Projecting the potential distribution of ticks in China under climate and land use change

Ticks are known as vectors of several pathogens causing various human and animal diseases including Lyme borreliosis, tick-borne encephalitis, and Crimean-Congo hemorrhagic fever. While China is known to have more than 100 tick species well distributed over the country, our knowledge on the likely distribution of ticks in the future remains very limited, which hinders the prevention and control of the risk of tick-borne diseases. In this study, we selected four representative tick species which have different regional distribution foci in mainland China. i.e., Dermacentor marginatus, Dermacentor silvarum, Haemaphysalis longicornis and Ixodes granulatus. We used the MaxEnt model to identify the key environmental factors of tick occurrence and map their potential distributions in 2050 under four combined climate and socioeconomic scenarios (i.e., SSP1-RCP2.6, SSP2-RCP4.5, SSP3-RCP7.0 and SSP5-RCP8.5). We found that the extent of the urban fabric, cropland and forest, temperature annual range and precipitation of the driest month were the main determinants of the potential distributions of the four tick species. Under the combined scenarios, with climate warming, the potential distributions of ticks shifted to further north in China. Due to a decrease in the extent of forest, the distribution probability of ticks declined in central and southern China. In contrast with previous findings on an estimated amplification of tick distribution probability under the extreme emission scenario (RCP8.5), our studies projected an overall reduction in the distribution probability under RCP8.5, owing to an expected effect of land use. Our results could provide new data to help identify the emerging risk areas, with amplifying suitability for tick occurrence, for the prevention and control of tick-borne zoonoses in mainland China. Future directions are suggested towards improved quantity and quality of the tick occurrence database, comprehensiveness of factors and integration of different modelling approaches, and capability to model pathogen spillover at the human-tick interface.     

Modelling the effect of temperature variation on the seasonal dynamics of Ixodes ricinus tick populations

Seasonal variation in temperature is known to drive annual patterns of tick activity and can influence the dynamics of tick-borne diseases. An age-structured model of the dynamics of Ixodes ricinus populations was developed to explore how changes in average temperature and different levels of temperature variability affect seasonal patterns of tick activity and the transmission of tick-borne diseases. The model produced seasonal patterns of tick emergence that are consistent with those observed throughout Great Britain. Varying average temperature across a continuous spectrum produced a systematic pattern in the times of peak emergence of questing ticks which depends on cumulative temperature over the year. Examination of the effects of between-year stochastic temperature variation on this pattern indicated that peak emergence times are more strongly affected by temperature stochasticity at certain levels of average temperature. Finally the model was extended to give a simple representation of the dynamics of a tick-borne disease. A threshold level of annual cumulative temperature was identified at which disease persistence is sensitive to stochastic temperature variation. In conclusion, the effect of changing patterns of temperature variation on the dynamics of I. ricinus ticks and the diseases they transmit may depend on the cumulative temperature over the year and will therefore vary across different locations. The results also indicate that diapause mechanisms have an important influence on seasonal patterns of tick activity and require further study.     

Projected effects of climate change on tick phenology and fitness of pathogens transmitted by the North American tick Ixodes scapularis

Ixodes scapularis is the principal tick vector of the Lyme borreliosis agent Borrelia burgdorferi and other tick-borne zoonoses in northeastern North America. The degree of seasonal synchrony of nymphal and larval ticks may be important in influencing the basic reproductive number of the pathogens transmitted by I. scapularis. Because the seasonal phenology of tick vectors is partly controlled by ambient temperature, climate and climate change could shape the population biology of tick-borne pathogens. We used projected monthly normal temperatures, obtained from the second version of the Canadian Coupled Global Climate Model (CGCM2) under emissions scenario A2 of the Intergovernmental Panel on Climate Change for a site in southern Ontario, Canada, to simulate the phenology of I. scapularis in a mathematical model. The simulated seasonal abundance of ticks then determined transmission of three candidate pathogens amongst a population of white-footed mice (Peromyscus leucopus) using a susceptible-infected-recovered (SIR) model. Fitness of the different pathogens, in terms of resilience to changes in tick and rodent mortality, minima for infection duration, transmission efficiency and particularly any additional mortality of rodents specifically associated with infection, varied according to the seasonal pattern of immature tick activity, which was different under the temperature conditions projected for the 2020s, 2050s and 2080s. In each case, pathogens that were long-lived, highly transmissible and had little impact on rodent mortality rates were the fittest. However, under the seasonal tick activity patterns projected for the 2020s and 2050s, the fitness of pathogens that are shorter-lived, less efficiently transmitted, and more pathogenic to their natural hosts, increased. Therefore, climate change may affect the frequency and distribution of I. scapularis-borne pathogens and alter their evolutionary trajectories.     

