Powassan virus

Powassan virus is a virus transmitted by ticks. The disease it causes is named after the town of Powassan, Ontario, where it was identified in a young boy who eventually died from it.

Vectors

The Powassan Virus (POWV) is normally found in the warm climate across Eurasia where it is part of the TBE-complex.^[1] The disease also exists in North America and can be transmitted with bites from the following species of Ixodes ticks: Ix. cookei, Ix scapularis, Ix. marxi, Ix spinipalpus. The Dermacentor ticks, Dermacentor andersonii and Dermacentor variabilis are also vectors of the POWV.^[2][3] There are a total of 6 known species of tick that act as vectors, with Ixodes cookei being the predominant species in Canada and the Northeastern United States and Ix. scapularis as a significant vector in Minnesota and Wisconsin.^[3] There are rare cases in which Ix. cookei attaches to humans, and as a result the case-patients with POWV have been mostly confirmed as having the strain of POWV, the Deer tick virus (DTV). The Deer Tick Virus plays a vital role in maintaining the POWV and is vectored by Ix. scapularis.^[1] Ix. scapularis is an important vector of the enzootic transmission cycle of the Deer Tick Virus.^[1] Ix. scapularis is also a primary vector for the agent of Lyme disease because they are a generalist feeder and readily bite humans. The Powassan virus is transmitted by ticks among small mammals in eastern Canada and the United States, where it has been responsible for 49 deaths in the U.S. between 2000–2011.^[4] In North America, the Powassan Virus has been noted as the only tick-borne Flavivirus with human pathogenicity so far.^[5]

Clinical

The Powassan virus is rarely diagnosed as a cause of encephalitis; however, when it is, Powassan encephalitis is severe, and neurologic sequelae are common.^[2] Powassan encephalitis has symptoms that are compatible with Acute disseminated encephalomyelitis, oftentimes making it difficult to diagnose.^[2] Powassan virus encephalitis is a challenge to diagnose because there are only a few laboratories that offer testing, the most effective being serologic testing.^[6] There are currently no medications or vaccines to treat or prevent the POWV. Victims of the Powassan virus generally show first symptoms after 1–3 weeks.^[3] The initial symptoms for the POWV include: fever, headache, nausea, occasional confusion, and weakness.^[7] With severe Powassan illnesses the victims should be hospitalized because the symptoms do worsen. If not treated symptoms could extend to meningoencephalitis, which may include: seizures, aphasia, cranial nerve palsies, paresis and altered mental status.^[3][7] Currently the best ways to treat POWV illnesses include: medications to reduce brain swelling, respiratory support and intravenous fluids.^[7] 10% of the POWV encephalitis cases are fatal and half of the survivors have permanent symptoms that affect their brain.^[8]

Evolution

The Powassan Virus is an RNA virus split into two separate lineages, Lineage I, labeled as the “prototype” lineage, and Lineage II, the Deer tick virus lineage.^[1] Lineage II has the most genetic variation which indicates that it is most likely the ancestral lineage that split as a result of positive natural selection.^[1] DTV is very closely related to Powassan virus and recent sequence analysis estimated that the two viruses diverged "approximately 200 years ago".^[9] Even though lineage II has been predominant in POWV positive tick pools, both lineages have had confirmed cases of human disease in North America and Russia^[3][7] The lineages share 84% nucleotide sequences and 94% amino acid sequence identity.^[1] Cross-neutralization occurs among flaviviruses due to the conservation of the envelope protein, this is what contributes to the fact that the two lineages are “serologically indistinguishable”.^[2] As a result, the lineages are part of the same viral species. The Powassan virus is also found in the Russian Far East (Primorsky Krai) and appears to have been introduced there 70 years ago.^[10]

Transmission

In North America, the lineages of the POWV are maintained in 3 main enzootic cycles.^[1] These cycles involve the ticks Ix. cookei, Ix. marxi, and Ix. scapularis, and small to medium-sized woodland mammals.^[1] Woodchucks and Ix. cookei, squirrels and Ix. marxi, and white-footed mice and Ix. scapularis are the enzootic cycles in which the virus is maintained.^[7] The POWV is transmitted from the bite of an infected tick and in humans the Ix. scapularis tick is notorious for being the attacker.^[11] The fastest transmission time of DTV from a nymph Ix. scapularis to a mouse was no more than 15 minutes.^[12] Based on the time interval for the other tick-borne diseases Lyme disease and anaplasmosis, the time interval for transmission of POWV is expected to be less than 12 hours.^[8] Once the POWV reaches humans it cannot be transmitted to a feeding tick, therefore humans are considered “dead-end” hosts.^[7]

References