A Geographic Information System Approach to Map Tick Exposure Risk at a Scale for Public Health Intervention.

Tick-borne disease control and prevention have been largely ineffective compared to the control of other vector-borne diseases. Although control strategies exist, they are costly or ineffective at large spatial scales. We need tools to target these strategies to places of highest tick exposure risk. Here we present a geographic information system (GIS) method for mapping predicted tick exposure risk at a 200 m by 200 m resolution, appropriate for public health intervention. We followed the approach used to map tick habitat suitability over large areas. We used drag-cloth sampling to measure the density of nymphal blacklegged ticks (Ixodes scapularis, Say (Acari: Ixodidae)) at 24 sites in Addison and Rutland Counties, VT, United States. We used a GIS to average habitat, climatological, land-use/land-cover, and abiotic characteristics over 100 m, 400 m, 1,000 m, and 2,000 m buffers around each site to evaluate which characteristic at which buffer size best predicted density of nymphal ticks (DON). The relationships between predictor variables and DON were determined with random forest models. The 100 m buffer model performed best and explained 37.7% of the variation in DON, although was highly accurate at classifying sites as having below or above average DON. This model was applied to Addison County, VT, to predict tick exposure risk at a 200 m resolution. This GIS approach to map predicted DON over a small area with fine resolution, could be used to target public health campaigns and land management practices to reduce human exposure to ticks.

Cross-kingdom analysis of nymphal-stage Ixodes scapularis microbial communities in relation to Borrelia burgdorferi infection and load.

The tick microbiota may influence the colonization of Ixodes scapularis by Borrelia burgdorferi, the Lyme disease bacterium. Using conserved and pathogen-specific primers we performed a cross-kingdom analysis of bacterial, fungal, protistan and archaeal communities of I. scapularis nymphs (N = 105) collected from southern Vermont, USA. The bacterial community was dominated by a Rickettsia and several environmental taxa commonly reported in I. scapularis, as well as the human pathogens B. burgdorferi and Anaplasma phagocytophilum, agent of human granulocytic anaplasmosis. With the fungal primer set we detected primarily plant- and litter-associated taxa and >18% of sequences were Malassezia, a fungal genus associated with mammalian skin. Two 18S rRNA gene primer sets, intended to target protistan communities, returned mostly Ixodes DNA as well as the wildlife pathogen Babesia odocoilei (7% of samples), a Gregarines species (14%) and a Spirurida nematode (18%). Data from pathogen-specific and conserved primers were consistent in terms of prevalence and identification. We measured B. burgdorferi presence/absence and load and found that bacterial beta diversity varied based on B. burgdorferi presence/absence. Load was weakly associated with bacterial community composition. We identified taxa associated with B. burgdorferi infection that should be evaluated for their role in vector colonization by pathogens.

Potential effects of blood meal host on bacterial community composition in Ixodes scapularis nymphs.

Tick microbiomes may play an important role in pathogen transmission. However, the drivers of microbiome variation are poorly understood, and this limitation has impeded mechanistic understanding of the functions of microbial communities for pathogen acquisition. The goal of this research was to characterize the role of the blood meal host in structuring the microbiome of Ixodes scapularis, the primary vector of Lyme disease in the eastern United States, and to determine if ticks that fed from different host species harbor distinct bacterial communities. We performed high-throughput 16S rDNA amplicon sequencing on I. scapularis nymphs that fed as larvae from known wildlife hosts: raccoon, Virginia opossum, striped skunk, red squirrel or gray squirrel. Using Analysis of Similarity, we found significant differences in the abundance-weighted Unifrac distance matrix among ticks fed from different host species (p = 0.048) and a highly significant difference in the weighted and unweighted Unifrac matrices for individuals within species (p < 0.01). This finding of associations between the blood meal host and I. scapularis microbiome demonstrates that the blood meal host may be a driver of microbiome variation that should be accounted for in studies of pathogen acquisition by ticks.

Seasonal, sub-seasonal and diurnal variation of soil bacterial community composition in a temperate deciduous forest.

