Using different methods of selection, three main types of echovirus type 9 variants were obtained from a single isolate derived from a naturally occurring strain. The three variants 88, 43 and 29 differed in a number of genetic characters as follows: virulence for infant mice (), capacity to grow at 40° as compared to 36.5° (), cytopathic effect on epithelial cell lines of human origin (), inhibition by rhesus monkey serum () and protection against heat inactivation by Al ions (). They also differed in the size and time of appearance of plaques on monkey kidney cell monolayers. The 88 variant was mouse-virulent, grew at 40° to the same degree as at 36.5°, was not cytopathogenic on epithelial cell lines of human origin, was not inhibited by normal rhesus monkey serum, not protected by Alions from heat inactivation and produced early large plaques on baboon kidney cell monolayers. Accordingly its genetic markers are . The genetic markers of the 43 variant are ; 43 differs from 88 only in the and markers. Though avirulent for mice, mouse-virulent variants could be selected from it by repeated passage in mice. 29 had the characters . It was distinctly different from both 88 and 43 by its marker and produced late and small plaques on baboon kidney cell monolayers. 29 was stable in its mouse avirulence and no virulent mutants could be obtained from it by repeated passage in mice. Progeny of 29 occurred in two ‘subforms’, and . The form was identical in these characters with both the and the prototype echovirus type 9, strains.


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