Powassan Virus Infections: A Systematic Review of Published Cases

Background: Powassan virus is an emerging neurotropic arbovirus transmitted by the tick Ixodes scapularis. This systematic review was conducted to aggregate data on its clinical manifestations, diagnostic findings, and complications. Methods: PubMed was searched until August 2023 using the term “Powassan”, to identify all published cases of Powassan virus infections, as per PRISMA guidelines. Results: Among the 380 abstracts identified, 45 studies describing 84 cases (70 adult, 14 pediatric) were included. Cases were reported from the USA and Canada. Complications included paralysis in 44.1% of adult and 42.6% of pediatric cases, cognitive deficits in 33.3% of adult and 25% of pediatric cases, while the mortality rate was 19.1% and 7.1% in the adult and pediatric populations, respectively. Correlation analysis revealed an association between mortality and age (r = 0.264, p = 0.029), development of paralysis (r = 0.252, p = 0.041), or respiratory distress or failure (r = 0.328, p = 0.006). Factors associated with persistent neurological deficits were development of ataxia (r = 0.383, p = 0.006), paralysis (r = 0.278, p = 0.048), speech disorder (r = 0.319, p = 0.022), and cranial nerve involvement (r = 0.322, p = 0.017). Other significant correlations included those between speech disorders and ataxia (r = 0.526, p < 0.001), and between paralysis and respiratory distress or failure (r = 0.349, p = 0.003). Conclusion: Powassan virus infections have significant morbidity and mortality and should be suspected in cases of encephalitis and possible tick exposure. PROSPERO registration number: CRD42023395991.


Introduction
Powassan virus is an arbovirus and a member of the Flaviviridae family.It is a singlestranded, positive-sense RNA virus, and has significant genetic similarities with another arbovirus, the deer tick virus, with a 84% nucleotide sequence identity [1].It is also closely related to the tick-borne encephalitis virus, which is found in Asia and Europe [2].It was originally isolated in 1958 from the brain of a child with fulminant encephalitis of unknown origin, from the town of Powassan in Ontario, Canada [3].
Powassan virus infection mainly manifests as encephalitis.However, the illness severity can vary significantly, from asymptomatic infection to the development of meningitis and encephalitis [4].Lymphocytic infiltration of perivascular neuronal tissue can be seen on pathology, with a predilection for gray matter, including the thalamus, midbrain, and Trop.Med.Infect.Dis.2023, 8, 508 2 of 22 cerebellum [5].In patients that develop meningoencephalitis, complications can include seizures, aphasia, cranial nerve palsies, and paresis [6], with approximately 10% of cases resulting in death [7].
The virus is transmitted by tick bites and has been isolated in various species, such as Ixodes cookei (groundhog tick), I. scapularis (black-legged or deer tick), I. marxi (squirrel tick), I. spinipalpus, and Dermacentor andersoni [8].Cases of Powassan virus reflect the ecology of its hosts and their interaction with humans.In general, cases begin increasing in April, peaking during the summer and continuing into the fall, before decreasing during the winter [9,10].Unlike Borrelia burgdorferi or Babesia microti (pathogens carried by I. scapularis), which require 48 h and 24 h of tick attachment for transmission to occur, respectively [11], Powassan virus transmission can occur 15-30 min after tick attachment, with symptoms appearing 1-4 weeks later [12].
Between 2004 and 2022, a total of 288 cases have been reported to the CDC, of which 267 (92.7%) pertained to neuroinvasive disease, 264 (91.7%) required hospitalization, and 36 (12.5%) resulted in death.The reported incidence of Powassan virus infection is increasing (Figure 1), with 44 incident cases noted in the USA in 2022 [9].
The virus is transmitted by tick bites and has been isolated in various species, such as Ixodes cookei (groundhog tick), I. scapularis (black-legged or deer tick), I. marxi (squirrel tick), I. spinipalpus, and Dermacentor andersoni [8].Cases of Powassan virus reflect the ecology of its hosts and their interaction with humans.In general, cases begin increasing in April, peaking during the summer and continuing into the fall, before decreasing during the winter [9,10].Unlike Borrelia burgdorferi or Babesia microti (pathogens carried by I. scapularis), which require 48 h and 24 h of tick attachment for transmission to occur, respectively [11], Powassan virus transmission can occur 15-30 min after tick attachment, with symptoms appearing 1-4 weeks later [12].
Between 2004 and 2022, a total of 288 cases have been reported to the CDC, of which 267 (92.7%) pertained to neuroinvasive disease, 264 (91.7%) required hospitalization, and 36 (12.5%) resulted in death.The reported incidence of Powassan virus infection is increasing (Figure 1), with 44 incident cases noted in the USA in 2022 [9].Given the increasing reported incidence of this devastating disease, we conducted a systematic review with the objective of identifying all published cases of Powassan virus infection, compiling aggregate data on its clinical manifestations, diagnostic findings and complications.

