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

Development of a low-dose fipronil deer feed: Bait-screening and range-finding to determine the optimal formulation to control blacklegged ticks (Ixodes scapularis) feeding on white-tailed deer (Odocoileus virginianus)

Lyme disease is the most prevalent vector-borne disease in the U.S., and acaricidal feeds administered to white-tailed deer (Odocoileus virginianus) have potential to disrupt blood feeding by the blacklegged tick, Ixodes scapularis. Two studies were conducted with the aim of determining an ideal formulation to deliver oral acaricides to white-tailed deer and finding the lowest fipronil dose level to effectively control I. scapularis. During formulation screening, various commercial attractants (baits) were presented to deer in large paddocks under semi-field conditions and motion-sensitive cameras were used to monitor consumption by deer and non-target species. During dose range-finding, deer were housed individually and presented a fipronil feed at one of five dose levels for 48 h (two deer untreated). At 24 h post-exposure, the deer were anesthetized, blood was drawn, and deer were manually infested with 20 I. scapularis mating pairs in feeding capsules. Tick mortality was monitored up to nine days post-attachment. Results of formulation screening indicated that loose/granular formulations were substantially more palatable to deer, relative to block formulations, and would be more effective under field conditions. Dose range-finding concluded fipronil feeds with doses of 25 ppm and higher would eliminate 100% of I. scapularis parasitizing deer. Additionally, 10 ppm fipronil feed controlled a considerable proportion of ticks, and results suggested 100% ticks could be eliminated if fipronil sulfone was present in plasma at ≥25.1 ppb. These results were paramount in developing a low dose fipronil deer feed for tick control and should provide valuable insights prior to execution of future field trials.

Deer management generally reduces densities of nymphal Ixodes scapularis, but not prevalence of infection with Borrelia burgdorferi sensu stricto

Human Lyme disease-primarily caused by the bacterium Borrelia burgdorferi sensu stricto (s.s.) in North America-is the most common vector-borne disease in the United States. Research on risk mitigation strategies during the last three decades has emphasized methods to reduce densities of the primary vector in eastern North America, the blacklegged tick (Ixodes scapularis). Controlling white-tailed deer populations has been considered a potential method for reducing tick densities, as white-tailed deer are important hosts for blacklegged tick reproduction. However, the feasibility and efficacy of white-tailed deer management to impact acarological risk of encountering infected ticks (namely, density of host-seeking infected nymphs; DIN) is unclear. We investigated the effect of white-tailed deer density and management on the density of host-seeking nymphs and B. burgdorferi s.s. infection prevalence using surveillance data from eight national parks and park regions in the eastern United States from 2014-2022. We found that deer density was significantly positively correlated with the density of nymphs (nymph density increased by 49% with a 1 standard deviation increase in deer density) but was not strongly correlated with the prevalence of B. burgdorferi s.s. infection in nymphal ticks. Further, while white-tailed deer reduction efforts were followed by a decrease in the density of I. scapularis nymphs in parks, deer removal had variable effects on B. burgdorferi s.s. infection prevalence, with some parks experiencing slight declines and others slight increases in prevalence. Our findings suggest that managing white-tailed deer densities alone may not be effective in reducing DIN in all situations but may be a useful tool when implemented in integrated management regimes.

Artificial infestation of white-tailed deer with ticks (Acari: Ixodidae) to study tick-host interactions

White-tailed deer (Odocoileus virginianus) are a main host for the adult life stages of tick species of medical and veterinary importance. Since white-tailed deer play a vital role in tick ecology, research has been conducted to understand this tick-host relationship. To date, research involving captive white-tailed deer and artificial infestation of these animals with ticks has focused on host suitability, the role of white-tailed deer in tick-borne diseases, and anti-tick vaccine research. The methodology reported for these studies was at times not descriptive and inconsistent regarding how and what region of the white-tailed deer was infested with ticks. Here, we propose a standardized method to artificially infest captive white-tailed deer with ticks for research purposes. The protocol describes a method proven effective to experimentally infest captive white-tailed deer with blacklegged ticks (Ixodes scapularis) to study tick-host interactions. The methods can be reliably transferred for experimental infestation of white-tailed deer by other multi-host and one-host tick species.

