Colorado Tick Fever Virus in the Far West: Forgotten, but Not Gone.

In the past few decades, reported human cases of Colorado tick fever in the western United States have decreased dramatically. The goal of this study was to conduct surveillance for Colorado tick fever virus (CTFV) in Dermacentor ticks in recreational sites in Colorado, Wyoming, and California to determine whether the virus is still present in Dermacentor ticks from these states. Surveillance focused on regions where surveys had been conducted in the 1950s, 1960s, and 1970s. Adult Rocky Mountain wood ticks (Dermacentor andersoni), Pacific Coast ticks (Dermacentor occidentalis), and winter ticks (Dermacentor albipictus) were tested by PCR. A subset of PCR-positive D. andersoni ticks (n = 7) were cultured in Vero cells. CTFV-positive Rocky Mountain wood ticks were found in all states: Colorado (58% prevalence), Wyoming (21%), and California (4%). Although no winter ticks tested positive, Pacific Coast ticks tested positive in one county (Siskiyou County, 15% prevalence) and were positive only in a location that also maintained Rocky Mountain wood ticks and golden mantled ground squirrels, a known CTFV host. In summary, CTFV is prevalent in D. andersoni and D. occidentalis in regions where they are sympatric in California and in D. andersoni in Colorado and Wyoming. Although the number of human CTFV cases has declined dramatically, this decrease in reported disease does not appear to be due to the disappearance or even the decline in prevalence of this virus in ticks in historically endemic regions of the country.

Detection and Isolation of Rickettsia tillamookensis (Rickettsiales: Rickettsiaceae) From Ixodes pacificus (Acari: Ixodidae) From Multiple Regions of California.

The western black-legged tick (Ixodes pacificus) is the most frequently identified human-biting tick species in the western United States and the principal vector of at least three recognized bacterial pathogens of humans. A potentially pathogenic Rickettsia species, first described in 1978 and recently characterized as a novel transitional group agent designated as Rickettsia tillamookensis, also exists among populations of I. pacificus, although the distribution and frequency of this agent are poorly known. We evaluated DNA extracts from 348 host-seeking I. pacificus nymphs collected from 9 locations in five California counties, and from 916 I. pacificus adults collected from 24 locations in 13 counties, by using a real-time PCR designed specifically to detect DNA of R. tillamookensis. DNA of R. tillamookensis was detected in 10 (2.9%) nymphs (95% CI: 1.6-5.2%) and 17 (1.9%) adults (95% CI: 1.2-3.0%) from 11 counties of northern California. Although site-specific infection rates varied greatly, frequencies of infection remained consistently low when aggregated by stage, sex, habitat type, or geographical region. Four novel isolates of R. tillamookensis were cultivated in Vero E6 cells from individual adult ticks collected from Alameda, Nevada, and Yolo counties. Four historical isolates, serotyped previously as 'Tillamook-like' strains over 40 yr ago, were revived from long-term storage in liquid nitrogen and confirmed subsequently by molecular methods as isolates of R. tillamookensis. The potential public health impact of R. tillamookensis requires further investigation.

Modeling future climate suitability for the western blacklegged tick, Ixodes pacificus, in California with an emphasis on land access and ownership.

In the western United States, Ixodes pacificus Cooley & Kohls (Acari: Ixodidae) is the primary vector of the agents causing Lyme disease and granulocytic anaplasmosis in humans. The geographic distribution of the tick is associated with climatic variables that include temperature, precipitation, and humidity, and biotic factors such as the spatial distribution of its primary vertebrate hosts. Here, we explore (1) how climate change may alter the geographic distribution of I. pacificus in California, USA, during the 21st century, and (2) the spatial overlap among predicted changes in tick habitat suitability, land access, and ownership. Maps of potential future suitability for I. pacificus were generated by applying climate-based species distribution models to a multi-model ensemble of climate change projections for the Representative Concentration Pathway (RCP) 4.5 (moderate emission) and 8.5 (high emission) scenarios for two future periods: mid-century (2026-2045) and end-of-century (2086-2099). Areas climatically-suitable for I. pacificus are projected to expand by 23% (mid-century RCP 4.5) to 86% (end-of-century RCP 8.5) across California, compared to the historical period (1980-2014), with future estimates of total suitable land area ranging from about 88 to 133 thousand km2, or up to about a third of California. Regions projected to have the largest area increases in suitability by end-of-century are in northwestern California and the south central and southern coastal ranges. Over a third of the future suitable habitat is on lands currently designated as open access (i.e. publicly available), and by 2100, the amount of these lands that are suitable habitat for I. pacificus is projected to more than double under the most extreme emissions scenario (from ~23,000 to >51,000 km2). Of this area, most is federally-owned (>45,000 km2). By the end of the century, 26% of all federal land in the state is predicted to be suitable habitat for I. pacificus. The resulting maps may facilitate regional planning and preparedness by informing public health and vector control decision-makers.

