Rocky Mountain spotted fever (RMSF) is a tickborne disease endemic in areas of the Americas. Persistent high incidence of the disease exists in northern Mexico, perpetuated by local populations of brown dog ticks (Rhipicephalus sanguineus sensu lato) and free-roaming dogs. Six cases of RMSF caused by Rickettsia rickettsii, including three deaths, were reported to the California Department of Public Health during July 2023-January 2024. All six patients were eventually determined to have had exposure to R. rickettsii in Tecate, Mexico, a municipality on the U.S. border that had not been previously described as a high-risk RMSF area. Identification and reporting of the cases were complicated by challenges in diagnosis. The serious nature of the disease and delays in initiating appropriate treatment can result in life-threatening consequences. Epidemiologic collaborations among local, state, federal, and international public health agencies were essential to identifying Tecate as the location of exposure. Further collaborations will be important for directing future prevention measures. Increased health care provider awareness of RMSF is critical on both sides of the border to facilitate earlier diagnosis and initiation of appropriate treatment.

Amblyomma maculatum sensu lato (s.l.) is an ixodid tick found in the semi-arid southwestern United States and northern Mexico where it is a parasite of medical and veterinary significance, including as a vector for Rickettsia parkeri, a cause of spotted fever rickettsiosis in the Americas. To describe the comprehensive natural history of this tick, monthly small mammal trapping and avian mist netting sessions were conducted at sites in Cochise County Arizona, within the Madrean Archipelago region where human cases of R. parkeri rickettsiosis and adult stages of A. maculatum s.l. were previously documented. A total of 1949 larvae and nymphs were removed from nine taxonomic groups of rodents and ten species of birds and were used in combination with records for adult stages collected both from vegetation and hunter-harvested animals to model seasonal activity patterns. A univoltine phenology was observed, initiated by the onset of the annual North American monsoon and ceasing during the hot, dry conditions preceding the following monsoon season. Cotton rats (Sigmodon spp.) were significantly more likely to be infested than other rodent taxa and carried the highest tick loads, reflecting a mutual affinity of host and ectoparasite for microhabitats dominated by grass.

Rocky Mountain spotted fever (RMSF) is an ongoing public health crisis in Mexico, particularly in states bordering the United States. The national highest incidence and mortality of RMSF occur in this region, resulting in a case-fatality rate that ranges annually between 10% and 50%, primarily affecting vulnerable groups such as children, elderly adults, and persons living in poverty. Multiple biological, environmental, and social determinants can explain its growing presence throughout the country and how it challenges the health system and society. It is necessary to integrate resources and capacities from health authorities, research centers, and society to succeed in dealing with this problem. Through a scientific symposium, a group of academicians, U.S. health officials, and Mexican health authorities met on November 8-10, 2023, in Hermosillo, Mexico, to discuss the current situation of RMSF across the country and the challenges associated with its occurrence. An urgent call for action to improve national capacity against RMSF in the aspects of epidemiological and acarological surveillance, diagnosis, medical care, case and outbreak prevention, health promotion, and research was urged by the experts. The One Health approach is a proven multidisciplinary strategy to integrate policies and interventions to mitigate and prevent the burden of cases, deaths, and suffering caused by RMSF in Mexico.

Rocky Mountain spotted fever (RMSF) is a severe tickborne disease that can reach epidemic proportions in communities with certain social and ecologic risk factors. In some areas, the case-fatality rate of brown dog tick-associated RMSF is up to 50%. Because of the spread of brown dog tick-associated RMSF in the southwestern United States and northern Mexico, the disease has the potential to emerge and become endemic in other communities that have large populations of free-roaming dogs, brown dog ticks, limited resources, and low provider awareness of the disease. By using a One Health approach, interdisciplinary teams can identify communities at risk and prevent severe or fatal RMSF in humans before cases occur. We have developed a conceptual framework for RMSF prevention to enable communities to identify their RMSF risk level and implement prevention and control strategies.

