Metatranscriptomic investigation of single Ixodes pacificus ticks reveals diverse microbes, viruses, and novel mRNA-like endogenous viral elements

Ticks are increasingly important vectors of human and agricultural diseases. While many studies have focused on tick-borne bacteria, far less is known about tick-associated viruses and their roles in public health or tick physiology. To address this, we investigated patterns of bacterial and viral communities across two field populations of western black-legged ticks (Ixodes pacificus). Through metatranscriptomic analysis of 100 individual ticks, we quantified taxon prevalence, abundance, and co-occurrence with other members of the tick microbiome. In addition to commonly found tick-associated microbes, we assembled 11 novel RNA virus genomes from Rhabdoviridae, Chuviridae, Picornaviridae, Phenuiviridae, Reoviridae, Solemovidiae, Narnaviridae and two highly divergent RNA virus genomes lacking sequence similarity to any known viral families. We experimentally verified the presence of these in I. pacificus ticks across several life stages. We also unexpectedly identified numerous virus-like transcripts that are likely encoded by tick genomic DNA, and which are distinct from known endogenous viral element-mediated immunity pathways in invertebrates. Taken together, our work reveals that I. pacificus ticks carry a greater diversity of viruses than previously appreciated, in some cases resulting in evolutionarily acquired virus-like transcripts. Our findings highlight how pervasive and intimate tick-virus interactions are, with major implications for both the fundamental biology and vectorial capacity of I. pacificus ticks.

IMPORTANCE

Ticks are increasingly important vectors of disease, particularly in the United States where expanding tick ranges and intrusion into previously wild areas has resulted in increasing human exposure to ticks. Emerging human pathogens have been identified in ticks at an increasing rate, and yet little is known about the full community of microbes circulating in various tick species, a crucial first step to understanding how they interact with each and their tick host, as well as their ability to cause disease in humans. We investigated the bacterial and viral communities of the Western blacklegged tick in California and found 11 previously uncharacterized viruses circulating in this population.

Surveillance of Rocky Mountain wood ticks (Dermacentor andersoni) and American dog ticks (Dermacentor variabilis) in Colorado

Ticks pose an emerging threat of infectious pathogen transmission in the United States in part due to expanding suitable habitat ranges in the wake of climate change. Active and passive tick surveillance can inform maps of tick distributions to warn the public of their risk of exposure to ticks. In Colorado, widespread active surveillance programs have difficulty due to the state's diverse terrain. However, combining multiple citizen science techniques can create a more accurate representation of tick distribution than any passive surveillance dataset alone. Our study uses county-level tick distribution data from Northern Arizona University, the Colorado Department of Public Health and the Environment, and veterinary surveillance in addition to literature data to assess the distribution of the Rocky Mountain wood tick, Dermacentor andersoni, and the American dog tick, Dermacentor variabilis. We found that D. andersoni for the most part inhabits counties at higher elevations than D. variabilis in Colorado.

Messaging Should Reflect the Nuanced Relationship between Land Change and Zoonotic Disease Risk

A hallmark of the media publicity surrounding COVID-19 has been the message that land change causes zoonotic diseases to spill over from wild animals to humans. The secondary peer-reviewed literature sends a similar message. However, as indicated in the primary peer-reviewed literature, the complexity of interacting variables involved in zoonotic disease spillover makes it unlikely for such a claim to be universally applicable. The secondary peer-reviewed literature and the mainstream media also differ markedly from the primary peer-reviewed literature in their lack of nuance in messaging about the relationship between land change and spillover risk. We advocate accurate, nuanced messaging for the sake of the local communities at greatest risk from zoonotic disease, for the sake of scientific credibility, and so that proportionate attention may be given to other possible drivers of spillover risk.

Estimating disease vector population size from citizen science data

Citizen science projects have the potential to address hypotheses requiring extremely large datasets that cannot be collected with the financial and labour constraints of most scientific projects. Data collection by the general public could expand the scope of scientific enquiry if these data accurately capture the system under study. However, data collection inconsistencies by the untrained public may result in biased datasets that do not accurately represent the natural world. In this paper, we harness the availability of scientific and public datasets of the Lyme disease tick vector to identify and account for biases in citizen science tick collections. Estimates of tick abundance from the citizen science dataset correspond moderately with estimates from direct surveillance but exhibit consistent biases. These biases can be mitigated by including factors that may impact collector participation or effort in statistical models, which, in turn, result in more accurate estimates of tick population sizes. Accounting for collection biases within large-scale, public participation datasets could update species abundance maps and facilitate using the wealth of citizen science data to answer scientific questions at scales that are not feasible with traditional datasets.

