Infection through food soiled by breath or nasal defluxion is a common cause. In our great swill stables during the prevalence of the plague, it was notorious that the disease advanced rapidly along the line of a feeding trough to the sixty or more animals using it, and that the rapidity of the advance was determined largely by the fact that the first animal was at the higher end of the trough. If at the lower end there was no upward current to carry infection to the others. Open pastures where the sick have fed and watering troughs or ponds are common sources of infection. The permanence of infection in and around large cities is largely due to the common pasturage by different herds in succession on the same unfenced lots waiting to be purchased for building. For this reason the plague always extended in summer when the cattle frequented these lots, and diminished in winter when they were strictly confined to stables and yards.
Mingling of different herds on great unfenced areas has been the main cause of the maintenance of the infection from time immemorial in the hills and forests of central Europe and on the boundless Steppes of Europe and Asia. This alone is chargeable with the permanence of the affection, in spite of all efforts for its extinction, in South Africa, Australia, Tasmania and New Zealand.
Contagion carried by attendants, cattle dealers and even dogs, is generally recognized, I have elsewhere quoted the case, in E. Lothian, Scotland, in which the son of the steward, who was cattleman on an infected farm, was the means of infecting first, his father’s cow, and later the whole of the stock on the place; also the case at Quincy, Mass., in which a farmer coming straight from the slaughter of sick cattle, infected his own herd; also the case of Mr. Jewell, of Long Island, who carried the infection from the herds visited, to his own family cow in a thoroughly secluded stable.
Infection through infected and emptied stables, was a common experience in dealing with the lung plague in America. Cows kept alone and only introduced after the predecessor had died habitually contracted the disease, though brought, through the most carefully guarded stockyards, from healthy districts. Stables have continued infecting for a year after having been vacated, (Friedberger and Fröhner).
Infection through manure spread on ground to which susceptible cattle had access was an occasional occurrence.
An active, unrestricted commerce is however the most prolific means of infection in cultivated countries. Slow as the disease progresses, and long as the animal diffuses infection, it soon attacks and kills or immunizes all the susceptible animals in the single herd, and if no other susceptible animal is bought or born into the herd, the germ in due time loses its pathogenic potency and infection is at an end. In this way many centres of infection started in herds on well fenced farms have worn themselves out. But the case was far otherwise in the city dairies and swill stables. The trade demanded that the stalls emptied by deaths should be filled up to consume the swill, or supply the milk route and thus fresh susceptible cattle were constantly exposed in the infected stable. The dairy cows were supplied by dealers who charged exorbitant prices for them and held a chattel mortgage for the amount. They had come through the infected dealer’s stable, and if they did not come up to the milk yield promised, they were either passed on to another dairy or returned to the dealer’s stable carrying back a new load of infection. The wagons used to carry the cows through the city were constantly infected and infecting. The swill stables became the final destination of the surviving cows that had gone dry and thus infection constantly gravitated into them.
In Europe where the great cattle and meat trade with England and the constant demand for cattle to consume the marc of the beet sugar and other factories, cause a continuous draught upon the infected districts the great western centres of enterprise and commerce have found it impossible to exterminate the plague.
In the country districts in England outbreaks were almost exclusively confined to the times when fresh cattle were purchased at one of the great fairs. Hence the late autumn, the period of laying in feeding cattle, witnessed the greatest extensions.
In Ireland, as shown by Ferguson, the whole cattle trade seemed to be arranged as if for the speedy and universal diffusion of infection. Calves, bought by jobbers from all sources, were bunched together with every opportunity for intercommunication and infection. As soon as they were a little further matured, they were assorted in lots and sent to a fair, where they were sold, and found their way to great common pastures, and this process was repeated again and again until they had reached full maturity. On the way to and from each market they were quartered over night in a public yard which had just been vacated by other animals, often diseased, traversed the same roads, and drank and fed from the same troughs which diseased ones had just used.
The introduction of apparently recovered cattle is generally credited with the infection of a fresh herd and Friedberger and Fröhner quote with approval Walley’s assertion that a recovered animal with a sequestrum in the lung can infect a herd into which it is taken. I have failed to produce even local exudate, subcutem or in the lung, from sequestra which were much more recent, but the animals operated on were rendered immune and inspired me to carry out a system of immunization by the use of the sterilized fresh exudate. The actual date of the expiration of virulence in the necrotic lung tissue has never been demonstrated and probably varies in different cases.