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Keywords:

  • Dermacentor reticulatus;
  • Ixodes canisuga;
  • Ixodes hexagonus;
  • Ixodes ricinus;
  • dogs;
  • tick;
  • Great Britain

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Current concerns over the potential impacts of climate change and the increased movement between countries of people and companion animals on the distribution of ectoparasites, highlight the need for accurate understanding of existing prevalence patterns. Without these future changes will not be detected. Here, the distribution and prevalence of tick infestations of domestic dogs in Great Britain were examined. A total of 173 veterinary practices were recruited to monitor tick attachment to dogs in their local areas between March and October 2009. Practices selected five dogs at random each week from those brought to the surgery and undertook a thorough, standardized examination for ticks. Each veterinary practice participated for 3 months before being replaced. Any ticks identified were collected and a sample sent to the investigators for identification, along with a clinical history of the dog. A total of 3534 dogs were examined; 810 dogs were found to be carrying at least one tick. Ixodes ricinus (Linnaeus) (Acari: Ixodidae) was identified in 72.1% of cases, Ixodes hexagonus Leach in 21.7% and Ixodes canisuga Johnston in 5.6% of cases. Five samples of Dermacentor reticulatus (Fabricius) (Acari: Ixodidae) were also found, adding to the growing evidence that an established population of D. reticulatus now exists in south-eastern England. Almost all the ticks found were adults. Overall, 19.2% of the veterinary practices reported no tick detections, 50% reported that ≥14.9% of the dogs seen were infested and 14.6% reported that >50% of the dogs inspected carried ticks. The estimated incidence of tick attachment was 0.013 per day in March (lowest) and 0.096 per day in June (highest). A number of risk factors affected the likelihood of tick attachment on dogs. Gundog, terrier and pastoral breed groups were more likely to carry ticks, as were non-neutered dogs. Dogs with shorter hair were less likely to have ticks, and dogs were most likely to carry a tick in June. This study is of value because, unusually, it presents the results of a randomized sample of dogs and gives a prevalence which is higher than those previously recorded in Great Britain.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Ticks are effective disease vectors, second only to mosquitoes in transmitting infectious disease (Le Bars, 2009). Approximately 10% of all known tick species act as vectors of various pathogens to humans and animals (Jongejan & Uilenberg, 2004). The potential impacts of climate changes and increased movement between countries of people and their companion animals on the distribution of ticks is therefore an area of increasing current concern. Historical records have been studied to determine the distribution of Ixodes ricinus (Linnaeus) in the British Isles (Pietzsch et al., 2005) and recent studies suggest that tick abundance is increasing in the U.K. (Kirby et al., 2004; Scharlemann et al., 2008). It has been estimated that the perceived distribution of ticks in Great Britain has expanded by 17% and abundances of ticks have increased at 73% of locations surveyed (Scharlemann et al., 2008). Although the latter result was based only on the perceptions of landowners, it matches patterns reported throughout Europe. The geographic range of I. ricinus is thought to have extended northwards and upwards to altitudes of up to 1100 m a.s.l. in Central Europe (Daniel et al., 2003); previously this tick had not been found above 700 m a.s.l. (Černýet al., 1965). Associated with this, tick-borne encephalitis has been found recently for the first time in mountainous regions of the Czech Republic (Daniel et al., 2004) and human cases of Lyme disease increased 30-fold between 1999 and 2008 in Scotland (Health Protection Scotland, 2009). The distribution of the European meadow tick, Dermacentor reticulatus (Fabricius), an important vector of canine babesiosis in Europe, is also believed to have extended northwards and populations have become established in Poland (Zygner et al., 2009), Belgium (Beugnet, 2009), Germany (Dautel et al., 2008), the Netherlands (Matjila et al., 2005) and southern England (Jameson & Medlock, 2009).

Tick abundance and activity might be expected to respond quickly to changes in weather patterns because the activity patterns of ticks are strongly influenced by environmental conditions. It has been suggested that, in order to avoid desiccation, questing activity in I. ricinus occurs only if the average daily temperature is ≥7 °C (Gardiner & Gettingby, 1983) and relative humidity (RH) in the tick's microhabitat is ≥80% (MacLeod, 1935). However, it is likely that local adaptation in temperature and humidity thresholds exists because I. ricinus nymphs have been found questing at temperatures as low as 2.5 °C and at 45% RH in central Europe (Hubálek et al., 2003).

