Human Seroprevalence of Tick-Borne Anaplasma phagocytophilum,Borrelia burgdorferi,and Rickettsia Species in Northern California

Abstract

There is a paucity of data on human exposure to tick-borne pathogens in the western United States. This study reports prevalence of antibodies against three clinically important tick-borne pathogens (Borrelia burgdorferi, Anaplasma phagocytophilum, and Rickettsia spp.) among 249 people in five counties in northern California. Individuals from Humboldt County were recruited and answered a questionnaire to assess risk of exposure to tick-borne pathogens. Samples from other counties were obtained from a blood bank and were anonymized. Seventeen (6.8%) samples were seropositive for antibodies against at least one pathogen: five for A. phagocytophilum, eight for B. burgdorferi, and four for Rickettsia spp. Women and people aged 26–35 had higher seroprevalence compared to other demographic groups. Santa Cruz County had no seropositive individuals, northern Central Valley counties had three seropositive individuals (all against A. phagocytophilum), and Humboldt County had 14 (all three pathogens), a significant, four-fold elevated risk of exposure. The Humboldt County questionnaire revealed that a bird feeder in the yard was statistically associated with exposure to ticks, and lifetime number of tick bites was associated with increasing age, time watching wildlife, and time hiking. Three-quarters of respondents were concerned about tick-associated disease, 81.0% reported experiencing tick bites, and 39.0% of those bitten reported a tick-borne disease symptom, including skin lesions (76.4%), muscle aches (49.1%), joint pain (25.5%), or fever (23.6%). Despite high levels of concern, many individuals who had been bitten by a tick were not tested for a tick-borne pathogen, including those with consistent symptoms. We highlight the need for further research and dissemination of information to residents and physicians in Northern California regarding tick-associated disease, so that appropriate medical attention can be rapidly sought and administered.

Introduction

Globally, tick-borne pathogens are an emerging health threat for humans and animals alike. Such is the case in northern California, where Anaplasma phagocytophilum, Borrelia burgdorferi, and Rickettsia spp. have been reported, posing considerable risk of morbidity and mortality (Brown and Lane 1992, Postic et al. 1998, Fedorova et al. 2014, Stephenson et al. 2017). A. phagocytophilum is responsible for granulocytic anaplasmosis. In humans, clinical presentation spans febrile illness (e.g., headache and myalgia) to severe or complicated disease that includes central nervous system (e.g., meningoencephalitis), renal-, respiratory failure, coagulopathy, and death (Dumler et al. 2005). B. burgdorferi is the etiological agent of Lyme borreliosis, the most common vector-borne disease in the United States (Centers for Disease Control 2018). Acute Lyme borreliosis is characterized by fever, rash (erythema migrans), joint and muscle pain, sometimes progressing to chronic or neurological disease (Nadelman et al. 1996, Seltzer et al. 2000). Rickettsia rickettsii causes Rocky Mountain spotted fever, a disease characterized by headache, myalgia and rash, which, if left untreated, can progress rapidly (∼5 days) to multisystem organ failure (notably with renal and respiratory compromise) conferring a high fatality rate (Drexler et al. 2017). Other rickettsial diseases that affect humans have been reported in the United States, including Pacific coast tick fever and typhus, and often are not differentiated clinically or on diagnostic testing (Padgett et al. 2016, California Department of Public Health 2019).

A. phagocytophilum, B. burgdorferi, and R. rickettsii are all bacteria that are transmitted to humans through the bite of ixodid ticks that have acquired the pathogen from infected mammalian hosts (Stephenson and Foley 2016, Stephenson et al. 2017). The temperate climate and diverse ecosystems in northern California provide optimal habitat for vector tick and reservoir host (largely small mammal) species in the sylvatic cycles of the three pathogens (Brown and Lane 1992, Postic et al. 1998, Fedorova et al. 2014, Foley and Piovia-Scott 2014, Foley et al. 2016, Stephenson and Foley 2016, Stephenson et al. 2017, and Hacker et al. 2018). Human risk of infection has been predicted using measures such as tick abundance and pathogen prevalence in vectors or reservoir hosts (Lane 1996, Eisen et al. 2006). The few studies in California that have evaluated incidence and seroprevalence in humans suggest that exposure is highest in coastal northern California, particularly in Humboldt and neighboring counties (Lane et al. 1992, Fritz et al. 2005, Stephenson et al. 2017, Yoshimizu et al. 2017). However, given a lack of active surveillance it is likely that burden of exposure and infection is underestimated (Foley et al. 1999).

