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Studies of the Virulence of Langat Virus Propagated in Chick Embryo or Hamster Kidney Tissue Cultures^*

When the eighth mouse brain passage of Langat virus was serially propagated in chick embryo tissue cultures, about 70% of the plaquepurified clones prepared from the 12th or 20th passages showed an increased virulence for mice as measured by peripheral infectivity. Such virus clones also caused more severe lesions of the central nervous system in rhesus monkeys inoculated intracerebrally than the virus did before chick embryo tissue culture passage. A serologic comparison of the original Langat virus present in the eighth brain suspension with the viruses present in the 12th or 20th chick embryo tissue cultures, as well as studies of control uninfected tissue culture fluids, showed that the virus present in these chick embryo tissue cultures was Langat virus. Therefore, this increase in virulence was due to Langat virus and not to some latent virus present in the tissue cultures or to a virus picked up by laboratory contamination. When the same original infected mouse brain suspension containing Langat virus was passed in hamster kidney tissue cultures only 3% of the plaque-purified isolates showed an increase in virulence. These results were obtained in a study of 100 plaque-purified isolates grown in chick embryo tissue cultures, 10 isolates being selected from each of 10 tissue culture tubes, and 100 plaque-purified isolates grown in hamster kidney tissue cultures, both types of virus clones being picked in a similar manner.

Further studies on the attenuated TP-21-9 isolate of Langat virus were reported. Of 29 monkeys that were inoculated intracerebrally with high doses of this isolate only one showed evidence of mild lesions in the brain. Histologic examination revealed mild to moderate lesions in the brains of 24 of 28 monkeys inoculated with similar concentrations of the original parent virus.

^* The work was supported by a grant from The Rockefeller Foundation.