Statewide surveillance of Ixodes scapularis (Acari: Ixodidae) for the presence of the human pathogen Borrelia miyamotoi (Spirochaetales: Spirochaetaceae), a relapsing fever spirochete in Pennsylvania, USA, 2019-2020

Borrelia miyamotoi disease is an emerging tick-borne human illness in the United States caused by Borrelia miyamotoi (Spirochaetales: Spirochaetaceae) bacterium. With Pennsylvania reporting thousands of tick-borne disease cases annually, determining the minimum infection rate (MIR) of B. miyamotoi in Ixodes scapularis (Say, Acari: Ixodidae) adults within Pennsylvania is of utmost importance. Active surveillance was performed from October 2019 to April 2020 to collect a minimum of 50 I. scapularis ticks from every county within Pennsylvania and then screened for B. miyamotoi via qPCR. Ticks were collected from all 67 counties with the majority of those being adult I. scapularis. Additional ticks collected were Dermacentor albipictus (Packard, Acari: Ixodidae), Haemaphysalis longicornis (Neumann, Acari: Ixodidae), and immature I. scapularis. Adult I. scapularis were pooled and tested for B. miyamotoi. MIR for positive B. miyamotoi pools and density of infected adult I. scapularis varied by county, with positive pools from 38 Pennsylvania counties. This is the first statewide evaluation of B. miyamotoi in Pennsylvania in questing adult I. scapularis. These prevalence and distribution data will aid health care practitioners within the state of Pennsylvania and the northeast United States to understand potential risk and bring awareness to the lesser known human Borrelia illness, Borrelia miyamotoi disease.

Cascading impacts of overstory structure in managed forests on understory structure, microclimate conditions, and Ixodes scapularis (Acari: Ixodidae) densities

Forest management practices designed to meet varied landowner objectives affect wildlife habitat and may interrupt the life-cycle stages of disease vectors, including the black-legged tick, Ixodes scapularis Say (Acari: Ixodidae). Ixodes scapularis transmits multiple pathogens including Borrelia burgdorferi, the causative agent of Lyme disease, which is the most common tick-borne disease in the United States. There is evidence that a range of active forest management practices (e.g., invasive plant removal, prescribed burning) can alter tick densities and pathogen transmission. However, few studies have investigated relationships between forest stand structural variables commonly manipulated by timber harvesting and tick ecology. Foresters may harvest timber to create certain forest structural conditions like the mean number of trees, or basal area, per hectare. This study used a spatially replicated experiment in a blocked design to compare forest stands with a range of overstory structures and document variations in the midstory, understory, and forest floor, as well as microclimate conditions within tick off-host habitat. Greater numbers of trees or basal area per hectare correlated with greater canopy closure but less understory cover, stabilized microclimate temperature, higher microclimate humidity, and greater I. scapularis nymph densities. A random forest model identified understory forest structure as the strongest predictor of nymph densities. There was no relationship between the number of trees or basal area per hectare and daily deer (Odocoileus virginianus Zimmermann) activity or nymphal infection prevalence. These findings provide a deeper understanding of tick-habitat associations within a forest stand and have the potential to inform forest management decisions.

Integrated Tick Management in South Central Wisconsin: Impact of Invasive Vegetation Removal and Host-Targeted Acaricides on the Density of Questing Ixodes scapularis (Acari: Ixodidae) Nymphs

