338 C. E. SCHWERDT 



comparisons are to be made among titers of different preparations of the 

 same virus. 



The effects of the external environment on the virus inoculum are well 

 known. Viruses are, after all, subject to the same denaturing activity of 

 chemical and physical agents as any protein or living protoplasm. They 

 exhibit varying degrees of resistance to inactivation by heat, unfavorable 

 ionic environment, and irradiation. Even exposure of inocula to daylight or 

 artificial illummation of ordinary intensity may inactivate some viruses. 

 Skinner and Bradish (1954) investigated such effects as possible sources of 

 error in the titration of the viruses of vesicular stomatitis, influenza, New- 

 castle disease, and fowlplague and in some cases found after 4 hours of 

 exposure titers as much as 3 to 5 log miits lower than those of the dark 

 controls. 



The selection of host species for assay as well as route of inoculation is 

 well known to influence the virus titer measured. To cite one example of 

 many for animal viruses, monkey, cotton rat, and white mouse yield different 

 titers when used as hosts for parallel assays of Lansing poliovirus. Titers 

 obtained in the first two species may exceed those obtained in the white 

 mouse by a factor as great as 2 log units (Bodian et al., 1950). Habel and Li 

 (1951) have compared titers of this same virus by intraspinal and intracere- 

 bral inoculation into the white mouse and found titers not only consistently 

 higher (0.7 log unit) but also more reproducible by the former route. The 

 method of inoculation may affect titers, as illustrated by the increased lesion 

 counts obtained with plant virus inoculated with the aid of a fine abrasive 

 (Kalmus and Kassanis, 1945). Steere (1955) has enumerated and review^ed 

 the many host factors which can influence plant virus titers, such as physiolo- 

 gical condition, age, and nutrition of the host plants, as well as genetic 

 differences among plants of the same species. Similar host factors are also 

 important in the assay of animal virus. For example, Fulton and Armitage 

 (1951) found the chorioallantoic membrane of the intact chick embryo more 

 sensitive to infection with influenza virus than surviving sections of membrane 

 in vitro. 



Finally, the volume of the inoculum may have a marked effect upon virus 

 titer. Sprunt (1941) made comparative titrations of vaccinia virus by-intra- 

 dermal inoculation into rabbits using 1.0- and 0.05-ml. inocula and observed 

 that the larger inocula yielded only a 3-fold higher 50 % end point than the 

 smaUer, instead of the 20-fold difference expected. Similarly, Bachrach et al. 

 (1957) found that plaque counts of foot-and-mouth disease virus on mono- 

 layer cultures of bovine kidney cells varied inversely with volume of inoculum. 

 Identical amounts of virus in 0.1- and 6.4-ml. volumes yielded plaque counts 

 in the relationship of 6 : 1, respectively. Sprunt believed that the smaller 

 intradermal inocula spread more readily than the larger and exposed more 



