.

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Notes



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  1.  
 Spring grass tetany appears to be the most frequent in N.W. Europe, 
although this is not a general rule. In the course of an enquiry covering 
eleven years 'T HART observed 940 cases of spring tetany, 206 cases of 
autumn tetany. *


    

    
 

  2.  
 
 Some authors even define a summer tetany. SEEKLES thus distinguishes 
three forms:

    
       1. An acute form, in the course of which the magnesium content of 
the serum falls within a few hours or a few days (this the author would 
classify as spring grass tetany).
    
       2. Hypomagnesaemia, developing particularly in regions where stock 
graze the whole year through. The magnesium content of the blood serum 
falls progressively during the autumn and winter to one-half-one-third of 
the normal summer value (this the author would classify as autumn and 
winter tetany).
    
       3. Hypomagnesaemia of the intermediate type in which the magnesium 
level in the blood serum does not fall as rapidly as in the acute form, 
but less slowly than in the second case. This form occurs in the Low 
Countries in dairy cows on summer pastures. That is to say, SEEKLES 
stresses, it occurs between attacks of spring and autumn grass tetany. He 
thinks this is a case of latent tetany and that any change in climate or 
alteration in the quality of the pasture grass can trigger the attack. *

     
       


    

    
 

  3.  
 
 It must be remembered, moreover, that winter grass not only has a high
       moisture content but is also very low in crude fibre and energy due 
to the low intensity of sunlight.
    
 MACKELLAR of the discussion on WHITE'S paper considers that 
autumn grass tetany is not only favoured by the abundance of moisture in 
this herbage but also by its relatively high protein and low energy 
(starch units) content. It also tends to cause scouring, which aggravates 
the effects of these imbalances. *


    

    
 

  4.  
 
 All the other triggering factors (excitement, etc.) naturally still 
remain. *



    

    
 

  5.  
 
 MACKELLAR  has pointed out that, in the area north of P]ymouth, winter 
grass tetany among cattle was often suddenly triggered off when the 
temperature fell to 0° C.
    

In general, winter tetany takes place mainly after a fall in temperature 
accompanied by showers of rain and cold winds. INGLIS has noted that 
winter tetany in sheep appears mainly at a low temperature accompanied by 
falls of snow which accentuate the effects of under-nourishment. 
SWAN found that strong, cold winds helped to trigger tetany among grazing 
animals. *



    

    
 

  6.  
 
 Note that these two "false" summers of the winter months, described by 
rural tradition as "the summer of St. Michael" (29th September) and "the 
Indian summer" (11th November), are responsible for a rise in temperature 
at the end of the season. *


    

    
 

  7.  
 
 CUTHBERTSON thinks that the triggering of winter tetany in cows with 
suckling calves is probably due to the hypomagnesaemia that follows 
abnormal grass growth subsequent to a rise in temperature in the course of 
the winter season. *


    

    
 

  8.  
 
 SEEKLES pointed out three types of grass tetany. From the 
point of view of the effectiveness of treatment, he considers that the 
injection of magnesium and calcium is generally efficient therapy in the 
acute, rapid type (1st spring type), but produces little result in the 
slowly developing type (2nd: autumn and winter type). In SEEKLES' third 
type (summer grass tetany) the method that has frequently proved effective 
is the sub-cutaneous injection of a solution of 10% magnesium chloride.
 *


    

    
 

  9.  
 
 Some authors (for example ROBSON) are of the opinion that this autumn 
and winter tetany in cows of the beef breeds, (with suckling calves) has 
assumed much more importance in recent years than spring grass tetany in 
cows of the dairy breeds. *


    

    
 

  10.  
 
 The external symptoms of transport and grass tetany are similar, 
moreover: the animal is anxious and excited; it grinds its teeth and foams 
at the mouth; it walks stiffly and unsteadily; finally, its hind-quarters 
collapse and it takes convulsions. Its temperature may rise to more than 
104° F. (40° C.). *


    

    
 

  11.  
 
 It has been seen that grazing green wheat can also prepare the way for 
transport tetany. *


    

    
 

  12.  
 
 DETREZ, moreover, in cows suffering from transport tetany, has noted 
the presence in the blood serum of large quantities of guanidine 
(40 mg./100 c.c. against 12 mg. in normal cows and 25 mg. in pregnant 
cows). It should be borne in mind that guanidine sensitizes the 
neuro-muscular system to the action of potassium ions. *