In this study, we performed a serosurvey for antibodies against A. phagocytophilum, B. burgdorferi, and Rickettsia spp. in 249 individuals from five counties in northern California: Humboldt, Santa Cruz, Tehama, Glenn, and Colusa. We sought both to document the extent of exposure as well as to investigate demographic factors (sex, age, and ethnicity) that may be associated with pathogen exposure and therefore seropositivity. In addition, we conducted additional surveillance in Humboldt County given prior reports that suggest that residents may be at particulalry high risk of exposure (Vector-Borne Disease Section 2014).

Materials and Methods

Acquisition of human blood samples

Human sera were purchased from Creative Testing Solutions (www.mycts.org) to include 24 samples from Santa Cruz County, 63 from Tehama, 27 from Glenn, and six from Colusa. They represented people who donated blood at regional blood centers between February and September in 2019. All donors were negative for routine infectious blood donor screening that includes human immunodeficiency virus, hepatitis B virus, hepatitis C virus, Trypanosoma cruzi, human T cell lymphoma virus 1 and 2, Treponema pallidum, and West Nile Virus. Standard blood donor consent allows for deidentified testing for research use. Data provided were race, sex, and age.

Prospective sampling of whole blood was conducted in Humboldt County yielding 174 samples. Two of these counties (Humboldt and Santa Cruz) were selected because of high risk (high Ixodes pacificus abundance and prior human and veterinary case reports (Furman and Loomis 1984, Eisen et al. 2006)), while the remaining counties had few or no reports of I. pacificus and human tick-borne disease cases. Due to low sample size, the three inland, low-risk counties (Tehama, Glenn and Colusa) were grouped as “North Valley”.

Humboldt County samples were collected as part of a previous study (Stephenson et al. 2017). Residents were recruited by word-of-mouth and by posting flyers in high-traffic areas (e.g., on grocery store bulletin boards) for five events between October 2012 and May 2013. An active effort was made to recruit Humboldt County residents, including forestry workers, wildlife biologists, college students interested in wildlife research, and Native Americans living near forests. After completing a questionnaire participants were asked to donate a blood sample; if consent was obtained, phlebotomy was performed by a licensed nurse.

Human diagnostic testing

Indirect immunofluorescence assays were used to evaluate A. phagocytophilum and Rickettsia spp. antibodies. Serum was diluted in phosphate-buffered saline (PBS) at 1:80 for A. phagocytophilum and 1:64 for Rickettsia spp. with a positive and negative control serum sample included in each run. For each sample, 25 ml of diluted serum was incubated on a slide (VMRD, Pullman, WA) well at 37°C for 30 min, followed by three washes with PBS. Next, 25 μL of fluorescein-labeled, anti-human IgG heavy and light chain conjugate (Kirkegaard & Perry Laboratories, Gaithersburg, MD) diluted in PBS at 1:100 were incubated on each well at 37°C for 30 min. Slides were again washed three times with PBS and counter-stained with eriochrome black. Samples were evaluated by ultraviolet microscopy, and those with strong fluorescence and consistent morphology of the antigen were considered positive. Samples were also tested for B. burgdorferi antibodies using a commercial C6 ELISA kit (Immunetics^®, Boston, MA), following the protocol provided by the manufacturer. A Lyme Index was calculated for each sample following methods provided by the manufacturer. Samples were considered positive if their Lyme Index was ≥1.10.

