Lyme Disease Risk Research Articles

Impacts of an Introduced Forest Pathogen on the Risk of Lyme Disease in California

Global changes such as deforestation, climate change, and invasive species have the potential to greatly alter zoonotic disease systems through impacts on biodiversity. This study examined the impact of the invasive pathogen that causes sudden oak death (SOD) on the ecology of Lyme disease in California. The Lyme disease bacterium, Borrelia burgdorferi, is maintained in the far western United States by a suite of animal reservoirs including the dusky-footed woodrat (Neotoma fuscipes) and deer mouse (Peromyscus maniculatus), and is transmitted by the western black-legged tick (Ixodes pacificus). Other vertebrates, such as the western fence lizard (Sceloporus occidentalis), are important tick hosts but are not reservoirs of the pathogen. Previous work found that higher levels of SOD are correlated with greater abundance of P. maniculatus and S. occidentalis and lower N. fuscipes abundance. Here we model the contribution of these tick hosts to Lyme disease risk and also evaluate the potential impact of SOD on infection prevalence of the tick vector. By empirically parameterizing a static model with field and laboratory data on tick hosts, we predict that SOD reduces an important index of disease risk, nymphal infection prevalence, leading to a reduction in Lyme disease risk in certain coastal woodlands. Direct observational analysis of the impact of SOD on nymphal infection prevalence supports these model results. This study underscores the important direct and indirect impacts of invasive plant pathogens on biodiversity, the transmission cycles of zoonotic diseases, and ultimately human health.

Predicting the speed of tick invasion: an empirical model of range expansion for the Lyme disease vector Ixodes scapularis in Canada

Summary1. Over the past two decades, Ixodes scapularis, the primary tick vector of the Lyme disease pathogen Borrelia burgdorferi in North America has expanded its range northward from the USA to colonize new regions in southern Canada. We have previously projected range increases for I. scapularis based on temperature suitability, but to what extent this is matched by actual tick range expansion is unknown.2. Since 1990, I. scapularis ticks have been collected across Canada offering a unique opportunity to track the range expansion of an arthropod vector. We used these data to model time‐to‐establishment for tick populations across Canada to identify factors influencing the rate of spread.3. Our results point to both long‐distance dispersal of ticks by migratory birds and local dispersal by resident hosts as important potential mechanisms underlying patterns of tick range expansion.4. Temperature (accumulated degree days > 0 °C) was the most important determinant of environmental suitability for tick population establishment, suggesting that climate warming may facilitate range expansion.5. Model projections suggest that I. scapularis range will expand c. 46 km year−1 in the coming decade, with climate warming expected to increase the rate of spread. This expansion is likely to result in a substantial increase in human Lyme disease risk, with the proportion of the human population of eastern Canada inhabiting areas with established tick populations increasing from 18% in 2010 to over 80% by 2020.6. This first empirical model of I. scapularis invasion supports theoretical range projections based on climate suitability and provides a unique data‐driven estimate of the speed of northward range expansion for I. scapularis at the continental scale.7. Synthesis and applications. By tracking I. scapularis invasion in Canada over the past two decades, we show that I. scapularis is rapidly expanding its range and is likely to colonize the most densely populated areas of southern Canada in the coming decade. These projections suggest that prompt action is necessary to prepare the Canadian public for a likely epidemic of Lyme disease, with emphasis on focusing surveillance activities to confirm the locations of emerging Lyme disease risk.

Passive Surveillance for I. scapularis Ticks: Enhanced Analysis for Early Detection of Emerging Lyme Disease Risk

Lyme disease (LD) is emerging in Canada because of the northward expansion of the geographic range of the tick vector Ixodes scapularis (Say). Early detection of emerging areas of LD risk is critical to public health responses, but the methods to do so on a local scale are lacking. Passive tick surveillance has operated in Canada since 1990 but this method lacks specificity for identifying areas where tick populations are established because of dispersion of ticks from established LD risk areas by migratory birds. Using data from 70 field sites in Quebec visited previously, we developed a logistic regression model for estimating the risk of I. scapularis population establishment based on the number of ticks submitted in passive surveillance and a model-derived environmental suitability index. Sensitivity-specificity plots were used to select an optimal threshold value of the linear predictor from the model as the signal for tick population establishment. This value was used to produce an "Alert Map" identifying areas where the passive surveillance data suggested ticks were establishing in Quebec. Alert Map predictions were validated by field surveillance at 76 sites: the prevalence of established I. scapularis populations was significantly greater in areas predicted as high-risk by the Alert map (29 out of 48) than in areas predicted as moderate-risk (4 out of 30) (P < 0.001). This study suggests that Alert Maps created using this approach can provide a usefully rapid and accurate tool for early identification of emerging areas of LD risk at a geographic scale appropriate for local disease control and prevention activities.

