Rickettsia spp. Infecting Lone Star Ticks (Amblyomma americanum) (Acari: Ixodidae) in Monmouth County, New Jersey

Tick-Borne Co-Infections: Challenges in Molecular and Serologic Diagnoses

Co-infections are a poorly understood aspect of tick-borne diseases. In the United States alone, nineteen different tick-borne pathogens have been identified. The majority of these agents are transmitted by only two tick species, Ixodes scapularis and Amblyomma americanum. Surveillance studies have demonstrated the presence of multiple pathogens in individual ticks suggesting a risk of polymicrobial transmission to humans. However, relatively few studies have explored this relationship and its impact on human disease. One of the key factors for this deficiency are the intrinsic limitations associated with molecular and serologic assays employed for the diagnosis of tick-borne diseases. Limitations in the sensitivity, specificity and most importantly, the capacity for inclusion of multiple agents within a single assay represent the primary challenges for the accurate detection of polymicrobial tick-borne infections. This review will focus on outlining these limitations and discuss potential solutions for the enhanced diagnosis of tick-borne co-infections.

Seasonality of Amblyomma americanum (Acari: Ixodidae) Activity and Prevalence of Infection with Tick-Borne Disease Agents in North Central Oklahoma

Background: Amblyomma americanum is the most common tick infesting both animals and humans in the southern United States and transmits a variety of zoonotic agents. The rise in tick-borne diseases (TBD) globally imparts a need for more active surveillance of tick populations to accurately quantify prevalence and risk of tick-borne infectious organisms. To better understand TBD risk in north central Oklahoma, this study aimed to describe the current seasonal activity of A. americanum in this region and investigate the seasonality of tick-borne infectious agents. Materials and Methods: Tick collections were performed twice a month for a duration of 2 years at a field site in Payne County, Oklahoma. Total nucleic acid was extracted from a subset of adult A. americanum and tested for Rickettsia spp., Ehrlichia spp., and Borrelia spp. using established PCR protocols. Results: Peak activity times for each life stage were observed, with adults primarily active 1 month earlier than historical seasonal trends describe, and male A. americanum active earlier in the year than female A. americanum. Rickettsia spp., Ehrlichia chaffeensis, Ehrlichia ewingii, and Borrelia lonestari were found in 26.4%, 6.1%, 2.5%, and 1.1% of adult A. americanum, respectively. No seasonal trend in spotted fever group Rickettsia spp. (SFGR) was observed in peak activity months. Conclusions: This study found an apparently shifting phenology for A. americanum adults in Oklahoma. While these results did not show a trend in SFGR, further investigation is needed to better understand the potential seasonality of infection prevalence within A. americanum across the expanding range of this vector, especially considering the extended activity of males in winter months.

First Report of Multiple Rickettsia sp., Anaplasma sp., and Ehrlichia sp. in the San Miguel Department of El Salvador from Zoonotic Tick Vectors

Neglected bacterial zoonoses are a group of Neglected Tropical Diseases (NTDs) that are commonly underdiagnosed and underreported due to their undifferentiated febrile illness symptomology. Spotted fever group rickettsioses (SFGR), a subset of tick-borne bacterial zoonoses, belong in this group. There is a dichotomy in the reporting and recognition of these pathogens in Central America: countries with reduced human development scores-like El Salvador-have little to no research or surveillance dedicated to these pathogens and the diseases they cause. This was the third-ever tick survey in El Salvador, highlighting the knowledge gap in this country. A total of 253 ticks were collected from 11 animals at two farm sites and one veterinary office. Standard and quantitative PCR were used to detect presence of SFGR, Ehrlichia, and Anaplasma sp. pathogens in ticks. Ehrlichia sp. were detected in 2.4% of all collected ticks and Anaplasma sp. were detected in 5.5% of all ticks. Rickettsia rickettsii was amplified in 18.2% of ticks, and amplicons similar to R. parkeri, and R. felis were found in 0.8% and 0.4%, of collected ticks, respectively. This is the first report of these pathogenic bacterial species in El Salvador. This study emphasizes the need for further surveillance and research including incorporating additional human seroprevalence and testing to understand the public health burden in this country.

