Field study of the relationship between skin-sensitizing antibody production in the cottontail rabbit, Sylvilagus floridanus, and infestation by the rabbit tick, Haemaphysalis leporispalustris (Acri: Ixodidae)

The resistance of cottontail rabbits to tick feeding appears correlated with the rabbits' development of skin-sensitizing antibodies. Resistance appeared to be greatest in adult rabbits which had been repeatedly infested with ticks. Rabbits with little exposure to ticks, usually the young cottontails, showed little or no skin-sensitizing antibody present in their blood and usually had relatively high tick loads when compared with adult rabbits. Models used to interpret the data show promise as tools for predicting tick population fluctuations and, perhaps, incidence of vector borne disease outbreaks. The existence of resistance to tick attachment has important implications for the host-parasite relationship. The research lends support to the hypothesis that the resistance may function as a homeostatic regulatory mechanism capable of maintaining the size of the tick population in equilibrium with the size of the rabbit population. In this way, host resistance may be advantageous to the parasite as well as to the host.     

Investigating the relationship between environmental factors and tick abundance in a small,highly heterogeneous region

The tick Ixodes ricinus (L.) is the most important vector of tick‐borne zoonoses in Europe. Apart from factors related to human behavior, tick abundance is a major driver of the incidence of tick‐borne diseases in a given area and related data represent critical information for promoting effective public health policies. The present study analyzed the relationship between different environmental factors and tick abundance in order to improve the understanding of I. ricinus autecology and develop spatial predictive models that can be implemented in tick‐borne disease prevention strategies. Ticks were sampled in 27 sites over a four‐year period and different environmental variables were studied. Five simple models were developed that explain a large part of variation in tick abundance. Precipitation seems to play the most important role, followed by temperature, woodland coverage, and solar radiation. Model equations obtained in this study may enable the spatial interpolation and extension of tick abundance predicted values to sites of the same area, in order to build regional predictive maps. They could also be useful for the validation of large‐scale spatial predictive maps.     

Continous alert for rickettsiosis in Sicily: molecular characterization of Rickettsia sp. obtained from ticks and human beings (1986-2001)

Several strains of Rickettsia sp. were isolated from patients in western Sicily with MSF (Mediterranean Spotted Fever) as well as ticks. Strains isolated were examined by PCR and identified as belonging to R. conorii sp. Importantly a strain of Israeli Spotted Fever Rickettsiae, obtained from a tick, was also identified. Our data prove that strains other than the classical R. conorii also circulate in Sicily.     

