Artificial feeding of ixodid ticks

Ixodid ticks are economically important as they cause direct damage to livestock and are vectors of several pathogens that cause diseases in humans and animals. Some of the important tick-borne pathogens of livestock are Theileria parva, T. annulata, Babesia bigemina, B. bovis, Anaplasma marginale and Cowdria ruminantium. These pathogens are responsible for causing enormous losses in livestock. Identification of factors that influence transmission and development of these pathogens in ticks will greatly facilitate development of rational strategies for control of tick-borne diseases. This research has been hampered by the lack of suitable artificial feeding methods. In this paper, Sam Waladde, Aian Young and Subhash Morzaria review recent developments in the artificial feeding of ixodid ticks and evaluate how this method can potentially be exploited. They use an example the transmission of an important livestock pathogen, T. parva, by Rhipicephalus appendiculatus.     

Ticks (Acari: Ixodoidea) in China: Geographical distribution,host diversity,and specificity

Ticks are obligate blood‐sucking ectoparasites, which not only directly damage through bites but also transmit many pathogens. China has a high diversity of tick species, 125 species have been reported, including 111 hard tick and 14 soft tick species. Many of the ticks are important vectors of pathogens, resulting in zoonoses. The dynamics of ticks are affected by both the host and habitat environment. However, systematic studies on the geographical distribution, host diversity, and specificity of ticks are limited in China. To achieve this goal, the relevant available data were summarized and analyzed in this study. Ticks are distributed in all parts of China and Xinjiang has the most records of ticks. The distribution of ticks in adjacent areas is similar, indicating that the habitat environment affects their distribution. Most ticks are widely distributed, whereas some species are endemic to their distributed regions. Ticks are parasitic on mammals, birds, and reptiles, of which mammals are the main host species. Overall, most ticks parasitize different hosts, only a few ticks have strict host specificity, such as ticks that are specifically parasitic on reptiles and bats. In addition, environmental changes and control efforts also influence the dynamics of ticks. These results can better reveal tick biological traits and are valuable for tick control.     

Using genomic approaches to unravel livestock (host)-tick-pathogen interactions

Ticks and tick-borne diseases are a major constraint on livestock farming in many developing countries, which has a huge impact on their economies. Genomic information is becoming more abundant for many of the species involved, which if exploited successfully could be used to develop new control strategies. Here, we review the genomic resources that are now available and discuss how this information is currently being harnessed or can be used in the future to explore the complex interplay that occurs between livestock hosts, tick vectors and tick-borne pathogens.     

The coevolution of ixodid ticks and terrestrial vertebrates

Paleontologic and zoogeographic data speak in favour of Mesozoic origin of ixodid ticks. The absence of strict restrictions for the feeding on unusual species of hosts has caused the domination of polyphagy and oligophagy over monophagy among ixodid ticks. The same peculiarities of ixodid ecology are responsible for a restricted part or absence of phylogenetic parallelism with hosts in their evolution. Primary food relations with reptiles are, apparently, preserved only in the genus Aponomma and in many species of Amblyomma while hosts for most species of other genera are mammals and, to a lesser extent, birds. The number of potential hosts in these species can be much greater than that of real ones. Restrictions in the distribution of some species are connected rather with direct effect of unfavourable environmental factors on their nonparasitic stages of the life cycle than with the absence of suitable hosts. During the evolution of natural landscapes and at a shorter stages under the influence of successions or anthropogenic factors ixodids easily adapt themselves to feeding on new species of hosts. So the differentiation of primary and secondary hosts of these parasites is rather difficult.     

Interactions between ticks and their hosts

Tick populations on hosts are limited by varied mechanisms expressed through natural host-parasite relationships. Tickborne disease losses in nature are minimal because of enzootic stability.Study of these natural interactions between ticks and their hosts provides guidelines for managing interactions with domestic livestock.     

Effects of deer density on tick infestation of rodents and the hazard of tick-borne encephalitis. I: empirical assessment

