1. A filterable virus has been isolated from Aëdes abnormalis group mosquitoes caught in the Semliki Forest in western Uganda. It has been called the Semliki Forest virus.

  2. The agent exerts its principal pathogenic properties on the central nervous system of susceptible hosts. It is virulent for, and lethal to, mice by various routes of inoculation, and for guinea-pigs, rabbits, and rhesus and red-tail monkeys by intracerebral inoculation. It is effective in very great dilution. When introduced extraneurally into resistant hosts, or even some susceptible hosts, it induces the formation of protective antibody without causing marked illness.

  3. In mice inoculated intracerebrally the virus is present in the blood and various other tissues in considerable concentration before the onset of objective illness. It has been demonstrated in the circulation of certain other inoculated animals, namely the red-tail and blue monkeys.

  4. The virus can be preserved by drying while frozen. It remains viable in saline at low temperatures for a number of days but gradually loses potency. It retains potency for several weeks when stored in serum in the refrigerator. It is not excessively sensitive to room or incubator temperatures. It survives heating at 60 C for ½ hour but not for 1 hour. It is inactivated by heating at 62 C for 30 minutes.

  5. Lesions caused by the virus in various animals have been described.

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