Whole blood samples from Humboldt County were additionally tested for tick-borne pathogen DNA. DNA was extracted from 100 μL of blood using a DNeasy Blood and Tissue Kit (Qiagen, Valencia, CA) according to the manufacturer's protocol. Real-time PCR (RT-PCR) was performed for B. burgdorferi sensu lato, A. phagocytophilum sensu lato, and Rickettsia spp. (Stenos et al. 2005, Drazenovich et al. 2006, Barbour et al. 2009). Samples were considered positive if the cycle threshold was <40 and there was a characteristic amplification curve. Three negative controls and a sequence-confirmed positive control were included in each run.

Humboldt County questionnaires

Participants from Humboldt County were asked to complete a questionnaire that included questions on demographic characteristics, tick bite and tick-borne pathogen testing history, and development of symptoms following any tick bite. Respondents who reported skin lesion or rash associated with a tick bite were shown a card with a photograph of four different skin lesions to match to lesions they had experienced. The questionnaire also inquired about contact with domestic animals and wildlife and participation in outdoor activities (e.g., gardening and dog walking). All questionnaire responses were categorical. All the works were performed under approval from the University of California, Davis Institutional Review Board.

Statistical analyses

Univariate logistic regressions were performed in “R” (R Core Development Team V 3.1.2) to assess relationships between the presence of antibodies to any tick-borne pathogen and the independent variables “sex” (female, male), “age” (17–25, 26–35, 36–65, and >65 years old), “ethnicity”, and “county” (Humboldt, Santa Cruz and North Valley). Even if samples were missing data for some predictors, they were still analyzed in univariate models for which potential predictors were available. The exponentials of the model coefficients and standard errors were used to calculate odds ratios and 95% confidence intervals. For questionnaire data, the dependent variable was the self-reported number of tick-bites during an individuals' lifetime: never, once, few times (2–5), sometimes (6–20), and many times (>20). The linear relationship between this outcome and predictor variables was tested using linear regression. Age and amount of time spent outdoors at work were included in the base model to control for these variables. Forward stepwise selection was used to construct the most parsimonious multivariable model based on the log-likelihood ratio.

Results

Human testing for tick-borne pathogens

A total of 249 sera samples were evaluated from five counties in Northern California, of which 129 (51.8%) were from Humboldt, 24 (9.6%) from Santa Cruz, and 96 (38.6%) from the three North Valley counties (Table 1). Just over half of the samples were from women. The most common age group was 36–65 years. In total, 17 (6.8%) samples tested positive for antibodies to a tick-borne pathogen, including five (2.0%) for A. phagocytophilum, eight (3.2%) for B. burgdorferi, and four (1.6%) for Rickettsia spp. There were no samples with antibodies for more than one pathogen.

Table 1.

Number of Samples Tested and Positive for Antibodies to Three Tick-Borne Pathogens, Seroprevalence, and Univariate Logistic Regression Results for 249 Human Subjects from Five Counties in Northern California