The temporal dynamics of soil bacterial communities are understudied, but such understanding is critical to elucidating the drivers of community variation. The goal of this study was to characterize how soil bacterial communities vary across diurnal, sub-seasonal and seasonal time-scales in a 5.8 m2 plot and test the hypothesis that bacterial diversity varies on each of these scales. We used 16S rDNA gene amplicon sequencing to quantify the alpha and beta diversity of soil bacteria as well as the Net Relatedness Index and Nearest Taxon Indices to assess the degree of phylogenetic clustering, and the extent to which community shifts were driven by stochastic vs. deterministic limitation. We found that species richness was highest in winter, lowest in fall and that communities were compositionally distinct across seasons. There was no evidence of diurnal-scale shifts; the finest temporal scale over which community shifts were detected using our DNA-based analysis was between sampling dates separated by 6 weeks. Phylogenetic analyses suggested that seasonal-scale differences in community composition were the result of environmental filtering and homogeneous selection. Our findings provide insight into temporal variation of soil bacterial communities across the hourly to seasonal scales while minimizing the potential confounding effect of spatial variation.

Shifts in microbial biomass and the bacteria: fungi ratio occur under field conditions within 3 h after rainfall.

Increases in the frequency of soil drying and extreme precipitation projected by climate models may have important consequences for soil microbial community composition. However, the microbial response may occur over short time scales not captured by traditional sampling methods. Following a 2-year rainfall exclusion experiment in a pine forest ecosystem, we used phospholipid fatty acid profiling to measure the hourly, daily, and weekly-scale response of soil microbial biomass and the bacteria/fungi ratio to a precipitation event. We compared this response to the rewetting of un-manipulated plots. Within 3 h of watering, we detected increases in fungal and bacterial biomass of 125% and 66%, respectively, in un-manipulated plots, but only small increases in biomass within drought plots. We detected a decrease in the bacteria/fungi ratio in un-manipulated plots and an increase in this ratio in the drought plots. This surprising result was likely caused by root mortality (resulting from the previous 2-year rain exclusion) and an increase in ammonium pools in the drought plots, both of which could have suppressed fungal growth. Whereas past research suggests that soil microbes are resistant to drying-rewetting stress and to changes in annual precipitation patterns, here we show that microbes are sensitive to soil drying, but highly resilient, recovering within hours or days of a rain event. We propose that more emphasis be placed on hourly-scale field measurements of soil microbial community structure in future climate change studies.

Ecological correlates of risk and incidence of West Nile virus in the United States.

West Nile virus, which was recently introduced to North America, is a mosquito-borne pathogen that infects a wide range of vertebrate hosts, including humans. Several species of birds appear to be the primary reservoir hosts, whereas other bird species, as well as other vertebrate species, can be infected but are less competent reservoirs. One hypothesis regarding the transmission dynamics of West Nile virus suggests that high bird diversity reduces West Nile virus transmission because mosquito blood-meals are distributed across a wide range of bird species, many of which have low reservoir competence. One mechanism by which this hypothesis can operate is that high-diversity bird communities might have lower community-competence, defined as the sum of the product of each species' abundance and its reservoir competence index value. Additional hypotheses posit that West Nile virus transmission will be reduced when either: (1) abundance of mosquito vectors is low; or (2) human population density is low. We assessed these hypotheses at two spatial scales: a regional scale near Saint Louis, MO, and a national scale (continental USA). We found that prevalence of West Nile virus infection in mosquito vectors and in humans increased with decreasing bird diversity and with increasing reservoir competence of the bird community. Our results suggest that conservation of avian diversity might help ameliorate the current West Nile virus epidemic in the USA.

Inter-annual associations between precipitation and human incidence of West Nile virus in the United States.

Higher-than-average precipitation levels may cause mosquito outbreaks if mosquitoes are limited by larval habitat availability. Alternatively, recent ecological research suggests that drought events can lead to mosquito outbreaks the following year due to changes in food web structure. By either mechanism, these mosquito outbreaks may contribute to human cases of West Nile Virus (WNV) in the recent United States outbreak. Using countylevel precipitation and human WNV incidence data (2002-2004), we tested the impacts of above and below-average rainfall on the prevalence of WNV in human populations both within and between years. We found evidence that human WNV incidence is most strongly associated with annual precipitation from the preceding year. Human outbreaks of WNV are preceded by above-average rainfall in the eastern United States and below-average rainfall in the western United States in the prior year. While no direct mechanism may be determined from this study, we hypothesize that differences in the ecology of mosquito vectors may be responsible for the opposite relationships between precipitation and WNV outbreaks between the eastern and western United States.