Tick abundance and diversity are substantially lower in thinned vs. unthinned forests in the New Jersey Pinelands National Reserve, USA

Forest thinning is a management tool used in the New Jersey Pinelands and elsewhere to improve forest health and resilience, mitigate wildfire risk, and manage for wildlife. Forest thinning leads to warmer drier microclimates, which have been shown in both field and laboratory studies to reduce tick survival and reproduction. To directly assess the effects of forest thinning on the abundance and diversity of ticks and on the prevalence of tick-borne human pathogens, we sampled ticks weekly from March to November 2021 at three replicated pairs of thinned and unthinned forest sites composed primarily of pitch-pine, shortleaf pine, and various oak species. We characterized microclimate in the understory and forest floor at each sampling plot by deploying multiple data loggers to monitor temperature and relative humidity throughout the study period. As expected, we found that thinned plots were significantly drier and warmer than unthinned plots. We also found that average questing tick abundance was 92% lower in thinned as compared with unthinned plots. Of the three main tick species collected in unthinned plots (Amblyomma americanum, Ixodes scapularis, and Dermacentor albipictus) only A. americanum and a single I. scapularis were collected in thinned plots. Prevalence of Ehrlichia species in A. americanum did not differ between treatments, and the sole I. scapularis collected in a thinned plot was infected with Borrelia burgdorferi sensu lato. However, the significant and much lower tick abundance in thinned plots indicates a lower risk of human-tick encounters. Our results add to the growing evidence that landscape and forest management can reduce local tick abundance, thereby reducing tick-borne disease risk.

Rickettsia amblyommatis in Ticks: A Review of Distribution, Pathogenicity, and Diversity

Rickettsia amblyommatis is a potentially pathogenic species of Rickettsia within the spotted fever group vectored by ticks. While many studies have been published on this species, there is debate over its pathogenicity and the inhibitory role it plays in diagnosing illnesses caused by other spotted fever group Rickettsia species. Many publications have recorded the high infection prevalence of R. amblyommatis in tick populations at a global scale. While this species is rather ubiquitous, questions remain over the epidemiological importance of this possible human pathogen. With tick-borne diseases on the rise, understanding the exact role that R. amblyommatis plays as a pathogen and inhibitor of infection relative to other tick-borne pathogens will help public health efforts. The goal of this review was to compile the known literature on R. amblyommatis, review what we know about its geographic distribution, tick vectors, and pathogenicity, assess relatedness between various international strains from ticks by phylogenetic analysis and draw conclusions regarding future research needed.

The Gulf Coast Tick, Amblyomma maculatum (Ixodida: Ixodidae), and Spotted Fever Group Rickettsia in the Highly Urbanized Northeastern United States

We report the multi-year collection of the Gulf Coast tick, Amblyomma maculatum Koch (Acaridae: Ixodida: Ixodidae) in Staten Island, New York City (NYC) as well as their detection in Brooklyn, NYC, and in Atlantic and Cumberland counties in southern New Jersey, USA. The first and most common detections were of adults, however in Freshkills Park on Staten Island larvae were also collected in a following year. The presence of larvae indicates that adults are successfully finding hosts in Staten Island. While it is still unknown how A. americanum reached Staten Island, immatures of this species often parasitize migratory birds, which are now often seen in Freshkills Park. We describe the landscape features of the area in Staten Island where populations were highest and larvae were detected, which could have facilitated the establishment of A. maculatum. Notably, we also report the presence of human pathogens Rickettsia parkeri in 5/10 (50%) of adults tested and R. felis in 1/24 (4.17%) of larvae tested. In addition to established populations in Staten Island we found evidence of A. maculatum in NJ and other NYC boroughs, suggesting current or future establishment is possible. The failure thus far to detect established populations in these areas may be due to inherent difficulties in detecting low density, spatially heterogeneous incipient populations, which could require targeted surveillance efforts for this species. We discuss the consequences to public health of the establishment of A. maculatum and detection of two additional rickettsial pathogens in the densely populated northeastern United States.