Attachment site selection of ticks on roe deer, <Emphasis Type="Italic">Capreolus capreolus</Emphasis>

The spatio-temporal attachment site patterns of ticks feeding on their hosts can be of significance if co-feeding transmission (i.e. from tick to tick without a systemic infection of the host) of pathogens affects the persistence of a given disease. Using tick infestation data on roe deer, we analysed preferred attachment sites and niche width of Ixodes ticks (larvae, nymphs, males, females) and investigated the degree of inter- and intrastadial aggregation. The different development stages showed rather consistent attachment site patterns and relative narrow feeding site niches. Larvae were mostly found on the head and on the front legs of roe deer, nymphs reached highest densities on the head and highest adult densities were found on the neck of roe deer. The tick stages feeding (larvae, nymphs, females) on roe deer showed high degrees of intrastadial spatial aggregation, whereas males did not. Male ticks showed large feeding site overlap with female ticks. Feeding site overlap between larval-female and larval-nymphal ticks did occur especially during the months May–August on the head and front legs of roe deer and might allow pathogen transmission via co-feeding. Tick density, niche width and niche overlap on roe deer are mainly affected by seasonality, reflecting seasonal activity and abundance patterns of ticks. Since different tick development stages occur spatially and temporally clustered on roe deer, transmission experiments of tick-borne pathogens are urgently needed.     

Thresholds for disease persistence in models for tick-borne infections including non-viraemic transmission,extended feeding and tick aggregation

Lyme disease and Tick-Borne Encephalitis (TBE) are two emergent tick-borne diseases transmitted by the widely distributed European tick Ixodes ricinus. The life cycle of the vector and the number of hosts involved requires the development of complex models which consider different routes of pathogen transmission including those occurring between ticks that co-feed on the same host. Hence, we consider here a general model for tick-borne infections. We assumed ticks feed on two types of host species, one competent for viraemic transmission of infection, the second incompetent but included a third transmission route through non-viraemic transmission between ticks co-feeding on the same host. Since a blood meal lasts for several days these routes could lead to interesting nonlinearities in transmission rates, which may have important effects.We derive an explicit formula for the threshold for disease persistence in the case of viraemic transmission, also for the case of viraemic and non-viraemic transmission. From this formula, the effect of parameters on the persistence of infection can be determined. When only viraemic transmission occurs, we confirm that, while the density of the competent host has always a positive effect on infection persistence, the density of the incompetent host may have either a positive effect, by amplifying tick population, or a negative ("dilution") effect, by wasting tick bites on an incompetent host. With non-viraemic transmission, the "dilution" effect becomes less relevant. On the other hand, if the nonlinearity due to extended feeding is included, the dilution effect always occurs, but often at unrealistically high host densities. Finally, we incorporated the effects of tick aggregation on the hosts and correlation of tick stages and found that both had an important effect on infection persistence, if non-viraemic transmission occurred.     

Effect of host populations on the intensity of ticks and the prevalence of tick-borne pathogens: how to interpret the results of deer exclosure experiments

Deer are important blood hosts for feeding Ixodes ricinus ticks but they do not support transmission of many tick-borne pathogens, so acting as dead-end transmission hosts. Mathematical models show their role as tick amplifiers, but also suggest that they dilute pathogen transmission, thus reducing infection prevalence. Empirical evidence for this is conflicting: experimental plots with deer removal (i.e. deer exclosures) show that the effect depends on the size of the exclosure. Here we present simulations of dynamic models that take into account different tick stages, and several host species (e.g. rodents) that may move to and from deer exclosures; models were calibrated with respect to Ixodes ricinus ticks and tick-borne encephalitis (TBE) in Trentino (northern Italy). Results show that in small exclosures, the density of rodent-feeding ticks may be higher inside than outside, whereas in large exclosures, a reduction of such tick density may be reached. Similarly, TBE prevalence in rodents decreases in large exclosures and may be slightly higher in small exclosures than outside them. The density of infected questing nymphs inside small exclosures can be much higher, in our numerical example almost twice as large as that outside, leading to potential TBE infection risk hotspots.     