Predicting the current and future distribution of the western black-legged tick, Ixodes pacificus, across the Western US using citizen science collections

In the twenty-first century, ticks and tick-borne diseases have expanded their ranges and impact across the US. With this spread, it has become vital to monitor vector and disease distributions, as these shifts have public health implications. Typically, tick-borne disease surveillance (e.g., Lyme disease) is passive and relies on case reports, while disease risk is calculated using active surveillance, where researchers collect ticks from the environment. Case reports provide the basis for estimating the number of cases; however, they provide minimal information on vector population or pathogen dynamics. Active surveillance monitors ticks and sylvatic pathogens at local scales, but it is resource-intensive. As a result, data are often sparse and aggregated across time and space to increase statistical power to model or identify range changes. Engaging public participation in surveillance efforts allows spatially and temporally diverse samples to be collected with minimal effort. These citizen-driven tick collections have the potential to provide a powerful tool for tracking vector and pathogen changes. We used MaxEnt species distribution models to predict the current and future distribution of Ixodes pacificus across the Western US through the use of a nationwide citizen science tick collection program. Here, we present niche models produced through citizen science tick collections over two years. Despite obvious limitations with citizen science collections, the models are consistent with previously-predicted species ranges in California that utilized more than thirty years of traditional surveillance data. Additionally, citizen science allows for an expanded understanding of I. pacificus distribution in Oregon and Washington. With the potential for rapid environmental changes instigated by a burgeoning human population and rapid climate change, the development of tools, concepts, and methodologies that provide rapid, current, and accurate assessment of important ecological qualities will be invaluable for monitoring and predicting disease across time and space.

Ecological Fallacy and Aggregated Data: A Case Study of Fried Chicken Restaurants, Obesity and Lyme Disease

Interdisciplinary approaches are merited when attempting to understand the complex and idiosyncratic processes driving the spillover of pathogens from wildlife and vector species to human populations. Public health data are often available for zoonotic pathogens but can lead to erroneous conclusions if the data have been spatially or temporally aggregated. As an illustration, we use human Lyme disease incidence data as a case study to examine correlations between mammalian biodiversity, fried chicken restaurants and obesity rates on human disease incidence. We demonstrate that Lyme disease incidence is negatively correlated with mammalian biodiversity, the abundance of fried chicken restaurants and obesity rates. We argue, however, that these correlations are spurious, representing both an 'ecologic fallacy' and Simpson's paradox, and are generated by the use of aggregated data. We argue that correlations based on aggregated data across large spatial scales must be rigorously examined before being invoked as proof of disease ecology theory or as a rationale for public health policy.

Borrelia miyamotoi Infections in Small Mammals, California, USA

Surveillance to investigate the wildlife–vector transmission cycle of the human pathogen Borrelia miyamotoi in California, USA, revealed infections in dusky-footed woodrats, brush mice, and California mice. Phylogenetic analyses suggest a single, well-supported clade of B. miyamotoi is circulating in California.

Using citizen science to describe the prevalence and distribution of tick bite and exposure to tick-borne diseases in the United States

Tick-borne pathogens are increasing their range and incidence in North America as a consequence of numerous factors including improvements in diagnostics and diagnosis, range expansion of primary vectors, changes in human behavior, and an increasing understanding of the diversity of species of pathogens that cause human disease. Public health agencies have access to human incidence data on notifiable diseases e.g., Borrelia burgdorferi, the causative agent of Lyme disease, and often local pathogen prevalence in vector populations. However, data on exposure to vectors and pathogens can be difficult to determine e.g., if disease does not occur. We report on an investigation of exposure to ticks and tick-borne bacteria, conducted at a national scale, using citizen science participation. 16,080 ticks were submitted between January 2016 and August 2017, and screened for B. burgdorferi, B. miyamotoi, Anaplasma phagocytophilum, and Babesia microti. These data corroborate entomologic investigations of tick distributions in North America, but also identify patterns of local disease risk and tick contact with humans throughout the year in numerous species of ticks and associated pathogens.

