The Ixodes scapularis Symbiont Rickettsia buchneri Inhibits Growth of Pathogenic Rickettsiaceae in Tick Cells: Implications for Vector Competence

Ixodes scapularis is the primary vector of tick-borne pathogens in North America but notably does not transmit pathogenic Rickettsia species. This tick harbors the transovarially transmitted endosymbiont Rickettsia buchneri, which is widespread in I. scapularis populations, suggesting that it confers a selective advantage for tick survival such as providing essential nutrients. The R. buchneri genome includes genes with similarity to those involved in antibiotic synthesis. There are two gene clusters not found in other Rickettsiaceae, raising the possibility that these may be involved in excluding pathogenic bacteria from the tick. This study explored whether the R. buchneri antibiotic genes might exert antibiotic effects on pathogens associated with I. scapularis. Markedly reduced infectivity and replication of the tick-borne pathogens Anaplasma phagocytophilum, R. monacensis, and R. parkeri were observed in IRE11 tick cells hosting R. buchneri. Using a fluorescent plate reader assay to follow infection dynamics revealed that the presence of R. buchneri in tick cells, even at low infection rates, inhibited the growth of R. parkeri by 86–100% relative to R. buchneri-free cells. In contrast, presence of the low-pathogenic species R. amblyommatis or the endosymbiont R. peacockii only partially reduced the infection and replication of R. parkeri. Addition of host-cell free R. buchneri, cell lysate of R. buchneri-infected IRE11, or supernatant from R. buchneri-infected IRE11 cultures had no effect on R. parkeri infection and replication in IRE11, nor did these treatments show any antibiotic effect against non-obligate intracellular bacteria E. coli and S. aureus. However, lysate from R. buchneri-infected IRE11 challenged with R. parkeri showed some inhibitory effect on R. parkeri infection of treated IRE11, suggesting that challenge by pathogenic rickettsiae may induce the antibiotic effect of R. buchneri. This research suggests a potential role of the endosymbiont in preventing other rickettsiae from colonizing I. scapularis and/or being transmitted transovarially. The confirmation that the observed inhibition is linked to R. buchneri's antibiotic clusters requires further investigation but could have important implications for our understanding of rickettsial competition and vector competence of I. scapularis for rickettsiae.

Tick-borne infections in Central Europe.

This expert opinion discusses how global warming and climate change impact (i) geographic expansion of indigenous tick species; (ii) tick survival and abundance in certain areas including urban parks and gardens; (iii) seasonal tick questing activities, increasing the number of infected hosts and the probability of pathogen transmission; (iv) emergence of new tick-borne disease foci; and (v) changing epidemiological patterns and increasing incidence of tick-borne diseases in endemic foci.

Climate change, ticks and tick-borne pathogens in Northern Europe.

This chapter focuses on the potential impacts of climate change on tick survival, reproduction and distribution as well as the prevalence of tick-borne pathogens in Northern Europe.

Possible Effects of Climate Change on Ixodid Ticks and the Pathogens They Transmit: Predictions and Observations

The global climate has been changing over the last century due to greenhouse gas emissions and will continue to change over this century, accelerating without effective global efforts to reduce emissions. Ticks and tick-borne diseases (TTBDs) are inherently climate-sensitive due to the sensitivity of tick lifecycles to climate. Key direct climate and weather sensitivities include survival of individual ticks, and the duration of development and host-seeking activity of ticks. These sensitivities mean that in some regions a warming climate may increase tick survival, shorten life-cycles and lengthen the duration of tick activity seasons. Indirect effects of climate change on host communities may, with changes in tick abundance, facilitate enhanced transmission of tick-borne pathogens. High temperatures, and extreme weather events (heat, cold, and flooding) are anticipated with climate change, and these may reduce tick survival and pathogen transmission in some locations. Studies of the possible effects of climate change on TTBDs to date generally project poleward range expansion of geographical ranges (with possible contraction of ranges away from the increasingly hot tropics), upslope elevational range spread in mountainous regions, and increased abundance of ticks in many current endemic regions. However, relatively few studies, using long-term (multi-decade) observations, provide evidence of recent range changes of tick populations that could be attributed to recent climate change. Further integrated ‘One Health’ observational and modeling studies are needed to detect changes in TTBD occurrence, attribute them to climate change, and to develop predictive models of public- and animal-health needs to plan for TTBD emergence.

Distribution and aggregation of engorging ticks on body parts of small rodent hosts

Small rodent species are the most important blood source of tick larvae, their role is crucial for tick survival, maintainance and spread in nature. From data of 1409 ticks collected from 282 individuals of three small rodent species live-trapped at a natural habitat, we drew conclusions about aggregation of ticks, the anatomical sites most frequently used for engorgement, different susceptibility of host species to tick infestation. We studied data of various age and sex groups of hosts, individuals of different reproductive status and recaptured rodents. The tick load of A. agrarius and A. flavicollis differed significantly, accumulation of ticks was shown on the auricles of both Apodemus species, and on the throat-neck region of A. agrarius. Old rodents carried more, lactating females less ticks, than the average. Data of recaptured hosts showed, that ticks invade the body via the toes and nose and they are aiming for the auricles. Voles were significantly more heavily infested with nymphs which fed in the auditory tracts, which were free of ticks in mice. The phenomenon of aggregation is important for understanding the ecology of tick-borne diseases, as only a minority of the hosts are responsible for disease risks, and it is particularly important for control strategies that apply acaricides to reservoir hosts.