Isolation of Borrelia miyamotoi and other Borreliae using a modified BSK medium.

Borrelia spirochetes are the causative agents of Lyme borreliosis (LB) and relapsing fever (RF). Despite the steady rise in infections and the identification of new species causing human illness over the last decade, isolation of borreliae in culture has become increasingly rare. A modified Barbour-Stoenner-Kelly (BSK) media formulation, BSK-R, was developed for isolation of the emerging RF pathogen, Borrelia miyamotoi. BSK-R is a diluted BSK-II derivative supplemented with Lebovitz's L-15, mouse and fetal calf serum. Decreasing the concentration of CMRL 1066 and other components was essential for growth of North American B. miyamotoi. Sixteen B. miyamotoi isolates, originating from Ixodes scapularis ticks, rodent and human blood collected in the eastern and upper midwestern United States, were isolated and propagated to densities > 108 spirochetes/mL. Growth of five other RF and ten different LB borreliae readily occurred in BSK-R. Additionally, primary culture recovery of 20 isolates of Borrelia hermsii, Borrelia turicatae, Borrelia burgdorferi and Borrelia mayonii was achieved in BSK-R using whole blood from infected patients. These data indicate this broadly encompassing borreliae media can aid in in vitro culture recovery of RF and LB spirochetes, including the direct isolation of new and emerging human pathogens.

West Nile virus in California, 2003-2018: A persistent threat.

The California Arbovirus Surveillance Program was initiated over 50 years ago to track endemic encephalitides and was enhanced in 2000 to include West Nile virus (WNV) infections in humans, mosquitoes, sentinel chickens, dead birds and horses. This comprehensive statewide program is a function of strong partnerships among the California Department of Public Health (CDPH), the University of California, and local vector control and public health agencies. This manuscript summarizes WNV surveillance data in California since WNV was first detected in 2003 in southern California. From 2003 through 2018, 6,909 human cases of WNV disease, inclusive of 326 deaths, were reported to CDPH, as well as 730 asymptomatic WNV infections identified during screening of blood and organ donors. Of these, 4,073 (59.0%) were reported as West Nile neuroinvasive disease. California's WNV disease burden comprised 15% of all cases that were reported to the U.S. Centers for Disease Control and Prevention during this time, more than any other state. Additionally, 1,299 equine WNV cases were identified, along with detections of WNV in 23,322 dead birds, 31,695 mosquito pools, and 7,340 sentinel chickens. Annual enzootic detection of WNV typically preceded detection in humans and prompted enhanced intervention to reduce the risk of WNV transmission. Peak WNV activity occurred from July through October in the Central Valley and southern California. Less than five percent of WNV activity occurred in other regions of the state or outside of this time. WNV continues to be a major threat to public and wild avian health in California, particularly in southern California and the Central Valley during summer and early fall months. Local and state public health partners must continue statewide human and mosquito surveillance and facilitate effective mosquito control and bite prevention measures.

Projected climate and land use change alter western blacklegged tick phenology, seasonal host-seeking suitability and human encounter risk in California.

Global environmental change is having profound effects on the ecology of infectious disease systems, which are widely anticipated to become more pronounced under future climate and land use change. Arthropod vectors of disease are particularly sensitive to changes in abiotic conditions such as temperature and moisture availability. Recent research has focused on shifting environmental suitability for, and geographic distribution of, vector species under projected climate change scenarios. However, shifts in seasonal activity patterns, or phenology, may also have dramatic consequences for human exposure risk, local vector abundance and pathogen transmission dynamics. Moreover, changes in land use are likely to alter human-vector contact rates in ways that models of changing climate suitability are unlikely to capture. Here we used climate and land use projections for California coupled with seasonal species distribution models to explore the response of the western blacklegged tick (Ixodes pacificus), the primary Lyme disease vector in western North America, to projected climate and land use change. Specifically, we investigated how environmental suitability for tick host-seeking changes seasonally, how the magnitude and direction of changing seasonal suitability differs regionally across California, and how land use change shifts human tick-encounter risk across the state. We found vector responses to changing climate and land use vary regionally within California under different future scenarios. Under a hotter, drier scenario and more extreme land use change, the duration and extent of seasonal host-seeking activity increases in northern California, but declines in the south. In contrast, under a hotter, wetter scenario seasonal host-seeking declines in northern California, but increases in the south. Notably, regardless of future scenario, projected increases in developed land adjacent to current human population centers substantially increase potential human-vector encounter risk across the state. These results highlight regional variability and potential nonlinearity in the response of disease vectors to environmental change.

