Bacteriological investigation has proved that numerous and varied microorganisms can be found in the milk or interstitial exudates in cases of mammitis, but only a few special forms have been proved to be specific: streptococcus of contagious mammitis of milch cows, and micrococcus of contagious gangrenous mammitis of ewes (Nocard).
Pathology. The pathogenic results produced by infective organisms depend on their number and power of reproduction, and on the activity of their life products.
The most immediate and regular result of acute mammary infection is coagulation of the milk within the udder by decomposition of the lactose, and the formation of lactic or even of butyric acid. The acini and excretory canals are dilated by coagula, and can no longer expel their products of secretion, so that the colonies of microorganisms develop there in full security. The active epithelial cells undergo granular degeneration and disappear, whilst the walls of the glands become infiltrated and large numbers of leucocytes are poured forth around the glandular culs-de-sac.
The tissues being thus affected, the virulent organisms penetrate from the acini into the interstitial tissue, and from this time onwards the lesions become mixed.
Inversely, should infection originate in the lymphatic spaces, a time arrives when the organisms make their way from the interstitial tissue into the acini, with a similar result in the end.
The development of the lesions may be arrested or may pass on to suppuration, or even gangrene, of the parenchymatous lobules. Cases happen in which infection is so rapid and severe that the successive stages cannot even be identified, and gangrene appears without any preliminary stages at all. Luckily the commonest forms are less serious.
Symptoms. Acute mammitis is characterised by its sudden appearance, more or less acute general symptoms (dulness, fever, and loss of appetite), and variable local symptoms. When the practitioner is able to follow the development of the disease throughout, he may sometimes distinguish well-marked signs, which permit the two clinical varieties to be distinguished.
A. Interstitial Mammitis.—This form, which might perhaps also be termed peri-mammitis when it primarily affects the subcutaneous lymphatic spaces, has also received the names of phlegmonous and lymphogenous mammitis.
It is characterised by alarming general symptoms, and particularly by a rise in temperature of 2°, 4°, or even 5° Fahr., with all its consequences, such as loss of appetite, stoppage of rumination, acceleration of breathing and circulation, slight tympanites, constipation, and by the thrusting of the hind limb on the affected side away from the centre line. The animals groan when forced to move.
These symptoms sometimes precede by a considerable interval the appearance of the local changes, which consist in painful swelling of one or two quarters, rarely of more.