Conservation of biodiversity as a strategy for improving human health and well-being

The Earth's ecosystems have been altered by anthropogenic processes, including land use, harvesting populations, species introductions and climate change. These anthropogenic processes greatly alter plant and animal communities, thereby changing transmission of the zoonotic pathogens they carry. Biodiversity conservation may be a potential win-win strategy for maintaining ecosystem health and protecting public health, yet the causal evidence to support this strategy is limited. Evaluating conservation as a viable public health intervention requires answering four questions: (i) Is there a general and causal relationship between biodiversity and pathogen transmission, and if so, which direction is it in? (ii) Does increased pathogen diversity with increased host biodiversity result in an increase in total disease burden? (iii) Do the net benefits of biodiversity conservation to human well-being outweigh the benefits that biodiversity-degrading activities, such as agriculture and resource utilization, provide? (iv) Are biodiversity conservation interventions cost-effective when compared to other options employed in standard public health approaches? Here, we summarize current knowledge on biodiversity-zoonotic disease relationships and outline a research plan to address the gaps in our understanding for each of these four questions. Developing practical and self-sustaining biodiversity conservation interventions will require significant investment in disease ecology research to determine when and where they will be effective.This article is part of the themed issue 'Conservation, biodiversity and infectious disease: scientific evidence and policy implications'.

Lyme disease ecology in a changing world: consensus, uncertainty and critical gaps for improving control

Lyme disease is the most common tick-borne disease in temperate regions of North America, Europe and Asia, and the number of reported cases has increased in many regions as landscapes have been altered. Although there has been extensive work on the ecology and epidemiology of this disease in both Europe and North America, substantial uncertainty exists about fundamental aspects that determine spatial and temporal variation in both disease risk and human incidence, which hamper effective and efficient prevention and control. Here we describe areas of consensus that can be built on, identify areas of uncertainty and outline research needed to fill these gaps to facilitate predictive models of disease risk and the development of novel disease control strategies. Key areas of uncertainty include: (i) the precise influence of deer abundance on tick abundance, (ii) how tick populations are regulated, (iii) assembly of host communities and tick-feeding patterns across different habitats, (iv) reservoir competence of host species, and (v) pathogenicity for humans of different genotypes of Borrelia burgdorferi Filling these knowledge gaps will improve Lyme disease prevention and control and provide general insights into the drivers and dynamics of this emblematic multi-host-vector-borne zoonotic disease.This article is part of the themed issue 'Conservation, biodiversity and infectious disease: scientific evidence and policy implications'.

Interacting effects of land use and climate on rodent-borne pathogens in central Kenya

Understanding the effects of anthropogenic disturbance on zoonotic disease risk is both a critical conservation objective and a public health priority. Here, we evaluate the effects of multiple forms of anthropogenic disturbance across a precipitation gradient on the abundance of pathogen-infected small mammal hosts in a multi-host, multi-pathogen system in central Kenya. Our results suggest that conversion to cropland and wildlife loss alone drive systematic increases in rodent-borne pathogen prevalence, but that pastoral conversion has no such systematic effects. The effects are most likely explained both by changes in total small mammal abundance, and by changes in relative abundance of a few high-competence species, although changes in vector assemblages may also be involved. Several pathogens responded to interactions between disturbance type and climatic conditions, suggesting the potential for synergistic effects of anthropogenic disturbance and climate change on the distribution of disease risk. Overall, these results indicate that conservation can be an effective tool for reducing abundance of rodent-borne pathogens in some contexts (e.g. wildlife loss alone); however, given the strong variation in effects across disturbance types, pathogen taxa and environmental conditions, the use of conservation as public health interventions will need to be carefully tailored to specific pathogens and human contexts.

This article is part of the themed issue ‘Conservation, biodiversity and infectious disease: scientific evidence and policy implications’.

Vaccines for Conservation: Plague, Prairie Dogs & Black-Footed Ferrets as a Case Study

The endangered black-footed ferret (Mustela nigripes) is affected by plague, caused by Yersinia pestis, both directly, as a cause of mortality, and indirectly, because of the impacts of plague on its prairie dog (Cynomys spp.) prey base. Recent developments in vaccines and vaccine delivery have raised the possibility of plague control in prairie dog populations, thereby protecting ferret populations. A large-scale experimental investigation across the western US shows that sylvatic plague vaccine delivered in oral baits can increase prairie dog survival. In northern Colorado, an examination of the efficacy of insecticides to control fleas and plague vaccine shows that timing and method of plague control is important, with different implications for long-term and large-scale management of Y. pestis delivery. In both cases, the studies show that ambitious field-work and cross-sectoral collaboration can provide potential solutions to difficult issues of wildlife management, conservation and disease ecology.