Tick borne encephalitis (TBE) is endemic to eastern and central Europe with broad temporal and spatial variation in infection risk. Although many studies have focused on understanding the environmental and socio-economic factors affecting exposure of humans to TBE, comparatively little research has been devoted to assessing the underlying ecological mechanisms of TBE occurrence in enzootic cycles, and therefore TBE hazard. The aim of this study was to evaluate the effect of the main ungulate tick hosts on the pattern of tick infestation in rodents and TBE occurrence in rodents and questing adult ticks. In this empirical study, we considered three areas where endemic human TBE occurs and three control sites having no reported human TBE cases. In these six sites located in Italy and Slovakia, we assessed deer density using the pellet group count-plot sampling technique, collected questing ticks, live-trapped rodents (primarily Apodemus flavicollis and Myodes glareolus) and counted ticks feeding on rodents. Both rodents and questing ticks were screened for TBE infection. TBE infection in ticks and rodents was positively associated with the number of co-feeding ticks on rodents and negatively correlated with deer density. We hypothesise that the negative relationship between deer density and TBE occurrence on a local scale (defined by the minimum overlapping area of host species) could be attributed to deer (incompetent hosts) diverting questing ticks from rodents (competent hosts), know as the 'dilution effect hypothesis'. We observed that, after an initial increase, the number of ticks feeding on rodents reached a peak for an intermediate value of estimated deer density and then decreased. Therefore, while at a regional scale, tick host availability has already been shown to be directly correlated with TBE distribution, our results suggest that the interactions between deer, rodents and ticks are much more complex on a local scale, supporting the possibility of a dilution effect for TBE.     

Effect of host lizard anemia on host choice and feeding rate of larval western black-legged ticks (Ixodes pacificus)

Although ticks are known to exhibit preferences among host species, there is little evidence that ticks select hosts within a species based on physiological condition. It may be beneficial for ticks to choose hosts that are easier to feed upon if the ticks can perceive indicative chemical or other signals from the host. For example, if ticks can detect host hematocrit they may choose hosts with high hematocrit, facilitating a faster blood meal. It may similarly be adaptive for ticks to avoid anemic hosts because it may be difficult for them to obtain an adequate meal and feeding duration may be extended. We tested the hypothesis that larval western black-legged ticks (Ixodes pacificus) detect host hematocrit using external cues and choose healthy over anemic hosts, allowing them to feed more quickly. We presented groups of larval ticks with pairs of healthy and anemic male western fence lizards (Sceloporus occidentalis), allowed them to select a host, and measured the feeding duration of the ticks. We found that the ticks did not exhibit a statistically significant preference for healthy over anemic lizards, but that the ticks fed to repletion significantly faster on healthy hosts than on anemic hosts. Larval ticks may not be able to detect external cues indicating the health of the host, at least not in terms of their hematocrit. The extended feeding duration likely reflects the extra time needed for the ticks to concentrate the blood meal of their anemic hosts.     

Predicting larval tick burden on white-footed mice with an artificial neural network

White-footed mice are important hosts for immature blacklegged ticks (Ixodes scapularis) and the most competent reservoir hosts for several tick-borne pathogens, including the agent of Lyme disease, in eastern North America. The distribution of larval ticks on individual mice tends to be highly heterogeneous, potentially resulting in few individual hosts causing the majority of host-to-tick transmission events. In this study, we created an artificial neural network (ANN) model using a 20 year data set from Millbrook, NY, to understand which attributes of mice or the environment predict high larval burden. Furthermore, we performed a sensitivity analysis to explore the importance of, and interactions between, the most influential attributes. Our analysis indicated that highest larval burden is predicted in warmer and drier than average years when host abundance is low, and that climatic conditions and host density are far more important in predicting larval burden than traits of individual mice, a finding that could have human health implications within the context of a warming climate. Practically, our results suggest that instead of basing tick-control treatments on particular attributes of hosts, treatments should be targeted based on climate factors. Additionally, our results highlight the importance of including variable interactions in models aiming to predict vector (tick) aggregation, and, most broadly, demonstrate the utility of ANNs in understanding aggregation of ticks and other vectors.     

Comparison of flagging,walking, trapping,and collecting from hosts as sampling methods for northern deer ticks,Ixodes dammini,and lone-star ticks,Amblyomma americanum (Acari: Ixodidae)

Ticks were sampled by flagging, collecting from the investigator's clothing (walking samples), trapping with dry-ice bait, and collecting from mammal hosts on Fire Island, NY, U.S.A. The habitat distribution of adult deer ticks,Ixodes dammini, was the same in simultaneous collections from the investigator's clothing and from muslin flags. Walking and flagging samples can both be biased by differences between investigators, so the same person should do comparative samples whenever possible. Walking samples probably give a more accurate estimate than flagging samples of the human risk of encountering ticks. However, ticks (such as immatureI. dammini) that seek hosts in leaf litter and ground-level vegetation are poorly sampled by walking collections. These ticks can be sampled by flagging at ground level.Dry-ice-baited tick-traps caught far more lone-star ticks,Amblyomma americanum, than deer ticks, even in areas where deer ticks predominated in flagging samples. In comparisons of tick mobility in the lab, nymphalA. americanum were more mobile than nymphalI. dammini in 84% of the trials. Therefore, the trapping bias may result from increased trap encounter due to more rapid movement byA. americanum, although greater attraction to carbon dioxide may also play a role. Tick traps are useful for intraspecific between-habitat comparisons.Early in their seasonal activity period, larvalI. dammini were better represented in collections from mouse hosts than in flagging samples. Apparently, sampling from favored hosts can detect ticks at low population levels, but often cannot be used to get accurate estimates of pathogen prevalence in questing ticks.     

