If the unfortunate results obtained by Martin, Reiter, and Chauveau, are insufficient to deter from the use of smallpox lymph which has been passed through the cow, the long experience with humanized vaccine, which in its inoculation from man to man for a century has shown no tendency to revert to smallpox virus should be a sufficient warning against such dangerous optimism.
No deduction can be safely drawn from the comparative mildness of most of the cases caused by reinoculation from cow or horse to man, inasmuch as that all forms of variola can be rendered less severe by resorting to inoculation, which was extensively practised to limit the ravages of smallpox before the days of vaccination, and is still largely resorted to in the case of sheeppox in Europe. In each of these diseases the mortality can easily be reduced to 2 per cent. instead of the 20 to 50 per cent. which are lost when the disease is contracted casually.
As occurring casually, cowpox like horsepox is rare. Yet in Denmark, a dairying country, 1,037 cases were reported in 1877–8, and 878 cases in 1888–9. I have found some outbreaks explainable, through the existence of vaccinations in the families of the milkers, and Bollinger says that in Germany, most outbreaks take place in spring, the time when children are vaccinated. He should have added that this is the usual time of parturition in the cow, the time when primipara are first subjected to the danger from the hands of the milker, and when the cow from the noninfected district is brought into an infected stable for the season’s milking. In a dairying district in Tompkins Co., N.Y., the affection appeared every spring, in the same barns, in heifers with their first calf and in newly bought cows. All older cows, bulls, steers and unimpregnated heifers escaped.
Symptoms. The period of incubation is two days, after inoculation, and though it may appear to extend to a week when the disease is contracted accidently, it is impossible in such cases to state the exact date of infection. The preliminary fever is not always present, or recognized, yet there may be slight encrease of temperature, partial impairment of appetite and rumination, extra firmness of the fæces, a higher color of the urine, and above all a slight diminution of milk, which is a little more watery and coagulates more readily, than the normal.
This is followed by heat and tenderness of the udder and the appearance on the teats of small, pale red nodules the size of a pea or larger. In one or two days more the nodule, largely encreased in size, presents in the centre a depressed or umbilicated bluish white portion, with a firm yellowish, reddish or reddish blue margin, and outside this a soft pink areola, shading off into the white skin. The epidermis is raised at points by a viscid, yellowish lymph, enclosed in a series of saccules (multilocular vesicle). The vesicle encreases to 8 or 10 lines in diameter by the eighth or tenth day, and exceptionally, the umbilication is effaced by the excessive production of lymph. If left unbroken a brownish shade appears in the centre and gradually extends toward the periphery, the contents becoming purulent, and the pustule gradually drying up to form a crust. The drying and thickening of the crust goes on until the fourteenth day and the crust is usually detached by the twentieth, leaving a pale rose colored, smooth, shallow depression, which forms the permanent pit left after the skin has healed. The primary scabs usually show the central umbilication, and always the conical projection in the center of the deep aspect, and corresponding to the pit.
Vesicles on the mammæ may pass through the above stages, but those on the teats are usually ruptured by the hands of the milker as soon as the liquid is thrown out, and this gives rise to troublesome sores, with complex infections, at times implicating the gland tissue so as to cause destructive mammitis with loss of one or more quarters, and in any case abraded and irritated at each milking, so that the animal resists handling, the milk is drawn off imperfectly, and dries up or the cow becomes an inveterate kicker. If the milker has not been vaccinated he is liable to contract the disease.
A succession of vesicles often appear on the same animal, so that they may be found in all different stages of vesicle, pustule and crust on the same bag at one time. The later eruptions may be the result of inoculation from the earlier ones, and tend to prolong the attack materially.
In inoculation of the bovine animal for the production of lymph for vaccination, the skin of the abdomen from the symphysis pubis to the umbilicus is shaved, or in other cases the skin between the thighs, or in still others the skin on each side over the loins, and the virus applied in 50 to 200 points, by preference scraped until liquid oozes, but without any escape of blood. In a warm room the eruption matures in four or five days, its form taking on an appearance approximating that seen on the hairy skin of the horse. The individual lesions are somewhat extended corresponding in form and size to the abrasion on which the lymph was applied, and usually present the appearance of a raised patch, covered by a grayish film of epidermis, on the removal of which there is seen a raw alveolated surface filled with the amber-colored lymph.
Differential Diagnosis. From aphthous fever, cowpox is clearly distinguished by (a) the multilocular structure of the vesicle, while that of aphthous fever is a single undivided cavity which can be drained completely by a single needle prick; (b) by the pitting or umbilication, the aphthous vesicle being uniformly rounded and convex; (c) by the absence of vesicles or sores on the mouth and feet, which are rarely wanting in the aphthous eruption; (d) by the comparative absence of hyperthermia and constitutional disturbance, which is better marked though still slight in aphthous fever, and (e) by the absence of the intense and subtle infection of aphthous fever, which quickly attacks a whole herd and extends with equal rapidity over sheep, goats and pigs, attacking all cloven-footed animals virtually without exception. The cowpox patient, on the contrary, does not necessarily attack the cow in the next stall unless milked by the same hands, and spares heifers, bulls, steers, sheep, goats and pigs.
From the rinderpest cutaneous eruption it is easily distinguished by the presence of lymph in the lesion, that of rinderpest being a mere epidermic concretion; by the absence of the intense fever, anorexia and general constitutional disturbance, and of the early and high mortality which characterize that disease; by the absence of rapid and uniform infection of other cattle irrespective of a common milker; by the immunity of heifers, steers and bulls, which are speedily prostrated by rinderpest, and by the absence of the congestions and epithelial concretions of the mucosæ which characterize rinderpest.