Phylogeographic reconstruction of the emergence and spread of Powassan virus in the northeastern United States.

Powassan virus is an emerging tick-borne virus of concern for public health, but very little is known about its transmission patterns and ecology. Here, we expanded the genomic dataset by sequencing 279 Powassan viruses isolated from Ixodes scapularis ticks from the northeastern United States. Our phylogeographic reconstructions revealed that Powassan virus lineage II was likely introduced or emerged from a relict population in the Northeast between 1940 and 1975. Sequences strongly clustered by sampling location, suggesting a highly focal geographical distribution. Our analyses further indicated that Powassan virus lineage II emerged in the northeastern United States mostly following a south-to-north pattern, with a weighted lineage dispersal velocity of ~3 km/y. Since the emergence in the Northeast, we found an overall increase in the effective population size of Powassan virus lineage II, but with growth stagnating during recent years. The cascading effect of population expansion of white-tailed deer and I. scapularis populations likely facilitated the emergence of Powassan virus in the northeastern United States.

Jamestown Canyon Virus in Collected Mosquitoes, Maine, United States, 2017-2019.

Jamestown Canyon virus (JCV) is a mosquito-borne arbovirus that circulates in North America. We detected JCV in 4 pools of mosquitoes collected from midcoastal Maine, USA, during 2017-2019. Phylogenetic analysis of a JCV sequence obtained from Aedes cantator mosquitoes clustered within clade A, which also circulates in Connecticut, USA.

Surge in Anaplasmosis Cases in Maine, USA, 2013-2017.

Incidence of human granulocytic anaplasmosis is rising in Maine, USA. This increase may be explained in part by adoption of tick panels as a frequent diagnostic test in persons with febrile illness and in part by range expansion of Ixodes scapularis ticks and zoonotic amplification of Anaplasma phagocytophilum.

Seroprevalence of Borrelia burgdorferi, B. miyamotoi, and Powassan Virus in Residents Bitten by Ixodes Ticks, Maine, USA.

We conducted a serosurvey of 230 persons in Maine, USA, who had been bitten by Ixodes scapularis or I. cookei ticks. We documented seropositivity for Borrelia burgdorferi (13.9%) and B. miyamotoi (2.6%), as well as a single equivocal result (0.4%) for Powassan encephalitis virus.

Fatal Deer Tick Virus Infection in Maine.

Deer tick virus (DTV), a genetic variant (lineage II) of Powassan virus, is a rare cause of encephalitis in North America. We report a fatal case of DTV encephalitis following a documented bite from an Ixodes scapularis tick and the erythema migrans rash associated with Lyme disease.

Human babesiosis, Maine, USA, 1995-2011.

We observed an increase in the ratio of pathogenic Babesia microti to B. odocoilei in adult Ixodes scapularis ticks in Maine. Risk for babesiosis was associated with adult tick abundance, Borrelia burgdorferi infection prevalence, and Lyme disease incidence. Our findings may help track risk and increase the focus on blood supply screening.

Partial trailside Japanese barberry (Ranunculales: Berberidaceae) removal did not reduce the abundance of questing blacklegged ticks (Acari: Ixodidae).

In a nature reserve in southern Maine, we removed invasive Japanese barberry (Berberis thunbergii de Candolle) along sections of forested recreational trails that ran through dense barberry infestations. Barberry thickets provide questing substrate and a protective microclimate for blacklegged ticks (Ixodes scapularis Say), and trail users could brush up against encroaching barberry and acquire ticks. Trailside barberry removal will reduce or eliminate encroaching tick questing substrate and could reduce trailside questing tick abundance by creating a microclimate more hostile to ticks. The same-day cut-and-spray treatment comprised mechanical cutting of barberry clumps (individual plants with numerous ramets) followed immediately by targeted herbicide application to the resulting root crowns. The treatment created trail shoulders to a lateral width of 1-2 m on both sides of 100-m trail sections, with initial treatment in the fall of 2013 and one retreatment in the summer of 2014. Our aim was to remove 90% of barberry clumps to achieve a 50% or better reduction in questing tick abundance on trail shoulders. However, by the fall of 2015, there were only 41% fewer barberry clumps on treated vs. untreated trail sections and there was no reduction in either adults or nymphs. We concluded that our barberry treatment protocol was not sufficiently aggressive since the resulting ecotone habitat on trail shoulders proved suitable for questing I. scapularis. In principle, cutting back barberry along trails should reduce trail user contact with questing deer ticks, but we were unable to demonstrate a reduction in trailside tick abundance.

