Nucleotide sequencing and serological evidence that the recently recognized deer tick virus is a genotype of Powassan virus

Abstract

Deer tick virus (DTV) is a recently recognized North American virus isolated from Ixodes dammini ticks. Nucleotide sequencing of fragments of structural and non-structural protein genes suggested that this virus was most closely related to the tick-borne flavivirus Powassan (POW), which causes potentially fatal encephalitis in humans. To determine whether DTV represents a new and distinct member of the Flavivirus genus of the family Flaviviridae, we sequenced the structural protein genes and 5′ and 3′ non-coding regions of this virus. In addition, we compared the reactivity of DTV and POW in hemagglutination inhibition tests with a panel of polyclonal and monoclonal antisera, and performed cross-neutralization experiments using anti-DTV antisera. Nucleotide sequencing revealed a high degree of homology between DTV and POW at both nucleotide (>80% homology) and amino acid (>90% homology) levels, and the two viruses were indistinguishable in serological assays and mouse neuroinvasiveness. On the basis of these results, we suggest that DTV should be classified as a genotype of POW virus.

Introduction

Deer tick virus (DTV) is a recently recognized North American virus isolated from deer ticks (Ixodes dammini) (Telford et al., 1997, Ebel et al., 1999). Preliminary nucleotide sequencing of fragments of structural and non-structural protein coding regions by those authors indicated that this virus was most closely related to the flavivirus Powassan (POW).

The Flavivirus genus of the family Flaviviridae consists of a closely related group of viruses that share similar structure and genome organization (Westaway et al., 1985). Traditionally, flaviviruses have been classified based on their serological reactivity in hemagglutination inhibition (HI) or neutralization assays (Westaway et al., 1985, Calisher et al., 1989). More recently, molecular phylogenetic analyses have been utilized to investigate the classification and evolution of flaviviruses (Marin et al., 1995, Zanotto et al., 1996, Kuno et al., 1998). Typically these molecular analyses have confirmed the serological classifications of the flaviviruses and, in some cases, have allowed finer discrimination of closely related viruses (Kuno et al., 1998).

POW is a tick-borne virus that is classified as a member of the tick-borne encephalitis (TBE) serogroup of the Flavivirus genus. Other members of the TBE serogroup include Russian Spring–Summer encephalitis and Langat (LGT) viruses.

POW virus is usually associated with small mammals such as squirrels and groundhogs (Monath and Johnson, 1992). Although symptomatic human infections with POW virus are rare (≈20 cases have been reported in North America since the first isolation of the virus in 1958 Goddard, 1997) they cause a non-specific fever, that can be followed by neurological disease with signs of meningitis and encephalitis (Artsob, 1988). POW encephalitis may be fatal, with a case fatality rate around 15%, and survivors have a high incidence (≈55%) of permanent neurological sequelae (Gholam et al., 1999). A serosurvey of several communities in northern Ontario, Canada, between 1959 and 1961 detected POW virus-neutralizing antibodies in up to 3% of individuals tested, suggesting that asymptomatic human infections with POW virus do occur (McLean et al., 1962). POW virus has been isolated in both eastern and western North America, and in parts of Russia (reviewed by Calisher 1994).

To determine whether DTV represents a new and distinct member of the Flavivirus genus, we have sequenced the structural protein genes (C, prM/M, E) and the 5′ and 3′ non-coding regions (NCRs) of this virus and compared these for conservation of characteristic nucleotide and amino acid sequence motifs. In addition, we compared the reactivity of DTV and POW virus with a panel of polyclonal and monoclonal antisera in HI and neutralization assays, and the infectivity of both viruses in mouse virulence experiments.