Distribution,density and migration dynamics of Ixodes ricinus in an area of the Jurassic mountains of Switzerland

In an area of the Jurassic mountains of Switzerland ecological aspects of the tick species Ixodes ricinus were studied. In particular, the influence of temperature, humidity and vegetation parameters on tick density and distribution were investigated several times in six different habitat types. This was carried out on two scales; on a small scale, within a habitat type and on a larger scale, where all six habitat types were included. By a marking experiment, aspects of the migration dynamics of the ticks were studied. The densities of the six habitat types differed significantly in most of the cases. An analysis of the distribution pattern within a habitat type showed a contagious distribution mostly for the forests and the forest edge, whereas a random distribution was found as well, mainly for the meadows. But these distribution patterns tend towards randomness with time. On a small scale none of the factors showed an influence on tick distribution, therefore the most important factor on this scale might be the hosts of Ixodes ricinus. On a larger scale, temperature and humidity do in some cases influence tick densities. The marking experiment suggests an extremely high turnover of the ticks in an area with an average host density, probably due to passive transportation by hosts and to activation of resting individuals.     

Functional conservation of an insect odorant receptor gene across 250 million years of evolution

Pest insects have a profound negative impact on agriculture and human health. Significant global losses of crops, stored agricultural products, timber and livestock can be attributed to damage and destruction by insects . Blood-feeding insects such as mosquitoes, flies and ticks transmit many of humanity's most devastating infectious diseases. Insect-borne diseases account for more than one million annual fatalities, and insect-associated illnesses surpass 300 million annual reported cases . The medical and economic impact of these animals can be ascribed in part to the sensitivity and selectivity of their olfactory systems, essential for location of their preferred plant and animal hosts.     

Factors affecting the distributions of the ticks Amblyomma hebraeum and A. variegatum in Zimbabwe: implications of reduced acaricide usage

The ticks Amblyomma hebraeum and A. variegatum are the main vectors of heartwater, a disease of ruminants caused by Cowdria ruminantium, in the agricultural areas of Zimbabwe. At present, A. hebraeum is widely distributed in the dry southern lowveld, and occurs in at least seven foci in the higher rainfall highveld. Amblyomma variegatum occurs in the Zambezi valley and surrounding dry lowveld areas in the northwest. The distribution of A. hebraeum has changed considerably over the past 70 years, while that of A. variegatum appears to have remained fairly static. The distribution patterns of both species in Zimbabwe display anomalous features; the ticks occur in areas of lowest predicted climatic suitability for survival and development and in areas where the densities of cattle, the most important domestic host, are lowest. The only factor favouring the survival of the species in the lowveld habitats in which they occur is the presence of alternative wildlife hosts for the adult stage. Their absence from more climatically favourable highveld habitats appears to have been the result of intensive acaricide treatment of cattle over a long period and a historic absence of significant numbers of wildlife hosts. Eradication of A. hebraeum and A. variegatum by intensive acaricide treatment of cattle can be achieved in the absence of significant numbers of alternative hosts, because of the long attachment and feeding periods of the adults of these tick species. However, eradication becomes impossible when alternative hosts for the adult stage are present, because a pheromone emitted by attached males attracts the unfed nymphal and adult stages to infested hosts. The unfed ticks are not attracted to uninfested hosts, such as acaricide-treated cattle.Regular acaricide treatment of cattle is expensive and so, for economic reasons, the Government of Zimbabwe is no longer enforcing a policy of strict tick control. It is likely that reduced tick control will result in the spread of Amblyomma ticks to previously uninfested areas. Added to this, recent introductions of various wildlife species to highveld commercial farming areas have created conditions in which the ticks could become established in higher rainfall areas. Amblyomma hebraeum is more likely to spread than A. variegatum, because its adults parasitize a wider range of wildlife hosts (warthogs, medium to large-sized antelope, giraffe, buffalo and rhinoceros), whereas adults of A. variegatum appear to be largely restricted to one wildlife species (buffalo) in Zimbabwe, the distribution of which is now confined to very limited areas of the country, as part of foot and mouth disease control measures. A model to predict the rate of spread of A. hebraeum through the highveld is described.Possible control options for dealing with the spread of Amblyomma ticks and heartwater to previous unaffected highveld areas, include (1) continuation of intensive acaricide treatment of cattle to prevent the spread, (2) establishment of a buffer zone of intensive tick control around affected areas to contain the spread and (3) allow the spread to occur and control heartwater by means of immunization. An economic analysis to determine the costs and benefits of the control options, which takes into account the development of Amblyomma-specific tick control technologies and improved heartwater vaccines, is recommended.Deceased.     

