The observations presented in this report were concerned with the nature and properties of the Turlock virus. The virus is highly pathogenic for the chick embryo, less so for the one day old chick. The suckling mouse, especially under nine days of age, is highly susceptible to extraneural inoculation of the virus whereas the adult mouse is highly resistant; animals of one month of age are so resistant that only an occasional individual succumbs to intraperitoneal inoculation of even maximum quantities of virus.
On isolation, the virus is virtually non-pathogenic for the adult mouse, at least for the strain of albino Swiss mouse employed in this work. However, virus recovered in the embryonated egg and passaged several times in the unweaned mouse, acquires the ability to produce fatal infections in older animals. Increasing age, however, appears to confer a certain amount of resistance to cerebral inoculation of the virus, as evidenced by the irregular mortality ratios encountered in animals of three weeks or more of age. Consequently, although intracerebral passage of the virus may be done in four- or five-week-old mice, experimental work in animals of this age cannot be conducted with any degree of accuracy.
The guinea pig and the rabbit appear to be resistant to infection with the Turlock virus, but respond to inoculation by producing relatively high levels of neutralizing antibody. The adult hamster is also resistant, whereas the immature hamster of several days of age is highly susceptible and succumbs to infection.
Preliminary work indicates that a complement-fixing antigen is present in tissues infected with the Turlock virus. The concentration in chick embryo tissues such as the brain or the amniotic membrane appears to be very low, since antigenic activity is barely demonstrable. Antigen prepared from unweaned-mouse brain, however, is considerably more potent and possesses a high degree of sensitivity. Whether the complement-fixing activity is referable to a so-called soluble antigen, or is attributable to the viral particle itself, remains to be ascertained.
The Turlock virus apparently does not belong to either immunologic group A or group B of the arthropod-borne virus group (Lennette et al., 1957). Although the Turlock virus is arthropod-borne, it seems further to differ from agents of this group by its large size, i.e., the ultrafiltration endpoint range of the virus was found qualitatively to be 240–280 millimicrons, which according to the Elford formula indicates a particle size in the range 120 millimicrons to 180 millimicrons. Because the ultrafiltration data presented here would place the Turlock virus amongst the “medium-sized” animal viruses, it is suggested that the particle-size estimates be checked by other methods of determining particle size.