Research brief

Accurate information about the spatial pattern of risk of infection with Borrelia burgdorferi, the tick-borne bacterium that causes Lyme disease, is hard to obtain with human case reporting because of under-reporting and misdiagnosis. Now, researchers have constructed a Lyme disease risk map for the eastern USA by using standardised field sampling to estimate the density of B burgdorferi-infected host-seeking Ixodes scapularis (the Lyme disease vector) throughout the tick's range. Their map, which identifies infection risk foci in the northeast and upper midwest regions, should help to guide surveillance, control, and prevention efforts.

Human risk of infection with Borrelia burgdorferi, the Lyme disease agent, in eastern United States.

The geographic pattern of human risk for infection with Borrelia burgdorferi sensu stricto, the tick-borne pathogen that causes Lyme disease, was mapped for the eastern United States. The map is based on standardized field sampling in 304 sites of the density of Ixodes scapularis host-seeking nymphs infected with B. burgdorferi, which is closely associated with human infection risk. Risk factors for the presence and density of infected nymphs were used to model a continuous 8 km×8 km resolution predictive surface of human risk, including confidence intervals for each pixel. Discontinuous Lyme disease risk foci were identified in the Northeast and upper Midwest, with a transitional zone including sites with uninfected I. scapularis populations. Given frequent under- and over-diagnoses of Lyme disease, this map could act as a tool to guide surveillance, control, and prevention efforts and act as a baseline for studies tracking the spread of infection.

Predicted Outcomes of Vaccinating Wildlife to Reduce Human Risk of Lyme Disease

Vaccination efforts for Lyme disease prevention in humans have focused on wildlife reservoirs to target the causative agent, Borrelia burgdorferi, for elimination in vector ticks. Multiple host species are involved in the transmission and maintenance of the bacterium, but not all host species can be vaccinated effectively. To evaluate vaccinating a subset of hosts in the context of host-tick interactions, we constructed and evaluated a dynamic model of B. burgdorferi transmission in mice. Our analyses indicate that on average, a mouse-targeted vaccine is expected to proportionally reduce infection prevalence among ticks by 56%. However, relative to mouse vaccination, human risk of exposure is dominated by the number of tick bites received per person, the proportion of tick blood meals taken from the highly reservoir-competent white-footed mouse relative to other hosts, and the average number of tick bites per mouse. Variation in these factors reduces the predictability of vaccination outcomes. Additionally, contributions of nonmouse hosts to pathogen maintenance preclude elimination of B. burgdorferi through mouse vaccination alone. Our findings indicate that to increase the impact of wildlife vaccination, reducing tick populations by acaricide application, in addition to targeting additional reservoir-competent host species, should be employed.

Estimating Lyme disease risk using pet dogs as sentinels

The reported number of cases of Lyme disease, Borrelia burgdorferi sensu lato, is thought to have increased in the UK over the past decade, but consistent surveillance data are lacking. Here the prevalence of B. burgdorferi in ticks attached to pet dogs was examined - using them as sentinels for human disease risk. Dogs give a good indication of the exposure of their human owners to infected ticks, since they largely share the same environment and visit the same outdoor areas. PCR was used to test 739 tick samples collected from 3534 dogs selected at random as they visited veterinary practices over a period of six months. Overall, the prevalence of infected ticks on all dogs was 0.5% giving an estimated 481 infected ticks per 100,000 dogs. The data suggest that the prevalence of Borrelia in the UK tick population is considerably higher than most recent estimates indicate.

