512 DISEASES OF THE HORSE. 
free from the disease, it certainly appears ‘after the introduction of 
an apparently healthy animal. : 
As one attack is usually self-protective, numbers of old horses, 
having had an earlier attack, are not capable of contracting it again; 
but, aside from this, young horses, especially those about four or five 
years of age, are much more predisposed to be attacked, while the 
older ones, even if they have not had the disease, are less liable to it. 
Again, the former age is that in which the horse is brought from the 
farm, where it has been free from the risk of exposure, and is sold to 
pass through the stables of the country taverns, the dirty, infected 
railway cars, and the foul stockyards and damp stables of dealers in 
our large cities. Overfed, fat, young horses which have just come 
through the sales stables are much more susceptible to contagion than 
the same horses are after a few months of steady work. 
Pilger, in 1805, was the first to recognize infection as the dirett 
cause of the licens. Roll and others studied the contagiousness of 
influenza, and, finding it so much more virulent and permanent in 
old stables than elsewhere, classed it as a “stall miasm.” The con- 
tagion will remain in the straw bedding and droppings of the animal 
and in the feed in an infected stable for a considerable time and if 
these are removed to other localities it may be carried in them. It 
may be carried in the clothing of those who have been in attendance 
on horses suffering from the disease. The drinking water in troughs 
and even running water may hold the virus and be a means of its 
communication to other animals, even at a distance. 
The studies of Dieckerhoff, in 1881, in regard to the contagion of 
influenza were especially interesting. He found that during a local 
enzootic, produced by the introduction of infected horses into an 
extensive stable otherwise perfectly healthy, the infection took place 
in what at first seemed to be a most irregular manner, but which was 
shown later to be dependent on the ventilation and currents of air 
through the various buildings. His experiments showed that the 
virus of influenza is excessively diffusible, and that it will spread 
rapidly to the roof of a building and pass by the apertures of ventila- 
tion to others in the neighborhood. The writer has seen cases that 
have appeared to spread through a brick wall and attack animals 
on the opposite side before others even in the same stable were 
affected. Brick walls, old woodwork, and the dirt which is too fre- 
quently left about the feed boxes of a horse stall will hold the con- 
tagion for several days, if not weeks, and communicate it to sus- 
ceptible animals when placed in the same locality. On two succes- 
sive mornings a 4-year-old colt belonging to the writer stood for 
about 10 minutes at the open door, fully 40 feet from the stalls, of a 