Emerging rickettsioses

Ticks are known to carry and transmit a number of microbial agents that cause diseases in humans and animals. Among these are members of the order Rickettsiales (alpha-proteobacteria), which include the genera Rickettsia and Ehrlichia. The most common and well-known Rickettsial human disease in Europe is Mediterranean Spotted Fever (MSF), caused by Rickettsia conorii. In recent years, a number of new Rickettsia species have been discovered in Europe, some of which have been shown to be pathogenic to humans. These discoveries have been facilitated by use of sequence-based molecular identification techniques. In Italy, it is generally believed that R. conorii is the only Rickettsia species present, and clinical tests for MSF rely on antigens raised against this bacterium. We are currently undertaking a molecular screening study of Rickettsiales-bacteria in ticks from various regions of Italy, to check for the potential presence of species from this order recently discovered in other parts of Europe. So far, we have identified a number of additional species in ticks collected from northern, central and southern regions. These include the known pathogens R. helvetica and R. slovaca as well as two species which may or may not be of medical relevance: R. monacensis and R. sp. IRS4. As a part of this survey, we have identified a novel alphaproteobacterium from the medically important tick Ixodes ricinus. This bacterium, tentatively named IricES1, has the unusual property of existing within the mitochondria, as well as the cytoplasm, of ovarian cells. To our knowledge, this is the only known example of a bacterium that is able to enter the mitochondria of animals. Our recently published electron microscopic data indicates that the bacterium enters mitochondria between the inner and outer membranes, and then proceeds to consume the inner mitochondrial matrix. We will present further data on this bacterium, including: 1) its phylogenetic position based on various molecular sequences, 2) its localization within the tick based on in situ hybridization; 3) its distribution among tick populations in Europe; 4) preliminary data on attempts at culturing this bacterium in a variety of cell types. Possible interactions between the bacterium and its host will be discussed. Ticks are known to carry and transmit a number of microbial agents that cause diseases in humans and animals. Among these are members of the order Rickettsiales (alpha-proteobacteria), which include the genera Rickettsia and Ehrlichia. The most common and well-known Rickettsial human disease in Europe is Mediterranean Spotted Fever (MSF), caused by Rickettsia conorii. In recent years, a number of new Rickettsia species have been discovered in Europe, some of which have been shown to be pathogenic to humans. These discoveries have been facilitated by use of sequence-based molecular identification techniques. In Italy, it is generally believed that R. conorii is the only Rickettsia species present, and clinical tests for MSF rely on antigens raised against this bacterium. We are currently undertaking a molecular screening study of Rickettsiales-bacteria in ticks from various regions of Italy, to check for the potential presence of species from this order recently discovered in other parts of Europe. So far, we have identified a number of additional species in ticks collected from northern, central and southern regions. These include the known pathogens R. helvetica and R. slovaca as well as two species which may or may not be of medical relevance: R. monacensis and R. sp. IRS4. As a part of this survey, we have identified a novel alphaproteobacterium from the medically important tick Ixodes ricinus. This bacterium, tentatively named IricES1, has the unusual property of existing within the mitochondria, as well as the cytoplasm, of ovarian cells. To our knowledge, this is the only known example of a bacterium that is able to enter the mitochondria of animals. Our recently published electron microscopic data indicates that the bacterium enters mitochondria between the inner and outer membranes, and then proceeds to consume the inner mitochondrial matrix. We will present further data on this bacterium, including: 1) its phylogenetic position based on various molecular sequences, 2) its localization within the tick based on in situ hybridization; 3) its distribution among tick populations in Europe; 4) preliminary data on attempts at culturing this bacterium in a variety of cell types. Possible interactions between the bacterium and its host will be discussed.     

Coronary involvement in Mediterranean spotted fever

Mediterranean spotted fever (MSF) is a tick-borne acute febrile disease caused by Rickettsia conorii and characterized by fever, a maculo-papular rash and a black eschar at the site of the tick bite. We describe the case of a 3-year-old boy with MSF who developed a transient right coronary artery ectasia. The patient was brought to the hospital after four days of fever and mild myalgia of the legs. The suspicion of MSF arose due to the presence of a maculo-papular skin rash and treatment with oral clarithromycin was started. After four days fever persisted and the differential diagnosis of Kawasaki syndrome was considered. Echocardiography showed a dilated right coronary artery with hyperreflective walls. Treatment with intravenous immunoglobulin was initiated while clarithromycin was continued. After one day the fever disappeared. An immunofluorescent antibody test performed after four weeks confirmed a R. conorii infection. A follow-up echocardiography was normal six weeks and six months later. We suggest that ectasia of the coronary arteries may be a manifestation of rickettsial vasculitis. Prospective studies are needed to understand the frequency and the possible consequences of this phenomenon in the course of MSF.     

Landscape features predict the current and forecast the future geographic spread of Lyme disease

Lyme disease, the most prevalent vector-borne disease in North America, is increasing in incidence and geographic distribution as the tick vector, Ixodes scapularis, spreads to new regions. We re-construct the spatial-temporal invasion of the tick and human disease in the Midwestern US, a major focus of Lyme disease transmission, from 1967 to 2018, to analyse the influence of spatial factors on the geographic spread. A regression model indicates that three spatial factors—proximity to a previously invaded county, forest cover and adjacency to a river—collectively predict tick occurrence. Validation of the predictive capability of this model correctly predicts counties invaded or uninvaded with 90.6% and 98.5% accuracy, respectively. Reported incidence increases in counties after the first report of the tick; based on this modelled relationship, we identify 31 counties where we suspect I. scapularis already occurs yet remains undetected. Finally, we apply the model to forecast tick establishment by 2021 and predict 42 additional counties where I. scapularis will probably be detected based upon historical drivers of geographic spread. Our findings leverage resources dedicated to tick and human disease reporting and provide the opportunity to take proactive steps (e.g. educational efforts) to prevent and limit transmission in areas of future geographic spread.     

