Confinement in the stall is an accessory cause, partly because stabled cattle are highly fed, partly because the air is hotter and fouler, and partly because there is no expenditure by exercise of the rich products of digestion.
High temperature is conducive to the malady, though the extreme colds of winter are no protection against it. Heat, however, conduces to fever, and fever means lessened secretion, which means a plethoric state of the circulation. The heats of summer are, however, often only a coincidence of the real cause, the mature rich pastures, and especially the clover ones, being the greater.
Electrical disturbances have an influence of a similar kind, disturbing the functions of the body and favoring sudden variations in the circulation. A succession of cases of the malady often accompany or precede a change of weather from dry to wet, from a low to a high barometric pressure.
Costiveness, which is the usual concomitant of fever, may in a case of this kind become an accessory cause, the retention in the blood of what should have passed off by the bowels tending to increase the fullness of the blood vessels and the density of the blood.
Mature age is a very strong accessory cause. The disease never occurs with the first parturition, and rarely with the second. It appears with the third, fourth, fifth, or sixth—after the growth of the cow has ceased and when all her powers are devoted to the production of milk.
Calving is an essential condition, as the disturbance of the circulation consequent on the contraction of the womb and the expulsion into the general circulation of the enormous mass of blood hitherto circulating in the walls of the womb fills to repletion the vessels of the rest of the body and very greatly intensifies the already existing plethora. If this is not speedily counterbalanced by a free secretion from the udder, kidneys, bowels, and other excretory organs, the most dire results may ensue. Calving may thus be held to be an exciting cause, and yet the labor and fatigue of the act are not active factors. It is after the easy calving, when there has been little expenditure of muscular or nervous energy and no loss of blood, that the malady is seen. Difficult parturitions may be followed by metritis, but they are rarely connected with parturition fever.
All these factors coincide in intensifying the one condition of plethora and point to that as a most essential cause of the affection. It is needless to enter here into the much-debated question as to the mode in which the plethora brings about the characteristic symptoms and results. As the results show disorder or suspension of the nervous functions mainly, it may suffice to say that this condition of the blood and blood vessels is incompatible with the normal functional activity of the nerve centers. How much is due to congestion of the brain and how much to bloodlessness may well be debated, yet in a closed box like the cranium, in which the absolute contents can not be appreciably increased or diminished, it is evident that, apart from dropsical effusion or inflammatory exudation, there can be only a given amount of blood; therefore, if one portion of the brain is congested, another must be proportionately bloodless; and as congestion of the eyes and head generally and great heat of the head are most prominent features of the disease, congestion of the brain must be accepted. This, of course, implies a lack of blood in certain other parts or blood vessels.
The latest developments of treatment indicate very clearly that the main cause is the production of poisonous, metabolic products (leucomains and toxins) by secreting cells of the follicles of the udder, acting on the susceptible nerve centers of the plethoric, calving cow. Less fatal examples of udder poisons are found in the first milk (colostrum), which is distinctly irritant and purgative, and in the toxic qualities of the first milk drawn from an animal which has been subjected to violent overexertion or excitement. Still more conclusive as to the production of such poisons is the fact that the full distention of the milk ducts and follicles, and the consequent driving of the blood out of the udder and arrest of the formation of depraved products, determines a speedy and complete recovery from the disease. This does not exclude the other causes above named, nor the influence of a reflex nervous derangement proceeding from the udder to the brain.
Symptoms.—It may be said that there are two extreme types of this disease, with intervening grades. In both forms there is the characteristic plethora and more or less sudden loss of voluntary movement and sensation, indicating a sudden collapse of nervous power; in one, however, there is such prominent evidence of congestion of head and brain that it may be called the congestive form par excellence, without thereby intimating that the torpid form is independent of congestion.
In the congestive form there is sudden dullness, languor, hanging back in the stall, or drooping the head, uneasy movements of the hind limbs or tail; if the cow is moved, she steps unsteadily, or even staggers; she no longer notices her calf or her feed; the eyes appear red and their pupils dilated; the weakness increases and the cow lies down or falls and after that is unable to rise. At this time the pulse is usually full, bounding, and the temperature raised, though not invariably so, the head, horns, and ears being especially hot and the veins of the head full, while the visible mucous membranes of nose and eyes are deeply congested.