The etiologic agent of Pacific Coast tick fever (PCTF), a moderately severe tickborne illness that resembles Rocky Mountain spotted fever (RMSF), was first isolated in 1966 from specimens of Dermacentor occidentalis (the Pacific Coast tick) obtained in California. For several decades, this bacterium was identified ambiguously as the unclassified spotted fever group Rickettsia species 364-D, Rickettsia 364, or Rickettsia philipii. However, none of these epithets satisfied criteria of formal bacterial nomenclature. Data developed from mouse serotyping studies performed 45 years ago, and multi-locus sequence typing several decades later, indicated that this bacterium was similar to, but distinct from isolates of Rickettsia rickettsii, the etiological agent of RMSF. We applied an integrative taxonomic approach, combining phenotypic, ecological, and clinical data with whole genome sequencing of 11 contemporary isolates of this pathogen to identify it as a distinct subspecies of R. rickettsii, and propose the name Rickettsia rickettsii subsp. californica subsp. nov.

We document a case of Rickettsia parkeri rickettsiosis in a patient in Connecticut, USA, who became ill after a bite from a Gulf Coast tick (Amblyomma maculatum). We used PCR to amplify R. parkeri DNA from the detached tick. The patient showed a 4-fold rise in IgG reactive with R. parkeri antigens.

Hard ticks (Acari: Ixodidae) have been historically identified by morphological methods which require highly specialized expertise and more recently by DNA-based molecular assays that involve high costs. Although both approaches provide complementary data for tick identification, each method has limitations which restrict their use on large-scale settings such as regional or national tick surveillance programs. To overcome those obstacles, the matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) has been introduced as a cost-efficient method for the identification of various organisms, as it balances performance, speed, and high data output. Here we describe the use of this technology to validate the distinction of two closely related Dermacentor tick species based on the development of the first nationwide MALDI-TOF MS reference database described to date. The dataset obtained from this protein-based approach confirms that tick specimens collected from United States regions west of the Rocky Mountains and identified previously as Dermacentor variabilis are the recently described species, Dermacentor similis. Therefore, we propose that this integrative taxonomic tool can facilitate vector and vector-borne pathogen surveillance programs in the United States and elsewhere.

Rocky Mountain spotted fever (RMSF), a severe and extraordinarily lethal infectious disease, has emerged as a widespread public health crisis among predominantly vulnerable populations in several countries of Latin America, particularly evident in northern Mexico. Historically, RMSF has gained less attention than many other tropical infectious diseases, resulting in insufficient allocations of resources and development of capabilities for its prevention and control in endemic regions. We argue that RMSF fulfills accepted criteria for a neglected tropical disease (NTD). The relative neglect of RMSF in most Latin American countries contributes to disparities in morbidity and mortality witnessed in this region. By recognizing RMSF as an NTD, an increased public policy interest, equitable and more appropriate allocation of resources, scientific interest, and social participation can ameliorate the impact of this potentially treatable disease, particularly in vulnerable populations.

Spotted fever rickettsiosis is rarely observed in solid organ transplant recipients, and all previously reported cases have been associated with tick bite months to years after transplantation. We describe a kidney transplant recipient in North Carolina, USA, who had a moderately severe Rickettsia parkeri infection develop during the immediate posttransplant period.

We describe 5 children who had Rocky Mountain spotted fever (RMSF) and manifested clinical symptoms similar to multisystem inflammatory syndrome in Sonora, Mexico, where RMSF is hyperendemic. Physicians should consider RMSF in differential diagnoses of hospitalized patients with multisystem inflammatory syndrome to prevent illness and death caused by rickettsial disease.

