Large scale spatial risk and comparative prevalence of Borrelia miyamotoi and Borrelia burgdorferi sensu lato in Ixodes pacificus.

Borrelia miyamotoi is a newly described emerging pathogen transmitted to people by Ixodes species ticks and found in temperate regions of North America, Europe, and Asia. There is limited understanding of large scale entomological risk patterns of B. miyamotoi and of Borreila burgdorferi sensu stricto (ss), the agent of Lyme disease, in western North America. In this study, B. miyamotoi, a relapsing fever spirochete, was detected in adult (n=70) and nymphal (n=36) Ixodes pacificus ticks collected from 24 of 48 California counties that were surveyed over a 13 year period. Statewide prevalence of B. burgdorferi sensu lato (sl), which includes B. burgdorferi ss, and B. miyamotoi were similar in adult I. pacificus (0.6% and 0.8%, respectively). In contrast, the prevalence of B. burgdorferi sl was almost 2.5 times higher than B. miyamotoi in nymphal I. pacificus (3.2% versus 1.4%). These results suggest similar risk of exposure to B. burgdorferi sl and B. miyamotoi from adult I. pacificus tick bites in California, but a higher risk of contracting B. burgdorferi sl than B. miyamotoi from nymphal tick bites. While regional risk of exposure to these two spirochetes varies, the highest risk for both species is found in north and central coastal California and the Sierra Nevada foothill region, and the lowest risk is in southern California; nevertheless, tick-bite avoidance measures should be implemented in all regions of California. This is the first study to comprehensively evaluate entomologic risk for B. miyamotoi and B. burgdorferi for both adult and nymphal I. pacificus, an important human biting tick in western North America.

Introduction

The relapsing fever spirochete, Borrelia miyamotoi, has recently been identified as a human pathogen in Russia, the Netherlands, Japan, and northeastern United States and is now considered an emerging infectious disease [1]–[6]. Borrelia miyamotoi was first identified in 1995 in Ixodes persulcatus ticks and blood from a Japanese field mouse (Apodemus argenteus) from the northern island of Japan [7]. Borrelia miyamotoi has since been detected in Ixodes tick species from other regions, including I. ricinus in Europe, I. scapularis in eastern North America [7]–[15], I. dentatus in mid-western North America [16], and I. pacificus in western North America [17]–[19]. Borrelia miyamotoi is one of three relapsing fever Borrelia species associated with ixodid ticks, along with B. lonestari [20] and B. theileri [21], [22]. Possible vertebrate reservoir hosts in North America include small rodents such as the white-footed mouse, deer, and ground dwelling birds such as wild turkeys [13], [16], [23], [24]. Unlike B. burgdorferi sensu stricto (ss), the agent of Lyme borreliosis, B. miyamotoi spirochetes can be maintained in ticks via transovarial and transtadial transmission and therefore do not necessarily require a vertebrate reservoir host to maintain the infection [8], [13], [25].

In both North America and Europe several members of the B. burgdorferi sensu lato complex, which includes B. burgdorferi ss, and B. miyamotoi have overlapping tick vectors and reservoir host species [13]. The prevalence of B. burgdorferi sensu lato (sl) in northwestern California I. pacificus nymphs averages 5%, [26] with prevalence locally as high as 20–40% [27], [28]. In southern California, the prevalence of B. burgdorferi sl in nymphal I. pacificus is much lower, generally less than 0.5% [29]. In a study conducted in the northeastern United States, B. burgdorferi ss was found to be 10 times more prevalent than B. miyamotoi in I. scapularis nymphs (20% versus 2%), and twice as prevalent in rodent reservoirs (12% versus 6%) [13]. In northern California (Mendocino County), the prevalence of B. miyamotoi in I. pacificus ticks has been detected at a prevalence similar to the Northeast in nymphs (1.7%) and slightly less in adults (0.7%) [18]. Ixodes pacificus is found throughout California, except in desert and high mountain regions. Like I. scapularis, I. ricinus, and I. persulcatus, I. pacificus adults and nymphs readily bite people.