Mapping and monitoring tick (Acari,Ixodida) distribution,seasonality, and host associations in the United Kingdom between 2017 and 2020

Tick-borne disease risk is intrinsically linked to the distribution of tick vector species. To assess risk and anticipate disease emergence, an understanding of tick distribution, host associations, and seasonality is needed. This can be achieved, to some extent, using passive surveillance supported by engagement with the public, animal health, and public health experts. The Tick Surveillance Scheme (TSS) collects data and maps tick distribution across the United Kingdom (UK). Between 2017 and 2020, 3720 tick records were received and 39 tick species were detected. Most records were acquired in the UK, with a subset associated with recent overseas travel. The dominant UK acquired species was Ixodes ricinus (Ixodida: Ixodidae, Linnaeus), the main vector of Lyme borreliosis. Records peaked during May and June, highlighting a key risk period for tick bites. Other key UK species were detected, including Dermacentor reticulatus (Ixodida: Ixodidae, Fabricius) and Haemaphysalis punctata (Ixodida: Ixodidae, Canestrini & Fanzago) as well as several rarer species that may present novel tick-borne disease risk to humans and other animals. Updated tick distribution maps highlight areas in the UK where tick exposure has occurred. There is evidence of increasing human tick exposure over time, including during the COVID-19 pandemic, but seasonal patterns remain unchanged.     

Tick antigens recognized by serum from a guinea pig resistant to infestation with the tick Rhipicephalus appendiculatus

Immune resistance to infestation by an ixodid tick, Rhipicephalus appendiculatus, the vector of the cattle disease East Coast Fever, was induced in a guinea pig by repeated tick infestation. This resistance is expressed as the ability of the host to interfere with tick feeding. Resistance to ixodid tick feeding is an acquired response mediated by host antibody. We report the use of antibodies from a resistant host animal, in immunoblotting, to characterize the tick antigens recognized. The major tick antigens identified had molecular weights of 120,000, 94,000, 88,000, 77,000, 58,000, 46,000, 35,000, 31,000, 28,000, 25,000, 20,000 and 16,000. Most of these antigens were found in tick salivary glands. The presence and concentration of many tick salivary antigens appeared to vary with relation to the tick feeding cycle. Many of the antigens present in salivary glands were also detected in tick cement. Tick gut extract, although a poorer source of antigens, contained more of the 31,000 dalton antigen than salivary glands. Larval and nymphal tick extract lacked many of the antigens present in adult ticks. The data suggest that tick resistance is a complex phenomenon probably elicited by several different tick antigens.     

Transmission of Ehrlichia chaffeensis from lone star ticks (Amblyomma americanum) to white-tailed deer (Odocoileus virginianus)

Amblyomma americanum is an aggressive ixodid tick that has been implicated as a vector for several bacterial agents. Among these is Ehrlichia chaffeensis, which causes human monocytic (or monocytotropic) ehrlichiosis. In this study, experimental tick transmission of E. chaffeensis from infected lone star ticks to deer was revisited, and the question of whether it would be possible to re-isolate the organism from deer was asked, because this had not been done previously. Here, we were able to transmit a wild strain of E. chaffeensis from acquisition-fed lone star ticks to white-tailed deer. Ehrlichia chaffeensis was re-isolated from one white-tailed deer on multiple days during the infection and from another deer on one day during the infection. Peak rickettsemias for E. chaffeensis-infected deer were 17 DPI with acquisition-fed ticks and 14 DPI with needle-inoculated deer. This study supports the role of the lone star tick and white-tailed deer as vector and reservoir host for E. chaffeensis, demonstrating culture re-isolation of E. chaffeensis in deer infected by experimental tick transmission for the first time.     

