Borrelia burgdorferi not confirmed in human-biting Amblyomma americanum ticks from the southeastern United States

Tick species infesting humans in the United States

The data for human tick encounters in the United States (US) presented in this paper were compiled with the goals of: (i) presenting quantitative data across the full range of native or recently established human biting ixodid (hard) and argasid (soft) tick species with regards to their frequency of infesting humans, based on published records of ticks collected while biting humans or crawling on clothing or skin; and (ii) providing a guide to publications on human tick encounters. Summary data are presented in table format, and the detailed data these summaries were based on are included in a set of Supplementary Tables. To date, totals of 36 ixodid species (234,722 specimens) and 13 argasid species (230 specimens) have been recorded in the published literature to infest humans in the US. Nationally, the top five ixodid species recorded from humans were the blacklegged tick, Ixodes scapularis (n=158,008 specimens); the lone star tick, Amblyomma americanum (n=36,004); the American dog tick, Dermacentor variabilis (n=26,624); the western blacklegged tick, Ixodes pacificus (n=4,158); and the Rocky Mountain wood tick, Dermacentor andersoni (n=3,518). Additional species with more than 250 ticks recorded from humans included Ixodes cookei (n=2,494); the Pacific Coast tick, Dermacentor occidentalis (n=809); the brown dog tick, Rhipicephalus sanguineus sensu lato (n=714); the winter tick, Dermacentor albipictus (n=465); and the Gulf Coast tick, Amblyomma maculatum (n=335). The spinose ear tick, Otobius megnini (n=69), and the pajaroello tick, Ornithodoros coriaceus (n=55) were the argasid species most commonly recorded from humans. Additional information presented for each of the 49 tick species include a breakdown of life stages recorded from humans, broad geographical distribution in the US, host preference, and associated human pathogens or medical conditions. The paper also provides a history of publications on human tick encounters in the US, with tables outlining publications containing quantitative data on human tick encounters as well as other notable publications on human-tick interactions. Data limitations are discussed. Researchers and public health professionals in possession of unpublished human tick encounter data are strongly encouraged to publish this information in peer-reviewed scientific journals. In future papers, it would be beneficial if data consistently were broken down by tick species and life stage as well as host species and ticks found biting versus crawling on clothing or skin.

Amblyomma americanum (Acari: Ixodidae) Ticks Are Not Vectors of the Lyme Disease Agent, Borrelia burgdorferi (Spirocheatales: Spirochaetaceae): A Review of the Evidence

In the early 1980s, Ixodes spp. ticks were implicated as the key North American vectors of Borrelia burgdorferi (Johnson, Schmid, Hyde, Steigerwalt and Brenner) (Spirocheatales: Spirochaetaceae), the etiological agent of Lyme disease. Concurrently, other human-biting tick species were investigated as potential B. burgdorferi vectors. Rashes thought to be erythema migrans were observed in patients bitten by Amblyomma americanum (L.) (Acari: Ixodidae) ticks, and spirochetes were visualized in a small percentage of A. americanum using fluorescent antibody staining methods, sparking interest in this species as a candidate vector of B. burgdorferi. Using molecular methods, the spirochetes were subsequently described as Borrelia lonestari sp. nov. (Spirocheatales: Spirochaetaceae), a transovarially transmitted relapsing fever Borrelia of uncertain clinical significance. In total, 54 surveys from more than 35 research groups, involving more than 52,000 ticks, have revealed a low prevalence of B. lonestari, and scarce B. burgdorferi, in A. americanum. In Lyme disease-endemic areas, A. americanum commonly feeds on B. burgdorferi-infected hosts; the extremely low prevalence of B. burgdorferi in this tick results from a saliva barrier to acquiring infection from infected hosts. At least nine transmission experiments involving B. burgdorferi in A. americanum have failed to demonstrate vector competency. Advancements in molecular analysis strongly suggest that initial reports of B. burgdorferi in A. americanum across many states were misidentified B. lonestari, or DNA contamination, yet the early reports continue to be cited without regard to the later clarifying studies. In this article, the surveillance and vector competency studies of B. burgdorferi in A. americanum are reviewed, and we conclude that A. americanum is not a vector of B. burgdorferi.