		Number of individuals tested (% of all individuals)	Anaplasma positive (n = 5)	Borrelia positive (n = 8)	Rickettsia positive (n = 4)	Positive for any pathogen (n = 17)
County	North Valley	96 (38.5%)	3 (3.1%)	0	0	3 (3.1%)
	North Valley	96 (38.5%)	3 (3.1%)	0	0	Referent
	Colusa	6 (2.4%)	1 (16.7%)	0	0	1 (16.7%)
	Glenn	27 (10.8%)	0	0	0	0
	Tehama	63 (25.3%)	2 (3.2%)	0	0	2 (3.2%)
	Humboldt	129 (51.8%)	2 (1.6%)	8 (6.2%)	4 (3.1%)	14 (10.9%)
	Humboldt	129 (51.8%)	2 (1.6%)	8 (6.2%)	4 (3.1%)	OR = 3.8, CI = 1.0–13.9, p = 0.04
	Santa Cruz	24 (9.6%)	0	0	0	0
	Santa Cruz	24 (9.6%)	0	0	0	OR <0.1, CI = 0.1–100, p = 0.99
Sex	Female	129 (51.8%)	4 (3.1%)	3 (2.3%)	3 (2.3%)	10 (7.8%)
	Female	129 (51.8%)	4 (3.1%)	3 (2.3%)	3 (2.3%)	Referent
	Male	111 (44.6%)	1 (0.9%)	4 (3.6%)	1 (0.9%)	6 (5.4%)
	Male	111 (44.6%)	1 (0.9%)	4 (3.6%)	1 (0.9%)	OR = 0.7, CI = 0.2–2.0, p = 0.47
	Declined to state	9 (3.6%)	0	1 (11.1%)	0	1 (11.1%)
	Declined to state	9 (3.6%)	0	1 (11.1%)	0	Not included
Age range (years)	17–25	56 (22.5%)	0	1 (1.8%)	2 (3.6%)	3 (5.4%)
	17–25	56 (22.5%)	0	1 (1.8%)	2 (3.6%)	Referent
	26–35	50 (20.1%)	0	5 (10.0%)	2 (4.0%)	7 (14.0%)
	26–35	50 (20.1%)	0	5 (10.0%)	2 (4.0%)	OR = 2.9, CI = 0.7–12.1, p = 0.14
	36–65	109 (43.8%)	3 (2.8%)	1 (0.9%)	0	4 (3.7%)
	36–65	109 (43.8%)	3 (2.8%)	1 (0.9%)	0	OR = 0.7, CI = 0.1–3.2, p = 0.61
	>65	31 (12.4%)	2 (6.5%)	1 (3.2%)	0	3 (9.7%)
	>65	31 (12.4%)	2 (6.5%)	1 (3.2%)	0	OR = 1.9, CI = 0.3–10.3, p = 0.45
	Declined to state	3 (1.2%)	0	0	0	0
Ethnicity	African American	1 (0.4%)	0	0	0	0 (NM)
	Asian	6 (2.41%)	0	0	0	0
	Caucasian	176 (70.68%)	5 (2.8%)	4 (2.3%)	4 (2.3%)	13 (7.4%)
	Latino	16 (6.43%)	0	0	1 (6.3%)	1 (6.3%)
	Native American	39 (15.66%)	0	0	2 (5.1%)	2 (5.1%)
	Other	4 (1.61%)	0	0	0	0
	Declined to state	7 (2.81%)	0	0	1 (14.3%)	1 (14.3%)

NM, no model; logistic regression did not converge.

No seropositive individuals were identified in Santa Cruz County for any of the pathogens evaluated (Table 1). In contrast, seropositivity was detected for all three pathogens in Humboldt County, with the most prevalent being B. burgdorferi. Three of the North Valley residents were seropositive for A. phagocytophilum, but neither antibodies to B. burgdorferi nor Rickettsia spp. were detected. Humboldt County residents had a significantly four-fold elevated risk of exposure to any tick-borne pathogen.

A higher proportion of tested women were seropositive than men, although this difference was not significant (Table 1). Indeed, women comprised 80.0% and 75.0% of seropositive individuals for A. phagocytophilum and Rickettsia spp. respectively. Seroprevalence was highest among those aged 26–35 years although age was not a significant risk. Notably, however, all A. phagocytophilum seropositive individuals were 36 years old or older, the Rickettsia spp. seropositive individuals were aged 17–35 years, and B. burgdorferi antibodies were present in all age groups (Table 1). There were no significant differences in seropositivity among ethnic groups with prevalence from 5.1% to 7.4% in Caucasians, Latinos, and Native Americans. These three ethnic groups had evidence of seropositivity for Rickettsia spp., but A. phagocytophilum and B. burgdorferi antibodies were only detected in Caucasians (Table 1).

Questionnaire results in Humboldt County

Demographic data for the 174 Humboldt County residents who provided answers to questionnaires are provided in Table 2. Respondents were aged 17 to 65 years old with a relatively even sex distribution. Humboldt County residents had concerns related to tick-associated diseases, with almost 80% responding that they were very concerned or moderately concerned. The majority answered yes when asked if they had ever experienced a tick bite, and of those that had been bitten, 39.0% reported at least one symptom characteristic of tick-associated diseases, including skin rash or lesion, muscle ache, joint pain, fever, and headache (Fig. 1). Half of those who reported a skin lesion or rash selected the photograph of a local erythematous lesion as most closely resembling their symptom, followed in descending frequency by an eschar lesion, erythema migrans lesion, and petechial rash. Only 25.5% of individuals who experienced symptoms following a tick bite underwent testing for tick-borne pathogens.

