PERSISTENCE AND TRANSMISSION OF TICK-BORNE VIRUSES - IXODES-RICINUS AND LOUPING-ILL VIRUS IN RED GROUSE POPULATIONS

The population dynamics of tick-borne disease agents and in particular the mechanisms which influence their persistence are examined with re ference to the flavivirus that causes louping-ill in red grouse and sh eep. Pockets of infection cause heavy mortality and the infection prob ably persists as a consequence of immigration of susceptible hosts. Se roprevalence is positively associated with temporal variations in vect ors per host, although variation between areas is associated with the abundance of mountain hares. The presence of alternative tick hosts, p articularly large mammals, provides additional hosts for increasing ti ck abundance. Grouse alone can not support the vectors and the pathoge n but both can persist when a non-viraemic mammalian host supports the tick population and a sufficiently high number of nymphs bite grouse. These alternative hosts may also amplify virus through non-viraemic t ransmission by the process of co-feeding, although the relative signif icance of this has yet to be determined. Another possible route of inf ection is through the ingestion of vectors when feeding or preening. T rans-ovarial transmission is a potentially important mechanism for vir us persistence but has not been recorded with louping-ill and Ixodes r icinus. The influence of non-viraemic hosts, both in the multiplicatio n of vectors and the amplification of virus through non-viraemic trans mission are considered significant for virus persistence.