Deer tick virus is a virus

Virus

A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

 causing tick-borne encephalitis.

Deer tick virus is a flavivirus

Flavivirus

Flavivirus is a genus of the family Flaviviridae. This genus includes the West Nile virus, dengue virus, tick-borne encephalitis virus, yellow fever virus, and several other viruses which may cause encephalitis....

 closely resembling Powassan virus

Powassan virus

The Powassan virus is a tick-borne encephalitis virus related to the classic TBE flavivirus. This disease had its earliest origins in the town of Powassan, Ontario, found in a young boy who eventually died from it....

 (to which it has 94% amino acid sequence identity).
Because they are so related, deer tick virus is thought to be a genotype

Genotype

The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...

 of Powassan virus, and also called Powassan virus lineage II.

In 1997, it was isolated from Ixodes scapularis

Ixodes scapularis

Ixodes scapularis is commonly known as the deer tick or blacklegged tick , and in some parts of the USA as the bear tick. It is a hard-bodied tick of the eastern and northern Midwestern United States...

(the deer tick, formerly Ixodes dammini) collected in Massachusetts

Massachusetts

The Commonwealth of Massachusetts is a state in the New England region of the northeastern United States of America. It is bordered by Rhode Island and Connecticut to the south, New York to the west, and Vermont and New Hampshire to the north; at its east lies the Atlantic Ocean. As of the 2010...

 and Connecticut

Connecticut

Connecticut is a state in the New England region of the northeastern United States. It is bordered by Rhode Island to the east, Massachusetts to the north, and the state of New York to the west and the south .Connecticut is named for the Connecticut River, the major U.S. river that approximately...

.

Phylogram