Assay of the Immunizing Effect of Scarlet Fever Toxin in the Rabbit

Abstract

Evaluation of agents for immunization against scarlet fever has been restricted to the direct experience in human beings, the criteria of success being (1) the Dick test, (2) antitoxin determination in the serum, (3) clinical protection. 1 - 3 The latter will, of course, always remain the ultimate criterion of success. However, it was felt that for the development of an optimal immunizing agent for scarlet fever it would be of considerable help if a method could be found that would allow an experimental evaluation in the animal.

The assay of the scarlet fever toxin and its antitoxin in the skin of the rabbit has been successfully employed for some time⁴, ⁵.

The following experiments will show that a dermal test in the rabbit will give an opportunity for a quantitative evaluation of both active and passive antitoxic protection.

Procedure. White rabbits were tested for sensitivity to 1 STD∗ of scarlet fever toxin and only such animals were employed that gave a distinct reaction. Such animals were injected with the immunizing agent, for example, scarlet fever toxin, and at the end of a period of immunization tested against the toxin. This testing was done with 1, 4, 16, 64, and 256 STD, occasionally also with 1024 STD. Appropriate controls were added. It was ascertained that healthy rabbits can be expected to stay Dick sensitive for the time period corresponding to that of immunization. In a control series, rabbits were injected with the double amount of the same nutrient broth that had been used for the preparation of the toxin. Prior experience on the Dick reaction in rabbits that had been injected with suspensions of toxigenic strains of streptococci has shown that antibacterial antibodies would not interfere.