Geographic uniformity of the Lyme disease spirochete (Borrelia burgdorferi) and its shared history with tick vector (Ixodes scapularis) in the Northeastern United States

Over 80% of reported cases of Lyme disease in the United States occur in coastal regions of northeastern and mid-Atlantic states. The genetic structure of the Lyme disease spirochete (Borrelia burgdorferi) and its main tick vector (Ixodes scapularis) was studied concurrently and comparatively by sampling natural populations of I. scapularis ticks along the East Coast from 1996 to 1998. Borrelia is genetically highly diverse at the outer surface protein ospC. Since Borrelia is highly clonal, the ospC alleles can be used to define clones. A newly designed reverse line blotting (RLB) assay shows that up to 10 Borrelia clones can infect a single tick. The clone frequencies in Borrelia populations are the same across the Northeast. On the other hand, I. scapularis populations show strong regional divergence (among northeastern, mid-Atlantic, and southern states) as well as local differentiation. The high genetic diversity within Borrelia populations and the disparity in the genetic structure between Borrelia and its tick vector are likely consequences of strong balancing selection on local Borrelia clones. Demographically, both Borrelia and I. scapularis populations in the Northeast show the characteristics of a species that has recently expanded from a population bottleneck. Major geological and ecological events, such as the last glacial maximum (18,000 years ago) and the modern-day expansion of tick habitats, are likely causes of the observed "founder effects" for the two organisms in the Northeast. We therefore conclude that the genetic structure of B. burgdorferi has been intimately shaped by the natural history of its main vector, the northern lineage of I. scapularis ticks.     

Tick‐borne viruses: Current trends in large‐scale viral surveillance

Ticks are ectoparasites that transmit pathogens, such as tick‐borne viruses, to their hosts. Tick‐borne viruses are diverse: they can be categorized into two orders, nine families, and at least 12 genera. Almost 25% of these viruses are infectious to humans and some are a serious threat to public health. The global rise in tick‐borne virus diseases has been linked to climate change which has reduced tick mortality in the winter and extended their active period. The spread of tick‐borne viral diseases to humans has received significant interest due to the increased threat to human life; epidemiological monitoring of tick‐borne viruses using molecular, immunological, and environmental methods is now a priority. Nevertheless, many tick‐borne diseases remain undiagnosed, which poses a challenge to public administration and health care officials. This review discusses three major tick‐borne RNA viruses that cause serious infection in humans: severe fever with thrombocytopenia syndrome (SFTS) virus, tick‐borne encephalitis (TBE), and Crimean–Congo hemorrhagic fever (CCHF) virus. Specifically, we discuss the epidemiological monitoring, vector control measures, molecular diagnostics, vaccines, and environmental determinants related to these viruses. Furthermore, we review the current surveillance of these tick‐borne viruses with a specific focus on diagnostic approaches that employ molecular interventions such as viral nucleic acid isolation, PCR‐based diagnostics, and high‐throughput sequencing technologies.     

A tick antioxidant facilitates the Lyme disease agent's successful migration from the mammalian host to the arthropod vector

The tick Ixodes scapularis is an efficient vector for microbes, including the Lyme disease agent Borrelia burgdorferi. Ticks engorging on vertebrates induce recruitment of inflammatory cells to the bite site. For efficient transmission to the vector, pathogens have to traffic through this complex feeding site while avoiding the deleterious effects of immune cells. We show that a tick protein, Salp25D, plays a critical role-in the mammalian host-for acquisition of Borrelia burgdorferi by the vector. Silencing salp25D in tick salivary glands impaired spirochete acquisition by ticks engorging on B. burgdorferi-infected mice. Immunizing mice against Salp25D also decreased Borrelia acquisition by I. scapularis. Salp25D detoxified reactive oxygen species at the vector-pathogen-host interface, thereby providing a survival advantage to B. burgdorferi at the tick feeding site in mice. These data demonstrate that pathogens can exploit arthropod molecules to defuse mammalian responses in order to successfully enter the vector.     

