Infection of sheep and monkeys with Langat virus: cross-protection against other viruses of the Russian spring-summer complex

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Abstract
1. The susceptibility of sheep to infection with Langat virus has been studied. No viraemia or symptoms were detected in sheep inoculated either subcutaneously or intracerebrally.2. Only those sheep inoculated intracerebrally with 106·0–108·0 mouse IC LD 50 of virus developed significant quantities of neutralizing antibody.3. Two-thirds of sheep vaccinated with varying doses of Langat virus withstood subcutaneous challenge of louping-ill virus followed by intracerebral starch. All the intracerebrally vaccinated sheep survived this form of challenge but no sheep, whether vaccinated subcutaneously or intracerebrally, withstood intracerebral challenge of louping ill.4. In a field trial, three of ten hoggs vaccinated with Langat virus and exposed to natural louping ill infection at Camlarg and Dalcairnie died of the disease compared with all eight of the non-vaccinated hoggs. At Knockgray, there was no louping ill infection, but 93% of the hoggs from this hirsel developed louping ill antibody after transfer to Camlarg.5. Monkeys infected intracerebrally, subcutaneously or orally with Langat virus showed a low titre viraemia without clinical symptoms or histological changes in the brain and developed high titres of antibody. Vaccinated monkeys challenged with either Central European tick-borne encephalitis or Kyasanur Forest disease viruses remained healthy compared with control monkeys which showed evidence of disease.