Etiology. This disease has long been known as caused by infection alone. Excluded from England in the middle of the eighteenth century it did not appear again until re-imported in the middle of the nineteenth, and then speedily overran the whole island except the breeding districts into which strange stock were never taken. In South America it was unknown until imported from the Old World into the Argentine Republic and then it made a wide extension and maintained itself where the stock was kept on unfenced ranches. In our fenced Northeastern states it died out and has not reappeared.
The infection is especially resident in the vesicles or aphthæ. From the mouth this is distributed, with the abundant drivelling saliva, on pastures, roads, feeding and drinking troughs, ponds, streams and halters, and readily communicates the disease to healthy stock following in the same places. From the feet and especially the interdigital space, it is left on the vegetation, buildings, yards, cars, boats and all other possible media to infect other stock in turn. From the teats it mingles with the milk so as to infect the young suckling and all animals and men to whom the milk may be given. It may become dried on litter and other light objects and carried by the winds, or it may be carried on the feet of men or animals including birds, but apart from this it is not readily diffused and oftentimes a broad highway may set a limit to its propagation.
The infecting microbe is not definitely known. Nosotti found a micrococcus in the lymph of the vesicle, which stained readily in aniline colors, was easily cultivated and pathogenic. Klein found a streptococcus which, similarly tested, presented an equal claim to be the causative factor. Bassianus and Siegel found in the blood and tissues of a person who died of foot and mouth disease a small oval bacillus, which they later obtained from the vesicles of three children who were suffering from the disease, and from animals attacked in two successive epizoötics. With this they first successfully inoculated a calf and from the pure cultures obtained from its blood, inoculated three calves and a young pig.
Löffler and Frosch, the recent commission on foot and mouth disease in Germany, report that no organisms could be seen nor cultivated from the lymph found in recent bullæ of the buccal mucosa, though this lymph proved virulent when inoculated on calves.
They found that the lymph became inert when dried for 24 hours at 31° C. (88° F.), while it retained its vitality and virulence after exposure for 9 months to a temperature of 0° C. They concluded that it could not penetrate through the unbroken skin nor mucosa, and that it was most effective when injected into the blood or peritoneal cavity. One attack conferred immunity for 5 months. Blood from immune animals, injected into susceptible ones, does not confer immunity, but 75 per cent. could be rendered immune if injected with a mixture of the lymph from the vesicle and double the same amount of the blood from the immune animal. Animals so treated become immune to 100 times the infecting dose. Filtered lymph was still virulent and the commission suggests that the microbe may be so small as to pass through the filter, and escape discovery by the most powerful lenses. An object one fifth the size of the smallest known bacillus—that of influenza—would be invisible under our best microscope.
By actual experiment the virus has been found in the nose, larynx, bronchia, stomach and intestines, but into all these the virulent lymph of the bullæ can find its way. In the intestines, indeed, in cases caused by feeding, bullæ have been found on the mucosa.
A most important question would be that of the virulence of the milk, but inasmuch as the vesicles appear on the teats and even on the openings of the milk ducts, and in bursting discharge their contents with the milk into the pail, the milk becomes per force infecting. The experiment of Hertwig and his students who infected themselves by drinking the warm milk by way of experiment, has been often repeated unwittingly by unwilling victims, and the many cases of calves, pigs and chickens contracting the disease by consuming the otherwise discarded milk leaves no room for doubt that this product is often infecting.
Among conditions contributing to a spread of infection, nothing is more potent than a free movement of ruminants, and swine whether determined by war, trade, or the intermingling of different herds on commons or unfenced ranges. In infected countries, in which cattle are distributed through large central markets there is always a wide extension after one of these fairs, the infection being narrowly circumscribed to herds receiving cattle from the fair, or those that have travelled on the same roads or fields after the market cattle. It has repeatedly happened that cattle shipped from the United States, where this disease has long been unknown, have been found diseased on their arrival at a British port, simply because they have been tied upon the passage with halters formerly used on infected Irish or Continental stock.
Symptoms in animals. There is first a period of incubation shorter in hot than in cold weather and varying from 36 hours to 6 days (exceptionally 15 days). It is altogether probable that prolonged incubation is really delayed infection, the virus having been attached to the feet for some time before it entered the tissues. Cattle usually show the disease two days after exposure in a public market, building or conveyance.
There is first moderate hyperthermia (102° to 103° F.), indicated by the clinical thermometer before there is any outward sign of ill health. There may be erection of the hair, tremors or distinct shivering, dryness and heat of the muzzle, redness and even tenderness of the buccal mucosa and teats, saliva drivels from the mouth or may show as a frothy mass at the commissures or margins of the lips, and there may be grinding of the teeth and a peculiar smacking of the tongue and hard palate which may be heard at a considerable distance. There are greatly impaired appetite and rumination. Tenderness of the feet is shown by halting or lameness and by the extension backward and shaking of the hind feet in turn.