Pathogens Manipulating Tick Behavior—Through a Glass, Darkly

Pathogens can manipulate the phenotypic traits of their hosts and vectors, maximizing their own fitness. Among the phenotypic traits that can be modified, manipulating vector behavior represents one of the most fascinating facets. How pathogens infection affects behavioral traits of key insect vectors has been extensively investigated. Major examples include Plasmodium, Leishmania and Trypanosoma spp. manipulating the behavior of mosquitoes, sand flies and kissing bugs, respectively. However, research on how pathogens can modify tick behavior is patchy. This review focuses on current knowledge about the behavioral changes triggered by Anaplasma, Borrelia, Babesia, Bartonella, Rickettsia and tick-borne encephalitis virus (TBEV) infection in tick vectors, analyzing their potential adaptive significance. As a general trend, being infected by Borrelia and TBEV boosts tick mobility (both questing and walking activity). Borrelia and Anaplasma infection magnifies Ixodes desiccation resistance, triggering physiological changes (Borrelia: higher fat reserves; Anaplasma: synthesis of heat shock proteins). Anaplasma infection also improves cold resistance in infected ticks through synthesis of an antifreeze glycoprotein. Being infected by Anaplasma, Borrelia and Babesia leads to increased tick survival. Borrelia, Babesia and Bartonella infection facilitates blood engorgement. In the last section, current challenges for future studies are outlined.

Impacts of Deciduous Leaf Litter and Snow Presence on Nymphal Ixodes scapularis (Acari: Ixodidae) Overwintering Survival in Coastal New England, USA

Blacklegged ticks (Ixodes scapularis Say) are the vector for pathogens that cause more cases of human disease than any other arthropod. Lyme disease is the most common, caused by the bacterial spirochete Borrelia burgdorferi (Johnson, Schmid, Hyde, Steigerwalt, and Brenner) in the northeastern United States. Further knowledge of seasonal effects on survival is important for management and modeling of both blacklegged ticks and tick-borne diseases. The focus of our study was on the impact of environmental factors on overwintering success of nymphal blacklegged ticks. In a three-year field study conducted in Connecticut and Maine, we determined that ground-level conditions play an important role in unfed nymphal overwintering survival. Ticks in plots where leaf litter and snow accumulation were unmanipulated had significantly greater survival compared to those where leaf litter was removed (p = 0.045) and where both leaf litter and snow were removed (p = 0.008). Additionally, we determined that the key overwintering predictors for nymphal blacklegged tick survival were the mean and mean minimum temperatures within a year. The findings of this research can be utilized in both small- and large-scale management of blacklegged ticks to potentially reduce the risk and occurrence of tick-borne diseases.

An instance of Boiga dendrophila dendrophila (Boie, 1827) (Reptilia: Colubridae) being parasitized by Amblyomma helvolum Koch, 1844 (Acari: Ixodidae), with comments about the attachment sites of this tick species

Ectoparasites, such as ticks, may exhibit preferences for particular attachment sites on various hosts, since the choice of attachment sites may affect tick survival. Herein we report an instance of a mangrove snake, Boiga dendrophila dendrophila, being parasitized by the tick Amblyomma helvolum, with comments on this tick’s attachment sites. Our collection of A. helvolum appears to be the first record of this species from Sandakan, a city on the northeast coast of Borneo, in the Malaysian state of Sabah.

“Ticking Bomb”: The Impact of Climate Change on the Incidence of Lyme Disease

Lyme disease (LD) is the most common tick-borne disease in North America. It is caused by Borrelia burgdorferi and transmitted to humans by blacklegged ticks, Ixodes scapularis. The life cycle of the LD vector, I. scapularis, usually takes two to three years to complete and goes through three stages, all of which are dependent on environmental factors. Increases in daily average temperatures, a manifestation of climate change, might have contributed to an increase in tick abundance via higher rates of tick survival. Additionally, these environmental changes might have contributed to better host availability, which is necessary for tick feeding and life cycle completion. In fact, it has been shown that both tick activity and survival depend on temperature and humidity. In this study, we have examined the relationship between those climatic variables and the reported incidence of LD in 15 states that contribute to more than 95% of reported cases within the Unites States. Using fixed effects analysis for a panel of 468 U.S. counties from those high-incidence states with annual data available for the period 2000–2016, we have found sizable impacts of temperature on the incidence of LD. Those impacts can be described approximately by an inverted U-shaped relationship, consistent with patterns of tick survival and host-seeking behavior. Assuming a 2°C increase in annual average temperature—in line with mid-century (2036–2065) projections from the latest U.S. National Climate Assessment (NCA4)—we have predicted that the number of LD cases in the United States will increase by over 20 percent in the coming decades. These findings may help improving preparedness and response by clinicians, public health professionals, and policy makers, as well as raising public awareness of the importance of being cautious when engaging in outdoor activities.