Epidemiology and Genetic Diversity of Anaplasma phagocytophilum in the San Francisco Bay Area, California

In California, the agent of human granulocytic anaplasmosis (HGA), Anaplasma phagocytophilum, is transmitted by western black-legged ticks (Ixodes pacificus). Cases of HGA are infrequent in California but do occur annually. We investigated nymphal and adult western black-legged tick populations in 20 recreational areas in California's San Francisco Bay Area (Marin, Napa, San Mateo, Santa Clara, Santa Cruz, and Sonoma counties). Overall, prevalence of A. phagocytophilum in adult ticks was 0.8% (11/1,465), and in nymphal ticks was 4.2% (24/568), though presence was patchy and prevalence varied locally. We detected significant sequence variation in our quantitative polymerase chain reaction (qPCR)-positive samples. This included four sequences that grouped within a clade that contains clinical human and veterinary isolates as well as four others that grouped with sequences from PCR-positive lizards from northern California. Tick populations in our study sites harbor genetically diverse strains of A. phagocytophilum, which may influence potential risk in the region.

Ecological Traits Driving the Outbreaks and Emergence of Zoonotic Pathogens

Infectious diseases that are transmitted from wildlife hosts to humans, such as the Ebola virus and MERS virus, can be difficult to understand because the pathogens emerge from complex multifaceted ecological interactions. We use a wildlife–pathogen system—prairie dogs (Cynomys ludovicianus) and the plague bacterium (Yersinia pestis)—to describe aspects of disease ecology that apply to many cases of emerging infectious disease. We show that the monitoring and surveillance of hosts and vectors during the buildup to disease outbreaks are crucial for understanding pathogen-transmission dynamics and that a community-ecology framework is important to identify reservoir hosts. Incorporating multidisciplinary approaches and frameworks may improve wildlife–pathogen surveillance and our understanding of seemingly sporadic and rare pathogen outbreaks.

Disease Risk & Landscape Attributes of Tick-Borne Borrelia Pathogens in the San Francisco Bay Area, California

Habitat heterogeneity influences pathogen ecology by affecting vector abundance and the reservoir host communities. We investigated spatial patterns of disease risk for two human pathogens in the Borrelia genus–B. burgdorferi and B. miyamotoi–that are transmitted by the western black-legged tick, Ixodes pacificus. We collected ticks (349 nymphs, 273 adults) at 20 sites in the San Francisco Bay Area, California, USA. Tick abundance, pathogen prevalence and density of infected nymphs varied widely across sites and habitat type, though nymphal western black-legged ticks were more frequently found, and were more abundant in coast live oak forest and desert/semi-desert scrub (dominated by California sagebrush) habitats. We observed Borrelia infections in ticks at all sites where we able to collect >10 ticks. The recently recognized human pathogen, B. miyamotoi, was observed at a higher prevalence (13/349 nymphs = 3.7%, 95% CI = 2.0–6.3; 5/273 adults = 1.8%, 95% CI = 0.6–4.2) than recent studies from nearby locations (Alameda County, east of the San Francisco Bay), demonstrating that tick-borne disease risk and ecology can vary substantially at small geographic scales, with consequences for public health and disease diagnosis.

Effects of land use on plague (Yersinia pestis) activity in rodents in Tanzania

Understanding the effects of land-use change on zoonotic disease risk is a pressing global health concern. Here, we compare prevalence of Yersinia pestis, the etiologic agent of plague, in rodents across two land-use types-agricultural and conserved-in northern Tanzania. Estimated abundance of seropositive rodents nearly doubled in agricultural sites compared with conserved sites. This relationship between land-use type and abundance of seropositive rodents is likely mediated by changes in rodent and flea community composition, particularly via an increase in the abundance of the commensal species, Mastomys natalensis, in agricultural habitats. There was mixed support for rodent species diversity negatively impacting Y. pestis seroprevalence. Together, these results suggest that land-use change could affect the risk of local transmission of plague, and raise critical questions about transmission dynamics at the interface of conserved and agricultural habitats. These findings emphasize the importance of understanding disease ecology in the context of rapidly proceeding landscape change.

