The Propagation of St. Louis Encephalitis Virus in Mouse Testicle

Abstract

The virus of St. Louis encephalitis displays so marked an affinity for nervous tissue that inoculation by any route other than the nasal or cerebral leads to infection only when relatively large amounts of virus are used. That this neurotropic tendency may be limited, however, was suggested by Webster and Clow, who found that the virus apparently multiplied to some extent in the spleen. With a view to modifying the tissue affinities of the virus, we resorted to testicular passage in the Swiss mouse. To initiate the testicular series, we used the Hubbard strain of virus, isolated in 1937. A 10% suspension in broth of a mouse brain from the 68th intracerebral passage was injected in 0.03 cc amounts into the testes of 4 mice. After 5 days, 3 animals were sacrificed and the testes removed with sterile precautions by the abdominal route. Two testicles were frozen and preserved (in each passage) in order to avert loss of the testicular virus in the event of bacterial contamination or other accident. The remaining testes were weighed, ground without abrasive and suspended in sufficient broth to make a 10% emulsion. The supernatant fluid obtained after allowing gross particles to settle out by gravity constituted the inoculum, which was always cultured in broth and on blood agar plates. The above procedure was adopted as a routine, passage being made at 5-day intervals, with groups of 4 mice, from 0.02-0.03 cc being injected into each testicle. By means of a 0.25 cc tuberculin syringe and a 27 gauge needle, this amount can be readily introduced into the testicle, although care must be exercised to avoid rupturing the organ. At each passage, 0.03 cc of the testicular suspension was injected intracerebrally into mice as a check on the presence of virus.