Understanding Habitats and Environmental Conditions of White-Tailed Deer Population Density and Public Health Data to Aid in Assessing Human Tick-Borne Disease Risk

The extent of tick-borne diseases (TBDs) in the United States is largely unknown and underreported. Equitable diagnostic and treatment options may vary by geographic location. Triangulating multi-modal data sources informed by a One Health approach provides robust proxies for human TBD risk. Using data from the Indiana Department of Natural Resources collected from hunters during the white-tailed deer (Odocoileus virginianus) hunting season and other sources, we employ a mixed-methods approach based on thematic mapping and mixed effects modelling to determine if deer population density aligns with official disease data at the county level from (1) positive canine serological reports for, anaplasmosis, and Lyme Disease (LD); (2) positive human cases of ehrlichiosis, anaplasmosis, LD, and Spotted Fever rickettsioses; and (3) tick infectivity. We propose the need for multimodal data analysis using a variety of potential proxies to better estimate disease risk and inform public health policy and practice. We find similar spatial distributions between deer population density and human and canine TBDs in northeastern and southern Indiana, which are rural and mixed geographic areas. Overall, LD is more prevalent in the northwest, central-western, and southeastern counties, while ehrlichiosis is more common in the southern counties. These findings hold true across humans, canines, and deer.

Development of a low-dose fipronil deer feed: evaluation of efficacy against two medically important tick species parasitizing white-tailed deer (Odocoileus virginianus) under pen conditions

BACKGROUND

Odocoileus virginianus (the white-tailed deer) is a key reproductive host for medically important tick species, including Ixodes scapularis and Amblyomma americanum. Orally administering a systemic acaricide to white-tailed deer has the potential to reduce tick reproduction, abundance and pathogen-infected tick bites. Prior studies have demonstrated considerable efficacy of a low-dose fipronil mouse bait in controlling larval I. scapularis parasitizing the pathogen reservoir, Peromyscus leucopus. No prior studies have evaluated the efficacy of a fipronil product in controlling ticks parasitizing white-tailed deer.

METHODS

A pen study was conducted to evaluate the efficacy of a fipronil deer feed in controlling I. scapularis and A. americanum adult ticks. Individually housed deer (n = 24) were exposed to deer feed containing 0.0025% fipronil (fipronil deer feed) for 48 h and 120 h, and a control group of deer were exposed to an untreated placebo. On post-exposure day 7 and day 21, all deer were parasitized with 20 mating pairs of feeding capsule-enclosed I. scapularis and A. americanum. Post-attachment, engorgement and mortality of ticks were recorded. The concentrations of fipronil in plasma, feces and tissues from euthanized deer were estimated using liquid chromatography-mass spectrometry.

RESULTS

The fipronil deer feed efficaciously controlled ticks parasitizing pen-reared white-tailed deer. Efficacy in reducing survivorship of blood-feeding female I. scapularis exceeded 90% in all instances except for when ticks parasitized 48-h treated deer at day 21 post-exposure (47.2%). Efficacy in reducing survivorship of A. americanum females exceeded 80% in all instances. In the 120-h exposure group there was 100% tick mortality at day 7 post-exposure for both tick species. A significant correlation was observed between reductions in tick survivorship and concentrations of fipronil sulfone in plasma. The results of tissue analysis suggest that a withdrawal period may be needed to allow for fipronil degradation prior to hunting season.

CONCLUSIONS

The results provide proof-of-concept for the use of a fipronil-based oral acaricide in controlling two medically important tick species infesting a key reproductive host. A field trial is necessary to confirm the efficacy and toxicology of the product in wild deer populations. Fipronil deer feed may provide a means of controlling multiple tick species parasitizing wild ruminants to be integrated into tick management programs.