As tick-borne disease incidence increases and pathogens expand into new areas, the need for effective tick management strategies is paramount. In this 5-yr study (2014-2018) conducted in south central Wisconsin, we assessed whether an integrated tick management approach, deployed during peak tick activity (May-August), was more effective at reducing black-legged ticks (Ixodes scapularis Say (Ixodida: Ixodidae)), than individual interventions. Using a factorial design, invasive vegetation removal (Amur honeysuckle, Lonicera maackii Ruprecht (Dipsacales: Caprifoliaceae) and common buckthorn, Rhamnus cathartica Linnaeus (Rosales: Rhamnaceae)) was coupled with deployments of permethrin-treated cotton nesting materials (tick tubes) that target the white-footed mouse (Peromyscus leucopus Rafinesque (Rodentia: Cricetidae)). Results show that the probability of encountering a larval tick by drag sampling was unaffected by treatments at the cumulative 5-yr level. However, vegetation removal significantly reduced larval encounters in 2014, 2015, and 2018, by 33%, 57%, and 61% respectively, and reduced the density of questing nymphal (DON) ticks by 45% in 2015 compared to controls. Despite the limited effect on DON, vegetation removal significantly reduced the cumulative 5-yr density of Borrelia burgdorferi sensu stricto infected nymphs (DIN) (70%) compared to controls as a result of decreased nymphal infection prevalence. Sites treated with tick tubes had lower DIN (66%) and DON (54%) across the study and nymphs were reduced every year following the initial year of deployment compared to controls. Combining treatments did not further reduce DIN or DONs. We conclude that long-term integration of tick tubes with invasive vegetation removal does not provide additional benefit over individual treatments alone.

Vertical Transmission: A Vector-Independent Transmission Pathway of Babesia microti in the Natural Reservoir Host Peromyscus leucopus

BACKGROUND

Babesia microti, a malaria-like pathogen, is increasing in mammal and human populations in endemic areas and is unlikely to be the sole result of horizontal pathogen transmission.

METHODS

Peromyscus leucopus mice, natural reservoir hosts, were infected via Ixodes scapularis nymphs. Infected parental females (n = 6) produced F1 offspring (n = 36) that were screened for B. microti using quantitative PCR. Xenodiagnostic larvae were fed on infected offspring to determine horizontal transmission and pathogen viability. Fifty engorged larvae were screened; the rest were allowed to molt and then screened to determine transstadial transmission. Infected F1 generation offspring were placed in breeding groups, producing 34 F2 offspring and screened for B. microti infection. Chronic infection was monitored in parental females since time of initial vector infection.

RESULTS

Vertical transmission of B. microti was 74% efficient in offspring born in the first 6 months. Horizontal transmission occurred in larvae (61% prevalence) and molted nymphs (58% prevalence); these nymphs were able to infect susceptible hosts. F2 generation offspring infection prevalence was 38%. Chronic infection persisted for 1 year in some adults.

CONCLUSIONS

These results demonstrate that vertical transmission is an important nonvector-mediated pathway of B. microti transmission in the natural reservoir host.

Risk Factors for Bites and Diseases Associated With Black-Legged Ticks: A Meta-Analysis

The emergence and spread of Lyme disease and other infections associated with black-legged ticks is causing a public health crisis. No human vaccines are currently available, and both diagnosis and treatment are sometimes ineffectual, leading to advocacy for self-directed preventative measures. These recommendations are widely communicated to the public, but there is limited evidence for their efficacy. We undertook a systematic review and mixed-effects meta-regression analysis of factors purported to increase or decrease risk of black-legged tick bites and tick-borne disease. Published articles used in the study spanned the years 1984–2018. Variables associated with increased probability of tick-borne disease, with odds ratios significantly greater than 1, included deer abundance, high density of nymph-stage black-legged ticks, landscapes with interspersed herbaceous and forested habitat, low human population density, gardens, cat ownership, and race. Contrary to recommendations, use of landscape-related tick control measures, such as clearing brush, trimming branches, and having a dry barrier between lawn and woods, tended to increase risk. Pet ownership increased bite risk. Bite risk was highest for children aged 5 years or less, with a secondary peak in persons aged 50–70 years. Although some widely disseminated recommendations are supported by the research analyzed, others require further evaluation. Additional research is also needed to understand the mechanisms underlying significant relationships.

Evidence of Powassan/deer tick virus in adult black-legged ticks (Ixodes scapularis) recovered from hunter-harvested white-tailed deer (Odocoileus virginianus) in Pennsylvania: A public health perspective

Studies reporting tick infection rates for Powassan virus (POWV), an emerging zoonotic arthropod-borne pathogen responsible for POWV disease in the Commonwealth of Pennsylvania, are limited. To determine the presence and ascertain a statewide prevalence of POWV, ticks were collected from 9,912 hunter-harvested white-tailed deer (Odocoileus virginianus) heads presented to six regional Pennsylvania Game Commission Chronic Wasting Disease sampling stations in early December of 2013, 2014 and 2015. Of the 2,973 ticks recovered, 1,990 (66.9%) were identified as adult Ixodes scapularis (black-legged tick). The 1,990 I. scapularis ticks were PCR-tested for the presence of POWV. The ticks had a statewide Powassan/deer tick virus infection rate of 0.05%, providing evidence of this pathogen in Pennsylvania's adult I. scapularis ticks and supporting the need for more comprehensive pathogen prevalence assessment strategies, as well as increased public health awareness for this emerging zoonotic arthropod-borne pathogen of public health concern.