Species Distribution Models and Ecological Suitability Analysis for Potential Tick Vectors of Lyme Disease in Mexico

Species distribution models were constructed for ten Ixodes species and Amblyomma cajennense for a region including Mexico and Texas. The model was based on a maximum entropy algorithm that used environmental layers to predict the relative probability of presence for each taxon. For Mexico, species geographic ranges were predicted by restricting the models to cells which have a higher probability than the lowest probability of the cells in which a presence record was located. There was spatial nonconcordance between the distributions of Amblyomma cajennense and the Ixodes group with the former restricted to lowlands and mainly the eastern coast of Mexico and the latter to montane regions with lower temperature. The risk of Lyme disease is, therefore, mainly present in the highlands where some Ixodes species are known vectors; if Amblyomma cajennense turns out to be a competent vector, the area of risk also extends to the lowlands and the east coast.

What Do We Need to Know About Disease Ecology to Prevent Lyme Disease in the Northeastern United States?: Table 1.

Lyme disease is the most commonly reported vector-borne disease in the United States, with the majority of cases occurring in the Northeast. It has now been three decades since the etiological agent of the disease in North America, the spirochete Borrelia burgdorferi, and its primary North American vectors, the ticks Ixodes scapularis Say and I. pacificus Cooley & Kohls, were identified. Great strides have been made in our understanding of the ecology of the vectors and disease agent, and this knowledge has been used to design a wide range of prevention and control strategies. However, despite these advances, the number of Lyme disease cases have steadily increased. In this article, we assess potential reasons for the continued lack of success in prevention and control of Lyme disease in the northeastern United States, and identify conceptual areas where additional knowledge could be used to improve Lyme disease prevention and control strategies. Some of these areas include: 1) identifying critical host infestation rates required to maintain enzootic transmission of B. burgdorferi, 2) understanding how habitat diversity and forest fragmentation impacts acarological risk of exposure to B. burgdorferi and the ability of interventions to reduce risk, 3) quantifying the epidemiological outcomes of interventions focusing on ticks or vertebrate reservoirs, and 4) refining knowledge of how human behavior influences Lyme disease risk and identifying barriers to the adoption of personal protective measures and environmental tick management.

Associations between Ixodes scapularis ticks and small mammal hosts in a newly endemic zone in southeastern Canada: Implications for Borrelia burgdorferi transmission

Immature Ixodes scapularis infestation and Borrelia burgdorferi infection of wild small mammals were studied from June to October in 2007 and from May to October in 2008 at 71 study sites in a zone where I. scapularis populations and environmental Lyme disease risk are emerging in southwestern Quebec. Seasonal host-seeking activity of immature I. scapularis was similar to patterns reported previously in Canada and the USA: nymphal activity peaked in spring while larval activity peaked in late summer. Synchronous activity of nymphs with some larvae was observed in late spring, which could favour establishment of B. burgdorferi strains that cause short-lived infections in their hosts. White-footed mice ( Peromyscus leucopus), deer mice ( P. maniculatus), chipmunks ( Tamias striatus), and red squirrels ( Tamiasciurus hudsonicus) carried 92.0% of the larvae and 94.2% of the nymphs collected. Adult male white-footed mice carried significantly larger numbers of both larval and nymphal I. scapularis than other species and classes of small mammals (different demographic groups or physiological status: age, sex, sexual activity). We conclude that seasonality and host association were comparable to previous studies in North America, even in the context of a newly endemic pattern of low infection prevalence and low densities of host-seeking and feeding I. scapularis in southwestern Quebec. Our studies suggest that B. burgdorferi transmission cycles are focused on adult male mice (which carried 35% of all feeding ticks collected in the study), so control methods targeting this class of hosts may be particularly effective. However, our study also suggested that habitats containing a diverse host structure may dilute transmission cycles by partitioning of nymphal and larval ticks on different host species.

Differential Risk for Lyme Disease along Hiking Trail

To estimate relative risk for exposure to ticks infected with Lyme disease–causing spirochetes in different land-use types along a trail in Germany, we compared tick density and spirochete prevalence on ruminant pasture with that on meadow and fallow land. Risk was significantly lower on pasture than on meadow and fallow land.

