In what is termed spontaneous vaccinia the udder is covered with a varying number of pustules, usually in different stages of development. Some are very small, whilst others have attained the size of sixpence and are already in course of cicatrisation.
When cow-pox is accidental or the result of inoculation, the eruption occurs exactly at the point of inoculation, whether the latter has been through an abrasion, a puncture, an incision, or any other skin injury, and the eruption may assume the most varied appearances, according to the nature of the primary lesion, although the mode in which the pustules themselves form never varies. Pustules experimentally produced by puncturing the parts may be taken as a type of inoculation. On the day following the operation nothing abnormal appears. On the third day there is a slight swelling around the point punctured, and this increases until the fifth day, when there is exudation, which converts the primary lesion into a vesico-pustule. On the sixth day the vesico-pustule becomes umbilicated at its centre, the exudation is abundant, and already vaccine might be collected.
This may be termed the period of crisis; the appearances are most characteristic. During the following days the vesicle is ruptured; the discharge continues from the ninth to the twelfth day, when the pustule diminishes in size and dries up. After the fifteenth day crusts of a brownish colour form; these separate between the twentieth and twenty-fifth days, leaving hard whitish-looking cicatrices, which permanently remain.
Moderate itching accompanies the development of the eruption, the principal functions are not disturbed, and fever only appears in the event of the eruption becoming confluent or extending over a large area. When the eruption is generalised the pustules or vesico-pustules are found mostly in the region of the elbow, the lower border of the neck, the flank and the inner surface of the limbs. They present exactly the same appearance as the pustules on the surface of the udder, but, being covered with hair, are less open to inspection.
In certain rare cases the eruption extends to the perineum and lips of the vulva. Signs of inflammation then develop on one or both sides, the tissues display œdematous infiltration and disseminated or confluent pustules. The lymphatic glands and vessels in the neighbourhood of the pustules are always swollen.
Causation. Cow-pox, or vaccinia, is a virulent disease transmissible by accidental or intentional inoculation. The discharge from the vesico-pustules and the crusts which afterwards cover them are virulent, and inoculation can be performed by simply scratching the skin. A first attack confers prolonged and sometimes perfect immunity, the operation being successful if only one pustule develops.
The disease is transmitted to healthy animals by milkers, by calves in sucking, or by the conveyance in whatsoever form of virulent material to sores or cuts.
The nature of the parasitic or microbic agent which produces the disease is still unknown. Some investigators have described intracellular parasites, others extra-cellular parasites, others, again, blood parasites, etc., but the exact cause has always eluded research.
It is, however, known that filtration of vaccine, pure or diluted, through porcelain removes the active material, which remains in the residue arrested by the filter.
Prolonged exposure to a temperature above 104° Fahr. (40° C.) greatly diminishes the activity of the vaccine. Simple desiccation has no action. Warming to 140° Fahr. (60° C.) for fifteen minutes also destroys its action completely.