Age has a marked influence, but this is subsidiary to the milking qualities. The disease rarely attacks a cow after the first or second calving when the system is as yet immature, and the milk yield has not reached its maximum: nor one that is past its prime and already failing in vital energy and milking qualities. The following table is from statistics compiled from veterinary records in Denmark and Bavaria:
| Age, yrs | Cases |
| —3 | —8 |
| 4 | 21 |
| 5 | 65 |
| 6 | 160 |
| 7 | 171 |
| 8 | 202 |
| 9 | 117 |
| 10 | 124 |
| 11 | 44 |
| 12 | 70 |
| 13 | 78 |
| and over. |
It will be noted that it is in the period of the most vigorous, mature life, from the 6th to the 10th year inclusive that the great majority suffer. In a judiciously managed dairy it is the best cows that are carried at these ages, and although the very best are kept on into old age they show a steadily decreasing number of cases as they begin to fail. The disease is all but unknown in primipara.
High Feeding. Heavy and rich feeding prior to calving and immediately after, is a most prominent cause of the affection. This is so well known to owners of milking breeds, that they usually hold to the principle that the cow that is a heavy milker, should be all but starved for a fortnight before calving and for a week after. In herds where this rule is acted on the disease is rare and may be altogether unknown, and when it is neglected the malady is often very destructive.
Plethora. High Condition. Heavy feeding and high condition usually go together, and the majority of the victims are fat or in good flesh, yet a certain number are actually thin. The predisposing condition is plethora rather than fat or flesh, and this may be present in the comparative absence of flesh. The cow that is from a stock famed as heavy milkers, does not tend to lay on flesh, but, on succulent diet especially, the greater part of the nutritive matter assimilated goes to the production of milk, and she remains thin in flesh no matter how heavily she may be fed. Many such cows never go dry, but give a liberal yield of milk up to the day of calving, and if measures are taken to dry them up, it is done at the expense of a sudden plethora, as the milk giving system does not at once accommodate itself to the laying up of fat and flesh.
The drying up of the milk secretion sometime before calving in a cow which is normally a heavy milker is therefore a potent factor.
Parturition is an almost indispensable factor as the disease occurs one to seven days after that act, and only in rare and somewhat doubtful cases before it.
Easy Delivery with little nervous outlay or loss of blood, and no exhaustion is a special feature. The attack almost never occurs after a difficult parturition with considerable loss of blood and much nervous exhaustion. This should to a large extent exclude such alleged factors as shock or wearing out of nervous energy. The nervous prostration which figures so prominently in the disease, seems to be less the result of wear and tear, than of the supply of an excess of blood, which is either over-enriched, or charged with some injurious toxic matter. At the same time there is a manifest susceptibility at the parturient period which is not present at other times, and the plethora or toxin takes occasion to operate when this predisposition renders such an attack possible. The Warm Summer Season has been claimed to induce a greater number of cases, and doubtless exposure to continuous heat, tends to prostrate the nervous system and predispose to congestion, this fails to take into account the still more important element of the rich spring and early summer pastures, where the already plethoric animal is left to feed without stint, or the tempting red clover, alfalfa and other fodder crops, rich in albuminoids, which are fed liberally in a succulent condition.
Chills in cold winter weather have been similarly invoked as driving the blood from the surface to collect in internal organs, including the brain. That chills do act in this way cannot be denied, but there is no demonstration that any number of cases have been materially affected by cold.
Idiosyncrasy. Constitutional Predisposition. This must be allowed, inasmuch as that it covers all those individual conditions, functional and structural, which belong to the heavy milker, or the animal with extraordinary powers of digestion and assimilation. The same shows in the predisposition to a second attack of an animal which has survived a first one. The structural changes in the nerve centres, which occur in the primary attack, leave traces, which render these parts more susceptible at the next calving. In my own experience the violence of the disease is liable to increase with successive attacks, so that a second or third cannot be hoped to be as mild as was the former one.