Pathogenesis. Beside sheep and goats which contract the disease by exposure, the following genera have been successfully inoculated: ox, dog, pig, horse.
Forms. Two typical forms are recognized: (a) the discrete, regular or benign, in which the vesicles remain relatively few, and well isolated from each other, and (b) the confluent, irregular or malignant in which the vesicles are generally diffused over the body, even on the parts covered by wool, and set so close together that they merge into each other forming extensive continuous lesions. Other forms are the hæmorrhagic, purple or black sheeppox, the volante or intermittent kind, etc.
Geographical Distribution. Formerly common in Central and Western Europe, it still prevails continuously in the Balkans, the Danubian Principalities, Italy, Spain, the South of France and Algiers. Like other forms of variola, its permanent home is in Asia.
Causes. Long before the advent of modern bacteriology, sheeppox was held to be always and everywhere the result of contagion alone. Whenever it entered a new locality it was as the result of the importation of an affected sheep or one of its products; insular places like England maintained a permanent immunity, though the disease prevailed on the other side of the narrow straits or channel; yet when imported (1847 and 1862) it demonstrated a general susceptibility of the flocks on exposure or inoculation; more distant lands (America, Australia, Tasmania, South Africa, etc.) in the absence of imported infection remain clear to the present day.
The infection is more intense and diffusible than that of cowpox and horsepox following in this the smallpox of man. Absolute contact is not necessary, in either case the infection is carried in the air either on dust or otherwise, and above all in a confined building, a crowded sheep fold or a dusty highway.
In all forms of variola the virulence is concentrated in the lymph of the vesicle, and in horsepox and cowpox it is largely confined to this, whilst in sheeppox in severe cases it must also at times infect the blood, as lambs are occasionally born with sheeppox. On this basis the infection of the secretions generally has been advocated, but it is to be supposed that in moderate cases these are contaminated after secretion. Nocard and Roux produced immunity by the transfusion of blood from the sick to the healthy, but in no case a variolous eruption. Even the serosity from the swollen lymph glands failed to convey the disease.
In view of the diffusible nature of the germ, however, we must recognize that all secretions may be quickly contaminated as soon as they are exposed, and therefore no product of a sick or suspected sheep can be held to be safe, and all should be treated as presumably infected. The most dangerous products and those most liable to convey the disease are wool, hides, litter, buildings, yards, covers, parks, railway cars, boats, manure, urine and milk.
Living creatures like men, dogs, cats, birds, vermin, flies and other predacious insects are occasional bearers of infection.
Receptivity must be considered in every case. In countries and districts habitually immune from sheeppox, all breeds appear to be equally susceptible, the only refractory specimens being sheep that have survived a first attack, and lambs born of ewes that had the disease (naturally or inoculated) during the later stages of gestation. New born lambs, on an exclusively milk diet, are alleged to be somewhat refractory. In a country where sheeppox has prevailed long and extensively, as in Algiers and Brittany, certain breeds of sheep seem to have attained to a large measure of immunity (Nocard). This is doubtless largely due to the survival of the more insusceptible strains of blood, as Algerian sheep carried into France lead to most virulent outbreaks among the native animals (Galtier).
This relative immunity is still more decided when we come to animals of other genera. Though the latest results of research seem to identify the sporidia found in the different forms of variola, yet the long habit of living in an environment found in the ovine race unfits the germ for pathogenic life in various other genera that have their own variolæ. Sacco and Villain reported inoculations from sheep to man, but as the first was experimenting with variola at the same time, and the second did not test his pustules by reinoculation the results are not convincing. Küchenmeister had a general eruption in sheep after intravenous injection of variola of man, but failed to test it by reinoculation. Voisin and Nocard on the other hand were uniformly unsuccessful in attempts to convey sheeppox to man, and the handling of variolous flocks from time immemorial must have led to many cases in man, had he been appreciably receptive. It is virtually the same with cowpox. Huzard vaccinated 2,000 sheep without producing immunity from sheeppox. Voisin had precisely similar results. He further inoculated infants with sheeppox, and later, successfully, with vaccine lymph.