336 INFLUENZA 



mined. It spreads rapidly among horses. The virus appears 

 to lose its virulence quickly outside of the animal body, but 

 within the body it sems to be preserved for a long time. 

 According to the observation of Jensen and Clark, stallions 

 vi'hich have had the disease may transmit it, to the mares they 

 serve, for months after apparent recovery. Dieckerhoff suc- 

 ceeded in transmitting the disease to healthy animals by subcu- 

 taneous and intra-venous injections of the blood of infected 

 horses, but Friedberger and Arloing failed to do so. Horses 

 are most susceptible. Sex, breed, stable management and feed- 

 ing appears to have little or no influence on their individual 

 susceptibility. 



Infection usually takes place from horse to horse. The 

 virus appears to be carried by infected human beings, litter, 

 harnesses and thermometers. In many cases one attack con- 

 fers immunity but a second infection or a relapse frequently 

 occurs. Toward the end of an epizootic the disea.se is usually 

 milder in form, probably due to a gradual attenuation of the 

 virus. 



§ 255. Symptoms. The period of incubation varies 

 from four to seven days after which the disease appears sud- 

 denly, and may attain its highest point of intensity within 

 twenty-four hours. The organs of circulation, nervous 

 centers, digestive and respiratory mucous membranes and con- 

 junctiva are especially affected. There is partial or entire loss 

 of appetite and depression. The temperature is characterized 

 by a sudden rise of from 3 to 4° F. or even more. It remains 

 high with but slight variations for from three to six days and 

 then falls rather quickly, often within twenty-four hours, to 

 the normal. At first the frequency of the pulse is but little 

 increased in comparison to the elevation of the temperature, 

 but later it rises to from 60 to 70 and in fatal cases from 80 to 

 100 or more. It generally continues high for some time even 

 after the temperature has fallen. The fever is characterized 

 by unevenness in distribution of the external temperature of 

 the body. The early rise of temperature, while the affected 

 horses appear to be healthy, is of much diagnostic value. 