Precipitation as a Possible Risk Factor for Development of Lyme Nephritis in Dogs

To determine whether increased precipitation in the months before diagnosis is predictive of Lyme nephritis in dogs, a symmetrical bidirectional case-crossover study was performed on 87 dogs with presumptive Lyme nephritis. Cases were geographically linked to the nearest precipitation monitoring station, and monthly total precipitation for the 4 mo preceding diagnosis was compared to monthly total precipitation 1 yr before and 1 yr after. Breeds affected included Labrador retrievers (n = 31), mixed-breed dogs (n = 19), and golden retrievers (n = 10), with a possible seasonal bimodal peak in spring and fall. In age-adjusted analysis, each additional inch of precipitation 3 mo before diagnosis was associated with an increase in the odds of developing Lyme nephritis (odds ratio 1.13, 95% confidence interval 1.01-1.25). The results suggest that increased precipitation is a possible risk factor for Lyme nephritis in dogs. These findings may provide useful information for the accurate diagnosis of dogs with protein-losing nephropathy and may guide future studies of risk factors for Lyme nephritis. The methodology employed here reinforces the concept that case-crossover designs have utility beyond acute exposures and outcomes and offer an attractive alternative for studies in companion animals.

White-Tailed Deer Spatial Distribution in Relation to '4-Poster' Tick Control Devices in Suburbia

Deer are keystone hosts for adult ticks and have enabled the spread of tick distributions. The ‘4-Poster’ deer bait station was developed by the United States Department of Agriculture to control ticks feeding on free-ranging deer. Although effective in certain scenarios, ‘4-Poster’ deer treatment stations require the use of bait to attract deer to one location, which may cause increased deer disease transmission rates and habitat damage. To better understand and manage the impact of baited ‘4-Poster’ stations on deer movements, we captured and GPS-monitored 35 deer as part of an integrated pest management project. Fifteen ‘4-Poster’ stations were deployed among three suburban county parks to control ticks. To quantify the effects of ‘4-Poster’ stations, we calculated deer movement metrics before and after feeders were filled with whole kernel corn, and we gathered information on visitation rates to feeders. Overall, 83.3% of collared deer visited a feeder and revisited approximately every 5 days. After feeders were refilled, collared deer were ~5% closer to feeders and conspecifics than before filling. Males used a higher percentage of available feeders and visited them more throughout the deployment periods. Although these nuanced alterations in behavior may not be strong enough to increase local deer abundance, in light of infectious diseases affecting deer populations and effective ‘4-Poster’ densities, the core range shifts and clustering after refilling bait may be a cause for concern. As such, trade-offs between conflicting management goals should be carefully considered when deploying ‘4-Poster’ stations.

Mapping distributions of the Lyme disease vector, Ixodes scapularis, and spirochete, Borrelia burgdorferi, in Kentucky using passive and active surveillance

Lyme disease is the most common tick-borne illness in the United States and is becoming more prevalent each year. It is transmitted to humans and animals through the bites of Ixodes scapularis ticks infected with Borrelia burgdorferi in the eastern United States, I. pacificus in the western U.S, and I. ricinus in Europe and Asia. In Kentucky, where Lyme disease is non-endemic, the number of reported human cases in 2010 totaled five. In 2019, that number had increased by over 300%. Identifying the distribution of I. scapularis populations infected with B. burgdorferi is important data for effective prevention strategies and an important first step in monitoring disease spread. In collaboration with the Kentucky Department for Public Health, we performed surveillance for I. scapularis throughout the state of Kentucky using both active and passive surveillance methods. Diagnostic testing for the identification of B. burgdorferi (sensu stricto) was also conducted. We identified 457 I. scapularis ticks from March 2019 to December 2020 from 32 counties in Kentucky. B. burgdorferi was detected in I. scapularis populations collected from 14 different counties. These results add to the little data that exists in Kentucky on I. scapularis and B. burgdorferi distribution.