In the U.K., the most common and extensively studied species is the sheep tick, I. ricnius, which is a vector of a range of rickettsial, bacterial and viral pathogens. The most notable of these that pose a threat to dogs are Anaplasma phagocytophilum, Borrelia burgdorferi and Babesia spp. (Gray, 1991; Robinson et al., 2009). The abundance of questing I. ricinus is associated with high host availability (especially grazing livestock) and high humidity in poorly draining habitats (Medlock et al., 2008). Milne (1944) also noted that geology with poor drainage, such as thick bedrock, leads to a moister microhabitat and a greater abundance of I. ricinus. A second species of clinical significance to dogs in the U.K. is the hedgehog tick, Ixodes hexagonus Leach, which feeds on domestic dogs and cats. Urban parks and gardens in particular may support significant populations of this tick (Ogden et al., 2000) because large proportions of them comprise the ecotonal habitat favoured by hedgehogs, Erinaceus europaeus (Erinaceomorpha: Erinaceidae) (Morris, 1991). The third most common tick species found on dogs is Ixodes canisuga Johnston, populations of which are usually associated with animals in boarding kennels (Ogden et al., 2000).

It is essential to obtain accurate initial surveillance data and to map existing prevalences and distributions because, without this information, it will not be possible to detect future changes in tick prevalence and distribution patterns. Many previous studies have examined distribution patterns of questing ticks in the field, sampled by blanket dragging, and implicitly assumed that this correlates directly with tick challenge and biting rates. Others have examined tick abundance on hosts, but almost always from animals that were known to be infested, thereby making it impossible to determine a true prevalence. The aims of the current study, therefore, were to consider the prevalence of tick infestation in a randomized sample of domestic dogs, to assess the seasonal and spatial distribution patterns of infestation, to determine the tick attachment rate and to consider the risk factors associated with attachment.

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Veterinary practice selection

From March to October 2009, veterinary practices in England, Scotland and Wales were enlisted through Merial Animal Health Ltd. No selection criteria were imposed other than that practices were required to be small animal or mixed practices that saw a minimum of five dogs per week and were willing to participate in the study for at least 3 months. A minimum sample size was calculated based on ticks present on dogs at only 5% of veterinary practices. At least 56 practices would be required at any one time to allow ticks to be detected with 95% confidence (Win Episcope 2.0, Thrusfield et al., 2001).

Study design

Participating veterinary practices were asked to examine five dogs per week for ticks over a 2- or 3-month period (March–May, June–August or September–October). These dogs were to be chosen at random without prior knowledge of whether or not they were carrying ticks. Geriatric and obese dogs were excluded because such dogs were likely to be exercised less than the general population and were therefore less likely to encounter ticks. Each practice was provided with a tick survey kit containing a protocol, questionnaires, sample pots and envelopes addressed to the researchers. At any one time, approximately 60 veterinary practices were involved in the survey; each practice participated for 3 months and was then replaced. It was felt that no practice would be likely to participate reliably for >3 months. Veterinary practices were categorized as rural or urban by plotting their postcodes using http://www.multimap.com. Practices were classified as being in a rural area if ≤25% of the screen was occupied by buildings at a scale of 1 : 50 000 (Bisdorff & Wall, 2006). The same screen was used each time to standardize this method.

The grooming protocol was intended to ensure that dog examinations were standardized. It required veterinarians and veterinary nurses to undertake a 3-min inspection of the hair. They were to check the head area for ticks, paying special attention to the ears, and then to check the neck and chest area, legs, axillae and between the toes. They were asked to brush their fingers through the dog's hair from head to tail and from tail to head, applying enough pressure to feel any small lumps. Finally, they were asked to use a nit or flea comb and part the hair down the length of the body to check visually for ticks. Questionnaires were to be completed for each inspection and the ticks found were to be removed and sent to the University of Bristol. The questionnaire comprised mostly closed questions asking for information about the location of the dog sampled (the postcode of the owner or, if this was not given, the postcode of the vet's practice), dog breed, sex, age, whether it had visited kennels or been abroad during the previous 2 weeks, when it had last been treated for ticks and the number of ticks found. The period of interest for kennel visits and travelling abroad was set at 2 weeks because this was considered to be the maximum period that I. ricinus and I. hexagonus ticks would be likely to remain attached to a host (Arthur, 1963).