Prevalence and Genetic Diversity of Deer Tick Virus (Powassan Virus, Lineage II) in Ixodes scapularis Ticks in Five Habitats at a Nature Reserve in Southern Maine, United States.

Deer tick virus (DTV), also known as Powassan virus lineage II, is a rising health concern due to increased recognition as a cause of human encephalitis. Since European tick-borne encephalitis virus persists in nature in enzootic foci (i.e., higher prevalence rates in small pockets in nature), we sought to determine whether DTV is also focally maintained in relation to habitat type, to better understand factors leading to human risk of exposure. From 2018 to 2021, questing Ixodes scapularis ticks were collected from five habitats at the Wells National Estuarine Research Reserve (WNERR) in Wells, ME: forest with invasive vegetation in the understory, edge, shrub, forest with native vegetation in the understory, and open field. Deer tick virus prevalence was greater in adult ticks (2.0%) than in nymphs (0.5%). Deer tick virus prevalence in adult ticks collected from forest with invasive vegetation was 3.2% compared to 0 to 1.7% in other habitat types. A hot spot analysis revealed a higher number of infected adults collected per hour on one of the transects located in forest with invasive vegetation. Phylogenetic analysis of 37 full-length DTV genomes sequenced in this study revealed four major clades from the WNERR, and there was high genetic diversity within each transect, suggesting frequent, short-range dispersal between habitats. Analysis of DTV sequences from other New England counties and states also indicated long-distance dispersal to and/or from the WNERR. This study provides preliminary evidence that DTV is focal and that the risk of encountering DTV-infected ticks in forest with invasive vegetation may be greater than in other habitat types.

Effect of a botanical acaricide on Ixodes scapularis (Acari: Ixodidae) and nontarget arthropods.

We tested the effectiveness of the rosemary oil-based insecticide, Eco-Exempt IC2, to control all stages of Ixodes scapularis (Say) in southern Maine. We selected plots in oak-pine forest where I. scapularis is endemic and recorded the abundance of ticks and nontarget arthropods before and after applications of IC2, bifenthrin (a synthetic pyrethroid), and water (reference treatment). Licensed applicators applied high-pressure spray treatments during the summer nymphal and fall adult seasonal peaks. Both acaricides sprayed during the summer nymphal season reduced nymphal I. scapularis/hour to zero. IC2 was as effective as bifenthrin in controlling nymphs through the rest of the nymphal season and also controlled adult ticks 9 mo postspray compared with 16 mo for bifenthrin, and both acaricides reduced larvae through 14 mo postspray. Both acaricides sprayed during the fall adult season reduced adult I. scapularis/hour to zero; IC2 controlled adult ticks 6 mo postspray compared with 1 yr for bifenthrin. Both fall-applied acaricides controlled nymphs 9 mo postspray and reduced larvae up to 10 mo postspray. Impacts on some nontarget arthropods was assessed. Colleoptera, Hymenoptera, and Collembola declined 1 wk postspray in acaricide-treated plots, and in IC2 plots all numbers rebounded by 20 d postspray. For bees and other flower-visiting insects there were no detectable reductions in nests produced, number emerged from nests, or number of foraging visits to flowering plants in IC2 or bifenthrin plots. IC2 was phytotoxic to the leafy portions of select understory plants that appeared to recover by the next growing season.

Phylodynamics of deer tick virus in North America.

The burden of ticks and the pathogens they carry is increasing worldwide. Powassan virus (POWV; Flaviviridae: Flavivirus), the only known North American tick-borne flavivirus, is of particular concern due to rising cases and the severe morbidity of POWV encephalitis. Here, we use a multifaceted approach to evaluate the emergence of the II POWV lineage, known as deer tick virus (DTV), in parts of North America where human cases occur. We detected DTV-positive ticks from eight of twenty locations in the Northeast USA with an average infection rate of 1.4 per cent. High-depth, whole-genome sequencing of eighty-four POWV and DTV samples allowed us to assess geographic and temporal phylodynamics. We observed both stable infection in the Northeast USA and patterns of geographic dispersal within and between regions. A Bayesian skyline analysis demonstrated DTV population expansion over the last 50 years. This is concordant with the documented expansion of Ixodes scapularis tick populations and suggests an increasing risk of human exposure as the vector spreads. Finally, we isolated sixteen novel viruses in cell culture and demonstrated limited genetic change after passage, a valuable resource for future studies investigating this emerging virus.

Microclimate conditions alter Ixodes scapularis (Acari: Ixodidae) overwinter survival across climate gradients in Maine, United States.

