BY FRANK P. FOSTER, M.D.
SYNONYMS.—Vaccina, Variolæ vaccinæ (Jenner), Cow-pox, Cow-pock, Kine-pox, Kine-pock; Fr. Vaccine; Ger. Kuhpocken, Schutzpocken, Impfpocken, Schutzblattern; It. Vaccina; Sp. Vacuna.
DEFINITION.—An eruptive disease characterized by a cutaneous lesion closely resembling that of small-pox, going through the stages of papulation, vesiculation, pustulation, incrustation, and cicatrization; differing from small-pox in the mildness or almost total absence of the constitutional symptoms, by being communicable only by inoculation, and by the fact that the lesions, as a rule, are developed only at the points of inoculation and in their immediate neighborhood.
This definition holds good for the great majority of cases, but in each of its parts we must take account of exceptions. For example, the lesion does not always follow the regular sequence of changes described. It may stop short at the stage of papulation, constituting the so-called raspberry excrescence, which will be further referred to hereafter; it may pass directly from the stage of vesiculation into that of incrustation, without any such change in its liquid contents as can properly be said to form a pustule; desquamation may take the place of incrustation; and, after an evolution otherwise normal, there may be no formation of a scar, simply because the destructive effect of the lesion has not extended deeper than the epidermis. The constitutional symptoms are sometimes severe, but they are always of very short duration. The disease is said to have been communicated otherwise than by inoculation in the case of some of the lower animals. Thus, Chauveau succeeded in producing some of its phenomena in the horse by causing the virus to be inhaled in the form of spray. It is doubtful, however, if it is possible to eliminate all sources of fallacy in such experiments. Finally, a generalized eruption is occasionally observed, although with great rarity. In stating these exceptions no reference is intended to cases in which complications occur.
NATURE OF THE DISEASE.—Many considerations warrant us in classing cow-pox among the varioliform diseases—chiefly its general resemblance to variola, and the fact that individuals who have been affected by it are thereby more or less fully protected against small-pox. It has been thought, indeed, that cow-pox was in reality but a modified form of small-pox; and this idea has been the basis of one of the theories that have been held as to the origin of vaccinia. Before enumerating and discussing those theories it will be well to mention that cow-pox is spoken of as spontaneous, casual, or inoculated, according to its mode of origin, known or assumed, in individual instances.
Spontaneous or original cow-pox is the name commonly applied to the disease as it is met with in the cow in instances in which its mode of origin is unknown. Strictly interpreted, this expression implies a belief that the affection is capable of being developed in a cow independently of contagion or infection—a notion that seems to be held by many physicians, but not, so far as the writer is aware, by those whose study of the subject has been such as to lend any considerable weight to their opinions. Ordinarily, however, the term spontaneous cow-pox is employed simply as a convenient expression to denote the disease as it occurs naturally in cows, without implying any belief or theory as to its mode of origin.
Casual cow-pox is the term applied in cases that have been contracted by accidental inoculation, whether in the cow or in man. It is manifest that the so-called spontaneous cases are really casual, unless we accept the doctrine that infection is not necessary to the development of the disease.
The term inoculated cow-pox implies that the affection has been produced by intentional inoculation. Here, again, we are confronted with an illogical expression, for a disease that is inoculated accidentally is still inoculated, as much as if it had been conveyed purposely. It may be said, indeed, that the casual disease is due to some other form of infection than inoculation, but for such an assertion there is not a particle of proof.
Passing from this unsatisfactory nomenclature to a consideration of the theories that have been held as to the nature of cow-pox, we are first met with that of its being a disease sui generis, like small-pox, measles, scarlet fever, and the like, and, like them, originating only by its own specific contagion, not being capable of development by a modification of any other contagion, however closely it may thus be counterfeited. This seems the most rational theory of the nature of cow-pox, but it cannot be demonstrated except by disproving all opposing theories; and that has not yet been accomplished.