FIG. 1.

Flow of questions related to tick and tick-borne pathogen exposure and answers given by 174 Humboldt County, California resident participants.

Table 2.

Demographic Characteristics and Results of Tick-Borne Pathogen Exposure (Serological) Screening in Residents of Humboldt County, California

	All respondents (n = 174)	Borrelia pos. (n = 8)	Anaplasmapos. (n = 2)	Rickettsia pos. (n = 4)	Any pathogen pos. (n = 14)
Age in years
18–25	38	1	0	2	3
26–35	45	5	0	2	7
36–65	73	1	2	0	3
65+	13	1	0	0	1
No response	5	0	0	0	0
Sex
Female	78	3	1	3	7
Male	83	4	1	1	6
No response	13	1	0	0	1
Median household income
Less than 25,000	73	4	0	3	7
25,000–49,999	40	3	2	1	6
50,000–99,999	30	0	0	0	0
100,000 and over	9	1	0	0	1
Prefer not to disclose	21	0	0	0	0
Education level
Some high school	9	0	0	0	0
High school graduate	22	1	0	0	1
Some college	58	2	1	1	4
Associates/Bachelors degree	57	4	1	2	7
Graduate/Professional degree	21	1	0	1	2
No response	7	0	0	0	0
Rural zip code
Yes	97	4	2	0	6
No	67	4	0	4	8
No response	10	0	0	0	0

After combining seroprevalence for all pathogens, the only significant questionnaire-reported activity that was associated with increased exposure to ticks was having a bird feeder in the yard (p = 0.036) (Table 3). For the outcome of the number of self-reported tick-bites that participants had received during their lifetime, the most parsimonious multivariable model included increasing age (p < 0.001), time spent bird or wildlife watching (p = 0.009), and time spent hiking (p = 0.028). Although time spent outdoors at work was not significant (p = 0.063), it was included in the base model to control for its effect.

Table 3.

Associations Between Predictor Variable and Outcome of (1) Being Seropositive for One of Three Tick-Borne Pathogens or (2) Number of Tick Bites Experienced Among Residents of Humboldt County, California Participating in Tick-Borne Disease Survey

						# tick bites experienced
		Seropositive for any tick-borne disease ^a		# tick bites experienced ^b Crude		Adjusted
Variable	Type	Beta ^c	p-value	Beta ^d	p-value	Beta ^e	p-value
Sex	Binary	−0.191	0.745	0.582	0.003	—	—
Direct contact with wildlife	Binary	0.829	0.188	0.619	0.002	—	—
Property in the woods	Binary	−0.930	0.170	0.440	0.025	—	—
Woodpile on property	Binary	−0.591	0.299	0.415	0.041	—	—
Bird feeder in yard	Binary	1.307	0.036^*	0.195	0.342	—	—
Own horses	Binary	−0.282	0.724	−0.154	0.566	—	—
Own dog	Binary	0.048	0.939	−0.123	0.578	—	—
Own cat	Binary	−0.536	0.362	0.432	0.026	—	—
Rural zip code	Binary	0.433	0.449	0.029	0.889	—	—
Work outdoors	Ordinal	−0.027	0.920	0.336	<0.001	0.175	0.063
Dog walking	Ordinal	0.252	0.208	−0.048	0.446	—	—
Gardening	Ordinal	0.158	0.485	0.077	0.315	—	—
Bird or wildlife watching	Ordinal	0.300	0.155	0.212	0.002	0.205	0.009
Outdoor sports	Ordinal	0.354	0.145	−0.067	0.421	—	—
Biking	Ordinal	0.047	0.847	−0.066	0.459	—	—
Hiking	Ordinal	0.175	0.484	0.275	<0.001	0.228	0.028
Fishing	Ordinal	−0.153	0.646	0.021	0.845	—	—
Camping	Ordinal	0.147	0.622	0.186	0.055	—	—
Hunting	Ordinal	−0.034	0.927	0.117	0.370	—	—
Income	Ordinal	−0.343	0.316	0.258	0.017	—	—
Education	Ordinal	0.229	0.436	0.148	0.117	—	—
Age	Ordinal	−0.346	0.266	0.385	<0.001	0.523	<0.001
Tick-bites experienced	Ordinal	−0.178	0.427	—	—	—	—

