The guinea pig is subject to cerebral and corneal inoculation of the herpes virus.
The effects of the inoculations vary with the strength or degree of virulence of the virus.
Weak strains of the virus are implanted on the cerebrum with difficulty and strong strains with ease.
Weak strains are quickly suppressed by the brain and strong strains may be passed indefinitely from brain to brain of the guinea pig. Strains of intermediate potency can be passed for a limited number of times only.
Weak strains induce keratoconjunctivitis without brain involvement, while strong strains invade the brain from the eye and produce fatal encephalitis. In the latter case, the brain contains active virus inoculable upon the cornea and into the brain of rabbits and guinea pigs. Strains of intermediate potency produce keratoconjunctivitis accompanied by mild symptoms of encephalitis, from which recovery results.
The guinea pig serves even more definitely than the rabbit to distinguish grades of virus according to strength or virulence. There is no difference of kind but only of degree of response to inoculation of herpes virus in the rabbit and the guinea pig.
The etiology of epidemic encephalitis has not, therefore, been brought appreciably nearer solution by experiments with herpes virus carried out in guinea pigs.