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Producing Disease Is Required: BCG Vaccine.—It has not been possible to produce an effective immunity to tuberculosis by inoculation with killed tubercle bacilli, and an avirulent strain of this bacterium that can be inoculated into susceptible animals, including man, without producing disease is required. Such a strain was developed at the Institut Pasteur from a bovine strain of the tubercle bacillus whose virulence was attenuated by continuous culture in the laboratory on a glycerin-bile-potato medium for 13 years. At the end of this time, in 1921, it was no longer able to produce disease.
Mycoplasmosis is a respiratory disease caused y the bacteria Mycoplasma gallisepticum. It is Iso known as air sac disease or chronic respira-jry disease. Those affected with the disease may how nasal discharge, watery eyes, and respira-)ry difficulty. This disease is often associated /ith other respiratory diseases. It is transmitted hiefly from infected hens to their chicks through le eggs. The disease can also be transmitted by ontact with infected individuals, but it spreads ery slowly in this manner. The disease can best e controlled by maintaining breeding flocks free f the disease by strict measures of isolation and mitation. Chicks hatched from such flocks can egin life free of the disease.
Marek's disease is primarily a disease of young chickens from 2 to 5 months of age. It is also known as jowl paralysis and neural lym-phomatosis. Nerve-tissue rumors that cause paralysis of both legs and wings are the most common form of this disease, but the tumors may also affect the viscera, eyes, and gonads. The disease is probably caused by a virus.
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