Insecticide Resistance Status of Aedes aegypti (Diptera: Culicidae) in California by Biochemical Assays.

Insecticide resistance in Aedes aegypti mosquitoes poses a major threat to public health worldwide. There are two primary biological mechanisms that can lead to insecticide resistance, target site and metabolic resistance, both of which confer resistance to specific classes of insecticides. Due to the limited number of chemical compounds available for mosquito control, it is important to determine current enzymatic profiles among mosquito populations. This study assessed resistance profiles for three metabolic pathways, α-esterases, ß-esterases, and mixed-function oxidases (MFOs), as well as insensitivity of the acetylcholinesterase (iAChE) enzyme in the presence of propoxur, among Ae. aegypti from the Central Valley and southern California. All field-collected Ae. aegypti demonstrated elevated MFOs and iAChE activity, indicating potential development of pyrethroid and organophosphate resistance, respectively. Although regional variations were found among α-esterase and ß-esterase activity, levels were generally elevated, further suggesting additional mechanisms for developing organophosphate resistance. Furthermore, mosquito samples from southern California exhibited a higher expression level to all three metabolic enzymes and iAChE activity in comparison to mosquitoes from the central region. These results could help guide future mosquito control efforts, directing the effective use of insecticides while limiting the spread of resistance.

Plague Epizootic Dynamics in Chipmunk Fleas, Sierra Nevada Mountains, California, USA, 2013-2015.

We describe Yersinia pestis minimum infection prevalence in fleas collected from Tamias spp. chipmunks in the Sierra Nevadas (California, USA) during 2013-2015. Y. pestis-positive fleas were detected only in 2015 (year of plague epizootic), mostly in T. speciosus chipmunks at high-elevation sites. Plague surveillance should include testing vectors for Y. pestis.

A Novel TaqMan Assay for Detection of Rickettsia 364D, the Etiologic Agent of Pacific Coast Tick Fever.

Pacific Coast tick fever is a febrile illness associated with the bite of Dermacentor occidentalis and results from an infection due to the intracellular pathogen Rickettsia 364D (also known by the proposed name "Rickettsia philipii"). Current molecular methods for the detection of this pathogen rely on the amplification of a conserved spotted fever group rickettsial gene (ompA) followed by DNA sequencing of the amplicon to identify the species. This work describes the development of a Rickettsia 364D-specific TaqMan assay to simplify and accelerate the detection and identification processes. The assay demonstrated a sensitivity of 1 genomic copy per 4-µl sample and is highly specific for Rickettsia 364D. The utility of this assay for ecological and diagnostic samples was evaluated using banked specimens collected in a single-blind manner and yielded a clinical sensitivity and specificity of 100%. In conclusion, we describe the development and evaluation of a novel TaqMan real-time PCR assay for the detection and identification of Rickettsia 364D suitable for ecological and diagnostic applications.

Identification of Molecular Determinants of Resistance to Pyrethroid Insecticides in Aedes aegypti (Diptera: Culicidae) Populations in California, USA.

The first breeding populations of Aedes aegypti (Linnaeus) were identified in California in 2013, and have since been detected in 13 counties. Recent studies suggest two introductions likely occurred, with genetically distinct populations in the central and southern regions of the state. Given the threat of dengue, chikungunya, and Zika virus transmission, it is imperative to understand if these populations harbor genes that could confer resistance to pyrethrin-based insecticides, known as pyrethroids, the most commonly used class of adulticides in the state. In 2017, the California Department of Public Health initiated a pesticide resistance screening program for Ae. aegypti to assess the presence of specific mutations on the sodium channel gene (V1016I and F1534C) associated with knockdown resistance to pyrethroids. Mosquitoes collected between 2015 and 2017 from 11 counties were screened for mutations using real-time polymerase chain reaction assays. Results revealed distinctly different resistance profiles between the central and southern regions. The central population displayed nearly fixed resistant mutations at both loci, whereas the southern population was more variable. The relative proportion of resistant alleles observed in sampled mosquitoes collected in southern California increased each year from 2015 through 2017, indicating potential increases in resistance across this region. The presence of these mutations indicates that these mosquitoes may be predisposed to surviving pyrethroid treatments. Additional biological and biochemical assays will help better elucidate the mechanisms underlying insecticide resistance in California Ae. aegypti and prompt the use of pesticides that are most effective at controlling these mosquitoes.

Phylogeography of Borrelia spirochetes in Ixodes pacificus and Ixodes spinipalpis ticks highlights differential acarological risk of tick-borne disease transmission in northern versus southern California.