Comparison of tick-borne microorganism communities in Ixodes spp. of the Ixodes ricinus species complex at distinct geographical regions

Characterizing the tick-borne microorganism communities of Ixodes ricinus (sheep tick) and Ixodes persulcatus (taiga tick) from the I. ricinus species complex in distinct geographical regions of Eastern Europe and European Russia, we demonstrated differences between the two ticks. Taiga ticks were more frequently mono- and co-infected than sheep ticks: 24.4 % (45/184 tested ticks) versus 17.5 % (52/297) and 4.3 % (8/184) versus 3.4 % (10/297), respectively. Ginsberg co-infection index values were significant at the various sites. Diversity of the tick-borne microorganism communities was estimated by the Shannon index, reaching values of 1.71 ± 0.46 and 1.20 ± 0.15 at the sheep-tick and the taiga-tick harbored sites, respectively. Richness of the tick-borne microorganism community in the sheep tick collection sites was about twice the value of the taiga tick collection sites. Future investigations are warranted to further characterize the peculiarities of the tick-borne microorganism communities among the ticks of the Ixodes ricinus complex.     

Impact of biodiversity and seasonality on Lyme-pathogen transmission

Lyme disease imposes increasing global public health challenges. To better understand the joint effects of seasonal temperature variation and host community composition on the pathogen transmission, a stage-structured periodic model is proposed by integrating seasonal tick development and activity, multiple host species and complex pathogen transmission routes between ticks and reservoirs. Two thresholds, one for tick population dynamics and the other for Lyme-pathogen transmission dynamics, are identified and shown to fully classify the long-term outcomes of the tick invasion and disease persistence. Seeding with the realistic parameters, the tick reproduction threshold and Lyme disease spread threshold are estimated to illustrate the joint effects of the climate change and host community diversity on the pattern of Lyme disease risk. It is shown that climate warming can amplify the disease risk and slightly change the seasonality of disease risk. Both the “dilution effect” and “amplification effect” are observed by feeding the model with different possible alternative hosts. Therefore, the relationship between the host community biodiversity and disease risk varies, calling for more accurate measurements on the local environment, both biotic and abiotic such as the temperature and the host community composition.     

Tick-borne infections of animals and humans: a common ground

A wide variety of pathogens is transmitted from ticks to vertebrates including viruses, bacteria, protozoa and helminths, of which most have a life cycle that requires passage through the vertebrate host. Tick-borne infections of humans, farm and companion animals are essentially associated with wildlife animal reservoirs. While some flying insect-borne diseases of humans such as malaria, filariasis and Kala Azar caused by Leishmania donovani target people as their main host, major tick-borne infections of humans, although potentially causing disease in large numbers of individuals, are typically an infringement of a circulation between wildlife animal reservoirs and tick vectors. While new tick-borne infectious agents are frequently recognised, emerging agents of human tick-borne infections were probably circulating among wildlife animal and tick populations long before being recognised as clinical causes of human disease as has been shown for Borrelia burgdorferi sensu lato. Co-infection with more than one tick-borne infection is common and can enhance pathogenic processes and augment disease severity as found in B. burgdorferi and Anaplasma phagocytophilum co-infection. The role of wild animal reservoirs in co-infection of human hosts appears to be central, further linking human and animal tick-borne infections. Although transmission of most tick-borne infections is through the tick saliva, additional routes of transmission, shown mostly in animals, include infection by oral uptake of infected ticks, by carnivorism, animal bites and transplacentally. Additionally, artificial infection via blood transfusion is a growing threat in both human and veterinary medicine. Due to the close association between human and animal tick-borne infections, control programs for these diseases require integration of data from veterinary and human reporting systems, surveillance in wildlife and tick populations, and combined teams of experts from several scientific disciplines such as entomology, epidemiology, medicine, public health and veterinary medicine.     

