Electron Micrography of the Western Strain Equine Encephalomyelitis Virus. ∗

A material behaving biologically as the Western strain equine encephalomyelitis virus has been isolated as previously noted 1 from extracts of chick embryo tissue diseased with this agent. Recent work in this laboratory has resulted in the isolation of the virus in preparations of high homogeneity as indicated by the character of sedimentation diagrams in the analytical ultracentrifuge. Among the studies made on the purified material was the examination of it by means of the electron microscope. The results obtained with this instrument are reported briefly in the present paper.

The Western strain of equine encephalomyelitis virus was cultured in 11 or 12-day-old chick embryos in a manner similar to that employed for the Eastern strain of this virus. 2 The diseased tissue was extracted in Ringer solution of pH 8.6 to 8.8 for 4 days between 5° and 8°C. The extracts, cleared by low-speed centrifugation and filtered with celite, were subjected to 1 or 2 ultracentrifugal cycles in alternate high (30,000 g) and low (17,000 g) centrifugal fields. The purified product in Ringer solution was cleared of aggregates by angle centrifugation (7000 g) for 5 to 10 min. Electron microscope preparations were made by applying to the collodion film thin layers of virus in undiluted Ringer solution or Ringer solution diluted with water to 0.03 M salt concentration. Some of the screens were allowed to dry and examined without further treatment. Others were dried and then washed with Ringer solution or with distilled water. The purified virus was examined the day of isolation and at intervals thereafter for 40 days. Paralled studies were made on changes in infectivity and sedimentation characters.

Electron micrographs of fresh virus preparations giving a sharply sedimenting boundary in the analytical ultracentrifuge showed the presence of circular images with little evidence of extraneous material.