Materials and Methods
This systematic review was conducted according to the international Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines [13].The study protocol has been published in the international prospective register of systematic reviews PROSPERO, (https://www.crd.york.ac.uk/prospero/display_record.php?Recor-dID=395991, accessed on 31/08/2023), registration number CRD42023395991.Given the increasing reported incidence of this devastating disease, we conducted a systematic review with the objective of identifying all published cases of Powassan virus infection, compiling aggregate data on its clinical manifestations, diagnostic findings and complications.

Materials and Methods
This systematic review was conducted according to the international Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines [13].The study protocol has been published in the international prospective register of systematic reviews PROSPERO, (https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=39 5991, accessed on 31 August 2023), registration number CRD42023395991.

Data Sources and Searches
A systematic literature review was conducted through PubMed, using the search term "Powassan [title/abstract]".No limitations were imposed on publication date or language, with the search concluding on August 31st, 2023.Screening of article titles and abstracts was based on selection criteria as presented in Table 1.To identify further relevant publications, references from the extracted articles were also reviewed.

Study Selection
The selection of studies for inclusion in this review was carried out by a team of three reviewers (L.K., V.R.V., D.K.), who applied the selection criteria outlined in Table 1 to determine each study's eligibility.Subsequently, the reviewers further assessed the relevance and quality of the studies that met the inclusion criteria.
Study design: case reports, case series, and cohort studies describing cases of Powassan virus infections.
Type of participants: adult and pediatric patients that are diagnosed with Powassan virus infection through immunoassays via the state or CDC.
Type of exposure: Powassan virus infection Type of outcome: The primary outcome was identifying and describing the clinical manifestations, results of laboratory and imaging studies, and outcomes of patients with Powassan virus infections.Secondary outcomes were evaluating whether the presence of specific clinical manifestations and laboratory and imaging findings was associated with specific outcomes.

Data Extraction and Quality Assessment
The following data were extracted from full-text articles included in this review: eligibility criteria, study design, patient demographic parameters, symptoms and signs on presentation, findings of cerebrospinal fluid (CSF) and imaging studies, treatment with steroids or intravenous immunoglobulin (IVIG), development of complications, and final outcome.
The risk of bias in the included studies was independently evaluated by three authors (L.K., V.R.V., D.K.) using the Murad scale, which is a modified version of the Newcastle-Ottawa Scale (NOS) [14] used for non-randomized trials.The assessment involved the evaluation of six evidence-based criteria, which assessed for selection, representativeness of cases, ascertainment of outcomes and exposure, and adequate reporting [15].The questions used for evaluating each study are presented in Supplementary Table S1.Discrepancies between the reviewers were resolved through group discussions until a consensus was reached.Based on the number of criteria met, a study was categorized as having low risk of bias (all 6 criteria met), moderate risk of bias (5 or 4 criteria met), or high risk of bias (3 or less criteria met) [16].The scores assigned to each study are presented in Supplementary Table S2.

Data Synthesis and Analysis
Data relating to the primary and secondary endpoints are extracted and presented descriptively.Statistical analysis was performed using IBM SPSS for Windows version 28.For continuous variables, means are reported for parametric variables and medians for non-parametric variables.Statistical analysis was also conducted in order to evaluate the effect of the presence of specific manifestations, laboratory and imaging findings on mortality or long-term neurological deficits.The data were analyzed by converting them into binary variables, and correlations between these variables were identified using Pearson correlation.p-values were calculated using Fisher's Exact Test (2-sided), and only p-values less than 0.05 were considered statistically significant.

Role of the Funding Source
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Results
The original PubMed search retrieved 373 articles, and citation searching led to the identification of an additional 25 articles.A total of 45 studies met the inclusion criteria and were included in the review (Figure 2).The extracted data are presented in Table 2, while the quality assessment of each study is provided in Supplementary Table S2.
For continuous variables, means are reported for parametric variables and medians for non-parametric variables.Statistical analysis was also conducted in order to evaluate the effect of the presence of specific manifestations, laboratory and imaging findings on mortality or long-term neurological deficits.The data were analyzed by converting them into binary variables, and correlations between these variables were identified using Pearson correlation.P-values were calculated using Fisher's Exact Test (2-sided), and only p-values less than 0.05 were considered statistically significant.

