AVIAN DIPHTHERIA AND BIRD POX 99 
Natural infection. Natural infection occurs under conditions 
which are not yet well understood. Apparently, the virus pene- 
trates through a slight lesion of the skin or mucous membranes and 
reaches the skin or the mucous membranes of the mouth by way of 
the blood stream. Infection by ingestion probably occurs. An in- 
stance of infection by a biting fly, Stomoxysis calritrans, has been 
reported. 
Experimental transmission. The infection may be readily 
“passed from the infected to susceptible birds by rubbing infective 
material into the skin or mucous membrane of the latter, especially 
if the parts be slightly scarified. 
In the pigeon the disease occurs on feathered portions of the body. 
To secure infection it is only necessary to pull out the feathers and 
rub in the virus lightly. After five or six days the skin shows a 
characteristic swelling. The follicles are greatly enlarged and upon 
pressure, whitish plugs are forced out. 
The virus is readily transmitted from the pigeon to the fowl but 
is transmitted from the fowl to the pigeon with great difficulty, if at 
all. Passage through the fowl appears to lower the virulence of the 
virus for the pigeon. 
Intravenous injection of the fowl with pox virus as a rule causes 
only diphtheritic lesions of the mucosa of the head, with no changes 
of the skin. This is true whether the virus which was injected, 
originated from the skin or the mucous membrane lesions. 
Inoculation of the comb with material from mucous membrane 
lesions does not always result in the occurrence of pox lesions. 
Some observers interpret these negative results as proving the ab- 
sence of pox virus in the diphtheritic material. Others regard the 
results as mere failures of pox virus to take. 
The virus is present in the circulating blood and internal organs 
in acute cases, as demonstrated by inoculation experiments. In 
some cases injection of liver and blood of birds affected with chronic 
lesions of the mucous membranes of the head, will reproduce the 
disease. When positive results occur, the incubation period is long 
and the cases induced are mild. In chronic cases, the virus in the 
blood appears to be weakened or lessened in amount. The virus 
from either skin or mucous membrane lesions, when filtered, will 
give positive results in most cases. Even if the material filtered is 
of high virulence, it will only produce mild cases of mucous mem- 
brane infection. 
The incubation period for both diphtheritic and pox lesions varies 
