100 DISEASES OF DOMESTICATED BIRDS 
from 3 to 12 days, depending upon the virulence of the virus and 
the method of inoculation. After injection of filtered virus, the 
period varies from 6 to 15 days. Isolated instances of incubation 
periods to 25 days have been observed. 
Mortality. The fatalities occurring in an infected flock vary 
within a wide range depending upon individual circumstances. 
Under favorable conditions, the disease may not spread to more than 
10 per cent of exposed birds, with the death rate proportionately 
low, while in other cases 80 per cent of the exposed may become in- 
fected, with deaths relatively numerous, ‘approaching in some in- 
stances 50 per cent of those affected. The virulence of the virus 
is variable and constitutes the main element for consideration in 
each outbreak. The mortality depends, to a large extent, on climatic 
conditions. Fowls are more severely affected during the fall and 
winter months and it is during this period that the heaviest losses 
are encountered. 
Course of disease. Avian diphtheria may follow an extremely 
acute course, affected birds dying without showing noticeable ex- 
ternal symptoms which would attract the casual observer. In less 
acute cases the disease may be resisted for several days after symp- 
toms are apparent, the fowl finally succumbing apparently as a re- 
sult of toxemia, or of starvation due to eye affection which inhibits 
feeding. In the milder, and especially in the more chronic cases, 
marked symptoms may persist for several weeks and finally result 
in recovery, in so far as outward appearances are indicative. 
Symptoms. The disease is manifested by three clinical forms: 
(1) a pure mucous membrane disease, (2) a pure skin disease, and 
(3) a combination of skin and mucous membrane affection. The 
disease usually first manifests itself in a discharge from the nostrils 
which soon becomes viscid and plugs the nasal passages. In some 
individuals the first evidence of disease presents itself in the eye. 
One or both eyes may show a sticky discharge that tends to gum the 
eyelids and obstruct vision. The exudate continues to form in the 
conjunctival sac, causing a swollen appearance of the eye and as- 
suming a cheesy character. This, through pressure and the accom- 
panying inflammation, may produce panophthalmia and loss of the 
eye. The exudate in the nostrils also becomes cheesy in consistency 
and causes a pressure on the cleft palate and distortion of the bony 
structures of the adjacent parts. The affected bird early shows ex- 
treme dullness and loss of vitality. The tendency is to mope around 
