[Experimental selection of weakly virulent strains of Aujeszky's disease virus]

An attempt was made to select slightly virulent strains by consecutive passages in cell cultures and use of extreme dilutions applying the temperature test (heating up to 50 degrees C for 1 hour). Initial and final fractions were compared for lethality and pruritus on rabbits and studied by the plate method. Levelling of the cytopathogenic titer after heating heated and unheated virus was achieved in laboratory samples isolated from lungs, and in two strains -- isolated from brains. This was achieved in the various strains following a varying number of passages, less in those isolated from lungs and more in those isolated from brains. Initial strains isolated from brains cause, while the lines produced by them do not cause, scratching in the infected rabbits. Temperature sensitive strains (identified by this test as virulent) produce mainly large (1.2 mm) plates, while the vaccine strain and the strains identified by the test as slightly virulent produce mainly small (0,8 mm) plates.