An important principle of the study was that vets were asked to complete the inspections on dogs selected at random and to return questionnaires on all inspections regardless of whether ticks were found or not. An identification number was assigned to each questionnaire and its date was recorded. Once the questionnaires were received, the data were entered into a spreadsheet. Any tick samples received were labelled with an identification code linking the sample to the relevant questionnaire. Each tick sample was examined in the laboratory and its sex, species and lifecycle stage recorded; in the case of Dermacentor, species identity was confirmed by the Natural History Museum (London), but for all others, a number of keys were consulted (Arthur, 1963; Walker, 1994; Baker, 1999).

Statistical analysis

Relationships between aspects of the clinical history of each dog and whether or not it was carrying ticks were assessed using binary logistic regression in spss Version 16.0 (SPSS, Inc., Chicago, IL, U.S.A.). Non-significant factors were removed in turn until a minimal model remained. Dog breed was categorized as one of the seven breed types listed by the Kennel Club (U.K.), namely: Gundog, Hound, Pastoral, Terrier, Toy, Utility or Working. Breed information from the Kennel Club was also used to determine the hair length (short, medium, long) and size (small, medium, large) of each dog. Missing values for age were replaced with the mean before analysis. Where data for other categories were missing, a category for ‘not known’ was entered.

The incidence of tick infestation was estimated as the observed prevalence of attached ticks divided by the assumed duration of tick attachment. Separate calculations were made for months with the lowest and highest tick prevalences to reflect the rate at which dogs are likely to acquire ticks at low- and high-risk times of year. Monte Carlo simulation was used, such that values were drawn at random from probability distributions reflecting parameter uncertainty, incidence was calculated, and the process repeated 10 000 times. Confidence intervals (CIs) of 90% for incidence were drawn directly from the simulation output. Distributions for prevalence were constructed as triangular distributions around overall prevalence for that month, with minimum and maximum values being the exact binomial 95% CIs. Tick attachment was assumed to follow a triangular distribution from zero to a peak at 1.5 days, close to the mean attachment duration recorded by Falco et al. (1996) and Hügli et al. (2009), and a maximum of 14 days (Arthur, 1963). The number of ticks present on infested dogs could also be used to estimate incidence, but this would be straightforward only if the attachment of each tick was independent. This hypothesis was assessed by comparing the variance and mean of tick numbers on infested dogs, such that independent acquisition should lead to a Poisson distribution, with variance equal to the mean.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Study population

During the course of the survey, a total of 173 veterinary practices participated; approximately 60 practices participated at any one time and this number did not change significantly over the course of the study. The proportions of veterinary practices from rural and urban areas were similar (49% and 51%, respectively). Between March and October 2009, the recruited veterinary practices inspected 3534 dogs. Of the dogs sampled, 810 were carrying at least one tick. Of these, 62.8% were inspected at rural veterinary practices and 37.2% at urban practices. All dogs inspected resided in the U.K.; only five had been abroad in the 2 weeks prior to inspection and only one of these five dogs carried a tick.

