The TBE virus belt spans in Eurasia from the United Kingdom in the west to Japan in the east. However, the TBE virus has not crossed the Bering Strait and is absent in North America. Nonetheless, a related flavivirus circulates in North America: the Powassan virus (POWV). For information on the phylogenetic relationship, see “The TBE Book”, 7th edition, chapter 4, Figure 1.
Recently, three cases of Powassan virus (POWV) infections were reported in Canada and the USA. One case was reported in Manitoba, Canada (Smith et al., 2024), while two other cases were observed in the USA, specifically in Massachusetts and New York (Madabhushi et al., 2024; Carrasco et al., 2024). These reports of Powassan encephalitis have prompted discussions about POWV and Powassan encephalitis within the TBE News series.
Powassan virus circulates in Canada, the United States, and Russia. POWV lineage 1 is mostly transmitted by the black-legged tick, Ixodes scapularis, and POWV lineage 2 is primarily spread by I. cookie (groundhog tick) and I. marxi (squirrel tick). These two lineages are serologically indistinguishable and can cause severe neuroinvasive disease. The aforementioned tick species are also vectors for other tick-borne pathogens such as Borrelia burgdorferi, Borrelia miyamotoi, Anaplasma phagocytophilium, and Babesia microti. While these pathogens generally require ticks to be attached to the host for at least 24 hours for transmission, the transmission time for POWV is much shorter, usually occurring within a few hours, but it may happen in as little as 15 minutes (see Snapshot week 45/2021). The incubation period for POWV is about one to five weeks, with illness typically beginning with non-specific influenza-like symptoms, including fever, headache, fatigue and gastrointestinal symptoms. The disease can progress to neurological symptoms, including confusion, seizures, and ataxia. Powassan disease is fatal in 10-15% of cases, and about 50% of patients experience long-term neurological sequelae.
The diagnosis of POWV infection requires a high degree of clinical suspicion and the appropriate testing, taking into account the phase of infection and the patient’s immune status. Complement fixation and hemagglutination inhibition tests are no longer routinely performed in the USA. Currently, serological screening tests are the first-line assays for detecting POWV infections, with various IgM and IgG ELISAs available (Klontz et al., 2024).
No causative antiviral drug treatment is available, so only supportive care is recommended. Additionally, no POWV vaccine currently exists, although a candidate vaccine based on recombinant glycoprotein E has been discussed in Snapshot week 25/2021.
The first human case in the USA was reported in 1970 and in Russia in 1978. Between 2004 and 2013, a total of 64 cases were reported to the CDC, and in the following decade, from 2014 to 2023, 270 cases were reported, with 21-44 cases annually from 2016 to 2022. Siegel et al. (2024) analyzed the presence of Powassan virus in ticks collected in the northeastern USA, finding that among 14,730 Ixodes scapularis ticks, 42 (0.29%) were positive for POWV. Of these, 38 ticks had bitten humans. Most POWV-positive tick bites remain asymptomatic or cause only mild disease, but the true rate of infections leading to neuroinvasive disease is unknown.
There is a lack of reporting of POWV infections, limited knowledge about the prevalence of the virus, and low awareness of POWV as a cause of severe disease. Surveillance is primarily passive, and the spread of the virus vector Ixodes scapularis is increasing. Thus, a significant disparity seems to exist between the perceived risk of acquiring a POWV infection and the actual rate of infections. Public health efforts to educate the public about the risks of POWV are needed to reduce the incidence of POWV infections. Further research is necessary to develop more accurate diagnostic tools, effective treatment, and potentially a vaccine.
Literature
Carrasco et al.
A fatal case of Powassan virus encephalitis. J Brown Hosp Med. 2024;3. doi:10.56305/001c.117405.
Klontz et al.
Laboratory Testing for Powassan Virus: Past, Present, and Future. J Infect Dis. 2024;230(Supplement_1):S70-S75. doi:10.1093/infdis/jiae197
Madabhushi et al.
A case of Powassan virus encephalitis in Massachusetts.
Ann Intern Med Clin Cases. 2024;3. doi:10.7326/aimcc.2023.1248.
Siegel et al.
Passive surveillance of Powassan virus in human-biting ticks and health outcomes of associated bite victims. Clin Microbiol Infect. 2024. doi:10.1016/j.cmi.2024.06.012
Smith et al.
Powassan virus encephalitis after a tick bite, Manitoba, Canada. Emerg Infect Dis. 2024;[published online ahead of print]. doi:10.3201/eid3009.23134.
Author: Dr. Michael Bröker
Compiled: August 2024