Prevalence of Tick-Borne Pathogens in Northeast Missouri

We evaluated Amblyomma americanum (lone star tick) and Dermacentor variabilis (American dog tick) in northeast Missouri for the presence of Borrelia, Ehrlichia, and Rickettsia bacteria and Heartland virus. We screened 436 individual adult lone star ticks (86% of all ticks collected) and infection rates were 6% for B. lonestari, 19% for E. chaffeensis, 3% for E. ewingii, 36% for R. amblyommatis, and 1% for R. montanensis. In the 189 individual American dog ticks, infection rates were 19% for E. chaffeensis, 15% for E. ewingii, 4% for R. amblyommatis, and 5% for R. montanensis. In addition, we screened 20 pools of adults and 30 pools of nymphs for the Heartland virus which was not detected. Understanding the presence and epidemiology of these causative (E. chaffeensis and E. ewingii) and suspected (B. lonestari, R. amblyommatis, and R. montanensis) agents in Missouri should increase awareness of potential tick-borne disease in the medical community.

Vector competence of Amblyomma americanum (Acari: Ixodidae) for Rickettsia rickettsii

Rickettsia rickettsii - the etiologic agent of Rocky Mountain spotted fever (RMSF) - is widely spread across the Americas. In the US, Dermacentor spp. ticks are identified as primary vectors of R. rickettsii and Rhipicephalus sanguineus s.l. has been implicated in transmission of this pathogen in several locations in the Southwest. Conversely, ticks of the genus Amblyomma are recognized vectors of RMSF in Central and South America, but not in the US. A. americanum is one of the most aggressive human-biting ticks in the US, whose geographical range overlaps with that of reported RMSF cases. Despite sporadic findings of R. rickettsii DNA in field-collected A. americanum and circumstantial association of this species with human RMSF cases, its vector competence for R. rickettsii has not been appropriately studied. Therefore, we assessed the ability of A. americanum to acquire and transmit two geographically distant isolates of R. rickettsii. The Di-6 isolate of R. rickettsii used in this study originated in Virginia and the AZ-3 isolate originated in Arizona. Under laboratory conditions, A. americanum demonstrated vector competence for both isolates, although the efficiency of acquisition and transovarial transmission was higher for Di-6 than for AZ-3 isolate. Uninfected larvae acquired the pathogen from systemically infected guinea pigs, as well as while feeding side by side with Rickettsia-infected ticks on non-rickettsiemic hosts. Once acquired, R. rickettsii was successfully maintained through the tick molting process and transmitted to susceptible animals during subsequent feedings. Guinea pigs and dogs infested with infected A. americanum developed fever, scrotal edema and dermatitis or macular rash. R. rickettsii DNA was identified in animal blood, skin, and internal organs. The prevalence of infection within tick cohorts gradually increased due to side-by-side feeding of infected and uninfected individuals from 33 to 49% in freshly molted nymphs to 71-98% in engorged females. Moreover, R. rickettsii was transmitted transovarially by approximately 28% and 14% of females infected with Di-6 and AZ-3 isolates, respectively. Hence, A. americanum is capable of acquiring, maintaining and transmitting R. rickettsii isolates originating from two different geographical regions of the US, at least under laboratory conditions. Its role in ecology and epidemiology of RMSF in the US deserves further investigation.

Borrelia burgdorferi DNA absent, multiple Rickettsia spp. DNA present in ticks collected from a teaching forest in North Central Florida

Tick-borne diseases are an emerging public health threat in the United States. In Florida, there has been public attention directed towards the possibility of locally acquired Borrelia burgdorferi sensu stricto, the causative agent of Lyme disease, in association with the lone star tick. The aim of this study was to determine the prevalence of ticks and the pathogens they carry and potentially transmit, such as B. burgdorferi, in a highly utilized teaching and research forest in North Central Florida. Ticks were collected by dragging and flagging methods over a four month period in early 2014, identified, and tested by PCR for multiple pathogens including Anaplasma, Borrelia, Rickettsia, and Ehrlichia species. During the study period the following ticks were collected: 2506 (96.5%) Amblyomma americanum L., 64 (2.5%) Ixodes scapularis Say, 19 (0.7%) Dermacentor variabilis Say, and 5 (0.2%) Ixodes affinis Neuman. Neither Borrelia spp. (0/846) nor Anaplasma spp. (0/69; Ixodes spp. only) were detected by PCR in any of the ticks tested. However, Rickettsia DNA was present in 53.7% (86/160), 62.5% (40/64), 60.0% (3/5) and 31.6% (6/19) of A. americanum, I. scapularis, I. affinis and D. variabilis, respectively. Furthermore, E. chaffeensis and E. ewingii DNA were detected in 1.3% and 4.4% of adult A. americanum specimens tested, respectively. Although receiving an A. americanum bite is likely in wooded areas in North Central Florida due to the abundance of this tick, the risk of contracting a tick-borne pathogen in this specific area during the spring season appears to be low. The potential for pathogen prevalence to be highly variable exists, even within a single geographical site and longitudinal studies are needed to assess how tick-borne pathogen prevalence is changing over time in North Central Florida.

