40 DISEASES OF DOMESTICATED BIRDS 
peatedly with cultures of the fowl cholera organism. The passive 
immunity induced by its injection into fowls is short, and its use- 
fulness is limited to flocks already infected or in grave danger of 
becoming so. 
Immunization with an aggressin has been accomplished in an 
experimental way by Weil. He produces a pleural exudate in a 
rabbit by injecting with fowl cholera bacilli. This exudate is car- 
bolized and heated for three hours at 44° C. A dose of 5 c.c. is said 
to immunize against subsequent injection of a lethal dose. 
Prevention. Sanitation with reference to known methods of 
dissemination should be the basis of combating the disease. The 
chief sources of infection within an infected flock are dead birds 
eaten by members of the flock, and contamination of food and water 
by droppings. On large poultry farms, measures to secure isola-. 
tion of the center of infection should be enforced promptly. This 
may involve temporary fencing, or moving the buildings, if portable. 
Daily inspection of the roosting houses at daybreak should be 
made to remove dead birds and to secure sick ones before they have 
opportunity to go afield. Constant watch during the day will reveal 
droopy birds which should be killed. 
Feeding troughs may be so arranged that the birds can only put 
in the head and do not have opportunity to contaminate the feed 
with their feet. The drinking water may be rendered safe by 
adding to it mercuric chloride in the proportion of one part to six 
thousand parts of water. Earthenware containers should be used. 
Disinfection of houses and adjacent areas should be carried out 
daily. Since the floor of the roosting house is highly contaminated 
by droppings, it should be disinfected if possible, before the birds 
leave the roost. At any rate, the birds should be excluded from the 
roosting house until it is possible to disinfect. 
Poultrymen, familiar with the ravages of cholera, have been 
known to ship sick and exposed birds to market, as the simplest 
way to avoid financial loss. Apart from the obvious effect in re- 
moving a center of infection, it has another effect in further spread- 
ing infection. Birds dying during the trip to the shipping point 
are thrown by the roadside, where they are liable to be partially 
eaten by any fowls encountering them. The disease may be intro- 
duced into a flock in a large number of ways such as introducing 
birds in the incubation period of the disease, the carrying in of 
infection on the shoes, by water, by animals, by pigeons or wild 