A mitochondrial import factor purified from rat liver cytosol is an ATP-dependent conformational modulator for precursor proteins.

Rat liver cytosol contained an activity that stimulated the import of wheat germ lysate-synthesized precursor proteins into mitochondria. The activity was purified 10,000-fold from the cytosol as a homogeneous heterodimeric protein. This protein (termed mitochondrial import stimulation factor or MSF) stimulated the binding and import of mitochondrial precursor proteins. MSF was also found to recognize the presequence portion of mitochondrial precursors and catalyze the depolymerization and unfolding of in vitro synthesized mitochondrial precursor proteins in an ATP-dependent manner; in this connection, MSF exhibited ATPase activity depending on the important-incompetent mitochondrial precursor protein. The mitochondrial binding and import-stimulating activities were strongly inhibited by the pretreatment of MSF with NEM, whereas the ATP-dependent depolymerization activity was insensitive to the NEM treatment, suggesting that the process subsequent to the unfolding was inhibited with the NEM treatment. We conclude that MSF is a multifunctional cytoplasmic chaperone specific for mitochondrial protein import.     

Disruption of pregnancy in rabbits serving as hosts for feeding ticks

The effect of tick infestation on reproductive success of mammals was studied by using rabbits bred after they had served as hosts for Dermacentor variabilis . Mating success, pregnancy incidence, litter size and neonatal viability were reduced in tick-exposed rabbits, and the proportional number of abnormalities found in preimplantation-stage embryos was much higher than in unexposed controls. Fecundity was restored by prolonging the interval between tick exposure and breeding or by treatment with an acaricide.     

The role of lizards in the ecology of Lyme disease in two endemic zones of the northeastern United States

We examined the role of lizards in the ecology of Lyme disease in New York and Maryland. We collected data on vector tick infestations, measured lizard "realized" reservoir competence for the Lyme disease spirochete Borrelia burgdorferi, and estimated lizard population density. These data were incorporated into a model that predicts a host's ability to influence the prevalence of B. burgdorferi in the tick population, a primary risk factor in the epidemiology of Lyme disease. Published data on other northeastern hosts were included in the model to provide a reference for interpreting the importance of lizards as hosts. The model results indicate that 5-lined skinks (Eumeces fasciatus) are dilution hosts, capable of reducing infection prevalence in the tick population by 10.7-51.5 percentage points, whereas eastern fence lizards (Sceloporus undulatus) are not dilution hosts in the areas studied. Owing to moderate burdens of larval ticks, relatively high population densities, and reservoir incompetence, E. fasciatus may play an important role in the ecology of Lyme disease by reducing vector infection prevalence and associated human risk of infection.     

Borrelia burgdorferi and Babesia microti: efficiency of transmission from reservoirs to vector ticks (Ixodes dammini)

In endemic regions, Peromyscus leucopus, the mouse reservoir of the Lyme disease spirochete (Borrelia burgdorferi) and the piroplasm causing human babesiosis (Babesia microti), is nearly universally infected with both agents. Paradoxically, spirochetal infection is nearly twice as prevalent as is babesial infection in populations of field-collected nymphal Ixodes dammini, the tick vector. In the laboratory, a similarly disproportionate rate of infection was observed among nymphal ticks, feeding as larvae, on either B. burgdorferi- or B. microti-infected mice. Ticks which fed on mice with concurrent spirochetal and babesial infections also exhibited twice the incidence of spirochetal infection over that of the piroplasm. These data suggest that the efficiency of acquisition and transstadial passage of B. burgdorferi and B. microti infection differ by a factor of two. This discrepancy may explain differences observed both in the prevalence of infection in ticks collected in the field, as well as the apparently greater risk of spirochetal infection to humans in endemic areas.