Local host-tick coextinction in neotropical forest fragments

Ticks are obligatory parasites with complex life cycles that often depend on larger bodied vertebrates as final hosts. These traits make them particularly sensitive to local coextinction with their host. Loss of wildlife abundance and diversity should thus lead to loss of tick abundance and diversity to the point where only generalist tick species remain. However, direct empirical tests of these hypotheses are lacking, despite their relevance to our understanding of tick-borne disease emergence in disturbed environments. Here, we compare vertebrate and tick communities across 12 forest islands and peninsulas in the Panama Canal that ranged 1000-fold in size (2.6–2811.3?ha). We used drag sampling and camera trapping to directly assess the abundance and diversity of communities of questing ticks and vertebrate hosts. We found that the abundance and species richness of ticks were positively related to those of wildlife. Specialist tick species were only present in fragments where their final hosts were found. Further, less diverse tick communities had a higher relative abundance of the generalist tick species Amblyomma oblongoguttatum, a potential vector of spotted fever group rickettsiosis. These findings support the host-parasite coextinction hypothesis, and indicate that loss of wildlife can indeed have cascading effects on tick communities. Our results also imply that opportunities for pathogen transmission via generalist ticks may be higher in habitats with degraded tick communities. If these patterns are general, then tick identities and abundances serve as useful bioindicators of ecosystem health, with low tick diversity reflecting low wildlife diversity and a potentially elevated risk of interspecific disease transmission via remaining host species and generalist ticks.     

A comprehensive multiple matrix model representing the life cycle of the tick that transmits the agent of Lyme disease.

An extension of Leslie matrix methodology was developed to describe the life cycle of Ixodes dammini, the tick that transmits the agent of Lyme disease in eastern North America. Thereby, we described the seasonally changing pattern of interactions of the tick with its various hosts in a well-studied site on Nantucket Island, Massachusetts. Particular numerical values representing the site were estimated mainly on the basis of published values and interpreted on the basis of experience. The model was used to predict seasonal abundance and annual rate of increase of this vector tick. Although these ticks quest for hosts during two consecutive feeding seasons, all but a few feed during the first such season. The relative distribution of developmental stages of the tick stabilized after 35 years. Abundance of the simulated population increased exponentially and doubled every 6 years. The simulations produced realistic seasonal feeding distributions of the ticks in its various developmental stages.     

Competition between strains of Borrelia afzelii in the host tissues and consequences for transmission to ticks

Pathogen species often consist of genetically distinct strains, which can establish mixed infections or coinfections in the host. In coinfections, interactions between pathogen strains can have important consequences for their transmission success. We used the tick-borne bacterium Borrelia afzelii, which is the most common cause of Lyme disease in Europe, as a model multi-strain pathogen to investigate the relationship between coinfection, competition between strains, and strain-specific transmission success. Mus musculus mice were infected with one or two strains of B. afzelii, strain transmission success was measured by feeding ticks on mice, and the distribution of each strain in six different mouse organs and the ticks was measured using qPCR. Coinfection and competition reduced the tissue infection prevalence of both strains and changed their bacterial abundance in some tissues. Coinfection and competition also reduced the transmission success of the B. afzelii strains from the infected hosts to feeding ticks. The ability of the B. afzelii strains to establish infection in the host tissues was strongly correlated with their transmission success to the tick vector. Our study demonstrates that coinfection and competition between pathogen strains inside the host tissues can have major consequences for their transmission success.Subject terms: Microbial ecology, Bacteria     

Altitudinal patterns of tick and host abundance: a potential role for climate change in regulating tick-borne diseases?

The impact of climate change on vector-borne infectious diseases is currently controversial. In Europe the primary arthropod vectors of zoonotic diseases are ticks, which transmit Borrelia burgdorferi sensu lato (the agent of Lyme disease), tick-borne encephalitis virus and louping ill virus between humans, livestock and wildlife. Ixodes ricinus ticks and reported tick-borne disease cases are currently increasing in the UK. Theories for this include climate change and increasing host abundance. This study aimed to test how I. ricinus tick abundance might be influenced by climate change in Scotland by using altitudinal gradients as a proxy, while also taking into account the effects of hosts, vegetation and weather effects. It was predicted that tick abundance would be higher at lower altitudes (i.e. warmer climates) and increase with host abundance. Surveys were conducted on nine hills in Scotland, all of open moorland habitat. Tick abundance was positively associated with deer abundance, but even after taking this into account, there was a strong negative association of ticks with altitude. This was probably a real climate effect, with temperature (and humidity, i.e. saturation deficit) most likely playing an important role. It could be inferred that ticks may become more abundant at higher altitudes in response to climate warming. This has potential implications for pathogen prevalence such as louping ill virus if tick numbers increase at elevations where competent transmission hosts (red grouse Lagopus lagopus scoticus and mountain hares Lepus timidus) occur in higher numbers.     