Leishmaniasis is an important travel-related parasitic infection in the United States. Treatment regimens vary by Leishmania species and require an accurate diagnosis. The sensitivity and specificity of diagnostic methods depend on the type and condition of specimen analyzed. To identify the best algorithm for detection of parasites in fresh and fixed tissue samples, we evaluated parasite cultures, two PCR methods, and Leishmania immunohistochemistry (IHC) in samples received by the CDC from 2012 through 2019. The sensitivity and specificity of IHC assays were evaluated in fresh specimens tested. Diagnostic accuracy for formalin-fixed tissue was evaluated by using PCR-based methods and IHC. Of 100 suspected cases with fresh tissue available, Leishmania spp. infection was identified by PCR in 56% (56/100) of specimens; from these, 80% (45/56) were positive by parasite culture and 59% (33/56) by IHC. Of 420 possible cases where only fixed specimens were available, 58% (244/420) were positive by IHC and/or PCR. Of these, 96% (235/420) were positive by IHC and 84% (204/420) by PCR-based methods. Overall parasite detection using all methodologies was similar for fresh and formalin-fixed tissue specimens (56% versus 58%, respectively). Although PCR-based methods were superior for diagnosis of leishmaniasis and species identification in fresh samples, IHC in combination with PCR increased the accuracy for Leishmania spp. detection in fixed samples. In conclusion, PCR is the most effective method for detecting Leishmania infection in fresh tissue samples, whereas for formalin-fixed samples, IHC and PCR-based methods should be used in combination.

The brown dog tick, Rhipicephalus sanguineus sensu lato (s.l.), is an important vector for Rickettsia rickettsii, causative agent of Rocky Mountain spotted fever. Current public health prevention and control efforts to protect people involve preventing tick infestations on domestic animals and in and around houses. Primary prevention tools rely on acaricides, often synthetic pyrethroids (SPs); resistance to this chemical class is widespread in ticks and other arthropods. Rhipicephalus sanguineus s.l. is a complex that likely contains multiple unique species and although the distribution of this complex is global, there are differences in morphology, ecology, and perhaps vector competence among these major lineages. Two major lineages within Rh. sanguineus s.l., commonly referred to as temperate and tropical, have been documented from multiple locations in North America, but are thought to occupy different ecological niches. To evaluate potential acaricide resistance and better define the distributions of the tropical and temperate lineages throughout the US and in northern Mexico, we employed a highly multiplexed amplicon sequencing approach to characterize sequence diversity at: 1) three loci within the voltage-gated sodium channel (VGSC) gene, which contains numerous genetic mutations associated with resistance to SPs; 2) a region of the gamma-aminobutyric acid-gated chloride channel gene (GABA-Cl) containing several mutations associated with dieldrin/fipronil resistance in other species; and 3) three mitochondrial genes (COI, 12S, and 16S). We utilized a geographically diverse set of Rh sanguineus s.l. collected from domestic pets in the US in 2013 and a smaller set of ticks collected from canines in Baja California, Mexico in 2021. We determined that a single nucleotide polymorphism (T2134C) in domain III segment 6 of the VGSC, which has previously been associated with SP resistance in Rh. sanguineus s.l., was widespread and abundant in tropical lineage ticks (>50 %) but absent from the temperate lineage, suggesting that resistance to SPs may be common in the tropical lineage. We found evidence of multiple copies of GABA-Cl in ticks from both lineages, with some copies containing mutations associated with fipronil resistance in other species, but the effects of these patterns on fipronil resistance in Rh. sanguineus s.l. are currently unknown. The tropical lineage was abundant and geographically widespread, accounting for 79 % of analyzed ticks and present at 13/14 collection sites. The temperate and tropical lineages co-occurred in four US states, and as far north as New York. None of the ticks we examined were positive for Rickettsia rickettsii or Rickettsia massiliae.