Similar to other relapsing fever group spirochetes, B. miyamotoi is not easy to culture. Although the original isolates were cultured from tick and rodent blood [7], subsequent efforts to culture spirochetes from ticks or from human case patients have been unsuccessful in other regions of the world [14], [30]. For suspected human cases of tick-borne relapsing fever, detection by blood smear is the standard diagnostic technique [31]. Two human cases of B. miyamotoi were diagnosed by direct detection of spirochetes in cerebrospinal fluid and follow-up polymerase chain reaction (PCR) [3], [4]. All other human cases have been diagnosed based on molecular detection of B. miyamotoi DNA in acute whole blood from patients [1], [6].

Recent studies have linked previously considered “non-pathogenic” bacterial species or viruses initially described from ticks to human illness, including B. bissettii [32] and Rickettsia philipii [33] in California, R. parkeri in southeastern North America [34], and deer tick virus in eastern North America [35] . The clarification of emerging tick-borne pathogen ecology, especially in the context of comparison to what we know about B. burgdorferi sl, provides an important contextual basis from which public health response and messaging can benefit. As part of on-going tick surveillance for B. burgdorferi sl in California, testing for B. miyamotoi was added to the statewide tick testing program. This additional surveillance enhances our understanding of B. miyamotoi ecology by determining the geographic range and prevalence in California.

Materials and Methods

Tick collections

From 2000 through 2012, California Department of Public Health (CDPH) staff and partner agencies collected Ixodes pacificus adults and nymphs, primarily in recreational areas, such as forest service campgrounds, hiking trails or picnic areas in national, state, or regional parks. No permit is required for CDPH staff or staff working under CDPH supervision to collect ticks in California per the California Health and Safety Code 116110–116112. These studies did not involve any endangered or protected species. Ticks were collected from vegetation, leaf litter, or other substrate (e.g., logs, tree trunks, rocks), using 1-meter2 white double nap flannel “flag” attached to a 1.5-meter wooden dowel. Ticks were either maintained alive within 10#dram (37 mL) polystyrene containers (Fisher Scientific, USA) retained in sealed plastic bags with moistened paper toweling at 3°C (adults) or retained in 70% ethanol (subset of nymphs). In lieu of sampling a prescribed area, collectors typically flagged for a minimum of an hour and submitted at least 20 ticks per 779 collection events; many collections were opportunistic and most were at novel locations. These collections are posted on a California statewide interactive map: http://cdphgis.maps.arcgis.com/apps/SocialMedia/index.html?appid=8d99fb1135d1424f9d8a8711acb7d459.

Tick testing protocols

Ticks were tested by one of two protocols: A) pools of up to ten ticks were tested by PCR, using primers targeting generic Borrelia followed by B. burgdorferi specific primers or B) individually tested by using fluorescein-labeled antibody to detect Borrelia species spirochetes visually with positive ticks subjected to follow-up nested PCR testing with primers that differentiated between B. burgdorferi sl group and relapsing fever group Borrelia [36]. Both protocols are described in detail below.

From 2000 to 2009, ticks were tested in pools as follows. Live or frozen I. pacificus adults or nymphs were submitted to the US Army, Public Health Command Region–West, Washington for testing. Up to ten ticks of the same sex, stage, and collection site were placed in tubes for DNA extraction. Total DNA was extracted from the pooled ticks using the IsoQuick Nucleic (Orca) Acid Extraction kit (MicroProbe Corp, Bothell, WA, USA). Initially, a nested PCR protocol that targets the flagellin (fla) gene (GenBank Accession number: X69611) was used to screen samples; this protocol was Borrelia generic [37]. The PCR assays were performed using the illustra PuReTaq Ready-T-Go PCR Beads system (GE Healthcare Biosciences, Pittsburg, PA, USA) under the following conditions: 30 seconds at 95°C, 30 seconds at 55°C, and 1 minute at 75°C. Both external and internal nested PCR reactions were run for 40 cycles. Borrelia burgdorferi (B31) DNA isolated from culture was used as positive controls. Borrelia positive tick pools were further screened with one of two additional primer sets that were species-specific for B. burgdorferi sl spirochetes. These primers include sets developed to amplify a segment of the P66 gene [38] and species-specific flagellin primers [39] where positive results indicated B. burgdorferi sl infection. If negative for these B. burgdorferi sl-specific primers, a 614 bp partial flagellin gene sequence was compared to selected Borrelia sequences (e.g., B. miyamotoi) in Genbank (using Clustal X,1.81) to determine species. Results from this testing protocol are stated as minimum infection prevalence (MIP) per stage and by location: (Number of positive tick pools/number of total ticks tested) × 100.

