This was recognized toward the end of the 18th century, in the valley of
the Severn, England, by Jenner, who believed it to be the origin of
cowpox, but failed apparently to distinguish it from ordinary
“_grease_”. To-day, when we must trace each case to a preëxistent one in
some animal, and ignore the question of primary origin, we must still
recognize that it passes readily from horse to cow, and from cow to
horse, through the hands of milkers and stable-men, but that it also has
the source of vaccinated persons in the families of those handling the
horses. Whether it was also often derived from the very prevalent small
pox in past times is doubtful, yet it appears to have been much more
common about the year 1800 than it is to-day. When once started in a
stable it passes readily from horse to horse, through the hands of
shoeing-smiths and grooms, including of course their aprons, brushes,
sponges and rubbers, and also through blankets, bandages, litter and
other things on which the virus has been received. The susceptibility of
the skin in the hollow of the pastern depends mainly on the frequency of
chaps and abrasions, and to a certain extent on contact with the
road-mud infected by other passing animals. At times the suffering
animal licks the affected part, and determines the eruption on the
mouth, lips and nose (see contagious pustular stomatitis). Megnin saw
cases affecting the external generative organs and transmitted between
the sexes by coition. (See vesicular eruption on the genitals). All such
cases should be identified by inoculation.
Hertwig, as early as 1830 recorded the existence of horsepox in Berlin,
where it has been seen frequently since, and noted its transmission to
man. Bouley gave evidence of its special prevalence at Paris, and of the
production of typical vaccine vesicles in man by its inoculation. More
recently it has been frequently observed and studied by inoculation and
otherwise.
_Symptoms._ These, whether seen in horse, cow or man, do not differ from
those of cow pox, with which it may be held to be identical. The
observations of Chauveau, Warlomont, and Pfeiffer that experimental
infection, intravenous, subcutaneous, intratracheal (inhalation), or by
feeding almost always failed to give the general eruption which
characterizes small pox and sheep pox. It remains strictly local except
when inoculated on other parts of the skin or mucosa. It must be added,
however, that small pox or sheep pox, when inoculated successfully on
the horse, as a rule remains confined to the seat of inoculation. The
constitution of the horse, like that of the cow, tends to resist its
virulence.
The affected horse may show preliminary febrile symptoms, but these are
usually too slight to be noticed. They are followed by heat, tenderness
and swelling of some part of the skin, usually in the hollow of the
pastern and perhaps the back part of the metatarsal region, with more or
less (sometimes extreme) lameness. On the swollen skin may be detected
nodules, which may pass early into vesicles and pustules. The eruption
varies, however, as developed on the comparatively hairless lips or
nose, or on the densely pilous pastern.
On the lips, or other part lightly covered with hair, one can easily
follow the successive formation of the round nodule, the distinct
(sometimes umbilicated) vesicle, with its clear translucent
straw-colored contents, and the pustule, which bursts, forming a sore,
or dries up forming a dense scab, like that of cowpox.
On the heels, or on any part thickly covered with hair, the vesicle or
pustule is rarely recognized, the exudate on the contrary takes place
mainly on the surface, which becomes encrusted with an abundant
yellowish concretion, matting the hairs together, and sometimes
literally covering them. This may be very misleading to the practitioner
who expects to see the succession of fully developed vesicle and
pustule, and overlooking the true nature of the malady he may allow it
to spread widely in a stable.
Describing his inoculation cases, Chauveau gives the following
successive phenomena:—
“From the fifth to the eighth day the points of inoculation become
distinctly papular. As far as about the tenth day, the papules encrease,
and become more prominent, taking the form of an extremely wide cone,
with a base of ⅓ to ½ inch. During this period these large conical
papules are resistant and painful on pressure, but show no elevation nor
change in the epidermis, save a slightly reddish reflection in animals
with white skins. Afterward supervenes a new stage which may be called
the _period of secretion_. This commences from the ninth to the twelfth
day. The epidermis, slightly raised upon all the papule, sweats out
numerous drops of a limpid, very slightly yellow serosity. These drops
soon concrete into yellowish, transparent crusts covering the whole
surface of the pustule:—a species of characteristic crystallization,
very different from the crusts that succeed the vaccine pustules in mare
and cow. The secretion, which continues several days, is terminated from
the thirteenth to the seventeenth day after inoculation. If then the
crust is raised there is exposed a humid, pink, granular surface not
projecting beyond the surrounding skin. This surface is hollowed out by
a very deep central cavity, a sort of umbilicus, in which is inserted,
like a nail, a projection from the deep surface of the crust.”
I would add that after recovery the hair in the seats of the nodules has
a lighter color and, on the shanks and higher, remains dappled for the
season.
No _treatment_ is demanded. The application of a solution of sodium
bisulphite once or twice a day, or continuously on a bandage, will
greatly modify the intensity of the inflammation, and ward off complex
infections. If the skin is left tender or with a disposition to crack,
treat it as advised under chapped heels.