Impact of white-tailed deer on the spread of Borrelia burgdorferi

There is a public perception that the white-tailed deer Odocoileus virginianus (Artiodactyla: Cervidae) is the main reservoir supporting the maintenance and spread of the causative agent of Lyme disease, Borrelia burgdorferi. This study examines the pathogen prevalence rate of Borrelia in adult Ixodes scapularis (Ixodida: Ixodidae), the black-legged tick, collected from white-tailed deer and compares it with pathogen prevalence rates in adult ticks gathered by dragging vegetation in two contiguous counties west of the Hudson Valley in upstate New York. In both Broome and Chenango Counties, attached and unattached ticks harvested from white-tailed deer had significantly lower prevalences of B. burgdorferi than those collected from vegetation. No attached ticks on deer (n = 148) in either county, and only 2.4 and 7.3% of unattached ticks (n = 389) in Broome and Chenango Counties, respectively, were harbouring the pathogen. This contrasts with the finding that 40.8% of ticks in Broome County and 46.8% of ticks in Chenango County collected from vegetation harboured the pathogen. These data suggest that a mechanism in white-tailed deer may aid in clearing the pathogen from attached deer ticks, although white-tailed deer do contribute to the spatial distribution of deer tick populations and also serve as deadend host breeding sites for ticks.

Phylogenetic analysis of basic helix-loop-helix transcription factors in the genome of a typical human-disease vector

Ixodes scapularis, the black-legged tick, is one of the most common human-disease vectors and transmits Borrelia species, such as B. burgdorferi, as well as Theileria microti, Anaplasma phagocytophilum, etc. As basic helix-loop-helix (bHLH) transcription factors have been recognized for many years as important regulators of various developmental processes, we performed phylogenetic analysis of the black-legged tick genome in order to identify the number and family of bHLH transcription factors. Because bHLH family members have been identified in many organisms, including silkworm and fruit fly, we were able to conduct this survey and identify 58 putative bHLH transcription factors. Phylogenetic analysis revealed that the black-legged tick has 26, 10, 9, 1, 9, and 1 member in groups A, B, C, D, E, and F, respectively, whereas two were orphan genes. This analysis also revealed that unlike silkworm and fruit fly, the black-legged tick has no Mesp, Mlx, or TF4 family members, but has one more MyoRb family member. The present study provides useful background information for future studies of the black-legged tick as a disease vector with the goal of prevention and treatment.

Insights into the development of Ixodes scapularis: a resource for research on a medically important tick species

Ticks (Acari: Ixodida) are arthropod ectoparasites dependent on a bloodmeal from a vertebrate host at each developmental stage for completion of their life cycle. This tick feeding cycle impacts animal health by causing damage to hides, secondary infections, immune reactions and diseases caused by transmission of pathogens. The genus Ixodes includes several medically important species that vector diseases, including granulocytic anaplasmosis and Lyme disease. I. scapularis, commonly called the black-legged or deer tick, is a medically-important tick species in North America and therefore was the first tick genome to be sequenced, thus serving as an important resource for tick research. This Primer focuses on the normal developmental cycle and laboratory rearing of I. scapularis. Definition of normal morphology, along with a consistent source of laboratory-reared I. scapularis, are fundamental for all aspects of future research, especially the effects of genetic manipulation and the evaluation of tick vaccine efficacy. Recent research important for the advancement of tick research, namely the development of tick cell culture systems for study of ticks and tick-borne pathogens, RNA interference for genetic manipulation of ticks and discovery of candidate antigens for development of tick vaccines, are briefly presented along with areas to target for future research.

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.