Beginning in 2007 the tick testing protocol was modified to test live individual ticks. Ixodes pacificus were tested by CDPH Vector-Borne Disease Section laboratory using a direct fluorescent antibody assay (DFA) using fluorescein-labeled Borrelia generic antibodies (Kirkegaard & Perry Laboratories, Inc, Gaithersburg, MD, USA) as previously described [36], [40]. For those ticks with visible spirochetes detected by DFA, DNA was extracted from reserved frozen tissue using the DNeasy Blood and Tissue kit (Qiagen, Germantown, MD, USA) and a nested PCR was performed. The first reaction of the nested PCR targeted the 16S–23S rRNA intragenic spacer region was used to screen for the presence of Borrelia (1,336 bp) and was followed by a nested reaction that separated B. burgdorferi sl (970 bp) from B. miyamotoi (450 bp) [40]. Each PCR reaction mix included 5 µl of extracted tick DNA amplified in a 50 µl reaction mix containing 5 µM of each primer, 5 µl of 10X Thermopol Buffer (New England Biolabs, Ipswich, MA, USA), 1 µl dNTP solution (New England Biolabs, Ipswich, MA, USA), 0.25 µl Taq DNA polymerase (New England Biolabs, Ipswich, MA, USA) and autoclaved sterile Milli-Q water. For each reaction, a negative control of autoclaved Milli-Q water was used and a pure DNA isolate of B. burgdorferi ss strain B31 antigen was used as a positive control. PCR products were separated using the E-gel agarose gel electrophoresis system on pre-cast 2% gels stained with SYBR safe (Invitrogen, Carlsbad, CA, USA). Results from this testing protocol are expressed as prevalence: (the number of positive ticks/number of ticks tested) × 100. Starting in 2007, a subset of ticks were screened with DFA and positive samples tested by nested PCR; from 2010–2012 all ticks were tested with this protocol.

All ticks that tested positive for Borrelia species by either Borrelia-generic PCR or by DFA were then subsequently characterized by either comparative sequence analysis or by distinct gel sizes. All positive ticks were found to be either B. burgdorferi sl or B. miyamotoi (Figure 1). Since the initial focus of the study was to ascertain B. burgdorferi prevalence, a subset of ticks was tested only to Borrelia genus level after testing negative for B. burgdorferi sl. Ticks that tested positive for B. burgdorferi were not further tested to specific genomospecies and are considered B. burgdorferi sl.

Figure 1

Partial 16S–23S rRNA intragenic spacer region of Borrelia miyamotoi (450 bp) and B. burgdorferi sl (970 bp).

Results

During 2000–2012, California Department of Public Health (CDPH) staff and partner agencies collected 24,635 adult and 3,252 nymphal I. pacificus from 47 counties in 779 separate collection events/sites (Table S1). From 2000 to 2009, the following I. pacificus were tested in pools of up to 10 ticks per pool: 18,599 adults in 2,184 pools and 1,064 nymphs in 110 pools from 34 counties (Table 1). Of the pooled ticks tested with the first protocol using PCR with flagellin primers, 108 pools of the adult I. pacificus were positive for Borrelia spirochetes (0.6% minimum infection prevalence (MIP); n = 2,184 pools of 18,599 ticks). Of these positive pools, 61 pools (0.3% MIP) were positive for B. burgdorferi sl and 19 (0.1% MIP) were positive for B. miyamotoi. Although the remaining 28 pools were not positive for B. burgdorferi sl, they were not sequenced due to lack of material. Of the 1,064 nymphal I. pacificus tested in 110 pools, 15 (1.4% MIP) of these pools were positive for Borrelia spirochetes. Borrelia burgdorferi sl was detected in 9 nymphal tick pools (0.9% MIP) and B. miyamotoi in 6 pools (0.6% MIP).

Table 1

California Ixodes pacificus adult and nymphal ticks, tested in pools or tested individually, 2000–2012.

