work has never been verified. More recently Proescher and Seil have described a diplococcus which they are inclined to regard as the virus of hog cholera, but as yet they have not submitted substantial proof to justify such a claim. Certain cell inclusions which in cholera-infected hogs appear in the epithelial cells of the conjunctival sac have also been regarded as possible possessors of pathogenic powers, but it now seems probable that these exist as an effect rather than as a cause. There are various organisms which, acting as secondary invaders, exert profound influence on the course of hog cholera and on the lesions which develop, but which should in no way be confused with the filterable virus that produces the disease. These will be considered in another chapter. CHAPTER III METHODS OF DISSEMINATION HOG cholera virus exists only in infected hogs and in material contaminated by their excretions, and this is the fundamental fact to which we must repeatedly refer in accounting for new outbreaks. There are numerous exceptions to the rule, but the individual outbreak can usually be traced to a definite source, and this fact is important in its relation to measures for control. Shipping infected animals is probably the one practice responsible for most new herd infections. It is not uncommon for a breeder to become discouraged when his hogs begin to die and to ship all seemingly well animals to a distant market. During the fall of the year especially one has but to stand for a few hours at the unloading chutes of some of our large stockyards in order to realize how nearly universal this practice has become. Thus most public stockyards harbor hog cholera virus, and all hogs unloaded in them and later taken to farms for feeding or breeding become potential sources of danger. In the eastern states garbage feeding is responsible for more outbreaks of hog cholera than all other factors combined, and in the country as a whole this practice plays an exceedingly important part in the spread of the virus from locality to locality. Many hogs are killed while they are in the incubation period of cholera, and pork that comes from their carcasses, even though it is fit for human food, will produce hog cholera when fed in small portions to hogs. Bits of this infected pork find their way into garbage which is fed to susceptible swine, and the cycle is complete. The use of hog cholera virus in the field in serum-virus immunization has now become a routine measure, and despite the advantages that result from this practice, it must in truth be said that it is responsible for many new outbreaks of hog cholera. The practice of giving feeding shoats serum-virus treatment and shipping them immediately to distant points operates to infect much new territory, and is often the cause of heavy losses among the hogs thus handled. "Vaccination cholera," as these "breaks" following serum-virus treatment are called, although it usually runs a less rapid course which invites secondary infection, is not fundamentally different from hog cholera contracted as a result of natural infection, but there is a marked tendency in some quarters to avoid the issue and attribute the deaths to causes other than hog cholera virus. The practice of taking breeding hogs to distant points to mate them is a fruitful source of new herd infections, and in more than one instance we have known the virus to be carried from one farm to another as a result of neighbors exchanging help during butchering time. Small streams to which many hogs have access may also become polluted and carry destruction to herds below the one in which the original infection occurs. Show hogs returned from fairs often contract hog cholera en route or during their contact with other hogs in the show ring, only to infect the herds they represent when they return home. Besides the regular channels of infection which we have already indicated, and which severally are responsible for most new outbreaks of hog cholera, there are almost an infinite number of casual carriers of the virus, such as crows, sparrows, buzzards, pigeons, and various predatory animals. These, by feeding in infected yards or on carcasses of hogs dead of cholera, may carry the infection to clean territory, but the probabilities are that in most localities the number of herds thus infected is relatively small. In recent experiments Dr. Marion Dorset has found it difficult to transmit hog cholera from herd to herd by employing attendants, pigeons and sparrows as agents of transmission, and in our own experiments we have failed in a surpris ing percentage of cases to infect yards with hogs sick of cholera so that susceptibles placed in them subsequently will contract the disease. In spite of these facts, though, we must in handling hog cholera be guided by the practically universal clinical experience which teaches that when hog cholera once finds its way into a farm herd it will eventually infect all individuals in it, irrespective of the fact that the herd may consist of several pens of hogs kept some distance apart. It is impossible, and indeed unnecessary, to discuss in detail the various influences which occasionally are instrumental in carrying hog cholera virus from herd to herd, and likewise it is impossible to assign to each influence a relative importance. It is much more important, in concluding this chapter, to call attention again to the fact that in the great majority of cases hog cholera virus travels in certain quite definite channels, and that new outbreaks are usually the direct or indirect result of shipping or moving infected hogs, or else they originate from the practice of garbage feeding, or that of using hog cholera virus indiscriminately in seeking to immunize against the disease. |