Partitioning the aggregation of parasites on hosts into intrinsic and extrinsic components via an extended Poisson-gamma mixture model

It is well known that parasites are often highly aggregated on their hosts such that relatively few individuals host the large majority of parasites. When the parasites are vectors of infectious disease, a key consequence of this aggregation can be increased disease transmission rates. The cause of this aggregation, however, is much less clear, especially for parasites such as arthropod vectors, which generally spend only a short time on their hosts. Regression-based analyses of ticks on various hosts have focused almost exclusively on identifying the intrinsic host characteristics associated with large burdens, but these efforts have had mixed results; most host traits examined have some small influence, but none are key. An alternative approach, the Poisson-gamma mixture distribution, has often been used to describe aggregated parasite distributions in a range of host/macroparasite systems, but lacks a clear mechanistic basis. Here, we extend this framework by linking it to a general model of parasite accumulation. Then, focusing on blacklegged ticks (Ixodes scapularis) on mice (Peromyscus leucopus), we fit the extended model to the best currently available larval tick burden datasets via hierarchical Bayesian methods, and use it to explore the relative contributions of intrinsic and extrinsic factors on observed tick burdens. Our results suggest that simple bad luck-inhabiting a home range with high vector density-may play a much larger role in determining parasite burdens than is currently appreciated.     

Methods currently used for the control of multi-host ticks: their validity and proposals for future control strategies

Ticks have a world distribution and pest status reputation of hampering livestock production through transmission of fatal disease such as theileriosis, anaplasmosis, babesiosis, etc. and bites which cause blood loss, pain and other debilitating effects. Control of ticks is largely more effective on the host. The multi-host ticks spend most of the time off the host with short feeding periods of between four and ten days. Chemical control using dips or sprays has been the traditional method of attempting to kill these ticks during the infestation period. In many situations control, using acaricides, has been quite successful. This has been possible through correct timing of the ticks' seasonal activity and feeding periods to determine the application interval, and efficacy of the acaricides. However, the rising costs of acaricides have made it almost impossible to use these chemicals on a regular basis according to the pest problem. This is particularly true in many Third World tropical countries in which tick-associated problems are more pronounced. This has necessitated the search for alternative tick control methods on an integrated approach to pest management. For this reason, vaccination against ticks and breeding for host resistance against ticks are being studied in the hope that future control strategies will involve only the economically effective acaricide application in conjunction with these and other methods.     

Microhabitat choice and host-seeking behaviour of the tuatara tick,

Understanding the factors that influence patterns of ectoparasite infestation within wildlife populations involves knowledge of the mechanisms that influence host infestation. For ectoparasitic ticks, knowing where ticks might occur in the off-host environment and how they locate their hosts is essential to understanding patterns of ectoparasite infestation. The tuatara tick (Amblyomma sphenodonti) parasitises the tuatara (Sphenodon punctatus) on Stephens Island, New?Zealand. We completed a series of laboratory microcosm experiments to examine daily activity patterns, microhabitat preferences and host-seeking behaviour by Amblyomma sphenodonti. Firstly, to determine the diel activity pattern of ticks, we observed the behaviour of ticks every 2?h over a 48-h period. We then tested the preferences of ticks for soil moisture, soil texture and shade by offering different pairs of substrate conditions. Last, to determine what cues ticks used to locate their hosts, we tested the response of ticks to filter paper infused with host scent or excrement. Ticks were most active at night. They also showed a significant preference for moister, coarser and shaded substrates 12 h after the start of the experiment. Ticks did not show an immediate response to either of the two host stimuli, but after 12 h showed a significant preference for host scent and avoided host excreta. We suggest that the microhabitat preferences of ticks reflect conditions within host refuges (burrows), and that the delayed response to host odour suggests ticks could use host scent to identify substrates frequented by hosts.     