The Pacific Coast tick (Dermacentor occidentalis Marx, 1892) is a frequently encountered and commonly reported human-biting tick species that has been recorded from most of California and parts of southwestern Oregon, southcentral Washington, and northwestern Mexico. Although previous investigators have surveyed populations of D. occidentalis for the presence of Rickettsia species across several regions of California, populations of this tick have not been surveyed heretofore for rickettsiae from Baja California, Oregon, or Washington. We evaluated 1,367 host-seeking, D. occidentalis adults collected from 2015 to 2022 by flagging vegetation at multiple sites in Baja California, Mexico, and Oregon and Washington, United States, using genus- and species-specific assays for spotted fever group rickettsiae. DNA of Rickettsia 364D, R. bellii, and R. tillamookensis was not detected in specimens from these regions. DNA of R. rhipicephali was detected in D. occidentalis specimens obtained from Ensenada Municipality in Baja California and southwestern Oregon, but not from Washington. All ompA sequences of R. rhipichephali that were amplified from individual ticks in southwestern Oregon were represented by a single genotype. DNA of the Ixodes pacificus rickettsial endosymbiont was amplified from specimens collected in southwestern Oregon and Klickitat County, Washington; to the best of our knowledge, this Rickettsia species has never been identified in D. occidentalis. Collectively, these data are consistent with a relatively recent introduction of Pacific Coast ticks in the northernmost extension of its recognized range.

BACKGROUND: Pathology and monkeypox virus (MPXV) tissue tropism in severe and fatal human mpox is not thoroughly described but can help elucidate the disease pathogenesis and the role of coinfections in immunocompromised patients. METHODS: We analyzed biopsy and autopsy tissues from 22 patients with severe or fatal outcomes to characterize pathology and viral antigen and DNA distribution in tissues by immunohistochemistry and in situ hybridization. Tissue-based testing for coinfections was also performed. RESULTS: Mucocutaneous lesions showed necrotizing and proliferative epithelial changes. Deceased patients with autopsy tissues evaluated had digestive tract lesions, and half had systemic tissue necrosis with thrombotic vasculopathy in lymphoid tissues, lung, or other solid organs. Half also had bronchopneumonia, and one-third had acute lung injury. All cases had MPXV antigen and DNA detected in tissues. Coinfections were identified in 5/16 (31%) biopsy and 4/6 (67%) autopsy cases. DISCUSSION: Severe mpox in immunocompromised patients is characterized by extensive viral infection of tissues and viremic dissemination that can progress despite available therapeutics. Digestive tract and lung involvement are common and associated with prominent histopathological and clinical manifestations. Coinfections may complicate mpox diagnosis and treatment. Significant viral DNA (likely correlating to infectious virus) in tissues necessitates enhanced biosafety measures in healthcare and autopsy settings.

Some of the most prevalent arthropod-borne pathogens impacting humans in the United States are transmitted by Ixodes ticks. However, little is known regarding the Rickettsia species that inhabit Ixodes scapularis in the United States. The aim of this study was to screen adult I. scapularis collected in central Oklahoma over an 8-yr period for the presence of tick-borne rickettsial pathogens or potential pathogens. During 2014-2021, 112 adult specimens of I. scapularis were collected from central Oklahoma. Amplicons for Rickettsia spp. were amplified from 53 (47.3%) of the samples. Of the positive ticks, 42 (79.2%) amplicon-positive Rickettsia samples were 100% identical to Rickettsia buchneri, 10 (18.9%) were 100% identical to R. tillamookensis strain Tillamook 23, and 1 (1.9%) specimen showed high identity for Rickettsia amblyommatis. This study highlights the importance of considering Rickettsia-specific assays when assessing Ixodes species ticks for potential pathogens.

The larva of the ixodid tick Dermacentor parumapertus Neumann, chiefly a parasite of hares and rabbits, is redescribed using scanning electron micrographs of seven specimens derived from two engorged females collected from two black-tailed jackrabbits, Lepus californicus, at the Black Gap Wildlife Management Area, Brewster County, southwestern Texas. The use of chaetotaxy for separating the genera of Ixodidae is discussed, and a morphological key is provided for separating the larvae of D. parumapertus from those of five other Dermacentor species with partly sympatric geographic ranges. © 2023 Systematic and Applied Acarology Society. All rights reserved.