Participant seropositive for antibodies to Anaplasma phagocytophilum or Rickettsia sp. by indirect immunofluorescence assays or Borrelia burgdorferi by ELISA.

Number of self reported tick bites experienced in the participant's lifetime.

Coefficient based on univariate logistic regression.

Coefficient based on univariate linear regression.

Coefficient based on multivariable linear regression.

Indicates a statistically significant p-value.

Discussion

This study, one of the largest (249 participants) human serosurveys of the three tick-borne pathogens in California to date, found that more than 5% of the sampled population (spanning five counties) were seropositive for at least one tick-borne pathogen. Although coexposure to tick-borne pathogens has previously been reported [e.g., (Belongia 2002)], none of the subjects in our study was seropositive for more than one agent.

Risk of tick-borne pathogen exposure was highly variable by geography. Specifically, Humboldt County residents had significantly higher rates of exposure than other tested areas, as has been reported (Vector-Borne Disease Section 2014). Contributing factors include the proximity to biodiverse forests, coupled with high frequency of participation of local residents in activities that can increase risk of tick exposure, such as forestry work, fishing, camping, and hunting (Stephenson et al. 2017). Our data and other studies suggest that there is no association between gender and seropositivity to the tested pathogens [e.g., (Grzeszczuk et al. 2004, Krause et al. 2014)]. However, males have historically been considered at greater risk of exposure to tick-borne pathogens given their association with activities that increase contact with ticks, such as forestry work, fishing, and hunting (Meldrum et al. 1992). The notion of gender-associated behaviors may well be based on overgeneralizations. Furthermore, behaviors such as gardening and dog walking also pose risk of tick-borne pathogen exposure and are likely shared by both sexes. All tested age groups showed some exposure to a tick-borne pathogen, with the highest prevalence in ages 26–35 years consistent with high participation in outdoor activities, sports, and travel.

When residents in Humboldt County were questioned regarding their tick bite history and subsequent symptoms, more than a third of bitten individuals reported experiencing symptoms. In prior work, 27% of tick-borne pathogen seropositive individuals in Northern California reported symptoms (Fritz et al. 1997), as did 15% of A. phagocytophilum seropositive individuals in New York (Aguero-Rosenfeld 2002). A poor match between bite histories, symptoms, and seroprevalence could occur because of underdiagnosis of infection, seropositive individuals lacking symptoms, serologic cross-reactivity with other pathogens, and low rates of infection (and seroconversion) even after tick bites.

Almost half of the respondents who experienced a skin lesion described it as local erythema based on photographs, while 17% described an erythema migrans lesion, which is often associated with B. burgdorferi infection. Eschar lesions and petechial rashes, which can be indicative of Rickettsia spp. infections (Chaudhry et al. 2009), were identified less commonly. Other reported symptoms such as headache and muscle ache were nonspecific to the three pathogens. Given that respondents commonly indicated a high level of concern related to tick-associated diseases, it is probable that some nonspecific symptoms were inaccurately ascribed to infection following a tick bite. Self-reported tick bites and seroreactivity, at least for Babesia, have been found not to be correlated (Leiby et al. 2002).

Only a quarter of people with tick bites underwent diagnostic testing for a tick-borne pathogen by a physician. Many people do not seek medical attention despite risk. Factors that may contribute to that decision include both individual perception of risk as well as the myriad of barriers to health care access (e.g., financial constraints and proximity to a clinic). Medical professionals may have elected not to perform testing if they did not suspect a tick-borne illness or chose to treat for infection with a tick-borne pathogen empirically without testing. The findings highlight the need for education to raise awareness regarding tick-borne illnesses in Northern California.

