Molecular, antegenic and biological studies of louping-ill virus variation in the British Isles

Abstract

Louping-ill (LI) virus is a member of the tick-borne encephalitis virus serocomplex in the genus Flavivirus. LI virus infection results in a biphasic encephalomyelitic disease which has been reported in many domestic animals and man. However, the vector of the virus, the sheep tick (Ixodes ricinus), is mainly associated with upland rough grazing pasture and therefore the disease most frequently affects sheep and wild red grouse (Lagopus lagopus scoticus). A vaccine, consisting of inactivated LI virus, has been available for sheep since the 1930s but the disease persists today. LI virus is the only flavivirus known to occur in the British Isles and exists at the end of a cline of tick-borne flaviviruses which has spread westward across Asia and Europe during the past 2,000 years. The spatial and temporal characteristics of LI virus evolution remain to be defined. Previous studies investigated the variation present among LI viruses in an attempt to understand the evolution of LI virus and further the knowledge of its epidemiology. The study presented here has extended this work with the investigation of antigenic, molecular and biological properties of 43 LI viruses collected from around the British Isles

Antigenic analysis using monoclonal antibodies identified two types of naturally occurring escape variants. The amino acid substitutions responsible for the alternative phenotypes were identified and shown to reside within the envelope (E) protein at residues 308 and 311. Investigation of the biological properties of the LI viruses in vitro and in vivo illustrated differences among them, some of which can be associated with genetic determinants

Louping-ill (LI) virus is a member of the tick-borne encephalitis virus serocomplex in the genus Flavivirus. LI virus infection results in a biphasic encephalomyelitic disease which has been reported in many domestic animals and man. However, the vector of the virus, the sheep tick (Ixodes ricinus), is mainly associated with upland rough grazing pasture and therefore the disease most frequently affects sheep and wild red grouse (Lagopus lagopus scoticus). A vaccine, consisting of inactivated LI virus, has been available for sheep since the 1930s but the disease persists today. LI virus is the only flavivirus known to occur in the British Isles and exists at the end of a cline of tick-borne flaviviruses which has spread westward across Asia and Europe during the past 2,000 years. The spatial and temporal characteristics of LI virus evolution remain to be defined. Previous studies investigated the variation present among LI viruses in an attempt to understand the evolution of LI virus and further the knowledge of its epidemiology. The study presented here has extended this work with the investigation of antigenic, molecular and biological properties of 43 LI viruses collected from around the British Isles

Louping-ill (LI) virus is a member of the tick-borne encephalitis virus serocomplex in the genus Flavivirus. LI virus infection results in a biphasic encephalomyelitic disease which has been reported in many domestic animals and man. However, the vector of the virus, the sheep tick (Ixodes ricinus), is mainly associated with upland rough grazing pasture and therefore the disease most frequently affects sheep and wild red grouse (Lagopus lagopus scoticus). A vaccine, consisting of inactivated LI virus, has been available for sheep since the 1930s but the disease persists today. LI virus is the only flavivirus known to occur in the British Isles and exists at the end of a cline of tick-borne flaviviruses which has spread westward across Asia and Europe during the past 2,000 years. The spatial and temporal characteristics of LI virus evolution remain to be defined. Previous studies investigated the variation present among LI viruses in an attempt to understand the evolution of LI virus and further the knowledge of its epidemiology. The study presented here has extended this work with the investigation of antigenic, molecular and biological properties of 43 LI viruses collected from around the British Isles.

Antigenic analysis using monoclonal antibodies identified two types of naturally occurring escape variants. The amino acid substitutions responsible for the alternative phenotypes were identified and shown to reside within the envelope (E) protein at residues 308 and 311. Investigation of the biological properties of the LI viruses in vitro and in vivo illustrated differences among them, some of which can be associated with genetic determinants

Molecular analysis of the viruses by determining the nucleotide and deduced amino acid sequence of the complete E gene or a representative fragment of the gene has enabled an extended investigation of genetic variation among LI viruses. There was a distinct correlation between genetic variation and geographic distribution, with clustering of viruses from particular areas implying the occurrence of microevolution within these regions. Grouping by geography is to be expected for tick-borne viruses with non-migratory hosts which are dispersed only sporadically into new geographic regions. The only exception to this is the Irish viruses which appear to represent two distinct virus populations existing in the same tick population.

Phylogenetic analysis of the sequence data implies that the ancestral LI virus was initially introduced into Ireland and at a later date into Great Britain via Wales. The virus was then transported to Scotland from where it was dispersed throughout Scotland, the north of England and Norway. More recently the virus was probably reintroduced into Ireland and also transported to the south-west of England. The nucleotide substitution rate was estimated for the LI viruses included in this study and used to calculate the dates when viral lineages diverged. This analysis implies that LI virus was introduced into the British Isles less than 600 years ago and that the most significant dispersal from Scotland occurred 100-200 years ago. The precise mode of LI virus distribution is not known, but the recent time-scale and the pattern of dispersal implicate the involvement of man and in particular the movement of livestock along specific transport routes