Relevance of Spatial and Temporal Trends in Nymphal Tick Density and Infection Prevalence for Public Health and Surveillance Practice in Long-Term Endemic Areas: A Case Study in Monmouth County, NJ

Tick-borne diseases are a growing public health problem in the United States, and the US northeast has reported consistently high case rates for decades. Monmouth County, New Jersey, was one of the earliest jurisdictions to report Lyme disease cases in 1979 and reports several hundred cases per year nearly 40 yr later. In the time since, however, tick-borne health risks have expanded far beyond Lyme disease to include a variety of other bacterial pathogens and viruses, and additional vectors, necessitating a continually evolving approach to tick surveillance. In 2017, Monmouth County initiated an active surveillance program targeting sites across three ecological regions for collection of Ixodes scapularis Say (Acari: Ixodidae) and Amblyomma americanum L. (Acari: Ixodidae) as well as testing via qPCR for associated bacterial pathogens. During the first five years of this program (2017-2021), we report high levels of spatiotemporal variability in nymphal density and infection prevalence in both species, limiting the granularity with which human risk can be predicted from acarological data. Nonetheless, broader patterns emerged, including an ongoing trend of A. americanum dominance, risks posed by Borrelia miyamotoi, and the frequency of coinfected ticks. We present some of the first county-level, systematic surveillance of nymphal A. americanum density and infection prevalence in the northeastern US. We also documented a temporary decline in Borrelia burgdorferi that could relate to unmeasured trends in reservoir host populations. We discuss the implications of our findings for tick-borne disease ecology, public health communication, and tick surveillance strategies in endemic areas.

Analyses of Bloodmeal Hosts and Prevalence of Rickettsia parkeri in the Gulf Coast Tick Amblyomma maculatum (Acari: Ixodidae) From a Reconstructed Piedmont Prairie Ecosystem, North Carolina

Host feeding patterns and the prevalence of infection with Rickettsia parkeri were determined for the primary vector, Amblyomma maculatum Koch as well as sympatric tick species A. americanum (Linnaeus) and Dermacentor variabilis (Say) collected from a reconstructed prairie in the Piedmont region of North Carolina during 2011 and 2012. The occurrence of R. parkeri among A. maculatum adults and nymphs was 36.9% (45/122) and 33.3% (2/6), respectively. Rickettsia parkeri was detected in a single male A. americanum 2.3% (1/43). A PCR-reverse line blot hybridization assay of a 12S rDNA fragment amplified from remnant larval and nymphal bloodmeals of host-seeking ticks was used to identify bloodmeal hosts. Of the tick samples tested, bloodmeal host identification was successful for 29.3% (12/41) of adult A. americanum and 39.2% (20/51) of adult D. variabilis. For A. maculatum, bloodmeal host identification was successful for 50% (61/122) of adults collected from vegetation and 100% (4/4) of nymphs removed from cotton rats (Sigmodon hispidus Say and Ord). The cotton rat was the most common bloodmeal host with 59.0% (36/61) identified for adult A. maculatum. No statistically significant association was observed, however, between bloodmeal host and pathogen prevalence for any tick species. While the cotton rat was an important bloodmeal host for A. maculatum nymphs, this vertebrate did not appear to be the primary source of R. parkeri infection for A. maculatum.

Utility of ultra-rapid real-time PCR for detection and prevalence of Rickettsia spp. in ticks

Background

Rickettsia spp. are important tick-borne pathogens that cause various human and animal diseases worldwide. A tool for rapid and accurate detection of the pathogens from its vectors is necessary for prevention of Rickettsioses propagation in humans and animals, which are infested by ticks. Therefore, this study was conducted to evaluate a molecular tool, ultra-rapid real-time PCR (UR-qPCR), for rapid and accurate detection of Rickettsia spp. from 5644 ticks in 408 pools collected from livestock and their surrounding environments in Gangwon and Jeju province in South Korea.

