Immunizing Value of Rickettsial Vaccines

Abstract

Recently developed methods for the cultivation of Rocky Mountain spotted fever and typhus rickettsiae in agar slant tissue cultures 1 and in the yolk sac 2 of the developing chick embryo have opened new possibilities for the preparation of vaccines against these infections. We have been making comparative studies (a) of the immunizing potencies of vaccines obtained from rickettsiae grown in these ways and (b) of the relative merits of these procedures for the large scale production of vaccines.

These comparative tests have been made using an “eastern” strain of spotted fever rickettsiae of relatively low virulence. It had been maintained in agar tissue cultures for at least one year before being used in the present experiments. It was uniformly infectious for guinea pigs but unlike more virulent strains of spotted fever the mortality rate among diseased pigs was not high. In spite of this difference in virulence vaccines made from it protected guinea pigs against many lethal doses of “western” spotted fever rickettsiae, and vice versa.

For the production of vaccines 3 this rickettsial strain was carried both on agar slant cultures and in developing eggs. When grown in culture large test tubes containing Tyrode-serum agar slants were inoculated with infected tissue according to the technic described by Zinsser, Wei and Fitzpatrick. 1 The tubes were incubated at 35°C for 5, 6, or 7 days. At the end of this period smears were made from bits of tissue detached from the agar slants, strained and examined for rickettsiae. Cultures showing good growth of rickettsiae were harvested by scraping off the tissue with a small spatula, care being taken not to disturb the agar surface itself.