The common human-biting tick, Ixodes pacificus, is the primary vector of the Lyme disease spirochete, Borrelia burgdorferi sensu stricto (ss) in western North America and has been found to harbor other closely-related spirochetes in the Borrelia burgdorferi sensu lato (sl) complex. Between 2008-2015, 11,066 adult and 3,815 nymphal I. pacificus and five adult and 144 nymphal Ixodes spinpalpis, a commonly collected wildlife tick, were collected from 42 California counties. Borrelia burgdorferi sl was detected in 1.2% and 3.8% I. pacificus adults and nymphs, respectively. Results from this study indicate genetic diversity and geographic structure of B. burgdorferi sl in California I. pacificus ticks, by sequence comparison of the16S rRNA gene, with B. burgdorferi ss, the agent of Lyme disease, found only in I. pacificus collected from the north and central coastal and Sierra Nevada foothill regions; B. burgdorferi ss was not detected in ticks tested from southern California. In contrast, Borrelia bissettiae, a member of the B. burgdorferi sl complex, was detected in both I. pacificus and I. spinipalpis, in the coastal region of both northern and southern California, but was absent from ticks in the Sierra Nevada foothills. In a similar pattern to B. bissettiae, Borrelia americana (a member of the B. burgdorferi sl complex) was detected in a single adult I. pacificus from the north coast and two I. spinipalpis nymphs from south-coastal California. This study highlights that the geographic area of Lyme disease acarological risk in California is the north-central and Sierra Nevada foothill regions of the state with little to no risk in the southern regions of the state.

Evidence for transmission of the zoonotic apicomplexan parasite Babesia duncani by the tick Dermacentor albipictus.

Babesiosis is a potentially fatal tick-borne zoonotic disease caused by a species complex of blood parasites that can infect a variety of vertebrates, particularly dogs, cattle, and humans. In the United States, human babesiosis is caused by two distinct parasites, Babesia microti and Babesia duncani. The enzootic cycle of B. microti, endemic in the northeastern and upper midwestern regions, has been well characterised. In the western United States, however, the natural reservoir host and tick vector have not been identified for B. duncani, greatly impeding efforts to understand and manage this zoonotic disease. Two and a half decades after B. duncani was first described in a human patient in Washington State, USA, we provide evidence that the enzootic tick vector is the winter tick, Dermacentor albipictus, and the reservoir host is likely the mule deer, Odocoileus hemionus. The broad, overlapping ranges of these two species covers a large portion of far-western North America, and is consistent with confirmed cases of B. duncani in the far-western United States.

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.

Rickettsia Species Isolated from Dermacentor occidentalis (Acari: Ixodidae) from California.

The Pacific Coast tick (Dermacentor occidentalis Marx, 1892) is one of the most widely distributed and frequently encountered tick species in California. This tick is the primary vector of an unclassified spotted fever group rickettsial pathogen, designated currently as Rickettsia 364D, the etiologic agent of a recently recognized tick-borne rickettsiosis known as Pacific Coast tick fever. Despite intensified interest in this pathogen, important questions remain regarding its taxonomic status and possible variations in genotype among different strains that could influence its pathogenicity. Only the extensively passaged prototypical isolate (strain 364-D) is widely available to rickettsiologists and public health scientists worldwide. To achieve a larger, more geographically diverse, and contemporary collection of strains, 1,060 questing adult D. occidentalis ticks were collected from 18 sites across six counties in northern and southern California in 2016 and 2017. Fourteen ticks (1.3%) yielded DNA of Rickettsia 364D and from these, 10 unique isolates from Lake and Orange counties were obtained. Additionally, Rickettsia rhipicephali was detected in 108 (10.2%) ticks, from which eight isolates were obtained, and Rickettsia bellii in six (0.6%), from which three isolates were obtained. The panel of recently acquired, low-passage strains of Rickettsia 364D derived from this study could enhance opportunities for investigators to accurately determine the taxonomic standing of this agent and to develop specific diagnostic assays for detecting infections with Rickettsia 364D in ticks and humans.

Modeling Climate Suitability of the Western Blacklegged Tick in California.

