Powassan virus is a Flavivirus transmitted by ticks, found in North America and in the Russian Far East. It is named after the town of Powassan, Ontario, where it was identified in a young boy who eventually died from it. It can cause encephalitis, an infection of the brain. No vaccine or antiviral drug exists. Prevention of tick bites is the best precaution.
Classification and occurrenceEdit
Powassan virus (POWV) is a Flavivirus named after the town of Powassan, Ontario, where it was identified in a young boy who might have died from it. The virus exists in North America. As of 2010[update], Powassan virus has been noted as the only tick-borne Flavivirus in North America with human pathogenicity.
Powassan virus is also found in the warm climate across Eurasia, where it is part of the tick-borne encephalitis virus-complex. It is found in the Russian Far East (Primorsky Krai) and appears to have been introduced there 70 years ago.
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 (DTV) lineage. 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. DTV is very closely related to Powassan virus and a sequence analysis showed that the two viruses diverged about 200 years ago. 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 The lineages share 84% nucleotide sequences and 94% amino acid sequence identity. 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.” As a result, the lineages are part of the same viral species.
The virus can be transmitted with bites from altogether six known species of ticks; the following four species of Ixodes ticks: Ixodes cookei, Ixodes scapularis, Ixodes marxi and Ixodes spinipalpus and the ticks Dermacentor andersoni and Dermacentor variabilis.
People with POWV have been mostly confirmed as having one strain of POWV, the deer tick virus. Ix. scapularis is an important vector for the Deer Tick Virus, which plays a vital role in maintaining the POWV. Ix. scapularis is also a primary vector for the agent of Lyme disease, because they are generalist feeders and readily bite humans.
In Canada and the Northeastern United States Ixodes cookei is the predominant species, while Ix. scapularis is a significant vector in Minnesota and Wisconsin. POWV is transmitted when an infected tick bites a mammal; in humans the tick is typically Ix. scapularis. In North America, the lineages of the POWV are maintained in three main enzootic cycles involving three different tick species and their respective small to medium-sized woodland mammals. POWV may infect Ix. cookei and woodchucks, or it may infect Ix. marxi and squirrels, and it can cycle between Ix. scapularis and white-footed mice.
Based on the time interval for other tick-borne diseases like Lyme disease and anaplasmosis, the time interval for transmission of POWV is expected to be less than 12 hours. Once the POWV reaches humans it cannot be transmitted to a feeding tick, therefore humans are considered "dead-end" hosts. As of 2004[update], the fastest transmission time of DTV from a Ix. scapularis nymph to a mouse was no more than 15 minutes.
Powassan virus infection is rarely diagnosed as a cause of encephalitis; however, when it is, Powassan encephalitis is severe, and neurologic sequelae are common. Powassan encephalitis has symptoms compatible with acute disseminated encephalomyelitis, oftentimes making it difficult to diagnose. Powassan virus encephalitis is a challenge to diagnose because there are only a few laboratories that offer testing, the most effective being serologic testing.
There are currently no medications or vaccines to treat or prevent the POWV. People affected by Powassan virus generally first show symptoms 1 to 3 weeks after infection. The initial symptoms include fever, headache, nausea, occasional confusion, and weakness. 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. Currently, the best ways to treat POWV illnesses include medications to reduce brain swelling, respiratory support and intravenous fluids. About 10% of POWV encephalitis cases are fatal and half the survivors have permanent symptoms that affect their brain. There were 33 confirmed cases of Powassan virus infection in the U.S. between 2001 and 2010.
A rare case of a five-month-old Connecticut infant boy contracting Powassan virus infection was published in 2017. He survived with normal motor and verbal development on follow-up at the age of 10 months, but a head MRI showed severely abnormal brain conditions, including scarring (gliosis) and softening (encephalomalacia) in the thalamus and basal ganglia on both sides, and volume loss and early mineralization in the left basal ganglia.
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