Seasonal activity patterns of the western black-legged tick, Ixodes pacificus, in relation to onset of human Lyme disease in northwestern California

Seasonal activity patterns of questing western black-legged ticks, Ixodes pacificus were investigated in northwestern California. Adult I. pacificus became active in the fall (late October/early November) and their appearance was associated with the first rain of the season. Following a peak in January, the abundance of adult ticks declined such that they were rare or absent by June/July. The nymphal tick activity season occurred from January through October, and larval activity occurred from April to June, but sometimes extended into October. Thus, potentially infectious ticks (nymphs and adults) present a year-round risk of Lyme disease transmission in northwestern California. The seasonality of Lyme disease cases in humans, based on the onset of erythema migrans, mirrored tick activity patterns and was year-round in cases infected in California. Peak incidence in humans occurs from May through July, and indicates that most disease transmission is from nymphal ticks. This study demonstrates that tick activity patterns are more extended than previously recognized in northwestern California.

Duration of plague (Yersinia pestis) outbreaks in black-tailed prairie dog (Cynomys ludovicianus) colonies of northern Colorado

Plague, caused by the bacterium Yersinia pestis, triggers die-offs in colonies of black-tailed prairie dogs (Cynomys ludovicianus), but the time-frame of plague activity is not well understood. We document plague activity in fleas from prairie dogs and their burrows on three prairie dog colonies that suffered die-offs. We demonstrate that Y. pestis transmission occurs over periods from several months to over a year in prairie dog populations before observed die-offs.

A meta-analysis suggesting that the relationship between biodiversity and risk of zoonotic pathogen transmission is idiosyncratic

Zoonotic pathogens are significant burdens on global public health. Because they are transmitted to humans from non‐human animals, the transmission dynamics of zoonoses are necessarily influenced by the ecology of their animal hosts and vectors. The ‘dilution effect’ proposes that increased species diversity reduces disease risk, suggesting that conservation and public health initiatives can work synergistically to improve human health and wildlife biodiversity. However, the meta‐analysis that we present here indicates a weak and highly heterogeneous relationship between host biodiversity and disease. Our results suggest that disease risk is more likely a local phenomenon that relies on the specific composition of reservoir hosts and vectors, and their ecology, rather than patterns of species biodiversity.

Distribution of the Lyme disease spirochete Borrelia burgdorferi in naturally and experimentally infected western gray squirrels (Sciurus griseus)

The dynamics of Borrelia burgdorferi infections within its natural hosts are poorly understood. We necropsied four wild-caught western gray squirrels (Sciurus griseus) that were acquired during a previous study that evaluated the reservoir competence of this rodent for the Lyme disease spirochete. One animal was infected experimentally, whereas the others were infected in the wild before capture. To investigate dissemination of B. burgdorferi and concurrent histopathologic lesions in different tissues, blood specimens, synovial and cerebrospinal fluid, ear-punch biopsies, and diverse tissue samples from skin and various organs were taken and examined by culture, polymerase chain reaction, and histology. Borrelia-positive cultures were obtained from three of the squirrels, that is, from skin biopsies (7 of 20 samples), ear-punch biopsies (2 of 8), and one (1 of 5) lymph node. Sequencing of amplicons confirmed B. burgdorferi sensu stricto (s.s.) infection in 9 of 10 culture-positive samples and in DNA extracted from all 10 positive cultures. The experimentally infected squirrel yielded most of the positive samples. In contrast, bodily fluids, all other organ specimens from these animals, and all samples from one naturally infected squirrel were negative for Borrelia for both assays. None of the necropsied squirrels exhibited specific clinical signs associated with B. burgdorferi. Similarly, necropsy and histological examination of tissues indicated the presence of underlying infectious processes, none of which could be ascribed conclusively to B. burgdorferi infection. Based on these results, obtained from a small number of animals investigated at a single time point, we suggest that B. burgdorferi s.s. infection in S. griseus may result in rather localized dissemination of spirochetes, and that mild or nonclinical disease might be more common after several months of infection duration. Since spirochetes could be detected in squirrels 7-21 months postinfection, we conclude that S. griseus can infect Ixodes pacificus ticks with B. burgdorferi s.s. trans-seasonally.