Role of the Funding Source
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Results
The original PubMed search retrieved 373 articles, and citation searching led to the identification of an additional 25 articles.A total of 45 studies met the inclusion criteria and were included in the review (Figure 2).The extracted data are presented in Table 2, while the quality assessment of each study is provided in Supplementary Table S2.

Demographics
A total of 84 cases were identified; 70 were adults and 14 were pediatric patients.Among the adult patients, 23 individuals (32.9%) were female.The mean age of the adult patients was 60.1 years.In the pediatric population, three patients (21.4%) were female, with a mean age of 4.8 years.The diagnosis was confirmed in all cases through testing of the serum samples or CSF for Powassan virus-neutralizing antibodies or genetic material.Specifically, immunoassays for Powassan virus IgM were used to identify 80 cases (95.2%)-either in serum for 61 cases (72.6%), in CSF for 38 cases (45.2%), or in both for 17

Demographics
A total of 84 cases were identified; 70 were adults and 14 were pediatric patients.Among the adult patients, 23 individuals (32.9%) were female.The mean age of the adult patients was 60.1 years.In the pediatric population, three patients (21.4%) were female, with a mean age of 4.8 years.The diagnosis was confirmed in all cases through testing of the serum samples or CSF for Powassan virus-neutralizing antibodies or genetic material.Specifically, immunoassays for Powassan virus IgM were used to identify 80 cases (95.2%)-either in serum for 61 cases (72.6%), in CSF for 38 cases (45.2%), or in both for 17 cases (20.2%).The virus was identified in the CSF through either PCR for 6 cases (7.1%) or metagenomic next-generation sequencing in 2 cases (2.4%).In total, genetic testing was the sole diagnostic method used in four cases (4.8%).

Geographic Distribution
All identified cases were recorded in North America, with 75 cases (89.3%) reported from the USA and 9 cases (10.7%) from Canada.The cases were mostly distributed between northeastern United States, midwestern states bordering Canada, and Canadian states on the border with the USA.Most cases were recorded in New York, Massachusetts, Minnesota, and Wisconsin with 13 cases each.Maine followed with 8 cases; Ontario, Connecticut, and North Dakota had 4 cases each; and Quebec had 3 cases.Vermont had 2 cases, while Michigan, Tennessee, New Jersey, Rhode Island, New Brunswick, Nova Scotia, and Newfoundland each had 1 recorded case (Figure 3).cases (20.2%).The virus was identified in the CSF through either PCR for 6 cases (7.1%) or metagenomic next-generation sequencing in 2 cases (2.4%).In total, genetic testing was the sole diagnostic method used in four cases (4.8%).

Geographic Distribution
All identified cases were recorded in North America, with 75 cases (89.3%) reported from the USA and 9 cases (10.7%) from Canada.The cases were mostly distributed between northeastern United States, midwestern states bordering Canada, and Canadian states on the border with the USA.Most cases were recorded in New York, Massachusetts, Minnesota, and Wisconsin with 13 cases each.Maine followed with 8 cases; Ontario, Connecticut, and North Dakota had 4 cases each; and Quebec had 3 cases.Vermont had 2 cases, while Michigan, Tennessee, New Jersey, Rhode Island, New Brunswick, Nova Scotia, and Newfoundland each had 1 recorded case (Figure 3).

Data on Individual Cases
Data on individual cases, including patient presentation, CSF, and imaging findings, as well as final outcome are presented in Table 1.

Data on Individual Cases
Data on individual cases, including patient presentation, CSF, and imaging findings, as well as final outcome are presented in Table 1.
The final diagnosis was encephalitis in 63 patients and meningoencephalitis in 16 patients; 5 patients had no final diagnosis reported.Furthermore, four patients had a concurrent tickborne infection.
In the pediatric population, fever was also the most commonly reported symptom, found in all 14 patients (100%).Seizures were the second-most frequently observed symptom (64.3%), followed by headache (50%).Hypersomnia, somnolence, lethargy, nausea, and vomiting were each reported in 42.9%.Weakness and fatigue were noted in 14.34% of patients.
Altered mental status was observed in 28.6% of pediatric cases, whereas meningeal signs, tremors, and twitching each occurred in 21.4%.Paralysis or paresis was noted in 14.3% of patients.A rash was present in 14.3% of patients, while ataxia and cranial nerve involvement were each present in 7.1% of cases.No cases of confusion, memory loss, myalgias, arthralgias, respiratory distress or failure, speech disorders, or visual symptoms were observed in the pediatric population.
The final diagnoses were encephalitis (10) and meningoencephalitis (4).No concurrent tickborne infections were noted in the pediatric population.

