My thanks to RR for the following:Am J Trop Med Hyg. 2012 Aug 13. [Epub ahead of print]
Prevalence and Genetic Characterization of Powassan Virus Strains Infecting Ixodes scapularis in Connecticut.
Anderson JF, Armstrong PM.
Department of Entomology and Center for Vector Biology and Zoonotic Diseases, The Connecticut Agricultural Experiment Station, New Haven, Connecticut.
A total of 30 Powassan virus (POWV) isolates from Ixodes scapularis collected from Bridgeport and North Branford, CT in 2008, 2010, 2011, and 2012 and one earlier isolate from Ixodes cookei collected in Old Lyme, CT 1978 were characterized by phylogenetic analysis of their envelope gene sequences. Powassan virus sequences segregated into two major groups termed the deer tick virus (DTV) and Powassan (POW) lineages. The lineage from I. cookei was POW. The remaining viruses from I. scapularis grouped with the DTV lineage. Powassan viruses from Bridgeport were nearly identical and clustered with a virus strain from a human in New York. Viruses from North Branford were homogeneous and grouped with viruses from Massachusetts, northwestern Connecticut, and Ontario. These findings suggest that POWV was independently introduced into these geographical locations in Connecticut and maintained focally in their respective environments. An improved method of isolation of POWV in vitro is described.
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20 AUGUST 2012
Connecticut's emerging virus
Anderson JF, Armstrong PM. Prevalence and Genetic Characterization of Powassan Virus Strains Infecting Ixodes scapularis in Connecticut. Am J Trop Med Hyg. 2012 Aug 13.
Powassan encephalitis is a relatively rare but serious infection that has been documented in Canada, the United States, and eastern Russia and is caused by Powassan virus (POWV), Flaviviridae: Flavivirus, the only known member of the tick-borne encephalitis serologic complex in North America. The index case was a child who died in the town of Powassan, Ontario, Canada in 1958. Since that time, there have been at least 70 human cases, primarily in the states of New York, Minnesota, Wisconsin, Maine, Vermont, and Pennsylvania and in the Canadian Provinces of Ontario, Quebec, and New Brunswick.
Despite the absence of reported human POWV cases in Connecticut, human disease has been recognized in the adjoining state of New York, and the virus has been isolated from ticks in Connecticut. In this study, we report 30 additional isolations of POWV from I. scapularis from two distinct geographical areas in Connecticut, an improved in vitro method for the isolation of this virus from ticks, the stability of two distinct genetic strains of POWV from two geographically separated populations over multiple years, and the focal nature of POWV.
Our isolation of POWV from 30 individual adult I. scapularis is the largest reported number of isolates of this virus-complex to date. All isolates belonged to the DTV [deer tick virus] lineage, but the isolates from the Bridgeport, CT site from 2008 to 2012 were genetically nearly identical to each other and clustered with POWV detected from a human in New York State. By contrast, the isolates from 2010 and 2011 from North Branford, CT were genetically similar to one other but distinctly different from the isolates from Bridgeport, CT and clustered with a previously published isolate from northwestern Connecticut, Massachusetts, and Ontario, Canada. The isolation of two distinct subclades of POWV from collection sites 40 km apart was surprising.
These findings suggest to us that POWV was separately and independently introduced into both of these geographical locations, and that both subclades were focal in their respective environments. Both of these subclades of the DTV lineage segregated from the POWV isolate from I. cookei collected in Old Lyme, CT in New London County. This isolate was similar to other POW lineage viruses from New York and Ontario, Canada. These findings support previous studies reporting on the focal nature and distinct transmission cycles of the POW and DTV genotypes.
Prevalence of Powassan encephalitis may be increasing, however reasons are unknown for the absence of reported human cases of POWV in Connecticut. Our two collecting sites were not near human habitation, and thus ticks had limited contact with humans, however I. scapularis is relatively common in Connecticut, and there are likely other locations where ticks are infected with POWV.
The absence of active surveillance for Powassan encephalitis in humans, and a possible lower infectiousness of the DTV lineage of POWV, compared with the virus causing tick-borne encephalitis may contribute to the lack of reporting of this encephalitic disease.