Influence of the Biotope on the Tick Infestation of Cattle and on the Tick-Borne Pathogen Repertoire of Cattle Ticks in Ethiopia

Background

The majority of vector-borne infections occur in the tropics, including Africa, but molecular eco-epidemiological studies are seldom reported from these regions. In particular, most previously published data on ticks in Ethiopia focus on species distribution, and only a few molecular studies on the occurrence of tick-borne pathogens or on ecological factors influencing these. The present study was undertaken to evaluate, if ticks collected from cattle in different Ethiopian biotopes harbour (had access to) different pathogens.

Methods

In South-Western Ethiopia 1032 hard ticks were removed from cattle grazing in three kinds of tick biotopes. DNA was individually extracted from one specimen of both sexes of each tick species per cattle. These samples were molecularly analysed for the presence of tick-borne pathogens.

Results

Amblyomma variegatum was significantly more abundant on mid highland, than on moist highland. Rhipicephalus decoloratus was absent from savannah lowland, where virtually only A. cohaerens was found. In the ticks Coxiella burnetii had the highest prevalence on savannah lowland. PCR positivity to Theileria spp. did not appear to depend on the biotope, but some genotypes were unique to certain tick species. Significantly more A. variegatum specimens were rickettsia-positive, than those of other tick species. The presence of rickettsiae (R. africae) appeared to be associated with mid highland in case of A. variegatum and A. cohaerens. The low level of haemoplasma positivity seemed to be equally distributed among the tick species, but was restricted to one biotope type.

Conclusions

The tick biotope, in which cattle are grazed, will influence not only the tick burden of these hosts, but also the spectrum of pathogens in their ticks. Thus, the presence of pathogens with alternative (non-tick-borne) transmission routes, with transstadial or with transovarial transmission by ticks appeared to be associated with the biotope type, with the tick species, or both, respectively.     

Molecular Detection of Tick-Borne Pathogens in Humans with Tick Bites and Erythema Migrans,in the Netherlands

BackgroundTick-borne diseases are the most prevalent vector-borne diseases in Europe. Knowledge on the incidence and clinical presentation of other tick-borne diseases than Lyme borreliosis and tick-borne encephalitis is minimal, despite the high human exposure to these pathogens through tick bites. Using molecular detection techniques, the frequency of tick-borne infections after exposure through tick bites was estimated.MethodsTicks, blood samples and questionnaires on health status were collected from patients that visited their general practitioner with a tick bite or erythema migrans in 2007 and 2008. The presence of several tick-borne pathogens in 314 ticks and 626 blood samples of this cohort were analyzed using PCR-based methods. Using multivariate logistic regression, associations were explored between pathogens detected in blood and self-reported symptoms at enrolment and during a three-month follow-up period.ResultsHalf of the ticks removed from humans tested positive for Borrelia burgdorferi sensu lato, Anaplasma phagocytophilum, Candidatus Neoehrlichia mikurensis, Rickettsia helvetica, Rickettsia monacensis, Borrelia miyamotoi and several Babesia species. Among 92 Borrelia burgdorferi s. l. positive ticks, 33% carried another pathogen from a different genus. In blood of sixteen out of 626 persons with tick bites or erythema migrans, DNA was detected from Candidatus Neoehrlichia mikurensis (n = 7), Anaplasma phagocytophilum (n = 5), Babesia divergens (n = 3), Borrelia miyamotoi (n = 1) and Borrelia burgdorferi s. l. (n = 1). None of these sixteen individuals reported any overt symptoms that would indicate a corresponding illness during the three-month follow-up period. No associations were found between the presence of pathogen DNA in blood and; self-reported symptoms, with pathogen DNA in the corresponding ticks (n = 8), reported tick attachment duration, tick engorgement, or antibiotic treatment at enrolment.ConclusionsBased on molecular detection techniques, the probability of infection with a tick-borne pathogen other than Lyme spirochetes after a tick bite is roughly 2.4%, in the Netherlands. Similarly, among patients with erythema migrans, the probability of a co-infection with another tick-borne pathogen is approximately 2.7%. How often these infections cause disease symptoms or to what extend co-infections affect the course of Lyme borreliosis needs further investigations.     

