The Ability of an Attenuated Isolate of Langat Virus to Protect Primates and Mice against Other Members of the Russian Spring-Summer Virus Complex

Abstract

Summary Spider monkeys inoculated with an attenuated isolate of Langat virus were protected to a great extent when challenged subcutaneously by 6 of 7 remaining members of the Russian spring-summer complex, as evidenced by the suppression of viremia. Although Powassan virus also manifested a lower viremia in monkeys previously inoculated with TP-21-9 virus than in control monkeys, the protection was not as great as that observed against Far Eastern Russian spring-summer encephalitis, louping ill, Omsk hemorrhagic fever, Negishi, or Kyasanur Forest disease viruses. This is in agreement with the observation that Powassan virus is not as closely related serologically to the Russian spring-summer group as other members. However, animals that received a triple vaccination involving the sequential inoculation of three attenuated strains (17D yellow fever, West Nile and Langat viruses) showed excellent protection when challenged by Powassan virus. The attenuated isolate of Langat virus also protected spider monkeys against central nervous system lesions in the brain and spinal cord, caused, under special conditions, by various members of the Russian spring-summer virus complex, and also protected mice against many lethal intraperitoneal doses of all known members of the Russian spring-summer complex. Formalin-inactivated Langat virus vaccine did not protect spider monkeys against challenges by heterologous members of the Russian spring-summer virus complex. Monkeys immunized orally with the attenuated Langat virus were protected against viremia produced by Kyasanur Forest disease virus. Spider monkeys inoculated 18 months previously with Langat virus were protected against viremia produced by Far Eastern Russian spring-summer encephalitis virus or Kyasanur Forest disease virus. The hypothesis that viremia precedes involvement of the central nervous system is discussed.