The serological response of sheep to infection with louping-ill virus

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Abstract
1. An H.A.I. test for louping ill in sheep is described and its potentiality investigated alongside a standard C.F. test.2. Sheep free from infection with, or which have not been vaccinated against, louping ill do not possess serum antibody demonstrable by the C.F., H.A.I. or neutralization techniques as described here.3. After the administration of commercial vaccine followed by infective virus, C.F., H.A.I. and neutralizing antibodies are detectable. The C.F. antibodies are the least stable and in many instances disappear entirely from the peripheral circulation within 4 months of their appearance; they also appear to be susceptible to fluctuation unassociated with the disease. H.A.I. antibodies elicited under the same circumstances reach a high level early, are maintained for longer periods than C.F. antibodies, and fall to a basic level which tends to be maintained.4. H.A.I. antibodies do not appear to be elicited in response to inoculation with vaccine only, in contrast to C.F. and neutralizing antibodies which are so elicited. It is suggested in these circumstances, measurement of H.A.I. antibody is the most useful of the serological tests available for surveys and for diagnostic purposes in sheep.We wish to thank Mr A. Sharman, B.Sc., M.R.C.V.S., formerly of the North of Scotland College of Agriculture, Aberdeen, under whose care and supervision the flock was maintained and who inoculated and bled the animals.We are particularly indebted to Dr C. E. Gordon Smith, of the London School of Hygiene and Tropical Medicine, for valuable criticism and advice.