The effect of landscape heterogeneity and host movement on a tick-borne pathogen

Landscape heterogeneity can be instrumental in determining local disease risk, pathogen persistence and spread. This is because different landscape features such as habitat type determine the abundance and spatial distributions of hosts and pathogen vectors. Therefore, disease prevalence and distribution are intrinsically linked to the hosts and vectors that utilise the different habitats. Here, we develop a simplified reaction diffusion model of the louping-ill virus and red grouse (Lagopus lagopus scoticus) system to investigate the occurrence of a tick-borne pathogen and the effect of host movement and landscape structure. Ticks (Ixodes ricinus), the virus-vector, are dispersed by a virally incompetent tick host, red deer (Cervus elephus), between different habitats, whilst the virus infects only red grouse. We investigated how deer movement between different habitats (forest and moorland) affected tick distribution and hence prevalence of infected ticks and grouse and hence, the effect of habitat size ratio and fragmentation on infection. When habitat type has a role in the survival of the pathogen vector, we demonstrated that habitat fragmentation can have a considerable effect on infection. These results highlight the importance of landscape heterogeneity and the proximity and size of adjacent habitats when predicting disease risk in a particular location. In addition, this model could be useful for other pathogen systems with generalist vectors and may inform policy on possible disease management strategies that incorporate host movements.     

Saliva,Salivary Gland,and Hemolymph Collection from Ixodes scapularis Ticks

Ticks are found worldwide and afflict humans with many tick-borne illnesses. Ticks are vectors for pathogens that cause Lyme disease and tick-borne relapsing fever (Borrelia spp.), Rocky Mountain Spotted fever (Rickettsia rickettsii), ehrlichiosis (Ehrlichia chaffeensis and E. equi), anaplasmosis (Anaplasma phagocytophilum), encephalitis (tick-borne encephalitis virus), babesiosis (Babesia spp.), Colorado tick fever (Coltivirus), and tularemia (Francisella tularensis) ^1-8. To be properly transmitted into the host these infectious agents differentially regulate gene expression, interact with tick proteins, and migrate through the tick ^3,9-13. For example, the Lyme disease agent, Borrelia burgdorferi, adapts through differential gene expression to the feast and famine stages of the tick''s enzootic cycle ^14,15. Furthermore, as an Ixodes tick consumes a bloodmeal Borrelia replicate and migrate from the midgut into the hemocoel, where they travel to the salivary glands and are transmitted into the host with the expelled saliva ^9,16-19.As a tick feeds the host typically responds with a strong hemostatic and innate immune response ^11,13,20-22. Despite these host responses, I. scapularis can feed for several days because tick saliva contains proteins that are immunomodulatory, lytic agents, anticoagulants, and fibrinolysins to aid the tick feeding ^{3,11,20,21,23}. The immunomodulatory activities possessed by tick saliva or salivary gland extract (SGE) facilitate transmission, proliferation, and dissemination of numerous tick-borne pathogens ^3,20,24-27. To further understand how tick-borne infectious agents cause disease it is essential to dissect actively feeding ticks and collect tick saliva. This video protocol demonstrates dissection techniques for the collection of hemolymph and the removal of salivary glands from actively feeding I. scapularis nymphs after 48 and 72 hours post mouse placement. We also demonstrate saliva collection from an adult female I. scapularis tick.     

An experimental test to compare potential and realised specificity in ticks with different ecologies

The majority of studies on ecological specialisation rely on data reflecting realised specificity, without considering species’ potential specificity. Most species of ticks, a large family of hematophagous ectoparasites, have a narrow host range in nature, but it is unclear whether this is due to host-driven adaptations or other processes (such as off-host abiotic environment). We investigated the potential specificity of two tick species with contrasting ecology by infesting three avian host species that occur in the same off-host macrohabitat but are unequally infested by the ticks in nature (i.e. have contrasting realised specificity). The endophilic specialist tick Ixodes arboricola resides inside the hosts’ nest and has high realised host specificity, whereas the exophilic generalist tick I. ricinus encounters hosts in the field and has very low realised specificity. As hosts, we used great tits (frequently infested by both tick species), blackbirds (frequently infested by I. ricinus but never by I. arboricola) and great spotted woodpeckers (no ticks of either species have been reported). If realised specificity is constrained by host-driven adaptations there should be no differences between potential and realised specificity, whereas if realised specificity is constrained by other processes potential specificity and realised specificity should be different. We found that attachment rates and weight during feeding of I. arboricola were lower on blackbirds than on great tits, whereas there were no such differences for I. ricinus. No ticks of either species attached to woodpeckers. These results indicate that realised host specificity of ticks is, at least partially, constrained by host-driven adaptations. This specificity therefore strongly depends on the ticks’ encounter rates with particular host types, which are affected by the ticks’ off-host ecological requirements, behaviour and life-history characteristics.     