Tick species identification

Of all the tick samples received, 90.4% contained adults, 12.2% contained nymphs and 2.0% contained larvae. All of the tick samples obtained comprised only one species: 72.1% of samples consisted of I. ricinus, 21.7% of I. hexagonus and 5.6% of I. canisuga. Ixodes ricinus was found throughout England, Scotland and Wales, whereas I. hexagonus appeared to be more commonly reported in southern and central England and south Wales, with relatively few cases in Scotland (Fig. 1A–C). Because of the relatively low numbers found, it is difficult to discern any geographic patterns in the distribution of I. canisuga. Five samples of D. reticulatus (0.68%) were also received; these came from locations in west Wales and south-eastern England (Fig. 1D). A significantly higher proportion of the dogs infested with I. ricinus were inspected in rural than in urban practices (16.1% vs. 10.8%, respectively; χ2 = 19.6, d.f. = 1, P < 0.001), but no such difference emerged in the proportions infested with I. hexagonus (χ2 = 1.7, d.f. = 1, P = 0.2) or I. canisuga (χ2 = 0.3, d.f. = 1, P = 0.6). The proportion of dogs found to be positive for I. ricinus peaked between May and July, although I. ricinus was found on dogs in every month of the survey period (Fig. 2). Numbers of I. hexagonus detected by month did not differ significantly from those of I. ricinus (Kolmogorov–Smirnov test, Dn = 0.63, P = 0.08). However, numbers of I. canisuga detected by month differed significantly from those of I. ricinus (Kolmogorov–Smirnov test, Dn = 0.75, P = 0.02), showing a less pronounced seasonal change.

image

Figure 1. Location of samples of (A) Ixodes ricinus, (B) Ixodes hexagonus, (C) Ixodes canisuga and (D) Dermacentor reticulatus, obtained from dogs during a national tick survey in England, Scotland and Wales. Locations were determined using the postcode of the dog owner or, if this was not available, the postcode of the reporting veterinary practice.

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image

Figure 2. Percentages by month (± 95% exact binomial confidence intervals) of dogs sampled carrying ticks of Ixodes ricinus (inline image), Ixodes hexagonus (inline image) or Ixodes canisuga (inline image).

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Tick attachment rate

The number of ticks per dog ranged from one to 82. The estimated incidence of tick attachment was 0.013 (90% CI 0.006–0.063) per day in March and 0.096 (90% CI 0.040–0.435) per day in June. The median number of ticks per dog was one, the mean was 2.26 (variance 28.32) and the variance : mean ratio was 12.54.

Risk factors affecting tick attachment on dogs

Logistic regression analysis indicated that visits to kennels, visits abroad, acaricide treatment in the previous 2 months, age, size and sex were not significant predictors of tick attachment on dogs. However, breed type significantly affected the likelihood of tick attachment, with hounds, toy, utility and working dogs having ticks less frequently than gundogs, terriers and pastoral dogs (Table 1). Dogs with medium hair length were 2.1 times more likely to have ticks than dogs with short hair (P < 0.001); however, there was no significant difference between long- and short-haired dogs. Dogs that had been neutered were less likely to have ticks than those that had not been neutered (P < 0.001). The month in which the dog was examined significantly affected tick attachment. Compared with March, the first month of the survey, dogs were more likely to have a tick attached in all subsequent months (P < 0.001) and tick attachment peaked in June (P < 0.001).

Table 1.  Significance, odds ratios and 95% confidence intervals (CIs) of the logistic regression between the presence or absence of ticks and a range of significant tick risk factors.
 SignificanceOdds ratio95% CI
  1. Hosmer–Lemeshow test: χ2 = 1.44, d.f. = 8, P = 0.994.

Breed type0.007  
 Hound0.0360.3870.159–0.941
 Toy0.0060.2990.125–0.713
 Utility0.0180.3530.149–0.835
 Working0.0160.3250.130–0.811
Hair length<0.001
 Medium<0.0012.0871.614–2.699
Seasonality<0.001
 April<0.0012.8642.015–4.072
 May<0.0016.5194.580–9.279
 June<0.00112.1168.807–16.668
 July<0.0017.3485.193–10.396
 August<0.0015.4253.563–8.259
 September<0.0013.4852.199–5.522
 October<0.0014.2262.409–7.413
Neuter status<0.0010.6670.569–0.804

Tick infestation prevalence

For a small number of veterinary practices, questionnaire return rates were uneven over time and all returns were positive; it seemed possible therefore that these practices had misunderstood the protocol and reported only positive cases. As a result, data from these practices (n = 43) were removed from the analyses of prevalence. The detection rate, plotted as a survivorship curve, shows that the median frequency of infestation in all dogs examined between March and October was 14.9% (Fig. 3). However, 25 of the 130 veterinary practices (19.2%) found no ticks, whereas 19 of the 130 practices (14.6%) reported that >50% of the dogs inspected carried ticks. Rural practices reported significantly higher infestation rates than urban practices (Kolmogorov–Smirnov, Z = 1.44, P = 0.03), with median prevalences of 17.1% and 14.3% of dogs, respectively. Median frequency was used in response to the non-normal distribution of results: a relatively large number of practices (n = 25) reported no cases of dogs with ticks.

image

Figure 3. Percentages of participating veterinary practices reporting different percentages of randomly selected dogs infested by at least one tick. The arrow shows the median frequency of dogs carrying ticks.