CSF Studies
Lumbar puncture was performed in 79 (94%) of patients, with findings summarized in Table 4.Among adult patients, 4 did not undergo lumbar puncture, while data were not available for 1 case.The median white blood cell (WBC) count was 119/mL, predominantly lymphocytic, with a median lymphocyte count of 59.2/mL, a median monocyte count of 1.6, and a median neutrophil count of 10.9/mL.The mean glucose level was 64.9 mg/dL, and the mean protein level was 88.2 mg/dL.Opening pressures were reported in three subjects, with a median value of 180 mm H 2 O. Powassan virus-neutralizing antibody titers were reported in 45 cases, with a median titer of 1:320.All 14 pediatric patients underwent lumbar puncture.The median WBC count was 137.5/mL, also with a lymphocytic predominance, consisting of a median lymphocyte count of 75.5/mL, a median monocyte count of 46, and a median neutrophil count of 48.1/mL.The mean glucose level was 65.3 mg/dL and the median protein level was 35.5 mg/dL.Data on opening pressures were available for four pediatric cases, with a median value of 140 mm of H 2 O. Powassan virus-neutralizing antibody titers were measured in 10 pediatric cases, with a median titer of 1:120.

Imaging Studies
Brain MRI results were available for 62 cases, specifically for 56 adults and 6 pediatric patients.Findings are summarized in Table 5.Among the adults, the most commonly affected location was the basal ganglia, observed in 32.1% of cases.This was followed by cerebral involvement (26.8%) and cerebellar involvement (21.4%).The thalamus was affected in 14.3% of cases, while the midbrain showed involvement in 8.9% of cases.Additionally, 8.9% of cases presented with leptomeningeal enhancement on MRI while 21.4% of cases showed a normal MRI result.
Among the pediatric patients, two had both basal ganglia lesions and thalamic lesions, one had basal ganglia lesions and cerebral lesions, and one had thalamic lesions and midbrain lesions, while two patients had a normal MRI result.

Treatment
The identified treatment modalities were the administration of corticosteroids and/or IVIG.Six adult patients received only corticosteroids, four received only IVIG, while another four received both treatments.An improvement was noted in 3 of the patients receiving only corticosteroids and in 3 of the patients receiving both steroids and IVIG.
Among the pediatric patients, two received corticosteroids, and both improved after treatment.No pediatric patients received IVIG.

Correlation Analysis
Pearson correlation analysis was conducted only in the adult population due to the small sample size in the pediatric population, which limited the feasibility of such an analysis.Factors significantly correlated with mortality were age (r = 0.264, p = 0.029), development of paralysis (r = 0.252, p = 0.041), or respiratory distress or failure (r = 0.328, p = 0.006).Factors associated with persistent neurological deficits were development of ataxia (r = 0.383, p = 0.006), paralysis (r = 0.278, p = 0.048), speech disorder (r = 0.319, p = 0.022), and cranial nerve involvement (r = 0.322, p = 0.017).Other significant correlations included those between speech disorders and ataxia (r = 0.526, p < 0.001), and between paralysis and respiratory distress or failure (r = 0.349, p = 0.003).