Evaluating the economic damage threshold for bont tick (Amblyomma hebraeum) control in Zimbabwe

Controlling ticks and tick-borne diseases by frequent applications of acaricides (e.g., dipping) is costly, and can leave treated livestock vulnerable to epizootics of tick-borne diseases should the system of applying acaricides break down. The concept of only applying acaricides on an infrequent (strategic) basis often relies on the target tick population displaying a seasonal cycle. However, as adult bont tick (Amblyomma hebraeum) infestations in Zimbabwe's lowveld do not have a strictly seasonal pattern of occurrence, it is recommended that tick control only be applied when bont tick infestations are equal to, or greater than, their economic damage threshold. The economic damage threshold is the minimum average weekly standard female tick burden sufficient to cause damage equal in dollar value to the costs of applying tick control. Assuming that each standard female tick represents a 10 gram weight loss, the economic damage threshold (standard female ticks/week) is equivalent to the ratio of the producer price of beef (liveweight equivalent): per head cost of dipping (Eqn (3)). To illustrate the application of the threshold methodology, it was assumed that the producer price of beef was Z$1.63/kg (U.S.$0.33/kg) and that tick control cost Z$0.29/hd/dip (U.S.$0.06/hd/dip). This gave a threshold of 18 standard female ticks/head/week. Using tick counts obtained from 20 Brahman cattle held at Mbizi in southeastern Zimbabwe, it was shown that for the 1988 calendar year there were only 32 weeks when the economic damage threshold was met or exceeded. This is substantially less that the 44 dippings per year that have been, until very recently, legally required in Zimbabwe. Sensitivity analysis showed that a 10% rise in the cost of dipping reduced to 23 (a 28% decrease) the number of weeks when tick burdens exceeded the economic damage threshold. By further assuming that an acaricide application and residual effects will cause a 3–5 week interval before the next application may be required, the number of weeks when the tick burden was equal to or greater than the threshold of 18 standard females/week fell to just 9–12 weeks. Three factors may cause an alteration in the economic damage threshold: i) tick burdens may cause damage to the udders; ii) secondary infestations (e.g., screw-worm) may cause economic damage; and iii) nutritional stress of the cattle may reduce the actual average per tick weight loss. Until further data becomes available, it is recommended that the economic damage threshold methodology be used as described here, and that farmers closely observe their herds during the suggested weekly tick sampling for udder damage and secondary infestations.     

Seasonal Population Dynamics of Ixodes Ticks and Tick-Borne Encephalitis Virus

Seasonality of the epidemic and epizootic processes of tick-borne encephalitis (TBE) depend on the period of activity of ixodid ticks Ixodes persulcatus Schulze and I. ricinus Linnaeus, which are the main reservoirs and vectors of TBE virus, and also on the process of their activation. The period of activity is the period during which the ticks occur in the active state. Activation is the transition into this state of ticks that moulted from the preceding stage and completed post-moulting development. For I. persulcatus, the first adult ticks generally emerge between April 10 and May 9. Under a variety of natural conditions, activation of adult I. persulcatus after wintering lasts for 45–86 days and this period may be even longer in certain areas of the Far East. The period during which one-half of the entire tick population becomes activated (AT₅₀) comprises no more than 10–20 days. In adult I. ricinus ticks the activation period may last even longer than in I. persulcatus. The data on duration of the period of activity and on activation of larval and nymphal stages of both tick species were considered. Ticks exhausting their nutrient reserves and failing to find a host die quickly. The period during which 50% of the entire tick population die under natural conditions is designated LT₅₀. The main types of I. persulcatus and I. ricinus seasonal activity within their species ranges were reviewed. Data on the relationship between TBE virus reproduction in a natural focus and physiological age, pattern of activation, and seasonal changes in age structure of the tick population were analyzed. Seasonal changes in the prevalence of infection among active unfed adult ticks in a natural population are determined by virus content in individual ticks at the moment of their activation and also by the duration of subsequent virus persistence (the rate of virus loss) in ticks. Apparently, the opportunity and frequency of horizontal TBE virus transmission under natural conditions, change during the season of tick activity.