Community ecology and disease risk: lizards, squirrels, and the Lyme disease spirochete in California, USA

Vector-borne zoonotic diseases are often maintained in complex transmission cycles involving multiple vertebrate hosts and their arthropod vectors. In the state of California, U.S.A., the spirochete Borrelia burgdorferi, which causes Lyme disease, is transmitted between vertebrate hosts by the western black-legged tick, Ixodes pacificus. Several mammalian species serve as reservoir hosts of the spirochete, but levels of tick infestation, reservoir competence, and Borrelia-infection prevalence vary widely among such hosts. Here, we model the host (lizards, Peromyscus mice, Californian meadow voles, dusky-footed wood rats, and western gray squirrels), vector, and pathogen community of oak woodlands in northwestern California to determine the relative importance of different tick hosts. Observed infection prevalence of B. burgdorferi in host-seeking I. pacificus nymphs was 1.8-5.3%, and our host-community model estimated an infection prevalence of 1.6-2.2%. The western gray squirrel (Sciurus griseus) was the only source of infected nymphs. Lizards, which are refractory to Borrelia infection, are important in feeding subadult ticks but reduce disease risk (nymphal infection prevalence). Species identity is therefore critical in understanding and determining the local disease ecology.

Inferring host-parasite relationships using stable isotopes: implications for disease transmission and host specificity

Identifying the roles of different hosts and vectors is a major challenge in the study of the ecology of diseases caused by multi-host pathogens. Intensive field studies suggested that grasshopper mice (Onychomys leucogaster) help spread the bacterium that causes plague (Yersinia pestis) in prairie dog colonies by sharing fleas with prairie dogs (Cynomys ludovicianus); yet conclusive evidence that prairie dog fleas (Oropsylla hirsuta) feed on grasshopper mice is lacking. Using stable nitrogen isotope analysis, we determined that many blood-engorged O. hirsuta collected from wild grasshopper mice apparently contained blood meals of prairie dogs. These results suggest that grasshopper mice may be infected with Y. pestis via mechanisms other than flea feeding, e.g., early phase or mechanical transmission or scavenging carcasses, and raise questions about the ability of grasshopper mice to maintain Y. pestis in prairie dog colonies during years between plague outbreaks. They also indicate that caution may be warranted when inferring feeding relationships based purely on the occurrence of fleas or other haematophagous ectoparasites on hosts. Stable-isotope analysis may complement or provide a useful alternative to immunological or molecular techniques for identifying hosts of cryptically feeding ectoparasites, and for clarifying feeding relationships in studies of host-parasite interactions.

Spatial analysis of plague in California: niche modeling predictions of the current distribution and potential response to climate change

BACKGROUND

Plague, caused by the bacterium Yersinia pestis, is a public and wildlife health concern in California and the western United States. This study explores the spatial characteristics of positive plague samples in California and tests Maxent, a machine-learning method that can be used to develop niche-based models from presence-only data, for mapping the potential distribution of plague foci. Maxent models were constructed using geocoded seroprevalence data from surveillance of California ground squirrels (Spermophilus beecheyi) as case points and Worldclim bioclimatic data as predictor variables, and compared and validated using area under the receiver operating curve (AUC) statistics. Additionally, model results were compared to locations of positive and negative coyote (Canis latrans) samples, in order to determine the correlation between Maxent model predictions and areas of plague risk as determined via wild carnivore surveillance.

RESULTS

Models of plague activity in California ground squirrels, based on recent climate conditions, accurately identified case locations (AUC of 0.913 to 0.948) and were significantly correlated with coyote samples. The final models were used to identify potential plague risk areas based on an ensemble of six future climate scenarios. These models suggest that by 2050, climate conditions may reduce plague risk in the southern parts of California and increase risk along the northern coast and Sierras.

CONCLUSION

Because different modeling approaches can yield substantially different results, care should be taken when interpreting future model predictions. Nonetheless, niche modeling can be a useful tool for exploring and mapping the potential response of plague activity to climate change. The final models in this study were used to identify potential plague risk areas based on an ensemble of six future climate scenarios, which can help public managers decide where to allocate surveillance resources. In addition, Maxent model results were significantly correlated with coyote samples, indicating that carnivore surveillance programs will continue to be important for tracking the response of plague to future climate conditions.