We detected the DNA of an Anaplasma bovis-like bacterium in blood specimens from 4 patients from the United States with suspected tickborne illnesses. Initial molecular characterization of this novel agent reveals identity to A. bovis-like bacteria detected in Dermacentor variabilis ticks collected from multiple US states.

In the published version of this article there was an error with the sequence data. Please see the corrected text below: | | | In the abstract | | Upon reanalysis with corrected 16S rRNA sequence, the closest full-length match to non-genome strains in Genbank at the time of this corrigendum was to ‘Candidatus Rickettsia senegalensis’ strain R184 (Genbank accession number OM311169) at 99.7 % identity. | | | In table 1 | | The 16S rRNA column analysis was performed with the chimeric 16S rRNA sequence and is thus in error. The top non-genome strain match and identity is as described for changes to the Abstract. The best match to a complete genome was to R. felis URRWXCal2 (Genbank CP000053) at 99.8 % identity. | | | In phylogeny | | 16S rRNA comparisons are in error and are corrected as for Abstract and Table 1. | | In summary, we have, through antibiotic clearance of Mycoplasma contamination of R. tillamookensis Tillamook 23T, ascertained that corrections need to be made to the published genome record for 16S and 23S rRNA sequences. Genbank records have been updated accordingly. These changes do not affect the status of R. tillamookensis as a distinct species or its phylogenetic placement among the transitional group of Rickettsia. | | The authors have provided a detailed explanation of this correction in the Supplementary File 1. | | The authors apologise for any inconvenience caused. |

The nematode Onchocerca lupi is an emerging human pathogen. Though its life cycle is not well studied, it likely infects humans after a bite from a black fly vector, which in turn acquires infective microfilariae from an infected canid. These microfilariae mature into an infective larval stage within the fly. Among six reported cases in the United States, five involved children, and all occurred in the southwest. In this report, we present a case of O. lupi infection with cervical spine invasion in a healthy 10-year-old girl. She presented with five months of neurological symptoms from a rural and medically underserved area, highlighting a need for clinical vigilance in such settings for this emerging infectious threat in the American southwest.

Rocky Mountain spotted fever (RMSF) is a life-threatening tick-borne disease documented in North, Central, and South America. In California, RMSF is rare; nonetheless, recent fatal cases highlight ecological cycles of the two genera of ticks, Dermacentor and Rhipicephalus, known to transmit the disease. These ticks occur in completely different habitats (sylvatic and peridomestic, respectively) resulting in different exposure risks for humans. This study summarizes the demographic, exposure, and clinical aspects associated with the last 40 years of reported RMSF cases to the California Department of Public Health (CDPH). Seventy-eight RMSF cases with onsets from 1980 to 2019 were reviewed. The incidence of RMSF has risen in the last 20 years from 0.04 cases per million to 0.07 cases per million (a two-fold increase in reports), though the percentage of cases that were confirmed dropped significantly from 72% to 25% of all reported cases. Notably, Hispanic/Latino populations saw the greatest rise in incidence. Cases of RMSF in California result from autochthonous and out-of-state exposures. During the last 20 years, more cases reported exposure in Southern California or Mexico than in the previous 20 years. The driver of these epidemiologic changes is likely the establishment and expansion of Rhipicephalus sanguineus sensu lato ticks in Southern California and on-going outbreaks of RMSF in northern Mexico. Analysis of available electronically reported clinical data from 2011 to 2019 showed that 57% of reported cases presented with serious illness requiring hospitalization with a 7% mortality. The difficulty in recognizing RMSF is due to a non-specific clinical presentation; however, querying patients on the potential of tick exposure in both sylvatic and peridomestic environments may facilitate appropriate testing and treatment.

