Preparation of Epidemic Typhus Vaccine A. P. BERKOWITZ (S. Afric. J. Med. Sci., 9, 109; 1944) has investigated the suitability of duck and turkey eggs for the large-scale preparation of epidemic typhus vaccine. Duck embryos live longer (8–9 days) after inoculation than chick embryo do (6–7 days). The growth of rickettsiæ in the yolk sac of the duck egg is far more prolific than in the yolk sac of the hen's egg; and the duck egg's yolk sac at the stage used (15–16 days old) is about twice as big as that of a 13–14 day hen's egg. Duck eggs inoculated by Cox's method gave, on the 7–8th day after inoculation with a South African strain of epidemic typhus, an average yield of 100–200 c.c. of vaccine, which is about five times the yield of hen's eggs. Stained smears show, in a high percentage of yolk sacs, an almost confluent sheet of rickettsiæ, "quite beyond anything seen even in the most heavily infected hen's eggs". Because the yield per yolk sac is so high, the residual proteins are correspondingly diluted, so that processing is easier. Preliminary protection tests in guinea pigs have indicated that the protective value of the duck egg vaccine is at least equal to that of the hen egg vaccine. A Middle East strain of epidemic typhus gave a similar extremely prolific growth in duck eggs. In turkey eggs the growth was just as prolific, and the yield of vaccine as great. It was found that the best conditions for primary incubation of duck eggs were a temperature of 102° F. in a completely humid atmosphere.