	No. Ticks Tested (in No. Pools) 2000–2009		No. Ticks Tested Individually 2007–2012
County	Adult Ticks Tested(in Pools)	Nymphal Ticks Tested(in Pools)	Adult Ticks TestedIndividually	Nymphal Ticks TestedIndividually
Alameda	0	0	73	0
Alpine	0	0	0	0
Amador	136 (16)	0	95	23
Butte	431 (48)	0	58	161
Calaveras	258 (38)	0	214	12
Colusa	0	0	5	0
Contra Costa	5 (2)	152 (17)	952	218
Del Norte	37 (7)	0	0	0
El Dorado	613 (73)	11 (3)	312	82
Fresno	30 (6)	0	2	0
Glenn	0	0	11	0
Humboldt	54 (7)	0	31	2
Imperial	0	0	0	0
Inyo	0	0	0	0
Kern	0	0	2	0
Kings	0	0	0	0
Lake	685 (76)	23 (5)	449	319
Lassen	0	0	0	0
Los Angeles	5061 (546)	2 (1)	476	3
Madera	145 (19)	0	0	2
Marin	0	0	406	240
Mariposa	84 (14)	0	106	3
Mendocino	0	0	54	17
Merced	0	0	0	0
Modoc	0	0	0	0
Mono	0	0	0	0
Monterey	561 (79)	0	49	2
Napa	99 (10)	0	1	101
Nevada	108 (15)	24 (4)	47	117
Orange	0	0	81	0
Placer	1718 (179)	33 (3)	10	39
Plumas	53 (8)	0	0	0
Riverside	1299 (187)	0	379	1
Sacramento	0	0	0	37
San Benito	0	0	47	4
San Bernardino	286 (34)	0	0	0
San Diego	24 (3)	0	23	0
San Francisco	0	0	0	0
San Joaquin	22 (8)	0	0	0
San Luis Obispo	230 (37)	0	0	0
San Mateo	0	0	316	39
Santa Barbara	716 (81)	0	0	0
Santa Clara	449 (47)	79 (7)	167	131
Santa Cruz	428 (57)	0	752	443
Shasta	1261 (133)	0	389	3
Sierra	0	0	12	0
Siskiyou	37 (4)	0	16	0
Solano	0	0	142	0
Sonoma	928 (113)	570 (59)	53	159
Stanislaus	63 (13)	0	107	0
Sutter	0	0	0	0
Tehama	4 (2)	0	0	0
Trinity	1681 (184)	3 (1)	56	0
Tulare	698 (82)	4 (2)	0	0
Tuolumne	268 (41)	163 (11)	12	3
Ventura	127 (15)	0	0	0
Yolo	0	0	0	0
Yuba	0	0	131	27
Totals	18599 (2184)	1064 (110)	6036	2188

The non-B. burgdorferi sl positive ticks were sequenced for the partial flagellin gene (614 bp) and BLAST analysis indicated that they most closely aligned with B. miyamotoi (96.9%) (GenBank accession number: AY024344.1) and more distantly with B. lonestari (91.0%). The non-B. burgdorferi sl positive ticks were also found to be more distantly aligned with B. burgdorferi sl (86.4%), B. garinii (86.5%) and B. afzelii (82.2%).

The prevalence as calculated with the individual tick-testing approach, provided higher prevalence estimates compared to the minimum infection prevalence pool testing approach. Beginning in 2007, the tick testing protocol was modified to test individual ticks. Ixodes pacificus (6,036 adults and 2,188 nymphs) from 38 counties were tested individually (Table 1), screened first by DFA, with Borrelia positives further characterized by nested PCR. Parallel to results of ticks tested in pools, B. burgdorferi sl and B. miyamotoi had similar prevalence in adult ticks with 37 individual ticks (0.6%) positive for B. burgdorferi sl and 51 (0.8%) positive for B. miyamotoi. Interestingly, nymphal I. pacificus infection prevalence was 2.5 times higher for B. burgdorferi sl (3.2%) than for B. miyamotoi (1.4%) (Table 2). A single nymph from Marin County tested positive for both B. burgdorferi sl and B. miyamotoi. While both DFA and nestedPCR are sensitive assays for detection of Borrelia, the inherent limitation of all diagnostic pathogen detection methods underestimate infection prevalence.

Table 2

Proportion of individual adult and nymphal Ixodes pacificus ticks with Borrelia burgdorferi sensu lato and B. miyamotoi detected in California, 2009–2012.