Mechanistic movement models reveal ecological drivers of tick-borne pathogen spread

Identifying ecological drivers of tick-borne pathogen spread has great value for tick-borne disease management. However, theoretical investigations into the consequences of host movement behaviour on pathogen spread dynamics in heterogeneous landscapes remain limited because spatially explicit epidemiological models that incorporate more realistic mechanisms governing host movement are rare. We built a mechanistic movement model to investigate how the interplay between multiple ecological drivers affects the risk of tick-borne pathogen spread across heterogeneous landscapes. We used the model to generate simulations of tick dispersal by migratory birds and terrestrial hosts across theoretical landscapes varying in resource aggregation, and we performed a sensitivity analysis to explore the impacts of different parameters on the infected tick spread rate, tick infection prevalence and infected tick density. Our findings highlight the importance of host movement and tick population dynamics in explaining the infected tick spread rate into new regions. Tick infection prevalence and infected tick density were driven by predictors related to the infection process and tick population dynamics, respectively. Our results suggest that control strategies aiming to reduce tick burden on tick reproduction hosts and encounter rate between immature ticks and pathogen amplification hosts will be most effective at reducing tick-borne disease risk.

Genetic diversity of Borrelia burgdorferi sensu stricto: Novel strains from Mexican wild rodents

Borrelia burgdorferi s.s. is a Gram-negative spirochaete, the aetiological agent of Lyme disease, the most common vector-borne disease in the Northern hemisphere. Reports on the presence of B. burgdorferi in central Mexico have been strongly criticized, since these were based only on unspecific serological methods. Furthermore, the worldwide genetic diversity of B. burgdorferi s.s. has not been evaluated. For this reason, the aim of the present study was to confirm the presence of B. burgdorferi in the central area of Mexico and to evaluate its relationship with regard to the global genetic diversity of B. burgdorferi s.s. To achieve this, fragments of the flagellin and the outer surface protein A genes were amplified from ear biopsies of the arboreal wild endemic mice Habromys schmidlyi. With these sequences, a concatenated Bayesian analysis was performed to confirm the identity of B. burgdorferi s.s. Afterwards, the global genetic diversity of this bacterial species was evaluated using our sequences and those available in GenBank. A prevalence of 10.4% (5/48) of H. schmidlyi infected with Borrelia sp. was detected, and the phylogenetic analyses confirmed the identity of B. burgdorferi s.s. Using both genes, the genetic diversity was low. However, genetic structuring analyses revealed that populations of western United States and those from Mexico formed slightly different genetic groups, separated from the populations of the rest of the world. Our study not only confirms the presence of this bacterium in central Mexico, but also shows the most southern record of this bacterium so far. It also highlights the importance of H. schmidlyi as a new potential host of this bacterial species. Our study also provides first genetic data on an incipient process of divergence in B. burgdorferi s.s. populations of eastern United States and central Mexico.

Lyme disease and relapsing fever in Mexico: An overview of human and wildlife infections

Lyme borreliosis and Relapsing fever are considered emerging and re-emerging diseases that cause major public health problems in endemic countries. Epidemiology and geographical distribution of these diseases are documented in the US and in Europe, yet in Mexico, studies are scarce and scattered. The aims of this study were (1) to present the first confirmatory evidence of an endemic case of Lyme disease in Mexico and (2) to analyze the epidemiological trend of these both diseases by compiling all the information published on Borrelia in Mexico. Two databases were compiled, one of human cases and another of wild and domestic animals in the country. The analysis included the evaluation of risk factors for the human population, the diversity of Borrelia species and their geographic distribution. Six Borrelia species were reported in a total of 1,347 reports, of which 398 were of humans. Women and children from rural communities were shown to be more susceptible for both Lyme borreliosis and Relapsing fever. The remaining reports were made in diverse mammalian species and ticks. A total of 17 mammalian species and 14 tick species were recorded as hosts for this bacterial genus. It is noteworthy that records of Borrelia were only made in 18 of the 32 states, mainly in northern and central Mexico. These results highlight the importance of performing further studies in areas where animal cases have been reported, yet no human studies have been done, in order to complete the epidemiological panorama for Lyme borreliosis and Relapsing fever. Finally, the search for Borrelia infections in other vertebrates, such as reptiles and amphibians is recommended to gain a more accurate view of Borrelia species and their distribution. The geographical approach presented herein justifies an intense sampling effort to improve epidemiological knowledge of these diseases to aid vector control and prevention programs.

