314 ACUTE GENERAL INFECTIOUS DISEASES 



country where it is kept alive by the unsanitary stables of 

 horse-dealers and in livery barns from whence it is from time 

 to time spread. It may also attack asses, mules and zebras. 



Etiology. — The most recent investigations indicate that the 

 cause of influenza is a filterable virus which appears in the 

 blood and probably other body fluids. Gaffky produced in- 

 fluenza in healthy horses by subcutaneous injections (5 ex.) 

 of defibrinated blood from naturally infected horses. The 

 period of incubation is from five to six days. When the blood 

 was injected intravenously the period of incubation was only 

 four days. In artificially infected horses the characteristic 

 symptoms were produced in forty hours. Similar experiments 

 made with filtered blood serum gave positive results. The 

 infectiousness of the blood was annihilated by the addition of 

 the citrate of ammonia. In no case was he able to obtain 

 growths on culture media from either the blood or blood 

 serum used. 



Natural Infection. — The disease is probably spread by the 

 nasal discharge and feces of infected horses. In all probability 

 apparently recovered cases may be "germ carriers" introduc- 

 ing the disease into stables and when brought in contact with 

 susceptible individuals. Indirectly the infection may be 

 carried by contaminated food, bedding, manure, stable uten- 

 sils, harness or in the clothing of grooms, horsemen and 

 veterinarians. Many sale and livery stables, due to their 

 lack of light, ventilation and cleanliness, may harbor the 

 infection for an indefinite period ("stable miasma") and all 

 horses, especially "green" horse's from the country, placed in 

 them fall victims of the disease. The same is true of railway 

 cars and stockyards which have not been properly disin- 

 fected. Public watering troughs may also harbor infection 

 and thus contribute to the spread of the disease. Although 

 influenza is commonly enzootic it not unrarely assumes an 

 epizootic form. While it appears at all seasons of the year, 

 in the late winter and early spring horses seem most predis- 

 posed. This is probably due to the' condition of the mucous 

 membranes, which are generally catarrhally inflamed from 

 "colds" in these seasons. Influenza will attack horses of all 

 ages, but it is not so common in colts less than a year old, nor 



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