Detection of Borrelia, Ehrlichia, and Rickettsia spp. in ticks in northeast Missouri

We evaluated Amblyomma americanum (lone star tick) and Dermacentor variabilis (American dog tick) in northeast Missouri for the presence of Borrelia, Ehrlichia, and Rickettsia bacteria. We collected actively questing ticks from four sites within Adair County, Missouri. A total of 15,162 ticks were collected, of which 13,980 were grouped in 308 pools (lone star ticks, 288 pools; American dog ticks, 20 pools) and tested for presence/absence of bacteria using polymerase chain reaction. Infection rates were calculated as the maximum likelihood estimation (MLE) with 95% confidence intervals (CI). Of the 308 pools tested, 229 (74.4%) were infected with bacteria and the overall MLE of the infection rate per 100 ticks was calculated as 2.9% (CI 2.61-3.21). Infection rates varied among life stages, 28.6% (CI 23.89-33.97) in adults, 7.0% (CI 5.10-9.86) in nymphs, and 1.0% (CI 0.75-1.20) in larvae. In the 116 adult lone star pools, infection rates were calculated for Borrelia lonestari (1.4%), Borrelia spp. (2.7%), Ehrlichia chaffeensis (6.1%), Ehrlichia ewingii (3.3%), Rickettsia amblyommii (18.3%), and Rickettsia montanensis (0.4%). Infection rates for the 52 nymphal lone star pools were calculated as B. lonestari (1.03%), Borrelia spp. (0.40%), E. chaffeensis (2.02%), E. ewingii (0.24%), and R. amblyommii (2.70%). In the 20 adult American dog tick pools, infection rates were determined as E. chaffeensis (9.47%), E. ewingii (5.47%), and R. montanensis (8.06%). Eight Borrelia samples were sequenced with five 99-100% identical to B. burgdorferi (s.l.) and three 99% identical to B. lonestari. Eight samples were sequenced for E. chaffeensis (all 99-100% identical) and one sample was sequenced for E. ewingii (99% identical). Seven samples were sequenced for Rickettsia and three were 99% identical to R. montanensis and four were 100% identical to R. amblyommii. This study demonstrates B. lonestari, E. chaffeensis, E. ewingii, R. amblyommii, and R. montanensis in northeast Missouri ticks for the first time. Understanding the presence and epidemiology of these causative (E. chaffeensis and E. ewingii) and suspected (B. lonestari and R. amblyommii) agents in Missouri should increase awareness of potential tick-borne disease in the medical community.

Avian migrants facilitate invasions of neotropical ticks and tick-borne pathogens into the United States

Migratory birds have the potential to transport exotic vectors and pathogens of human and animal health importance across vast distances. We systematically examined birds that recently migrated to the United States from the Neotropics for ticks. We screened both ticks and birds for tick-borne pathogens, including Rickettsia species and Borrelia burgdorferi. Over two spring seasons (2013 and 2014), 3.56% of birds (n = 3,844) representing 42.35% of the species examined (n = 85) were infested by ticks. Ground-foraging birds with reduced fuel stores were most commonly infested. Eight tick species were identified, including seven in the genus Amblyomma, of which only Amblyomma maculatum/Amblyomma triste is known to be established in the United States. Most ticks on birds (67%) were neotropical species with ranges in Central and South America. Additionally, a single Ixodes genus tick was detected. A total of 29% of the ticks (n = 137) and no avian blood samples (n = 100) were positive for infection with Rickettsia species, including Rickettsia parkeri, an emerging cause of spotted fever in humans in the southern United States, a species in the group of Rickettsia monacensis, and uncharacterized species and endosymbionts of unknown pathogenicity. No avian tick or blood samples tested positive for B. burgdorferi, the etiologic agent of Lyme disease. An extrapolation of our findings suggests that anywhere from 4 to 39 million exotic neotropical ticks are transported to the United States annually on migratory songbirds, with uncertain consequences for human and animal health if the current barriers to their establishment and spread are overcome.