The submaxillary gland virus of guinea pigs was serially transmitted from brain to brain in young guinea pigs. The source of virus was the submaxillary glands of six groups of stock animals. Brain to brain transfer was effected in two series, in one to the second generation and in the other to the third. The transmission was evidenced by the presence of nervous symptoms and death and by a typical microscopic pathology of the brain. Only certain attempts were successful, ten of twenty-three brain to brain injections being fatal with the specific histopathology present in five. A few observations suggest that the virus may be present spontaneously in the gland and experimentally in the brain without cellular changes being demonstrable, or before they are evident.
While we were able to transmit the virus from brain to brain with fatal results by single injections of small doses, this was not readily accomplished and the transmission failed after two or three passages. We were unable to show any perceptible increase in virulence or adaptation of the virus to the brain tissue of the natural host.
The histopathology was that of a meningoencephalitis. The inflammatory reaction irregularly involved the meninges, the underlying brain substance, and the perivascular tissue of the meninges and upper cortical layer. These structures were infiltrated with mononuclear cells, many of which contained a typical acidophilic inclusion. Congestion of cerebral capillaries uniformly occurred and various degrees of recent hemorrhage were frequently found. Necrosis was noted only when associated with an occasional area of extensive hemorrhage. Similar changes were observed in sections of the spinal cord.
When sufficient time (15 days or more) elapsed between cerebral inoculation and death, typical cellular inclusions were seen in the salivary glands, whereas none was found in animals that died earlier (7 to 9 days). Under the first mentioned conditions, inclusions were demonstrated in the parotid and mucous portion of the submaxillary glands, although in spontaneously infected animals, we failed to find the mucous portion involved and other workers report that the parotid is spared.
About one-third of the stock guinea pigs examined showed cellular inclusions in both the nucleus and cytoplasm of epithelial duct cells of the serous part of the submaxillary gland. From an analysis of the results of brain to brain inoculations, it was evident that spontaneous infection and resistance to cerebral inoculation increased with age. The 3rd week of life is the period of choice for such experimentation.