The tropical lineage within the Rhipicephalus sanguineus species complex is cause for growing concern in the U.S. based on its prominent role in creating and perpetuating multiple recently identified outbreaks of Rocky Mountain spotted fever in the southwestern United States and northern Mexico. This lineage is undergoing a northward range expansion in the United States, necessitating the need for enhanced surveillance for Rh. sanguineus. To inform more focused surveillance efforts we use species distribution models (SDMs) to predict current (2015-2019) and future (2021-2040) habitat for the tropical lineage. Models using the MaxEnt algorithm were informed using geolocations of ticks genetically confirmed to be of the tropical lineage, for which data on 23 climatic and ecological variables were extracted. Models predicted that suitability was optimal where temperatures are relatively warm and stable, and there is minimal precipitation. This translated into habitat being predicted along much of the coast of southern states including California, Texas, Louisiana, and Florida. Although the endophilic nature of tropical Rh. sanguineus somewhat violates the assumptions of SDMs, our models correctly predicted known locations of this tick and provide a starting point for increased surveillance efforts. Furthermore, we highlight the importance of using molecular methods to distinguish between ticks in the Rh. sanguineus species complex.

Whipple's disease was formally described more than 100 years ago; nonetheless, only recently have investigators identified, cultivated, and characterized the causative agent, Tropheryma whipplei, and deciphered some of its complex associations with human hosts. The acquisition of knowledge about T. whipplei, a Gram-positive bacterium in the class Actinobacteria, accelerated at an extraordinarily rapid pace during the first two decades of the 21st century, to include the recognition of asymptomatic carriage, localized disease, and acute infections caused by this peculiar bacterium. This review discusses current knowledge of the microbiology and epidemiology of T. whipplei, the expanding clinical spectrum of disease caused by the pathogen, its treatment, and historical and diagnostic assays that facilitate the diagnosis of the various diseases caused by this enigmatic bacterium. The findings and conclusions in this article are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention. 2022 Elsevier Inc.

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.

To the Editor.We read with concern the manuscript by Gill et al1 that described autopsy findings for 2 adolescents who died after COVID-19 vaccination. The authors correctly stated that the Centers for Disease Control and Prevention (CDC) performed testing for SARS-CoV-2 and found no evidence of SARS-CoV-2 infection in autopsy tissues from the decedents. However, the authors did not include findings from far more thorough immunohistochemical and molecular testing performed by the CDC for each patient. We believe that all of the CDC's findings should be considered carefully, and that the omission of these data has important implications for the conclusions of this paper.

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. The Author(s) 2022.

Tick-borne rickettsial infections are serious, common, and difficult to diagnose. Among the most important factors leading to failure to diagnose and treat tick-borne rickettsioses effectively is a lack of consideration of the potential diagnosis by primary caregivers and emergency department physicians in patients presenting with undifferentiated acute febrile illness during tick season. This situation exists because of insufficient primary and continuing medical education of medical students, primary care and emergency medicine residents, and practicing physicians regarding tick-borne rickettsioses specific to the region where they practice. Delayed initiation of treatment with an appropriate antibiotic is associated with adverse outcomes including increased rates of hospitalization, admission to an intensive care unit, and mortality. The earliest symptoms are nonspecific, consisting of fever, headache, myalgias, and nausea and/or vomiting. Laboratory abnormalities are typically absent at this time when the therapeutic response to an appropriate antibiotic would be optimal. There is a mistaken idea among a substantial portion of physicians that the best antibiotic available, doxycycline, should not be administered to children 8 years of age or younger or during pregnancy. For all of the above reasons, there is unnecessary morbidity and mortality caused by tick-borne rickettsioses. This report proposes measures to address these critical issues regarding tick-borne rickettsioses.

A unique genotype of Rickettsia parkeri, designated R. parkeri strain Black Gap, has thus far been associated exclusively with the North American tick, Dermacentor parumapertus. The compete genome consists of a single circular chromosome with 1,329,522 bp and a G+C content of 32.5%.