Risk Factors of Lyme Disease: An Intersection of Environmental Ecology and Systems Science

Lyme disease (LD) cases have been on the rise throughout the United States, costing the healthcare system up to $1.3 billion per year, and making LD one of the greatest threats to public health. Factors influencing the number of LD cases range from environmental to system-level variables, but little is known about the influence of vegetation (canopy, understory, and ground cover) and human behavioral risk on LD cases and exposure to infected ticks. We determined the influence of various risk factors on the risk of exposure to infected ticks on 22 different walkways using multinomial logistic regression. The model classifies the walkways into high-risk and low-risk categories with 90% accuracy, in which the understory, human risk, and number of rodents are significant indicators. These factors should be managed to control the risk of transmission of LD to humans.

Amblyomma americanum (Acari: Ixodidae) Ticks Are Not Vectors of the Lyme Disease Agent, Borrelia burgdorferi (Spirocheatales: Spirochaetaceae): A Review of the Evidence

In the early 1980s, Ixodes spp. ticks were implicated as the key North American vectors of Borrelia burgdorferi (Johnson, Schmid, Hyde, Steigerwalt and Brenner) (Spirocheatales: Spirochaetaceae), the etiological agent of Lyme disease. Concurrently, other human-biting tick species were investigated as potential B. burgdorferi vectors. Rashes thought to be erythema migrans were observed in patients bitten by Amblyomma americanum (L.) (Acari: Ixodidae) ticks, and spirochetes were visualized in a small percentage of A. americanum using fluorescent antibody staining methods, sparking interest in this species as a candidate vector of B. burgdorferi. Using molecular methods, the spirochetes were subsequently described as Borrelia lonestari sp. nov. (Spirocheatales: Spirochaetaceae), a transovarially transmitted relapsing fever Borrelia of uncertain clinical significance. In total, 54 surveys from more than 35 research groups, involving more than 52,000 ticks, have revealed a low prevalence of B. lonestari, and scarce B. burgdorferi, in A. americanum. In Lyme disease-endemic areas, A. americanum commonly feeds on B. burgdorferi-infected hosts; the extremely low prevalence of B. burgdorferi in this tick results from a saliva barrier to acquiring infection from infected hosts. At least nine transmission experiments involving B. burgdorferi in A. americanum have failed to demonstrate vector competency. Advancements in molecular analysis strongly suggest that initial reports of B. burgdorferi in A. americanum across many states were misidentified B. lonestari, or DNA contamination, yet the early reports continue to be cited without regard to the later clarifying studies. In this article, the surveillance and vector competency studies of B. burgdorferi in A. americanum are reviewed, and we conclude that A. americanum is not a vector of B. burgdorferi.

Brave New Worlds: The Expanding Universe of Lyme Disease

Projections around the globe suggest an increase in tick-vectored disease incidence and distribution, and the potential for emergence of novel tick-borne pathogens. Lyme disease is the most common reported tick-borne illness in the Unites States and is prevalent throughout much of central Europe. In recent years, the worldwide burden of Lyme disease has increased and extended into regions and countries where the disease was not previously reported. In this review, we discuss the trends for increasing Lyme disease, and examine the factors driving Lyme disease expansion, including the effect of climate change on the spread of vector Ixodid ticks and reservoir hosts; and the impacts of increased awareness on disease reporting and diagnosis. To understand the growing threat of Lyme disease, we need to study the interplay between vector, reservoir, and pathogen. In addition, we need to understand the contributions of climate conditions to changes in disease risk.