Stage	B. burgdorferi s.l.,% (No.)	B. miyamotoi % (No.)	N
I. pacificus adults	0.6% (37)	0.8% (51)	6036
I. pacificus nymphs	3.2% (70)	1.4% (30)	2188

While B. burgdorferi sl and B. miyamotoi were detected in I. pacificus in those regions of California where this tick species is abundant, there is variation in overall prevalence of both agents (Figure 2). Borrelia burgdorferi sl was more prevalent in northern and central coastal California and in the Sierra Nevada foothills: the highest MIP for B. burgdorferi sl in adult ticks was reported from Napa (4.04), Mariposa (3.57), Santa Clara (3.12), San Luis Obispo (1.74), and Tuolumne (1.5) counties. The highest prevalence was found in Placer (10.0), El Dorado (3.2), Nevada (2.12), San Mateo (1.58), and Santa Clara (1.2) counties (Figure 3). Similarly, B. miyamotoi was also highly prevalent in northern and coastal regions as well as the Sierra foothills: the highest B. miyamotoi MIP of adults was recorded in Monterey (1.1), Nevada (0.92), Santa Cruz (0.47), Lake (0.44) and Sonoma (0.22) counties and highest prevalence of B. miyamotoi in adults was recorded in San Mateo (6.33), Siskiyou (6.25), Lake (1.78), Butte (1.72), and Solano counties (1.41) (Figure 4).

Figure 2

Borrelia burgdorferi sl and/or B. miyamotoi detected by California counties, 2000–2012.

Figure 3

Adult and nymphal B. burgdorferi sl pooled results (minimum infection prevalence) and individual tick results (prevalence), 2000–2012.

Figure 4

Adult and nymphal B. miyamotoi pooled results (minimum infection prevalence) and individual tick results (prevalence), 2000–2012.

Few B. burgdorferi sl and B. miyamotoi positive ticks were detected in southern California during this study. Despite testing over 5,000 adult ticks from Los Angeles County, only one B. burgdorferi sl positive tick was detected from Catalina Island. Similarly, Riverside County had no B. burgdorferi sl infected ticks detected and only two positive for B. miyamotoi despite over 1,600 adult ticks tested.

Similar to adult ticks, B. burgdorferi sl in nymphs was more commonly detected in northern California and Sierra foothill counties with the highest recorded B. burgdorferi sl prevalences in nymphal I. pacificus from Placer (10.0), Nevada (2.13), Butte (1.72), San Mateo (1.58), and Santa Clara (1.2) counties. For B. miyamotoi, the counties with the highest prevalence in nymphs were Nevada (6.84), Amador (4.85), Placer (2.56), San Mateo (2.56) and El Dorado (2.44) counties. All nymphal ticks were tested individually (prevalence results only). No nymphal I. pacificus tested positive for B. burgdorferi sl or B. miyamotoi from southern California, a region where collection of this stage is difficult.

Discussion

This 13-year comparative study documents both B. burgdorferi sl and B. miyamotoi prevalences in I. pacificus ticks over a large geographic region and denotes the potential relative risk of contracting these two Borrelia human pathogens from nymphal and adult tick bites. These data suggest a similar risk of exposure to B. miyamotoi as to B. burgdorferi sl from adult ticks in western North America. Moreover, there is a higher risk of acquiring B. burgdorferi sl than B. miyamotoi from nymphal ticks. Both B. burgdorferi sl and B. miyamotoi are found in many regions of California but are most abundant in the north coastal and foothill regions of the state. This is the first study to provide a comprehensive estimate of entomologic risk for B. burgdorferi sl and B. miyamotoi over a large region of western North America as most studies have focused on only the region of California considered to be endemic for Lyme disease (north-western portion of state).

Borrelia miyamotoi is now recognized as a human pathogen in Europe and the eastern United States. One of the primary objectives for this study was to assess B. miyamotoi prevalence in I. pacificus nymphs and adults, the two stages of ticks that commonly bite people in western North America. Soon after being described in ticks in the eastern United States [8], CDPH-VBDS detected B. miyamotoi in California in pools of adult I. pacificus collected in 2000 from Sonoma, Monterey, and San Luis Obispo counties [17]. It was subsequently detected in I. pacificus collected in Mendocino County [18] and more recently in San Mateo and Santa Clara counties [19]. While B. miyamotoi has been detected in the white-footed mouse, deer, and wild turkeys in other parts of North America, this spirochete has not been detected in potential vertebrate reservoirs in California – future research is needed to understand the transmission ecology of B. miyamotoi in California.

