An organism of the Pasteurella group (Pasteurella avicida) has been shown to possess considerable pathogenicity for rabbits when injected into the trachea. 1 cc. of bouillon containing 1/100 cc. of a 24 hour bouillon culture introduced by means of a curved metal tube passed through the larynx gives rise to a rapidly fatal septicemia in which the lungs are not particularly involved. Rabbits may be made completely resistant to this intratracheal infection by means of intratracheal, intraperitoneal, or subcutaneous injections of killed cultures. Vaccination and subsequent injection of living culture beneath the skin resulted in extensive abscess formation about the site of inoculation. Vaccines sprayed on the mucous membranes of the nose and throat failed to raise appreciably the resistance against subsequent intratracheal injection.

Rabbits whose resistance has been stimulated within narrow limits by a small dose of vaccine beneath the skin become more resistant to the experimental respiratory infection. The character of the disease is changed from a rapidly fatal septicemia without lung involvement to a more prolonged pneumonia. The blood remains sterile for varying periods. Consolidations of the more dependent lobes occur with considerable regularity. In two instances a characteristic empyema developed in the course of the experiments.

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