Varicella.—In varicella the stage of invasion is usually much shorter than in smallpox, the prostration less marked, and the lumbar pains of the latter disease are absent. The eruption in varicella comes out in successive crops and runs a shorter course, so that lesions in various stages of development may be seen side by side. The temperature does not necessarily fall on the appearance of the eruption, and there may be a more or less marked rise with each fresh crop of vesicles, the temperature curve presenting thus a remittent character. The eruption itself presents marked differences in the character and the course of the individual lesions, as well as in their distribution. The clear vesicles shoot up from the surface, as it were, without warning; or there may be for a brief period only a circumscribed erythema like that which usually precedes the appearance of an urticarial wheal. The vesicles of varicella have usually a somewhat obtusely conical shape, while those of smallpox are distinctly hemispherical. The characteristic umbilication of the smallpox vesicle is wanting in varicella. It is true the varicella vesicle often shows a depression at its apex; but this false umbilication, as it is called, is due to the rupture of the vesicle and the escape of some of its fluid or to a partial drying of its watery contents, and occurs only after the vesicle has existed for some time. The vesicle of varicella appears much more superficial in its seat, and its roof is much thinner, so that it ruptures readily. Very moderate pressure with the finger suffices to break it. When ruptured in this way the vesicle usually collapses completely, contrasting in this respect with the smallpox vesicle, from which, owing to the multilocular character of the lesion, all the fluid does not escape.
In varicella the distribution of the lesions over the surface is far more erratic than in smallpox. The very decided tendency to grouping of lesions upon the face and about the wrists so characteristic of smallpox does not occur in varicella, in which the vesicles may appear even more extensively on the trunk than upon the face. In varicella the palms and the soles, except in infants, are almost never affected; while in smallpox these regions are practically never exempt. It is true that in the extraordinarily mild cases of smallpox, such as have constituted the majority of cases during the past two years throughout the West, lesions may or may not be present on the palms and soles; but in the severe and moderately severe cases, such as have characterized the recent epidemic in New York, the soles and especially the palms have practically without exception shown the lesions. The localization of smallpox lesions on the palms and soles deserves far more emphasis than is generally accorded it in the textbooks, many of which even fail to mention it all. It may be put down as a safe rule that a case showing an extensive eruption of vesicles or pustules, however suspicious in other respects, is not smallpox if the palms and soles are free.
Acne.—Among the skin diseases proper there are a few whose appearance upon hasty examination may occasion some confusion with smallpox. Acne pustulosa presents only a superficial resemblance to variola, but in cases where it is accidentally associated with an acute febrile disease, like grippe, for instance, it may give rise to some diagnostic difficulty. In these cases, however, inquiry will develop the fact that the acne lesions have been present before the inception of the febrile disease; and the presence of comedos, the limitation of the lesions to the face, chest, and back, together with the absence of any lesions on the palms and soles, will serve to exclude smallpox.
Impetigo Contagiosa.—In impetigo contagiosa there might under similar circumstances be a momentary doubt as to the nature of the illness. Impetigo lesions have no typical distribution on the surface, the mucous membranes are always exempt; the vesicle itself is extremely superficial, ruptures very readily, and is at once replaced by a crust, so that lesions in various stages, vesicles, pustules, and crusts may always be seen at the same time.
Zoster.—Zoster is, as a rule, readily distinguished by the definite grouping of the lesions in the tract supplied by one or more nerves, its asymmetrical distribution, and the more or less severe neuralgic pain that precedes or accompanies the eruption. It must be remembered, however, that in zoster, in addition to the typical grouped lesions, there are occasionally seen a few isolated vesico-pustules scattered promiscuously over the entire surface; and the difficulty of diagnosis may be increased by the occurrence of a moderate temperature movement. In these cases, to which attention was first called by Teneson, the history of the case, the presence of characteristic herpetic groups, and the evolution and course of the individual lesions will suffice to clear the diagnosis.
Drug Eruptions.—The ingestion of bromides, iodides, and quinine is sometimes followed by an eruption which may create some confusion in diagnosis. In general the drug eruptions may be distinguished by the absence of fever and of the subjective symptoms of smallpox. The bromide and the iodide acne never occur on the palms and soles, where there are no sebaceous glands, and the lesions lack the evolution and course of the variolous eruption. The erythematous and purpuric eruption of quinine may be confused with the hemorrhagic form of smallpox; but here, too, the history of the course of the illness and the absence of fever will obviate the difficulty.
Syphilis.—Of all the diseases of the skin it is the pustular syphilide which most resembles the lesions of smallpox. Dermatologists and experts in variola are agreed that the pustular syphilide may be absolutely indistinguishable from smallpox so far as the appearance and distribution of the lesions is concerned. Furthermore, the pustular syphilide is frequently accompanied by a decided febrile movement. The differential diagnosis can be made in these cases only by the closest inquiry into the history of the case and by careful observation of the course of the disease. The characteristic history of an acute illness of short duration followed by a remission on the appearance of the eruption will of course be wanting in syphilis. The syphilitic eruption is more sluggish in its evolution as well as in the course of its subsequent changes; and though there may be lesions of syphilis on the mucous membrane of the mouth, they will lack the characteristic appearance of the vesicles and pustules of smallpox in this region. The palms and soles are not apt to show any lesions in this form of syphilis; and finally some other forms of syphilitic manifestation are very often present in this polymorphic disease to give the clue to the real nature of the eruption.
In conclusion, the fact should be emphasized that there are cases of smallpox of so mild a character, with general symptoms so slight and eruption so sparse and ill-defined, as to make a positive diagnosis extremely difficult. It is a good plan to employ vaccination in such cases as a test. Within three or four days the experienced observer will be able to determine whether the vaccination is successful or not; a negative result will of course have but a moderate value, but a positive result will serve to definitely exclude the diagnosis of smallpox. In all cases of doubt, whether before or after the eruption has appeared, the physician owes it to himself not less than to the patient and the community to frankly explain to the patient or his family the difficulty in arriving at a diagnosis, and to express his suspicions that the case may be one of smallpox. It need hardly be said that such a case should be as strictly isolated as if the diagnosis of smallpox were already established.