The incidence and geographic range of vector-borne diseases have been expanding in recent decades, attributed in part to global climate change. Blacklegged ticks (Ixodes scapularis), the primary vector for multiple tick-borne pathogens in North America, are spreading rapidly beyond their historic post-colonial range and are thought to be constrained mainly by winter temperature at northern latitudes. Our research explored whether winter climate currently limits the distribution of blacklegged ticks and the pathogens they transmit in Maine, U.S.A., by contributing to overwinter mortality of nymphs. We experimentally tested tick overwinter survival across large-scale temperature and snowfall gradients and assessed factors contributing to winter mortality in locations where blacklegged tick populations are currently established and locations where the blacklegged tick has not yet been detected. We also tested the hypothesis that insulation in the tick microhabitat (i.e., by leaf litter and snowpack) can facilitate winter survival of blacklegged tick nymphs despite inhospitable ambient conditions. Overwinter survival was not significantly different in coastal southern compared to coastal and inland northern Maine, most likely due to sufficient snowpack that protected against low ambient temperatures at high latitudes. Snow cover and leaf litter contributed significantly to overwinter survival at sites in both southern and northern Maine. To further assess whether the current distribution of blacklegged ticks in Maine aligns with patterns of overwinter survival, we systematically searched for and collected ticks at seven sites along latitudinal and coastal-inland climate gradients across the state. We found higher densities of blacklegged ticks in coastal southern Maine (90.2 ticks/1000 m2) than inland central Maine (17.8 ticks/1000 m2) and no blacklegged ticks in inland northern Maine. Our results suggest that overwinter survival is not the sole constraint on the blacklegged tick distribution even under extremely cold ambient conditions and additional mechanisms may limit the continued northward expansion of ticks.

Use of Cervid Serosurveys to Monitor Eastern Equine Encephalitis Virus Activity in Northern New England, United States, 2009-2017.

Vertebrate surveillance for eastern equine encephalitis virus (EEEV) activity usually focuses on three types of vertebrates: horses, passerine birds, and sentinel chicken flocks. However, there is a variety of wild vertebrates that are exposed to EEEV infections and can be used to track EEEV activity. In 2009, we initiated a pilot study in northern New England, United States, to evaluate the effectiveness of using wild cervids (free-ranging white-tailed deer and moose) as spatial sentinels for EEEV activity. In Maine, New Hampshire, and Vermont during 2009-2017, we collected blood samples from hunter-harvested cervids at tagging stations and obtained harvest location information from hunters. U.S. Centers for Disease Control and Prevention processed the samples for EEEV antibodies using plaque reduction neutralization tests (PRNTs). We detected EEEV antibodies in 6 to 17% of cervid samples in the different states and mapped cervid EEEV seropositivity in northern New England. EEEV antibody-positive cervids were the first detections of EEEV activity in the state of Vermont, in northern Maine, and northern New Hampshire. Our key result was the detection of the antibodies in areas far outside the extent of documented wild bird, mosquito, human case, or veterinary case reports of EEEV activity in Maine, New Hampshire, and Vermont. These findings showed that cervid (deer and moose) serosurveys can be used to characterize the geographic extent of EEEV activity, especially in areas with low EEEV activity or with little or no EEEV surveillance. Cervid EEEV serosurveys can be a useful tool for mapping EEEV activity in areas of North America in addition to northern New England.

Multitarget test for emerging Lyme disease and anaplasmosis in a serosurvey of dogs, Maine, USA.

To determine if the range of deer ticks in Maine had expanded, we conducted a multitarget serosurvey of domestic dogs (Canis lupus familiaris) in 2007. An extension of exposure to Borrelia burgdorferi to the northern border and local transmission of Anaplasma phagocytophilum throughout southern areas was found.

Support for deer herd reduction on offshore Islands of Maine, U.S.A.

