344 ACUTE GENERAL INFECTIOUS DISEASES 



The infectiousness of the blood was annihilated by the addi- 

 tion 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 proba- 

 bihty apparently recovered cases may be "germ carriers" 

 introducing the disease into stables and when brought in 

 contact with susceptible individuals. Indirectly the infec- 

 tion may be carried by contaminated food, bedding, manure, 

 stable utensils, harness or in the clothing of grooms, horse- 

 men 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" horses from the country, placed 

 in them fall victims of the disease. The same is true of rail- 

 way 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 

 aged horses. Horses three to flve years of age are most fre- 

 quently affected. One attack produces lasting immunity. 

 Horses which have had influenza may, however, take infec- 

 tious pneumonia, and vice versa. The infection is most 

 commonly taken in with the food and water. Infection via 

 the respiratory tract is highly probable but not proved. 



Symptoms. — The period of incubation is usually less than a 

 week. A minimum period of one day and a maximum of ten 

 days are recognized. The initial symptoms in many cases are 

 moderately developed, but in the majority they are quite 

 intense. Usually the first symptoms noticed by the owner or 

 caretaker are loss of appetite, dulness, and marked languor. 

 The patient stands with head down, eyes closed, ears drooped. 



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