Transmission cycles of Borrelia burgdorferi and B. bissettii in relation to habitat type in northwestern California

This study was undertaken to determine which rodent species serve as primary reservoirs for the Lyme disease spirochete Borrelia burgdorferi in commonly occurring woodland types in inland areas of northwestern California, and to examine whether chaparral or grassland serve as source habitats for dispersal of B. burgdorferi- or B. bissettii-infected rodents into adjacent woodlands. The western gray squirrel (Sciurus griseus) was commonly infected with B. burgdorferi in oak woodlands, whereas examination of 30 dusky-footed woodrats (Neotoma fuscipes) and 280 Peromyscus spp. mice from 13 widely spaced Mendocino County woodlands during 2002 and 2003 yielded only one infected woodrat and one infected deer mouse (P. maniculatus). These data suggest that western gray squirrels account for the majority of production by rodents of fed Ixodes pacificus larvae infected with B. burgdorferi in the woodlands sampled. Infections with B. burgdorferi also were rare in woodrats (0/47, 0/3) and mice (3/66, 1/6) captured in chaparral and grassland, respectively, and therefore these habitats are unlikely sources for dispersal of this spirochete into adjacent woodlands. On the other hand, B. bissettii was commonly detected in both woodrats (22/47) and mice (15/66) in chaparral. We conclude that the data from this and previous studies in northwestern California are suggestive of a pattern where inland oak-woodland habitats harbor a B. burgdorferi transmission cycle driven primarily by I. pacificus and western gray squirrels, whereas chaparral habitats contain a B. bissettii transmission cycle perpetuated largely by I. spinipalpis, woodrats and Peromyscus mice. The dominant role of western gray squirrels as reservoirs of B. burgdorferi in certain woodlands offers intriguing opportunities for preventing Lyme disease by targeting these animals by means of either host-targeted acaricides or oral vaccination against B. burgdorferi.

Effects of weather and plague-induced die-offs of prairie dogs on the fleas of northern grasshopper mice

Plague, the disease caused by the bacterium Yersinia pestis, can have devastating impacts on black-tailed prairie dogs (Cynomys ludovicianus Ord). Other mammal hosts living on prairie dog colonies may be important in the transmission and maintenance of plague. We examined the flea populations of northern grasshopper mice (Onychomys leucogaster Wied) before, during, and after plague epizootics in northern Colorado and studied the influence of host and environmental factors on flea abundance patterns. Grasshopper mice were frequently infested with high numbers of fleas, most commonly Pleochaetis exilis Jordan and Thrassis fotus Jordan. Flea loads changed in response to both environmental temperature and rainfall. After plague-induced prairie dog die-offs, flea loads and likelihood of infestation were unchanged for P. exilis, but T. fotus loads declined.

Identifying the reservoir hosts of the Lyme disease spirochete Borrelia burgdorferi in California: the role of the western gray squirrel (Sciurus griseus)

We investigated the role of the western gray squirrel (Sciurus griseus) as a reservoir host of the Lyme disease spirochete Borrelia burgdorferi. A survey of 222 western gray squirrels in California showed an overall prevalence of B. burgdorferi infection of 30%, although at a county level, prevalence of infection ranged from 0% to 50% by polymerase chain reaction. Laboratory trials with wild-caught western gray squirrels indicated that squirrels were competent reservoir hosts of the Lyme disease bacterium and infected up to 86% of feeding Ixodes pacificus larvae. Infections were long-lasting (up to 14 months), which demonstrated that western gray squirrels can maintain B. burgdorferi trans-seasonally. Non-native eastern gray squirrels (Sciurus carolinensis) and fox squirrels (Sciurus niger) were infrequently infected with B. burgdorferi.

The potential role of swift foxes (Vulpes velox) and their fleas in plague outbreaks in prairie dogs

Swift foxes (Vulpes velox) have been proposed as potential carriers of fleas infected with the bacterium Yersinia pestis between areas of epizootics in black-tailed prairie dogs (Cynomys ludovicianus). We examined antibody prevalence rates of a population of swift foxes in Colorado, USA, and used polymerase chain reaction (PCR) assays to examine their flea biota for evidence of Y. pestis. Fifteen of 61 (24%) captured foxes were seropositive, and antibody prevalence was spatially correlated with epizootic plague activity in prairie dog colonies in the year of, and previous to, the study. Foxes commonly harbored the flea Pulex simulans, though none of the fleas was positive for Y. pestis.

