We have studied certain properties, additional to those previously described (3), of the virus of vesicular stomatitis of horses, and of the characteristic biological reactions of the virus of equine encephalomyelitis.
It has been found that the virus of stomatitis, ordinarily dermotropic, can acquire neurotropism and the neurotropic encephalomyelitis virus can, in turn, be rendered dermotropic in its action. The neurotropism in both instances is associated with definite, although not pronounced, viscerotropism.
Both viruses can bring about a similar infection in the white mouse, rat, guinea pig, rabbit, and rhesus or cynomolgus monkeys. Of these animals, rabbits show the lowest degree of susceptibility and mice the highest, especially after intracerebral inoculation. The mouse is the best animal for work with these viruses because of the uniform and rapidly lethal encephalitis which can be induced in it. Moreover, the mouse is highly sensitive to the instillation of the viruses in the nasal passages: 1 to 10 million dilution sufficing to induce a fatal encephalitis. The uninjured nasal mucosa of mice appears, therefore, to be as susceptible to experimental infection as the traumatized brain or pads of animals.
The microscopic changes accompanying the reactions to both viruses reveal, in rapidly lethal infections, pronounced destructive lesions in the cells of the central nervous system. When the experimental disease is more protracted in its course, however, these lesions are associated with beginning productive, inflammatory reactions, consisting chiefly of mononuclear infiltrations. In the latter instances, characteristic, intranuclear inclusion bodies can be more readily observed.
Both viruses can be cultivated with facility in the medium of minced chicken embryonic tissue suspended in Tyrode's solution, although 24 to 48 hour old chicks are refractory to artificial infection.
No cross-immunity reactions occur between the two strains of stomatitis virus or between them and the encephalomyelitis strain.
The viruses are evidently similar in many biological properties. In view of the fact that the horse is the natural host for both, it is suggested that they may be generically related. They are not, of course, identical since cross-immunity between them does not exist. The absence of cross-immunity does not, however, exclude the possibility of a generic relationship, for there are at least three immunologically distinct types of foot-and-mouth disease, two of vesicular stomatitis, and two of equine encephalomyelitis (14) virus.