Canine Serology as Adjunct to Human Lyme Disease Surveillance

To better define areas of human Lyme disease risk, we compared US surveillance data with published data on the seroprevalence of Borrelia burgdorferi antibodies among domestic dogs. Canine seroprevalence >5% was a sensitive but nonspecific marker of human risk, whereas seroprevalence <1% was associated with minimal risk for human infection.

Canine Serology as Adjunct to Human Lyme Disease Surveillance

To better define areas of human Lyme disease risk, we compared US surveillance data with published data on the seroprevalence of Borrelia burgdorferi antibodies among domestic dogs. Canine seroprevalence >5% was a sensitive but nonspecific marker of human risk, whereas seroprevalence <1% was associated with minimal risk for human infection.

Reduction in Lyme Disease Risk as an Economic Benefit of Habitat Restoration

Habitat restoration is costly and it is often necessary to justify the costs with evidence of benefits to society. These benefits are difficult to quantify because they are measured in terms of ecosystem services rather than currency. This paper introduces a somewhat novel restoration‐related ecosystem service, a reduction in the risk of tick‐borne disease, and incorporates it into a cost/benefit analysis of the restoration of a rare habitat. We use a cost‐of‐illness study to calculate the costs averted by preventing Lyme disease (LD), and a contingent‐valuation survey to estimate the benefit of biodiversity protection. The restoration, removal of an invasive tree, reduced the risk of LD by approximately 98%. Cost‐of‐illness studies show that the restoration would be financially justifiable if it averted 75 cases of LD per year. Given the local LD rate and the visitation rate to the preserve, the habitat restoration can plausibly be justified solely on the benefit of LD cases averted. However, as we do not know how many cases of LD are contracted in the preserve, we also establish the perceived value of protecting biodiversity in a contingent‐valuation survey. Results show that residents were willing to pay a significant fraction of the net cost of restoration to protect biodiversity. When these benefits are taken into account, the number of cases of disease that must be averted to justify remediation is reduced. This exercise spotlights an underappreciated ecosystem service that, when appropriate, can help establish the cost effectiveness of restoration.

Repeated holdout cross-validation of model to estimate risk of Lyme disease by landscape characteristics

We previously modeled Lyme disease (LD) risk at the landscape scale; here we evaluate the model's overall goodness-of-fit using holdout validation. Landscapes were characterized within road-bounded analysis units (AU). Observed LD cases (obsLD) were ascertained per AU. Data were randomly subset 2,000 times. Of 514 AU, 411 (80%) were selected as a training dataset to develop parameter estimates used to predict observations in the remaining 103 (20%) AU, the validation subset. Predicted values were subtracted from obsLD to quantify accuracy across iterations. We calculated the percentage difference of over- and under-estimation to assess bias. Predictive ability was strong and similar across iterations and datasets; the exact number of obsLD cases per AU were predicted almost 60% of the time. However, the three highest obsLD AU were under-predicted. Our model appears to be accurate and relatively unbiased, however is conservative at high disease incidence.

Strain diversity of Borrelia burgdorferi in ticks dispersed in North America by migratory birds

The role of migratory birds in the dispersal of Ixodes scapularis ticks in the northeastern U.S. is well established and is presumed to be a major factor in the expansion of the geographic risk for Lyme disease. Population genetic studies of B. burgdorferi sensu stricto, the agent of Lyme disease in this region, consistently reveal the local presence of as many as 15 distinct strain types as designated by major groups of the ospC surface lipoprotein. Recent evidence suggests such strain diversity is adaptive to the diverse vertebrate hosts that maintain enzootic infection. How this strain diversity is established in emergent areas is unknown. To determine whether similar strain diversity is present in ticks imported by birds, we examined B. burgdorferi strains in I. scapularis ticks removed from migrants at an isolated island site. Tick mid-guts were cultured and isolates underwent DNA amplification with primers targeting ospC. Amplicons were separated by gel electrophoresis and sequenced. One hundred thirty-seven nymphal ticks obtained from 68 birds resulted in 24 isolates of B. burgdorferi representing eight ospC major groups. Bird-derived ticks contain diverse strain types of B. burgdorferi, including strain types associated with invasive Lyme disease. Birds and the ticks that feed on them may introduce a diversity of strains of the agent of Lyme disease to emergent areas.