Results

The UR-qPCR of Rickettsia DNA showed a limit of detection of 2.72 × 10¹ copies of Rickettsia DNA and no cross reaction with other tick-borne pathogens, namely Anaplasma phagocytophilum, Ehrlichia chaffeensis, E. canis, Toxoplasma gondii, and Borrelia burgdorferi. In addition, the PCR assay also showed possibility of various Rickettsia species detection including R. monacensis, “Candidatus R. longicornii”, R. japonica, R. roultii, and R. tamurae. The collected ticks were identified with major species belonged to Haemaphysalis longicornis (81.62%), followed by H. flava (15.19%), and Ixodes nipponensis (3.19%). Rickettsia detection from tick samples using the UR-qPCR showed that the minimum infection rate (MIR) of Rickettsia in collected ticks was 1.24‰ and that all positive pools contained H. longicornis, equal to the MIR of 1.39‰ of this species. Additionally, MIR of Rickettsia spp. detected in ticks collected in Gangwon and Jeju was 1.53‰ and 0.84‰, respectively. Furthermore, the sequencing results of the 17 kDa protein antigen gene and ompA gene showed that Rickettsia spp. sequences from all pools were related to “Candidatus R. longicornii” and “Candidatus R. jingxinensis”.

Conclusions

The UR-qPCR system was demonstrated to be useful tool for accurate and rapid detection of Rickettsia from its vector, ixodid ticks, within 20 min. The data on Rickettsia spp. in ticks detected in this study provide useful information on the distribution of Rickettsia in previously unstudied Korean provinces, which are important for the prevention and control of the spread of rickettsioses in both animals and humans in the country.

Association between Growth Rate and Pathogenicity of Spotted Fever Group Rickettsia

Rickettsia parkeri and Rickettsia amblyommatis are spotted fever group Rickettsia (SFGR) associated with Amblyomma ticks. R. parkeri is a recognized human pathogen that causes an eschar-associated febrile illness, while R. amblyommatis has not been confirmed as a causative agent of human disease. We hypothesized that the rate of replication is one of the factors contributing to rickettsial pathogenicity. In this study, growth and infectivity of R. parkeri and R. amblyommatis in mammalian (Vero E6) and tick-derived (ISE6) cell lines were assessed and compared over a 96-hour time course of infection using quantitative real-time polymerase chain reaction and microscopy. The pathogenic R. parkeri displayed a significantly higher level of infection in both Vero E6 and ISE6 cells than R. amblyommatis at 72 hours post-inoculation (hpi). Distinct growth profiles between rickettsial species with known and uncertain pathogenicity were identified. R. parkeri burdens were significantly greater than those of R. amblyommatis from 24 to 96 hpi. The relative fold changes of load were significantly higher in the pathogenic agent than in R. amblyommatis from 48 hpi onward and reached the maximum fold increase of ~2002- and ~296-fold in Vero E6 cells and ~1363- and ~161-fold in ISE6 cells, respectively, at 96 hpi. The results from the present study demonstrate that growth rate is associated with the pathogenicity of rickettsiae. Understanding SFGR growth characteristics in mammalian and tick cells will provide insight into rickettsial biology and pathogenesis.

Highlights of Medical Entomology, 2020

Medical Entomology as a field is inherently global - thriving on international and interdisciplinary collaborations and affected dramatically by arthropod and pathogen invasions and introductions. This past year also will be remembered as the year in which the SARS-CoV-2 COVID-19 pandemic affected every part of our lives and professional activities and impacted (or changed, sometimes in good ways) our ability to collaborate and detect or respond to invasions. This incredible year is the backdrop for the 2020 Highlights in Medical Entomology. This article highlights the broad scope of approaches and disciplines represented in the 2020 published literature, ranging from sensory and chemical ecology, population genetics, impacts of human-mediated environmental change on vector ecology, life history and the evolution of vector behaviors, to the latest developments in vector surveillance and control.