Feeding of Ticks on Animals for Transmission and Xenodiagnosis in Lyme Disease Research

Transmission of the etiologic agent of Lyme disease, Borrelia burgdorferi, occurs by the attachment and blood feeding of Ixodes species ticks on mammalian hosts. In nature, this zoonotic bacterial pathogen may use a variety of reservoir hosts, but the white-footed mouse (Peromyscus leucopus) is the primary reservoir for larval and nymphal ticks in North America. Humans are incidental hosts most frequently infected with B. burgdorferi by the bite of ticks in the nymphal stage. B. burgdorferi adapts to its hosts throughout the enzootic cycle, so the ability to explore the functions of these spirochetes and their effects on mammalian hosts requires the use of tick feeding. In addition, the technique of xenodiagnosis (using the natural vector for detection and recovery of an infectious agent) has been useful in studies of cryptic infection. In order to obtain nymphal ticks that harbor B. burgdorferi, ticks are fed live spirochetes in culture through capillary tubes. Two animal models, mice and nonhuman primates, are most commonly used for Lyme disease studies involving tick feeding. We demonstrate the methods by which these ticks can be fed upon, and recovered from animals for either infection or xenodiagnosis.     

Indirect evidence of density‐dependent population regulation in Aponomma hydrosauri (Acari: Ixodidae), an ectoparasite of reptiles

Abstract The extent to which density‐dependent processes regulate natural populations is the subject of an ongoing debate. We contribute evidence to this debate showing that density‐dependent processes influence the population dynamics of the ectoparasite Aponomma hydrosauri (Acari: Ixodidae), a tick species that infests reptiles in Australia. The first piece of evidence comes from an unusually long‐term dataset on the distribution of ticks among individual hosts. If density‐dependent processes are influencing either host mortality or vital rates of the parasite population, and those distributions can be approximated with negative binomial distributions, then general host–parasite models predict that the aggregation coefficient of the parasite distribution will increase with the average intensity of infections. We fit negative binomial distributions to the frequency distributions of ticks on hosts, and find that the estimated aggregation coefficient k increases with increasing average tick density. This pattern indirectly implies that one or more vital rates of the tick population must be changing with increasing tick density, because mortality rates of the tick's main host, the sleepy lizard, Tiliqua rugosa, are unaffected by changes in tick burdens. Our second piece of evidence is a re‐analysis of experimental data on the attachment success of individual ticks to lizard hosts using generalized linear modelling. The probability of successful engorgement decreases with increasing numbers of ticks attached to a host. This is direct evidence of a density‐dependent process that could lead to an increase in the aggregation coefficient of tick distributions described earlier. The population‐scale increase in the aggregation coefficient is indirect evidence of a density‐dependent process or processes sufficiently strong to produce a population‐wide pattern, and thus also likely to influence population regulation. The direct observation of a density‐dependent process is evidence of at least part of the responsible mechanism.     

Adaptive timing of detachment in a tick parasitizing hole-nesting birds

In non-permanent parasites with low intrinsic mobility such as ticks, dispersal is highly dependent on host movements as well as the timing of separation from the hosts. Optimal detachment behaviour is all the more crucial in nidicolous ticks as the risk of detaching in non-suitable habitat is high. In this study, we experimentally investigated the detachment behaviour of Ixodes arboricola, a nidicolous tick that primarily infests birds roosting in tree-holes. We infested great tits with I. arboricola larvae or nymphs, and submitted the birds to 2 experimental treatments, a control treatment in which birds had normal access to nest boxes and an experimental treatment, in which the birds were prevented access to their nest boxes for varying lengths of time. In the control group, most ticks detached within 5 days, whereas in the experimental group, ticks remained on the bird for as long as the bird was prevented access (up to 14 days). This prolonged attachment caused a decrease in survival and engorgement weight in nymphs, but not in larvae. The capacity of I. arboricola larvae to extend the duration of attachment in non-suitable environments with no apparent costs, may be an adaptation to unpredictable use of cavities by roosting hosts during winter, and at the same time may facilitate dispersal of the larval instars.     

Persistence of tick-borne virus in the presence of multiple host species: tick reservoirs and parasite mediated competition.

Tick-borne viruses in tropical and temperate parts of the world have a significant impact on human, livestock and wildlife hosts both directly, through mortality/morbidity, and economically. Since the ticks have multiple life stages and can utilize a large range of host species our understanding of the dynamics of these infections is often not clear. In this paper we consider the impact of a population which is a tick host but non-viraemic on one which is both a tick host and viraemic. We present two simple deterministic models and use joint threshold density curves to illustrate the basic reproductive ratios of both the ticks and the virus. We find that the non-viraemic hosts can have considerable impact on the viraemic host. Either they amplify the tick population and cause the virus to persist, or they dilute the infection and cause it to die out. A general model framework is presented here but a special case of this model describes the red grouse-hare-Louping-ill system.     

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.