“Having a bird feeder” was the only significant association with seropositivity for one of the three pathogens. When we used number of tick bites as the outcome of a statistical model, we found that increasing age, time spent observing birds or wildlife, and time spent hiking significantly increased a participant's risk. Having a bird feeder may be a proxy for nature and wildlife enthusiasts and not the actual exposure activity, although western gray squirrels may be attracted to these feeders, and they are the reservoir for B. burgdorferi in California (Lane et al. 2005). Notably, both occupational and recreational activities put participants at risk of tick bites and exposure to tick-borne pathogens. This is in contrast to findings in the northeast, which show that most exposure occurs at a person's residence (Eisen and Eisen 2016).

This study does have some important limitations. Serological assays detect exposure that may have happened over months but do not confirm active infection and may not be species-specific (Bakken et al. 2006). Rickettsial serology can detect any of the closely related spotted-fever group rickettsias, R. felis, or even typhus group rickettsia (Kaplan and Schonberger 1986, La Scola and Raoult 1997). R. rickettsii infection is rare in California, while R. felis has been documented in cat fleas (Ctenocephalides felis) infesting dogs and various flea species from wildlife in Humboldt County (Stephenson et al. 2017). R. montanensis and R. rhipicephali have been found in Dermacentor spp. ticks in Humboldt County and R. philipii was documented in a nearby county (Shapiro et al. 2010, Parola et al. 2013). Similarly, genospecies within the B. burgdorferi sensu lato complex exist in California such as B. bissettiae and B. burgdorferi sensu stricto; their pathogenicity and cross-reactivity on the C6 ELISA are not well understood (Rudenko et al. 2011, Stephenson and Foley 2016). In addition, although Lyme borreliosis diagnostics are fraught with inadequacies in both sensitivity and specificity, the C6 ELISA has better sensitivity and comparable specificity to prior 2-tier testing (Wormser et al. 2013). Geographical analysis cannot confirm origin of exposure as travel history was not obtained from study participants. The study populations from Humboldt and the other counties may differ given that most were blood bank donors whereas Humboldt County residents agreed to participate in a research study. Finally, as the seroprevalences of pathogen antibodies were low even when combined, the analyses may suffer from type 2 error.

Conclusions

Our data on risk factors and antibody prevalence for three medically important tick-borne pathogens in 259 northern Californians provide an invaluable baseline for ongoing studies of patterns in epidemiology. Without an adequate index of risk, efforts to prevent and treat infection are inhibited. Infection with tick-borne pathogens can to some extent be avoided, for example, using personal protection such as wearing long pants, and disease is best resolved when treated rapidly after onset of symptoms (O'reilly et al. 2003, Halperin 2015). In the west, exposure is patchily distributed and multiple outside activities in higher risk areas serve to expose people to ticks. Future research should continue to focus on rural northern California; data from such studies can help inform when residents should seek medical care, what to do in the event of a tick bite, and when diagnostic testing is warranted.

Footnotes

Acknowledgments

We thank the UC Davis student chapter of the Humane Society Veterinary Medical Association, Michael Clifford, undergraduate volunteers of the Foley laboratory, the Humboldt State student chapter of the western section of The Wildlife Society, the Greater Rural Rescue Society, the Hoopa Valley Tribe, Aaron Pole, Deborah Giraud, Anne Kjemtrup, Jennifer McQuiston, and Kerry Padgett. JEF and ELP acknowledge funding from the Pacific Southwest Regional Center of Excellence for Vector-Borne Diseases funded by the U.S. Centers for Disease Control and Prevention (Cooperative Agreement 1U01CK000516) and the UC Davis School of Veterinary Medicine. This study was also supported by the UC Davis Center for Companion Animal Health, John Muir Institute for the Environment, the Center for Vector-borne Diseases, and LymeDisease.org.

Author Disclosure Statement

No competing financial interests exist.

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