Discussion
This systematic review examines Powassan virus infections, an emerging arbovirus increasingly identified as a causative agent of encephalitis in the USA [9].The review provides a comprehensive description of reported cases, including the prevalence of symptoms, signs, CSF, and imaging findings in these patients, as well as data on outcomes and prognostic factors.
Adult patients appeared to present with typical findings of viral encephalitis, with fever, headache, fatigue, and encephalopathy being among the most common findings.Some notable findings were the presence of speech disorders, visual symptoms, paresis, ataxia, and cranial nerve involvement.These findings may serve as clues for suspecting this pathogen, distinguishing it from other causes of encephalitis, such as HSV [58].Furthermore, imaging studies suggest a predilection for the basal ganglia, which has been associated with Flaviviridae infections [59].
Powassan virus infections were associated with significant morbidity and mortality.Among 288 cases reported to the CDC between 2004 and 2022, 36 (12.5%) had a fatal outcome [9].That mortality rate is lower than the rate calculated from the cases included in this review (14/78, 17.9%), suggesting the presence of publication bias.Furthermore, neurological deficits occurred at a significant rate, both cognitive and movement-related, which became persistent and long-term in the majority of adult cases.Correlation analysis identified age and complications such as paralysis and respiratory distress as associated with mortality, while the development of ataxia, paralysis or paresis, cranial nerve involvement, or speech disorders was associated with the development of long-term neurological deficits.
Notably, the pediatric patients frequently experienced seizures, which were the secondmost commonly reported symptom.It is notable that the prevalence of this symptom was only 1.4% in the adult population, compared to 64.3% in the pediatric population.Imaging studies were conducted less often, but when performed, they also identified the basal ganglia as a commonly affected site.The prognosis was also unfavorable in this group, as long-term neurological deficits were common and persistent.
Identification of exposure to ticks can play a crucial role in making the diagnosis.However, this task may be unreliable, as even in cases of Lyme disease, which requires prolonged tick attachment for transmission to occur, patients rarely recall tick bites [60].Consequently, clinicians should consider other indicators, such as the presence in or travel to an endemic area, as well as the engagement in activities that increase the likelihood of exposure to ticks, such as hiking, camping, gardening, or hunting [61].Such exposures should raise the clinician's suspicion regarding the possibility of Powassan virus as the underlying cause of the patient's condition.
Furthermore, the geographic distribution of cases of Powassan virus infections appears to be expanding.From the original case in Ontario [3], cases have been reported as far west as North Dakota [31] and as far south as Tennessee [8].This may be due to increased awareness and increased testing and diagnosis.It may also be due to the expanding habitat of its host.Indeed, studies evaluating the impact of climate change on I. scapularis have shown that there is a progressive expansion of its range, estimated to be approximately 46 km/year.This process is expected to continue, especially northward, as more areas become warmer and more suitable habitats for the tick [62].Furthermore, climate change is leading to warmer winters, which may translate to increased tick survival and possibly more infections [63].
There are limitations to this study.First and foremost, the information included in each case was dependent on what was reported by each study, and in several cases, the data were incomplete.Furthermore, the mortality rate calculated in this systematic review exceeds that reported by the CDC, suggesting that there is publication bias favoring the more severe cases.Information on co-morbidities, which may affect mortality and morbidity, was not extracted.Therefore, there is a significant selection bias that may increase the prevalence of more severe symptoms, signs, complications, and long-term sequalae including mortality, which may have been overestimated.In addition, although this study did not encompass reports of deer tick virus, it is noteworthy that the term "Powassan virus" is occasionally used as an umbrella term encompassing both Powassan virus and deer tick virus; consequently, the authors lack the means to ascertain whether the publication might have inadvertently documented a case of deer tick virus as a Powassan virus infection.Lastly, only articles published in the English language were evaluated; therefore, publications on Powassan virus outside of the US and Canada may have not been included.

Conclusions
In conclusion, Powassan virus encephalitis is associated with considerable mortality and poses a risk of long-term neurological complications.Clinicians should maintain a high index of suspicion for this disease in patients presenting with encephalitis, particularly those with a known or suspected tick exposure.Additionally, individuals who exhibit a combination of speech disorders, paralysis, ataxia, or cranial nerve dysfunction, along with basal ganglia lesions on imaging, should raise suspicion for Powassan virus.Given the increasing incidence and expanding geographic range of the virus, it is expected that the disease will become more prevalent in the coming years.Comprehensive clinical reporting, along with surveillance studies, and further research are warranted to better understand

Figure 1 .
Figure 1.Incident cases of Powassan virus infection in the USA, as reported by the CDC [9].

Figure 1 .
Figure 1.Incident cases of Powassan virus infection in the USA, as reported by the CDC [9].

Figure 3 .
Figure 3. Map depicting the distribution of Powassan virus cases reported in this review along with the distribution of Ixodes scapularis in the USA[17] and Canada[18].

Figure 3 .
Figure 3. Map depicting the distribution of Powassan virus cases reported in this review along with the distribution of Ixodes scapularis in the USA[17] and Canada[18].

Table 1 .
Inclusion and exclusion criteria for this study.

Table 2 .
Data on each individual published case.

Table 3 .
Prevalence of clinical manifestations of Powassan virus infections.

Table 4 .
Findings of cerebrospinal fluid analysis.

Table 5 .
Summary of MRI Findings.

Table 6 .
Summary of outcomes.