Lone Star Ticks (Amblyomma americanum):: An Emerging Threat in Delaware

Public health messaging in the eastern United States has historically underemphasized the risks posed by lone star ticks (Amblyomma americanum), focusing instead on blacklegged ticks (Ixodes scapularis). This gap persists despite mounting evidence that lone star ticks also play an important role in disease ecology as confirmed vectors for a wide variety of tick-borne pathogens. These pathogens include several distinct bacterial agents that cause ehrlichiosis and tularensis in humans and dogs, a protozoal agent that causes cytauxzoonosis in cats, and emerging viruses such as Heartland, Bourbon, and Tacaribe. Lone star ticks are additionally linked to Rocky Mountain spotted fever, southern tick-associated rash illness, and alpha-gal syndrome, a condition marked by immune reactions to ingestion of mammalian meat. Moreover, their distribution in North America is expanding due to changing climatic factors and land use patterns. Lone star ticks are the most commonly encountered tick in Delaware, especially in Sussex and Kent Counties, and make up the vast majority of ticks collected in the first two years of the state’s tick surveillance program. Given the magnitude of lone star ticks’ medical and veterinary import, it is vital for healthcare professionals and health educators to devote more attention to this emerging threat.

Detection of Rickettsia Species, and Coxiella-Like and Francisella-Like Endosymbionts in Amblyomma americanum and Amblyomma maculatum from a Shared Field Site in Georgia, United States of America

Two abundant species of aggressive ticks commonly feed on humans in Georgia: the Gulf Coast tick (Amblyomma maculatum) and the Lone Star tick (A. americanum). A. maculatum is the primary host of Rickettsia parkeri, "Candidatus Rickettsia andeanae," and a Francisella-like endosymbiont (AmacFLE), whereas A. americanum is the primary host for R. amblyommatis, Ehrlichia chaffeensis, E. ewingii, and a Coxiella-like endosymbiont (AamCLE). Horizontal transmission of R. parkeri from A. maculatum to A. americanum by co-feeding has been described, and R. amblyommatis has been found infrequently in A. maculatum ticks. We assessed the prevalence of these agents and whether exchange of tick-associated bacteria is common between A. maculatum and A. americanum collected from the same field site. Unengorged ticks were collected May-August 2014 in west-central Georgia from a 4.14 acre site by flagging and from humans and canines traversing that site. All DNA samples were screened with quantitative PCR assays for the bacteria found in both ticks, and the species of any Rickettsia detected was identified by species-specific TaqMan assays or sequencing of the rickettsial ompA gene. Only R. amblyommatis (15) and AamCLE (39) were detected in 40 A. americanum, while the 74 A. maculatum only contained R. parkeri (30), "Candidatus Rickettsia andeanae" (3), and AmacFLE (74). Neither tick species had either Ehrlichia species. Consequently, we obtained no evidence for the frequent exchange of these tick-borne agents in a natural setting despite high levels of carriage of each agent and the common observance of infestation of both ticks on both dogs and humans at this site. Based on these data, exchange of these Rickettsia, Coxiella, and Francisella agents between A. maculatum and A. americanum appears to be an infrequent event.

New Jersey-Wide Survey of Spotted Fever Group Rickettsia (Proteobacteria: Rickettsiaceae) in Dermacentor variabilis and Amblyomma americanum (Acari: Ixodida: Ixodidae)

For the last decade, the New Jersey (NJ) Department of Health has reported between 42 and 144 new cases each year of "spotted fever group rickettsiosis" (SFGR), a statistic that reflects uncertainty regarding which rickettsial agents (Proteobacteria: Rickettsiaceae: Rickettsia) are infecting NJ residents. To identify the Rickettsia circulating in NJ ticks, we used a combination of conventional and real time PCR approaches to screen 560 Dermacentor variabilis Say and 245 Amblyomma americanum L. obtained from a 1-day state-wide surveillance in May 2018 and an additional 394 D. variabilis collected across NJ in 2013-2018. We found zero D. variabilis infected with Rickettsia rickettsii, the agent of Rocky Mountain spotted fever and, on average, 1.3% infected with presumed nonpathogenic Rickettsia montanensis. We also found zero A. americanum infected with R. rickettsii, and 20% infected with Rickettsia amblyommatis, a prevalence somewhat lower than in more southern states. Overall, we conclude that it is unlikely that R. rickettsii vectored by D. variabilis is a primary cause of SFGR cases in NJ and discuss our findings in the context of known facts and current limitations. We conclude that understanding the causes of SFGR east of the Mississippi will require collaboration among medical doctors, public health authorities, and medical entomologists to follow up presumptive human cases of SFGR with detailed histories of exposure, species-specific molecular assays, and active surveillance of putative vectors and the pathogens they may carry.