Seroprevalence Rates and Transmission of Plague (Yersinia pestis) in Mammalian Carnivores

Exposure to plague (Yersinia pestis) by flea-bites or consumption of infected rodents is common in mammalian carnivores in North America. Most carnivore species exhibit seroprevalence rates ranging from 3% to 100% in areas where plague occurs. Seroprevalence is highest in mustelids, intermediate in ursids, felids, and canids, and lowest in procyonids, probably reflecting variation in exposure rates as a function of dietary habits. Although conventional wisdom suggests that carnivores may only be important in plague ecology as vectors of infective fleas, animal-to-human (zoonotic) transmission suggests that mammalian carnivores can act as infectious hosts. Furthermore, a review of clinical investigations reveals that plague can be harvested from canid and felid hosts, and suggests the possibility of plague transmission between carnivores. Further study of plague transmission by carnivores in both wild and laboratory conditions is needed to understand the possible role of carnivores as wildlife reservoirs of plague.

Host usage and seasonal activity patterns of Ixodes kingi and I. sculptus (Acari: Ixodidae) nymphs in a Colorado prairie landscape, with a summary of published North American host records for all life stages

We examined host usage and seasonal activity patterns of the nymphal stage of the ticks Ixodes kingi and I. sculptus within a prairie rodent community in north-central Colorado. Ixodes kingi was commonly encountered on both northern grasshopper mice (Onychomys leucogaster) and thirteen-lined ground squirrels (Spermophilus tridecemlineatus), whereas I. sculptus frequently infested S. tridecemlineatus but was absent from O. leucogaster. Low numbers of ticks of both species were collected from deer mice (Peromyscus maniculatus) and Ord's kangaroo rats (Dipodomys ordii). Nymphal loads of I. kingi and I. sculptus increased dramatically on commonly infested rodent species from spring (May-June) to summer (July-August). Further, rodents trapped on prairie-dog towns tended to experience increased nymphal loads of I. kingi (O. leucogaster, S. tridecemlineatus) but decreased loads of I. sculptus (S. tridecemlineatus) following plague epizootics among prairie dog populations. A summary of published North American host records revealed that I. kingi has been recorded from humans, domestic animals (cat, dog), 17 species of carnivores, 40 species of rodents, and four species of lagomorphs, and that I. sculptus has been recorded from humans, domestic animals (cat, dog, goat), 13 species of carnivores, 34 species of rodents, and three species of lagomorphs. In accordance with our observations from Colorado, I. kingi commonly has been found to infest heteromyid and murid rodents (such as grasshopper mice), whereas I. sculptus most frequently has been collected from ground-dwelling sciurid rodents, especially Spermophilus ground squirrels. The potential roles of I. kingi and I. sculptus as enzootic vectors of human pathogens, particularly the agents of tularemia (Francisella tularensis), Q fever (Coxiella burnetii), and Colorado tick fever (CTF virus), are discussed.

Epizootiology of blood parasites in an Australian lizard: a mark-recapture study of a natural population

The dynamics of a naturally endemic blood parasite (Hepatozoon hinuliae) were studied in a lizard (Eulamprus quoyii) host population, using 2 years of longitudinal data. We investigated how parasite abundance in the population varied over time, examined whether certain host sub-populations were more prone to infection, and compared parasite loads in relation to host reproductive behaviour. We recorded blood parasite infections of 331 individuals, obtained in 593 captures. Prevalence (the proportion of the host population infected) of blood parasites was high; approximately 66% of the lizard population was infected. Probability of infection increased with host age and size, but did not differ between the sexes. Within individuals, parasite load (the intensity of infection within individuals) did not vary over time, and was independent of host reproductive behaviour. Parasite load was significantly higher in males compared to females.

Spatial and discrimination learning in rodents infected with the nematode Strongyloides ratti

Recent work has shown that mice with subclinical parasitic infections suffer impaired spatial learning and memory, as assayed in an open-field water maze. Although the mechanism underlying this effect is not clear, the phenomenon has been reported following infection with both a protozoan parasite (Eimeria vermiformis) and a gastrointestinal nematode (Heligmosomoides polygyrus). In a variety of experiments, we examined the effects of a different gastrointestinal nematode, Strongyloides ratti, on the ability of rats and mice to learn a spatial or a discrimination task. Animals were tested at various stages post-infection, with different levels of infection, using different lines of S. ratti and with varying experimental protocols. All animals learned the tasks, but we found no evidence of an effect of S. ratti infection on learning or memory. Even rats infected with approximately 5000 S. ratti larvae, a dose which has an impact on rat body size, showed no deficit in learning ability. Various reasons for the conflict between our results and those previously reported for E. vermiformis and H. polygyrus are discussed. Our results show that impaired learning and memory following parasitic infection is not a ubiquitous or at least easily replicated phenomenon.