444 INFLUENZA IN HORSES 
to be carried by infected human beings, litter, harnesses and ther- 
mometers. In many cases one attack confers immunity but a second 
infection or a relapse frequently occurs. Toward the end of an 
epizodtic the disease is usually milder in form, probably due to a 
gradual attenuation of the virus. Much work still needs to be done 
on the etiology of this affection. 
The period of incubation varies from two to seven days. Siedam- 
grotzky places it at from four to five days and in rare cases from 12 to 
24 hours. In some cases symptoms have not appeared until from two 
to five weeks after the latest known exposure. 
Symptoms. The disease appears suddenly and may attain its 
highest point of intensity within twenty-four hours. The organs of 
circulation, nervous centers, digestive and respiratory mucous mem- 
branes and conjunctiva are especially affected. There is partial or 
entire loss of appetite and depression. The temperature rises sud- 
denly 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. 
Usually the nervous depression coexists with the fever. The 
animal may hold its head down and appear to be comatosed. Extreme 
muscular debility is frequently associated with this stupor. Tremors 
may occur, the hind legs may give way while walking and paralysis 
of the hind quarters appears in a few cases. 
The oral mucous membrane is greatly congested, hot, dry or covered 
with mucus. There is sometimes difficulty in swallowing. The 
animal frequently yawns. There is usually constipation in the begin- 
ning of the disease which may be accompanied with colic. The feces 
are formed into small hard balls and are covered with masses of mucus. 
Later diarrhea with considerable tenesmus usually occurs. The feces 
are of a thin, pulpy and even fluid consistence and sometimes have a 
fetid odor. At the beginning of the attack the urine is alkaline but it 
becomes acid when the intestinal lesions are developed. It rarely 