Making sense of unfamiliar risks in the countryside: The case of Lyme disease

The focus of this paper is on how popular representations of the countryside provide countryside users with a discursive framework to make sense of unfamiliar countryside-based risks, taking Lyme disease as an example. Sixty-six semi-structured interviews were conducted with 82 visitors in Richmond Park, New Forest, and Exmoor National Park in the UK. The data were analysed using thematic analysis and was informed by social representations theory. The analysis indicated that a lay understanding of the risk of Lyme disease was filtered by place-attachment and the social representations of the countryside. Lyme disease was not understood primarily as a risk to health, but was instead constructed as a risk to the social and restorative practices in the context of the countryside. The findings suggest that advice about zoonoses such as Lyme disease is unlikely to cause panic, and that it should focus on the least intrusive preventative measures.

Assessment of human health risk for lyme disease in a peri-urban park in southern Québec

&#x0D; &#x0D; &#x0D; Introduction: climate change has contributed to the spread of the hard tick Ixodes scapularis into increasingly northern latitudes, and subsequently has caused the spread of the lyme dis-ease causing bacterium, Borrelia burdorferi, into these northern areas. The spread of these ticks into the region of southern Québec is highly likely within the near future. As a result, new human populations are being exposed to these ticks and are at risk for contracting lyme disease. intent: This exploratory study examines the spatial and behavioral factors associated with human activity in longueuil regional park in relation to risk for lyme disease. Methods: we conducted exit surveys of park-goers to determine spatial and behavioral patterns of park use, as well as lyme disease awareness. results and conclusion: we found higher awareness of ticks in female park-goers, park-goers over 50, and high-frequency park-goers. our results, importantly, imply a discrepancy between peoples' awareness of tick bite precautions, and their perception of tick bite risk. we hope that these findings may help future research on the spread of lyme disease into Canada, as well as in the formulation of public health policy.&#x0D; &#x0D; &#x0D;

How Big Is the Lyme Problem? Using Novel Methods to Estimate the True Number of Lyme Disease Cases in British Columbia Residents from 1997 to 2008

Lyme disease (LD) is rare in British Columbia (BC) and, despite being a reportable condition since 1994, may be underreported. Here we review all provincial laboratory and clinical databases to determine the number of LD cases reported in BC from 1997 to 2008. We analyzed demographic characteristics of LD cases and used capture-recapture methodology to estimate the true number of cases in BC for this period. From 1997 to 2008, 93 confirmed cases of LD were reported in BC. Conservative capture-recapture estimates place the true number of LD cases in BC during this period at 142 (95% confidence interval: 111-224), indicating up to 40% underreporting of this rare disease. Despite this underreporting, BC continues to have low endemic risk of LD. Strategies are needed to increase both physician awareness and the use of preventive measures in the BC population, including for those traveling to other endemic areas.

Regional Variation in Immature Ixodes scapularis Parasitism on North American Songbirds: Implications for Transmission of the Lyme Pathogen, Borrelia burgdorferi

Borrelia burgdorferi, the etiological agent of Lyme disease, is transmitted among hosts by the black-legged tick, Ixodes scapularis, a species that regularly parasitizes various vertebrate hosts, including birds, in its immature stages. Lyme disease risk in the United States is highest in the Northeast and in the upper Midwest where I. scapularis ticks are most abundant. Because birds might be important to the range expansion of I. scapularis and B. burgdorferi, we explored spatial variation in patterns of I. scapularis parasitism on songbirds, as well as B. burgdorferi infection in bird-derived I. scapularis larvae. We sampled birds at 23 sites in the eastern United States to describe seasonal patterns of I. scapularis occurrence on birds, and we screened a subset of I. scapularis larvae for presence of B. burgdorferi. Timing of immature I. scapularis occurrence on birds is consistent with regional variation in host-seeking activity with a generally earlier peak in larval parasitism on birds in the Midwest. Significantly more I. scapularis larvae occurred on birds that were contemporaneously parasitized by nymphs in the Midwest than the Northeast, and the proportion of birds that yielded B. burgdorferi-infected larvae was also higher in the Midwest. We conclude that regional variation in immature I. scapularis phenology results in different temporal patterns of parasitism on birds, potentially resulting in differential importance of birds to B. burgdorferi transmission dynamics among regions.