Over the past three decades, citizens of Maine in the northeastern United States have experienced increasing blacklegged tick (Ixodes scapularis) abundance and rising incidence of Lyme and other tick-borne diseases. White-tailed deer (Odocoileus virginianus) overabundance has been considered one cause of the high incidence of tick-borne diseases on offshore islands of New England. Most of Maine's 15 offshore, unbridged island communities have a history of concern about ticks, Lyme disease, and white-tailed deer overabundance, but have been challenged to keep deer numbers down through hunting or culls. This history has led to perennial, often divisive community debates about whether and how to reduce the size of their deer herds. In 2016 we conducted a convenience sample survey of year-round and summer residents of Maine's offshore islands to quantify the level of concern about Lyme disease, and assess the motivations and level of support for deer herd reduction. Among respondents, 84 % agreed Lyme disease was a problem on their island and 61 % supported deer herd reduction. Agreement that Lyme disease was a problem was associated with having acquired tick-borne disease as well as with tick bites without disease. Respondents ranked deer overabundance as a top cause of tick abundance and tick-borne disease and supported deer herd reduction as an approach to reduce the risk of Lyme disease. Other problems associated with deer overabundance (vehicle collisions, damage to landscaping, and damage to forests) also motivated support for deer reduction. Approval of doe permits, an expanded archery season, and sharpshooting as reduction methods was greater than an expanded firearms season. Respondents felt responsibility for tick control fell to the town for the most part, and recognized that multiple factors have contributed to the tick problem in Maine, not just deer.

A Generalized Additive Model Correlating Blacklegged Ticks With White-Tailed Deer Density, Temperature, and Humidity in Maine, USA, 1990-2013.

Geographical range expansions of blacklegged tick [Ixodes scapularis Say (Acari: Ixodidae)] populations over time in the United States have been attributed to a mosaic of factors including 20th century reforestation followed by suburbanization, burgeoning populations of the white-tailed deer [Odocoileus virginianus Zimmerman (Artiodactyla: Cervidae)], and, at the northern edge of I. scapularis' range, climate change. Maine, a high Lyme disease incidence state, has been experiencing warmer and shorter winter seasons, and relatively more so in its northern tier. Maine served as a case study to investigate the interacting impacts of deer and seasonal climatology on the spatial and temporal distribution of I. scapularis. A passive tick surveillance dataset indexed abundance of I. scapularis nymphs for the state, 1990-2013. With Maine's wildlife management districts as the spatial unit, we used a generalized additive model to assess linear and nonlinear relationships between I. scapularis nymph abundance and predictors. Nymph submission rate increased with increasing deer densities up to ~5 deer/km2 (13 deer/mi2), but beyond this threshold did not vary with deer density. This corroborated the idea of a saturating relationship between I. scapularis and deer density. Nymphs also were associated with warmer minimum winter temperatures, earlier degree-day accumulation, and higher relative humidity. However, nymph abundance only increased with warmer winters and degree-day accumulation where deer density exceeded ~2 deer/km2 (~6/mi2). Anticipated increases in I. scapularis in the northern tier could be partially mitigated through deer herd management.

Trial of a minimal-risk botanical compound to control the vector tick of Lyme disease.

We compared the application of IC2, a minimal-risk (25B) botanical compound containing 10% rosemary oil, with bifenthrin, a commonly used synthetic compound, and with water for the control of Ixodes scapularis Say (= Ixodes dammini Spielman, Clifford, Piesman & Corwin), on tick-infested grids in Maine, in an area where Lyme disease is established and other tick-borne diseases are emerging. High-pressure sprays of IC2, bifenthrin, and water were applied during the peak nymphal (July) and adult (October) seasons of the vector tick. No ticks could be dragged on the IC2 grids within 2 wk of the July spray, and few adult ticks were found in October or the following April. Similarly, no adult ticks could be dragged 1.5 wk after the October IC2 spray, and few the following April. No ticks were found on the bifenthrin grids after either spray through the following April, whereas substantial numbers of ticks remained throughout on the grids sprayed with water. Thus, IC2 appears to be an effective, minimum-risk acaricide to control the vector tick of Lyme disease.

Decoupling of Blacklegged Tick Abundance and Lyme Disease Incidence in Southern Maine, USA.

Lyme disease is caused by the bacterial spirochete Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt, and Brenner (Spirocheatales: Spirochaetaceae) which is transmitted through the bite of an infected blacklegged tick Ixodes scapularis Say (Ixodida: Ixodidae). Maine, USA, is a high Lyme disease incidence state, with rising incidence of Lyme disease and other tick-borne illnesses associated with increasing I. scapularis abundance and northward range expansion. Members of the public submitted ticks to a tick identification program (1990-2013). From these passive surveillance data, we characterized temporal trends in I. scapularis submission rate (an index of abundance), comparing Maine's northern tier (seven counties) versus southern tier (nine counties). In the northern tier, the I. scapularis submission rate increased throughout the duration of the time series, suggesting I. scapularis was emergent but not established. By contrast, in the southern tier, submission rate increased initially but leveled off after 10-14 yr, suggesting I. scapularis was established by the mid-2000s. Active (field) surveillance data from a site in the southern tier-bird tick burdens and questing adult tick collections-corroborated this leveling pattern. Lyme disease incidence and I. scapularis submission rate were temporally correlated in the northern but not southern tier. This suggested a decoupling of reported disease incidence and entomological risk.