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Despite the extensive work currently underway to elucidate patterns of tick abundance (Daniel et al., 2003; Materna et al., 2008; Vanwambeke et al., 2010), relatively few studies of dogs have systematically evaluated infestation prevalence using a longitudinal randomized study of infestation rates on the scale reported here. Although a number of studies across Europe have aimed to determine which species of tick infest domestic dogs, such as in Hungary (Földvári & Farkas, 2004), Greece (Papazahariadou et al., 2003), and Great Britain (Ogden et al., 2000), few have also inspected dogs at random to determine tick infestation rates. However, in Italy Lorusso et al. (2010) followed a cohort of randomly selected dogs over time to evaluate the seasonal dynamics of Rhipicephalus sanguineus (Latreille) (Ixodida: Ixodidae) in an animal shelter.

Ixodes ricinus was the most common tick species identified here, which supports previous work showing it to be the tick species that most often attaches to dogs in Great Britain (Ogden et al., 2000). Dogs visiting rural veterinary practices were infested with I. ricinus more frequently than those attending urban practices, which is consistent with the fact that rural areas contain larger proportions of the typical I. ricinus habitat of woodland and uplands and alternative hosts (Gray, 1991) compared with urban areas. Notably, there was no difference between rural and urban practices in the prevalences of infestation by I. hexagonus and I. canisuga, which suggests that urban environments provide appropriate habitat and sufficient host abundance to support populations of these ticks.

The spatial distribution of I. ricinus was found to be relatively uniform across England, Scotland and Wales (Fig. 1A). However, this map does not provide information on where I. ricinus is absent and no attempt has been made to differentiate the pattern of I. ricinus from the pattern of reporting veterinary surgeries. Despite this, it is still possible to gain important information on the distribution of I. hexagonus as this species is absent from areas where cases of I. ricinus were reported (Fig. 1B). In fact, only five of 60 I. hexagonus specimens were found in northern England and Scotland, which suggests this species is not commonly found as far north as I. ricinus. Many of the cases of I. canisuga were clustered around London, but too few samples of this species were collected to provide any real insight into the spatial distribution of this species (Fig. 1C). The identification of D. reticulatus from areas in eastern England was notable (Fig. 1D); Jameson & Medlock (2009) reported an established population of D. reticulatus in Southend, Essex. The ticks in this survey were found in Cheshunt, Hertfordshire and Chelmsford, Essex, supporting the opinion that this species has become established in southeast England.

A number of studies worldwide have attempted to determine rates of tick infestation in a variety of domesticated animals. Ferede et al. (2010) carried out a similar cross-sectional, randomized study and found a mean infestation rate of donkeys in a region of Ethiopia of 32%. Furthermore, Yukari & Umar (2002) found cattle, sheep and goats in Turkey to have tick infestation rates of 21.8%, 25.4% and 15.8%, respectively. A recent study in a rural community in northeastern Brazil estimated the tick infestation rate on dogs to be 58.8% (Dantas-Torres et al., 2009). In the present study, the median frequency of dogs carrying ticks in Great Britain was found to be 14.9%. In this study, veterinarians were asked to select animals to examine at random without prior knowledge of whether the dog was carrying a tick or not. This suggests that many dogs carry ticks without their owners' knowledge and thus tick attachment to dogs and the potential risk for undiagnosed tick-borne diseases may be much higher than previously thought.

