Growth Curves of Influenza Virus based on Hemagglutination Titers in Individual Embryonated Eggs *

Examination of growth curves, which reflect changes in quantity of the products of viral multiplication over a period of time, offers a promising approach to further understanding of the phenomenon of viral reproduction. Henle et al.' have described growth curves based upon the titers of pooled allantoic fluids from groups of eggs sacrificed at various intervals of time after inoculation of influenza virus. They have demonstrated the step-like nature of the rises in titer by means of "one-step" curves, which they obtained by utilizing the interfering action of non-infectious virus injected subsequent to the infectious inoculum. Blumenthal et al.' have recently described a multiple-step growth curve based on the infectivity titers of a series of pooled fluids. Hoyle7 has studied curves based upon the titers not only of pooled fluids, but also of serial samples from individual eggs. Growth curves in individual eggs, which may be obtained relatively easily by the technique described by Green and Freymann,' have an advantage in revealing significant characteristics which may be obscured when pooled fluids are used. Individual curves have been used recently by McClelland and van Rooyen8 as a means of studying the effect of specific agents on viral multiplication. The present paper presents growth curves of the PR8 strain of influenza A virus based on serial titrations of hemagglutinin in the allantoic fluids of single infected eggs. These data confirm the findings of previous observers regarding the step-like nature of the curves and regarding an effect of the size of viral inoculum upon the lag period before appearance of detectable hemagglutinin. In addition, there are described a multiple step curve, an effect of age of egg upon the lag period, and some unusual variations of the ''normal" curves. It is realized, of course, that titers of hemagglutinating capacity may not be directly proportional to the amount of infectious virus present in tissues or fluids of the host, and that these curves are but one reflection of the reproductive phenomenon.