Ixodes pacificus Cooley & Kohls (Acari: Ixodidae), the primary vector of Lyme disease spirochetes to humans in the far-western United States, is broadly distributed across Pacific Coast states, but its distribution is not uniform within this large, ecologically diverse region. To identify areas of suitable habitat, we assembled records of locations throughout California where two or more I. pacificus were collected from vegetation from 1980 to 2014. We then employed ensemble species distribution modeling to identify suitable climatic conditions for the tick and restricted the results to land cover classes where these ticks are typically encountered (i.e., forest, grass, scrub-shrub, riparian). Cold-season temperature and rainfall are particularly important abiotic drivers of suitability, explaining between 50 and 99% of the spatial variability across California among models. The likelihood of an area being classified as suitable increases steadily with increasing temperatures >0°C during the coldest quarter of the year, and further increases when precipitation amounts range from 400 to 800 mm during the coldest quarter, indicating that areas in California with relatively warm and wet winters typically are most suitable for I. pacificus. Other consistent predictors of suitability include increasing autumn humidity, temperatures in the warmest month between 23 and 33°C, and low-temperature variability throughout the year. The resultant climatic suitability maps indicate that coastal California, especially the northern coast, and the western Sierra Nevada foothills have the highest probability of I. pacificus presence.

Detection and characterization of a novel spotted fever group Rickettsia genotype in Haemaphysalis leporispalustris from California, USA.

The rabbit tick, Haemaphysalis leporispalustris Packard, is known for its association with Rickettsia rickettsii as it harbors both virulent and avirulent strains of this pathogen. In this manuscript we report findings and preliminary characterization of a novel spotted fever group rickettsia (SFGR) in rabbit ticks from California, USA. Rickettsia sp. CA6269 (proposed "Candidatus Rickettsia lanei") is most related to known R. rickettsii isolates but belongs to its own well-supported branch different from those of all R. rickettsii including strain Hlp2 and from Rickettsia sp. 364D (also known as R. philipii) and R. peacockii. This SFGR probably exhibits both transovarial and transstadial survival since it was found in both questing larvae and nymphs. Although this rabbit tick does not frequently bite humans, its role in maintenance of other rickettsial agents and this novel SFGR warrant further investigation.

Chromosome and Large Linear Plasmid Sequences of a Borrelia miyamotoi Strain Isolated from Ixodes pacificus Ticks from California.

Borrelia miyamotoi, a relapsing fever group spirochete, is an emerging tick-borne pathogen. It has been identified in ixodid ticks across the Northern Hemisphere, including the West Coast of the United States. We describe the chromosome and large linear plasmid sequence of a B. miyamotoi isolate cultured from a California field-collected Ixodes pacificus tick.

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'.

Detection and Establishment of Aedes aegypti and Aedes albopictus (Diptera: Culicidae) Mosquitoes in California, 2011-2015.

In 2011, a thriving population of Aedes albopictus (Skuse), the Asian tiger mosquito, was discovered within three cities in Los Angeles County over an estimated 52-km2 urban area. Two years later in 2013, Aedes aegypti (L.), the yellow fever mosquito, was detected within several urban areas of Madera, Fresno, and San Mateo counties. State and local vector control agencies responded with an aggressive effort to eradicate or interrupt the spread of these two invasive mosquitoes; however, known populations continued to expand outward and new infestations were identified at an accelerated pace in central and southern California. By the end of 2015, one or both species had been detected within the jurisdictional boundaries of 85 cities and census-designated places in 12 counties. Herein we report on the discovery and widespread establishment of Ae. aegypti and Ae. albopictus in urban areas of coastal, central, and southern California between 2011 and 2015 and discuss the subsequent rapid changes to the activities and priorities of vector control agencies in response to this unprecedented invasion.

The Eco-epidemiology of Pacific Coast Tick Fever in California.

Rickettsia philipii (type strain "Rickettsia 364D"), the etiologic agent of Pacific Coast tick fever (PCTF), is transmitted to people by the Pacific Coast tick, Dermacentor occidentalis. Following the first confirmed human case of PCTF in 2008, 13 additional human cases have been reported in California, more than half of which were pediatric cases. The most common features of PCTF are the presence of at least one necrotic lesion known as an eschar (100%), fever (85%), and headache (79%); four case-patients required hospitalization and four had multiple eschars. Findings presented here implicate the nymphal or larval stages of D. occidentalis as the primary vectors of R. philipii to people. Peak transmission risk from ticks to people occurs in late summer. Rickettsia philipii DNA was detected in D. occidentalis ticks from 15 of 37 California counties. Similarly, non-pathogenic Rickettsia rhipicephali DNA was detected in D. occidentalis in 29 of 38 counties with an average prevalence of 12.0% in adult ticks. In total, 5,601 ticks tested from 2009 through 2015 yielded an overall R. philipii infection prevalence of 2.1% in adults, 0.9% in nymphs and a minimum infection prevalence of 0.4% in larval pools. Although most human cases of PCTF have been reported from northern California, acarological surveillance suggests that R. philipii may occur throughout the distribution range of D. occidentalis.