It seems, therefore, pending further field tests on a larger territory, that the following facts are fairly well established by the above experiments.
1. Healthy turkeys may be raised in an incubator from eggs of infected birds. In the above experiments all remained well to August 14, the end of the hen exposure test, when they were 12 weeks and 4 days old. The first death occurred 2 weeks after the beginning of exposure to actual disease, when they were more than 14 weeks old.
2. Hens from a blackhead farm and from a farm free from turkeys did not convey the disease to the incubator turkeys on uninfected land.
3. The infection is either not transmitted at all or only under exceptional conditions by turkeys in the early acute stage. It is probably carried and shed by those birds which have successfully passed through an attack.
Any definite statement concerning the mode of transmission of the infection cannot be made. The vehicle is unknown. The nature of the disease makes it probable, however, that it is introduced with the food, that it lodges first in one or both ceca, and that fecal matter is the vehicle.
During the entire season, portions of the small intestines of all the turkeys that died or were killed were sectioned and examined both with reference to the possible presence of coccidia and of any preliminary stage of Amœba meleagridis. Sections were studied from the upper (duodenum), middle, and lower portions. A few coccidia cysts were found in two turkeys and are referred to more in detail elsewhere. In a third turkey an intracellular parasite was seen which is very minute and which differs from those usually met with in birds. It is tentatively placed with the coccidia. It was not seen in any other case although searched for to obtain more material for study. It may be that we have an aberrant parasite to deal with which comes from the insects eaten and obtains lodgment in rare cases only. The existence of any earlier stages of the blackhead parasites in the small intestine whence they move down into the ceca is contradicted by the focal lesions found in the ceca and by the fact that in many cases only one cecum is attacked and this only in a single, restricted area. If the parasites multiply higher up we should expect both ceca to become infected.
There is some evidence pointing to a greater resistance of older birds than is usually presented by young turkeys. Thus, the total mortality (including those chloroformed while sick) following exposure on the infected farms and to the yearling turkey-hens was nine out of nineteen exposed, or a trifle less than 50 per cent. This figure is usually exceeded among those exposed immediately after hatching. The surviving turkeys are still well at the present time (January 1917).
The other kind of evidence is derived from the histological examination of the lesions in the liver and ceca. In these organs the process was rather early associated with extensive infiltration of roundish cells, while the tendency to necrosis was relatively slight. The lesions of the walls of the ceca were characterized by a marked thickening of the wall with little or no necrosis and exudation of fibrin.
In repeating tests of this kind it will be well for the experimenter to bear in mind that they are no longer laboratory experiments but conform to natural occurrences. The infection is not isolated in pure culture but is associated in the body of the carrier with unknown factors, both normal and pathological. The virus, if such it may be called, may be mixed with different injurious agents in different localities. It may be favored by various protozoan or higher parasites accidentally present, and by digestive disturbances due to improper feeding.
To utilize animals infected in the natural way as a source of virus, is to put the experiments into nature's hands as far as possible without losing control of the main conditions. Such experiments cannot, therefore, be completely reproduced at will as is possible in the laboratory with pure cultures, and the results may vary from place to place. They are practical rather than scientific tests in the sense that the practical test may involve unknown factors largely eliminated in the scientific or laboratory test.