Most dogs carried only a single tick and this supports studies of the seasonal incidence of I. ricinus on wild small-mammal hosts, which has been reported at a median intensity of one tick per host (L’Hostis et al., 1996). The estimated incidence of tick attachment can be used as an index of risk for individual dogs. However, it is appreciated that the assumed average duration of tick attachment will vary across tick species and lifecycle stages. Nevertheless, this analysis gives a useful first attempt to quantify attachment rates and risk. Thus, in June, dogs in the study population had a daily chance of 0.096 of acquiring a tick, which means that they might be expected to acquire a tick or ticks on average once every 10 days or so. In March, the predicted risk is more than seven times lower, at around 0.013 per day. The CIs around these estimates are broad as a result of the very limited data available on the duration of tick attachment on dogs (Falco et al., 1996; Hügli et al., 2009). Further work in this area is needed to refine these estimates of incidence. Notably, tick distribution was highly aggregated and variances in the number of ticks on infested dogs greatly exceeded the mean.

Logistic regression analysis revealed that a number of factors affect the likelihood of a dog carrying a tick. Dogs with medium-length hair were more than twice as likely to carry ticks than dogs with short hair. Owners of dogs with shorter hair are more likely to notice and remove ticks than are owners of longer-haired dogs. These dogs may also be more effective at self-grooming and may remove ticks themselves. Furthermore, it is also possible that longer hair is more likely to brush against questing ticks than short hair. However, no difference could be found between short- and long-haired dogs, although this may represent a product of the disparity between the sample sizes of the two groups (n = 1401 and n = 177, respectively). Dogs that had been neutered were less likely to carry a tick than dogs that had not been neutered. This may be related to age; however, when age was entered into the analysis as an independent factor, it was found not to have a significant effect on the likelihood of a dog carrying a tick. Higher risk in non-neutered animals may therefore reflect management factors such as exercise regime. Certain breed types were found be less likely to have ticks (hounds, toy, utility and working dogs). The reasons behind this finding may well reflect a complex interaction between different factors, including management factors not included in the questionnaire.

Time of year significantly affected the likelihood of ticks attaching to dogs and there was a clear pattern of seasonality relating to tick attachment. Tick questing activity in the U.K. may be highly variable from year to year, depending on weather patterns (Randolph et al., 2002), but populations of adults and nymphs most commonly show a strong peak in questing activity in late spring and early summer and, at some sites, a second smaller peak in activity in autumn (Lees & Milne, 1951; Gray, 1991; Randolph et al., 2002). Here, tick attachment increased from March to peak between May and July. In June, dogs were more than 12 times as likely to carry a tick as in March, the beginning of the tick season. Attachment by I. ricinus declined in September, but then showed a slight resurgence in October. Although the number of ticks attached was higher than might have been typically expected in July and August, the attachment pattern is not dissimilar to the yearly patterns of tick questing activity in the U.K. observed in blanket-dragging surveys and is within the variance expected as a result of year-to-year weather variation. The reason(s) why almost all ticks collected were adults cannot be determined from the present study; although it may be that dogs are fed on primarily by adult ticks, it is also possible that nymphal and larval attachment was underestimated in the clinical inspections.

The prevalence of ticks in rural areas differed from that in urban areas in that it was higher and more evenly spread in rural areas and lower and more aggregated in urban areas. This result can be explained by differences in the types of habitat available to dog walkers in the two areas. In urban areas, some parks and gardens available for exercising dogs may create hotspots for ticks and tick populations may also be maintained by visiting wildlife. By contrast, other dogs in cities may live in areas lacking in parks and therefore may not come into contact with tick habitats at all. In rural areas, habitats are much more homogeneous and the likelihood of picking up a tick will be higher and more evenly distributed between veterinary practices.

The key finding of the current study is that the prevalence of tick infestation in dogs in Great Britain was much higher than expected, in both urban and rural environments. Clearly, many cases of tick infestation are likely to go unnoticed by owners and are only detected by a thorough clinical examination. This has important implications for the potential transmission of tick-borne disease, not only in dogs, but also in humans who use the same habitat. Further work is needed to more fully understand the association between tick abundance, habitat use and the prevalence and distribution of tick-borne diseases.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

We would like to thank the territory managers at Merial Animal Health Ltd, who helped recruit the veterinary practices to take part in the tick survey, and all the veterinary practices themselves. We are grateful to Dr Anne S. Baker, Natural History Museum (London), for examining specimens of Dermacentor. We would also like to thank the Natural Environment Research Council and Merial Animal Health for funding this work.

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  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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