The tick-borne pathogen Rickettsia parkeri causes a mild rickettsiosis, with cases reported from several countries to its known distribution in the Americas. Molecular analyses have identified a clear distinction between strains of R. parkeri sensu stricto (s. s.) and R. parkeri sensu lato (s. l.) as well as separation between North American and South American R. parkeri s. s. strains. To expand on this previous work, we developed a multilocus sequence typing analysis with two aims: first, to investigate the genetic diversity within strains of North American R. parkeri s. s., and second, to further the understanding of the genetic relationships between R. parkeri s. s. and R. parkeri s. l. Sixty-four R. parkeri isolates and 12 R. parkeri-positive tick lysates were analyzed using a novel typing scheme consisting of four coding regions and two intergenic regions. A concatenated Bayesian phylogeny that identified eight clades was constructed: three represent the R. parkeri s. l. strains, and five represent the R. parkeri s. s. strains. The clades appear to be generally phylogeographically organized and associated with specific tick vectors. However, while one of the four R. parkeri s. s. North American clades appears to be limited to the southwestern United States, the other North American clades exhibit broad dispersal, most notably seen in the largest group, which includes representative samples extending from northern Mexico to Delaware. This work highlights the increasingly recognized geographic range of R. parkeri in the Americas and suggests a potential public health risk for these areas. IMPORTANCE Since 1937, when Rickettsia parkeri was originally identified in Amblyomma maculatum group ticks, the recognized range and associated vectors for this pathogen have expanded significantly. In recent years, R. parkeri has been identified in 12 tick species from seven countries in the Americas. Herein, we provide evidence that the greatest genetic diversity within R. parkeri exists in North America, where one R. parkeri sensu lato and four R. parkeri sensu stricto genotypes are present. While one distinct R. parkeri sensu stricto genotype exists only in the southwestern United States, three genotypes are broadly distributed in the eastern United States, with the largest of these found across the known range of R. parkeri in North America. In contrast, the South American R. parkeri sensu stricto samples represent a single genotype and are completely clonal at the loci analyzed, irrespective of their country of origin.

A previously unrecognized Rickettsia species was isolated in 1976 from a pool of Ixodes pacificus ticks collected in 1967 from Tillamook County, Oregon, USA. The isolate produced low fever and mild scrotal oedema following intraperitoneal injection into male guinea pigs (Cavia porcellus). Subsequent serotyping characterized this isolate as distinct from recognized typhus and spotted fever group Rickettsia species; nonetheless, the isolate remained unevaluated by molecular techniques and was not identified to species level for the subsequent 30 years. Ixodes pacificus is the most frequently identified human-biting tick in the western United States, and as such, formal identification and characterization of this potentially pathogenic Rickettsia species is warranted. Whole-genome sequencing of the Tillamook isolate revealed a genome 1.43 Mbp in size with 32.4 mol% G+C content. Maximum-likelihood phylogeny of core proteins places it in the transitional group of Rickettsia basal to both Rickettsia felis and Rickettsia asembonensis. It is distinct from existing named species, with maximum average nucleotide identity of 95.1% to R. asembonensis and maximum digital DNA-DNA hybridization score similarity to R. felis at 80.1%. The closest similarity at the 16S rRNA gene (97.9%) and sca4 (97.5%/97.6% respectively) is to Candidatus 'Rickettsia senegalensis' and Rickettsia sp. cf9, both isolated from cat fleas (Ctenocephalides felis). We characterized growth at various temperatures and in multiple cell lines. The Tillamook isolate grows aerobically in Vero E6, RF/6A and DH82 cells, and growth is rapid at 28 °C and 32 °C. Using accepted genomic criteria, we propose the name Rickettsia tillamookensis sp. nov., with the type strain Tillamook 23. Strain Tillamook 23 is available from the Centers for Disease Control and Prevention Rickettsial Isolate Reference Collection (WDCM 1093), Atlanta, GA, USA (CRIRC accession number RTI001(T)) and the Collection de Souches de l'Unité des Rickettsies (WDCM 875), Marseille, France (CSUR accession number R5043). Using accepted genomic criteria, we propose the name Rickettsia tillamookensis sp. nov., with the type strain Tillamook 23 (=CRIRC RTI001=R5043).