Determining Effects of Winter Weather Conditions on Adult Amblyomma americanum (Acari: Ixodidae) Survival in Connecticut and Maine, USA.

The lone star tick (Amblyomma americanum L.) is native to the United States, with its primary range encompassing the Southeast and portions of the Midwest. It is an aggressive ectoparasite that actively seeks out hosts through detection of carbon dioxide and vibrations and can transfer ehrlichiosis-causing bacteria as well as a carbohydrate that causes alpha-gal syndrome (red meat allergy) in humans. It has become of increasing concern as its range has recently expanded into coastal regions of the Northeast. Historically, harsh northeastern winter weather conditions made these areas inhospitable for A. americanum survival, but a warming climate coupled with increased host availability seem to have facilitated their range expansion. We developed a study to observe the effects of weather conditions on adult A. americanum overwintering survival. The study was conducted over three years in Connecticut and Maine. Ground-level conditions were manipulated to determine the effects of differing combinations of natural insulative barriers (leaf litter and snow accumulation) on adult A. americanum survival. We determined that there was a significant difference in survival between the two states, between years in Maine, and between sexes within Connecticut. However, presence or absence of snow and/or leaf litter had no impact on survival. Overall, we found a positive correlation between mean hourly temperature and adult survival in Maine, where temperatures were consistently below freezing. The results of this study can be included in an adaptive, predictive analytic model to accommodate the expected fluctuations and range expansion of A. americanum that will most likely accompany an increase in temperatures throughout the Northeast.

Prevalence and Genetic Characterization of Deer Tick Virus (Powassan Virus, Lineage II) in Ixodes scapularis Ticks Collected in Maine.

Deer tick virus (DTV) is a genetic variant of Powassan virus (POWV) that circulates in North America in an enzootic cycle involving the blacklegged or "deer tick," Ixodes scapularis, and small rodents such as the white-footed mouse. The number of reported human cases with neuroinvasive disease has increased substantially over the past few years, indicating that POWV may be of increasing public health importance. To this end, we sought to estimate POWV infection rates in questing I. scapularis collected from four health districts in Maine (York, Cumberland, Midcoast, and Central Maine). Infection rates were 1.6%, 1.7%, 0.7%, and 0%, respectively, for adults collected from April to November in 2016. Adults collected in October and November in 2017 from York and Cumberland counties had slightly higher rates of 2.3% and 3.5%, respectively. There was no difference in the number of males verses the number of females infected. All positive samples were of the DTV (lineage II) variant. Phylogenetic analysis was performed on 8 of the 15 DTV sequences obtained in 2016. Deer tick virus from the coastal regions were genetically similar and clustered with virus strains isolated from I. scapularis from New York State and Bridgeport, CT. The two inland viruses were genetically nearly identical and grouped with viruses from Massachusetts, Connecticut, and New York. These results are the first reported infection rates and sequences for POWV in questing ticks collected in Maine and will provide a reference point for future POWV studies.

Impacts of Deciduous Leaf Litter and Snow Presence on Nymphal Ixodes scapularis (Acari: Ixodidae) Overwintering Survival in Coastal New England, USA.

Blacklegged ticks (Ixodes scapularis Say) are the vector for pathogens that cause more cases of human disease than any other arthropod. Lyme disease is the most common, caused by the bacterial spirochete Borrelia burgdorferi (Johnson, Schmid, Hyde, Steigerwalt, and Brenner) in the northeastern United States. Further knowledge of seasonal effects on survival is important for management and modeling of both blacklegged ticks and tick-borne diseases. The focus of our study was on the impact of environmental factors on overwintering success of nymphal blacklegged ticks. In a three-year field study conducted in Connecticut and Maine, we determined that ground-level conditions play an important role in unfed nymphal overwintering survival. Ticks in plots where leaf litter and snow accumulation were unmanipulated had significantly greater survival compared to those where leaf litter was removed (p = 0.045) and where both leaf litter and snow were removed (p = 0.008). Additionally, we determined that the key overwintering predictors for nymphal blacklegged tick survival were the mean and mean minimum temperatures within a year. The findings of this research can be utilized in both small- and large-scale management of blacklegged ticks to potentially reduce the risk and occurrence of tick-borne diseases.