In North America, seven genomospecies have been identified within the B. burgdorferi sl complex, including B. burgdorferi ss, B. americana, B. andersoni, B. bissettii, B. californiensis, B. carolinensis, B. kurtenbachii, and B. garinii [42]–[47]. The life-cycles of these Borrelia species are complex and involve members of the hard tick genus Ixodes and numerous vertebrate reservoirs [48]. To date, only B. burgdorferi ss, B. americana, B. bissettii, and B. californiensis have been described in I. pacificus from California, along with various uncharacterized Borrelia species [29], [42], [43].

Lyme borreliosis is the most commonly reported vector-borne disease in North America [49]. Thus far, a total of 18 Borrelia genomospecies have been described in Ixodid ticks worldwide [50], with the causative agent of Lyme borreliosis being designated as B. burgdorferi ss and other closely related species composing the B. burgdorferi sl complex. In the western United States, I. pacificus is the only tick species that transmits B. burgdorferi ss to humans, and the nymphal stage is the primary vector [51]. Ixodes pacificus is widely distributed in the far west from southern British Columbia to northwestern Baja California and as far east as Utah [52]. In California, I. pacificus has been recorded in 56 of 58 counties (CDPH unpublished results).

Borrelia miyamotoi belongs in the relapsing fever group, a separate clade from B. burgdorferi sl [30]. While relapsing fever group Borrelia are predominately associated with argasid ticks (soft ticks) such as Ornithodoros species, B. miyamotoi is the only relapsing fever group spirochete associated with ticks in the Ixodes ricinus species complex. In California, B. hermsii is the most commonly reported relapsing fever group pathogen, with an average of eight human cases reported per year [17]. This agent is transmitted to people by soft ticks, Ornithodoros hermsi, typically in rodent-infested mountainous cabins. Other relapsing fever group Borrelia species in California include: B. parkeri, transmitted by O. parkeri, primarily in California’s Central Valley, B. turicatae, transmitted by O. turicatae, and B. coreaceae, transmitted by O. coriaceus. Other than a handful of cases of B. parkeri in the Central Valley in the early 1900 s [53], none of the later species are typically associated with human infection in California.

The distribution of B. miyamotoi in California appears to mirror the range of I. pacificus and similar to B. burgdorferi sl, it is most prevalent in north-coastal and foothill regions of California where ticks are associated with hardwood conifer woodland habitats [26], [54]. In most counties in northern California, and often within the same collection sites, both B. burgdorferi sl and B. miyamotoi were detected. Results from this study suggest that although B. miyamotoi is detected in I. pacificus from diverse regions of California, both B. miyamotoi and B. burgdorferi sl have lowest prevalence in southern California. This may parallel the findings of a lower density of infected nymphs (DIN) in south-eastern North America and thus a lower risk of exposure to Lyme disease [55]. A recent study of over 2,000 adult I. pacificus in Los Angeles County found a similar low prevalence of B. burgdorferi sl, with only 0.04% I. pacificus positive for the human pathogen, B. burgdorferi ss [29]. Nymphal ticks are difficult to collect in southern California, likely due to environmental conditions, and thus few nymphs were tested from southern California in this study as well as in previous studies [29].

In this study, ticks were tested both in pools as well as individually. When infection prevalences are low, e.g., 1–5%, expected results for both approaches would be comparable. Here, the individual testing approach yielded a higher prevalence of Borrelia spp. in ticks than those tested in pools. This result may not necessarily be reflective of testing ticks in pools but could be because a higher proportion of ticks post-2007, when more ticks were tested individually, were from northern and foothill regions, as compared to southern California.

