Infections in the chick-embryo with two recently isolated human strains of influenzal virus and two strains of swine-virus have been studied with relation to the influence of the temperature of incubation and the age of the host. The findings indicate that the temperature is of considerable importance and that 35–36 C is more suitable than 39–40 C for titrational purposes, as titrations carried out at 39–40 C gave very low and irregular 50-percent-infectivity endpoints. The explanation seems to be that a larger dose of virus was needed to infect at the higher temperature, and further that in many infected eggs the concentration of virus was not high enough to give positive red-cell agglutination. Both these factors contributed to the low endpoints obtained in the titrations at 39–40 C. This would probably be less conspicuous with strains having a higher agglutinin/infectivity ratio. Whether some intermediate temperature might be more suitable than 35–36 C has not been tested. Incubation at 37 C as used by Hirst for purposes of titration and isolation is obviously satisfactory. How 37 C compares with 35–36 C as to susceptibility of the embryo and amount of virus produced would have to be clarified by direct comparison.

The strains tested killed the embryos regularly at the lower temperature and infections and deaths were prevented by immune sera. The swine-strains also killed at 39–40 C but a few survivors were found with the human strains at that temperature. The report (24) that H. influenzæ suis is necessary in addition to the swine-virus if high mortality is to result from the infection is probably to be explained by differences in the strains of virus used.

The 7- and 12-day embryos apparently did not differ markedly in their resistance although the 7-day eggs were probably slightly more susceptible.

The route of inoculation (on the chorioallantoic membrane or into the amnion) and the amount of virus used did not greatly affect the rate of increase of virus in the fluids with strains TW 772 and V 15 strain 2. Strain TW 771, on the other hand, seemed to depend more on the route of inoculation and the size of the inoculum, the concentration rising quicker after a large inoculum.

Finally, the effect of Na-sulfadiazine on the infection was tested. It was found that 20 mg of Na-sulfadiazine per embryo did not reduce the number of positive infections in critical virus-dilutions; or in other words, titrations in eggs containing this amount of the drug did not show a lower infectivity-endpoint than did titrations in normal control eggs. Should it be desired to use an antibacterial agent with inocula that are not completely free of bacteria, Na-sulfadiazine would seem to be suitable as far as the virus is concerned.

This content is only available via PDF.