Unlike B. burgdorferi ss, which is not transmitted from infected adult female to eggs, B. miyamotoi is transovarially transmitted, thus larval ticks may be infected [8], [13], [25]. Furthermore, ixodid ticks can be considered a reservoir for this agent and B. miyamotoi does not need to rely on vertebrate reservoir hosts to maintain local infection prevalence. Previously reported Borrelia positive I. scapularis, I. pacificus, and I. ricinus larvae, detected by direct or indirect fluorescence antibody tests, were most likely detections of B. miyamotoi [25]. DFA-positive larvae from field-collected female I. pacificus, previously attributed to B. burgdorferi, were likely infected with B. miyamotoi [25], [56]. Furthermore, B. miyamotoi is difficult to culture and Borrelia obtained from larval ticks have been uncultivable [56]. Additional transmission studies with B. burgdorferi-infected laboratory I. pacificus colonies showed no evidence of transovarial transmission [25], [57]. While the infection of larvae with B. miyamotoi suggests human risk from larval tick bites, I. pacificus larvae are rarely reported as an ectoparasite of people (CDPH, unpublished results) [58].

With improved molecular detection methods, there is interest in cataloging the bacterial and viral communities within ticks and other biting arthropods, as some of these may be potential human pathogens [48], [59]. Out of approximately 800 adult I. pacificus from seven northern California counties, tested with 18 primer pairs in a combined PCR with electrospray ionization mass spectrometry (Ibis Biosciences), the prevalence of B. burgdorferi sl and B. miyamotoi was found to be similar to this study; in addition, Rickettsia and Anaplasma were detected among other bacteria and endosymbionts (Crowder, C and Eshoo, M, personal communication). Similarly, with increasing use of multiplex assays, there is interest and capabilities to detect co-infections in ticks. Ixodes scapularis adult ticks have been found to harbor co-infections of B. burgdorferi sl and B. miyamotoi in New York state and Canada [60], [61]. In our study, only one nymph from Marin County tested positive for both B. burgdorferi sl and B. miyamotoi.

Although B. burgdorferi sl group spirochetes were not further characterized to genomospecies here, it is likely that many positive ticks from California may harbor B. bissettii, B. americana, B. andersonii, or B. californiensis in addition to B. burgdorferi ss [45]. The diversity of Borrelia species in California I. pacificus should be taken into account when testing ticks for B. burgdorferi sl for surveillance purposes; if testing methods are not specific enough, the prevalence of B. burgdorferi ss in a tick population may be overestimated. Furthermore, additional analyses regarding the genomospecies should be taken into account to help elucidate the complex vector-reservoir-Borrelia strain ecology in California.

In California, 0.8% of adult and 1.4% of nymphal I. pacificus were infected with B. miyamotoi, suggesting similarly low risk of exposure to this spirochete for each stage. This is similar to surveillance results in Europe where the prevalence in adult and nymphal ticks collected in Estonia was estimated at 0.8% and 1%, respectively [15]. In comparison, the risk of acquiring B. burgdorferi ss is higher following a nymphal versus adult I. pacificus bite in California [49]. As determined by this study, statewide estimates of B. burgdorferi sl infection of 3.3% in nymphs and 0.7% in adults support this higher risk. Results presented here suggest comparable risk of transmission for B. burgdorferi sl and B. miyamotoi following an adult I. pacificus bite; however, the risk of exposure is 2.5 times higher for B. burgdorferi sl than for B. miyamotoi following a nymphal I. pacificus bite in California. Although this study includes a large sample size of ticks over a large geographic region, sampling was not conducted using methods such as a grid to facilitate calculation of index of risk such as density of infected nymphs (DIN) [55]. Thus, it should be noted that relative prevalences of these Borrelia may differ locally and temporally.

Laboratories that test ticks removed from humans or test ticks as part of a regional risk assessment program in western North America should ensure that their tests are specific for B. burgdorferi ss so as to not inflate risk estimates for Lyme borreliosis. Furthermore, due to the increased interest in diverse potentially pathogenic Borrelia species, including those within the B. burgdorferi sl group, more specific characterization may also prove to be informative when describing human risk for emerging tick-borne pathogens. With the knowledge that B. miyamotoi is present in California ticks, at a comparable prevalence to other parts of the world where human cases of B. miyamotoi have been reported, physicians should keep tick-borne relapsing fever as part of their differential diagnosis when evaluating patients with compatible disease following an I. pacificus bite and if suspected, contact their infectious disease specialist or local health officer for guidance in testing. Furthermore, these results highlight the continuing need for the general public to be familiar with tick bite prevention measures to avoid exposure to the diverse pathogens ticks carry.

Collection Locations for Ixodes pacificus.

(XLSX)

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