Background

As haematophagous obligatory parasites of reptiles, birds, and mammals, ticks are among the most important vectors of pathogens affecting livestock, companion animals, and humans worldwide [1, 2]. Ticks transmit viruses, bacteria, and protozoa during blood feeding, which can compromise the health of the vertebrate host [3]. A variety of factors influence the susceptibility of companion animals to TBD, including exposure to questing ticks, the pet’s lifestyle, and ectoparasite control [4]. Some TBD of companion animals are zoonotic [5, 6], which in some circumstances may also place human owners at risk of infection. Furthermore, companion animals can act as sentinels for emerging TBD [7–9]. In 2013, it was estimated that there are a total of 4.2 million pet dogs, and 3.3 million pet cats in Australia [10].

Of the 896 recognised tick species worldwide [11] there are 70 species endemic to Australia: 14 soft tick (family Argasidae) and 56 hard tick (family Ixodidae) species [12]. While the majority of these ticks are unique to Australia, there are five species that have been introduced since European colonisation in the last 250 years with poultry (e.g. Argas persicus), horses (e.g. Otobius megnini), cattle (e.g. Haemaphysalis longicornis and Rhipicephalus australis), and dogs (e.g. Rhipicephalus sanguineus) [12]; however, R. sanguineus may have been introduced earlier than this [13].

To date, 20 ixodid species have been recorded on dogs, cats and horses in Australia (Table 1). Dogs are the primary hosts of R. sanguineus; however, native ticks such as Ixodes cornuatus, Ixodes holocyclus, and Ixodes tasmani are known to parasitise domestic dogs, as well as cats. Ixodids that usually feed on cattle (H. longicornis and R. australis) also feed on horses but, as with dogs and cats, horses can also be parasitised by native Australian ticks [14].

Table 1 Ticks (Acari: Ixodidae) previously recorded on dogs (Canis lupus familiaris), cats (Felis catus), and horses (Equus ferus caballus) in Australia
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As a result of its geographical isolation and robust biosecurity regulations, Australia is considered free of many of the TBD endemic to countries overseas. There are currently two TBD of dogs recognised in Australia; canine infectious cyclic thrombocytopenia (CICT) and canine babesiosis. Anaplasma platys is the causative agent of CICT and was detected in dogs in central Australia in the early 2000s [15, 16]. Canine babesiosis is caused by Babesia vogeli and Babesia gibsoni in Australia. Babesia vogeli has been detected in dogs from northern Australia [17, 18] and New South Wales [18], and is transmitted by R. sanguineus [19, 20]. Babesia gibsoni has been detected in dogs from south-eastern Australia [21]. Evidence in Japan suggests that H. longicornis, which is also distributed in Australia, is a vector of B. gibsoni [22], and there is also one report of direct transmission of the piroplasm between dogs in Australia [23].

Although cytauxzoonosis is a major TBD of cats in the United States [24, 25], neither cytauxzoonosis nor any other TBD of cats are known to occur in Australia. Equine piroplasmosis was first diagnosed in Australia in 1976 [26]. The disease was later confirmed to be caused by the protozoan pathogen Babesia equi [27], which has since been redescribed as Theileria equi [28]. The presence of T. equi in horses in Australia was considered to have occurred due to the importation of infected horses during the twentieth Century [29], however, the disease remained localised and Australia is now free of equine piroplasmosis [29, 30].

Tick infestations can cause other health problems in companion animals. Tick paralysis manifests as ascending paralysis and local neurological deficits [31–33]. The Australian tick species known to frequently cause tick paralysis in eastern and south-eastern Australia are I. cornuatus and I. holocyclus [34]. Tick paralysis caused by Ixodes hirsti has also been reported in cats [35]. Additionally, heavy or repeated infestations of ticks can cause anaemia in the host animal, which is associated with blood loss during tick feeding [36]. Immunosuppression [37], secondary infections at the bite site [38], and localised dermatitis [39] can also result from tick infestations.

The present study aimed to determine the tick species that are associated with dogs, cats and horses in Australia, and is part of broader research investigating tick-borne pathogens.

Methods

Sample collection

Individual ticks (n = 4765) were collected during 2012–2015 from a total of 837 companion animal hosts (n = 4191 from 643 dogs; n = 345 from 42 horses; n = 229 from 152 cats) from New South Wales (NSW), the Northern Territory (NT), Queensland (QLD), South Australia (SA), Tasmania (TAS), Victoria (VIC), and Western Australia (WA). Ticks were removed from animals by staff at veterinary clinics, and by various persons throughout Australia in response to a nationwide advertising campaign. The ticks were preserved in 70 % ethanol and were sent to Murdoch University for analysis. For each submission received, the source, approximate geographic location of collection site, host, and date of collection was recorded.

Ethics statement

The Murdoch University Animal Ethics Committee sanctioned the opportunistic removal of ticks from animal hosts. The use of questionnaires was approved by the Murdoch University Human Research Ethics Committee (Permit No. 2011/005).

Tick identification

Individual ticks were examined with an Olympus SZ61 stereomicroscope (Olympus, Center Valley, PA, USA) with a Schott KL 1500 LED light source (Schott AG, Mainz, Germany). Photographs were taken with an Olympus SC30 digital camera and analysis getIT software (Olympus, Center Valley, PA, USA). The instar, sex, and species were morphologically identified [14, 40] and the data were recorded with Microsoft® Excel® for Mac 2011, version 14.5.2.

Sample mapping

The sample collection locations were geo-referenced using the open source software QGIS, version 2.10.1 [41] with the latest Australian coordinate system: Geocentric Datum of Australia 1994 (GDA94) [42]. Layers were styled with a categorised renderer, with layer symbology classified according to tick species, and a point displacement renderer was used to visualise overlapping points around a centre symbol on rendering circles [43].

Questionnaire design

A questionnaire was designed in conjunction with Bayer Australia Ltd to obtain information about the age, sex, weight, habitat, use of tick control products and clinical signs of tick paralysis [44] of dogs, cats and horses that were presented to veterinary clinics (Additional file 1). The companion animal owners completed the questionnaires while at the veterinary clinic. A total of 433 questionnaires from 30 veterinary clinics were collected by Bayer Australia Ltd area managers, and sent to Murdoch University.

Statistical analysis

The database of tick identification results, sample information, and questionnaire data was generated and summarised with Microsoft® Excel® for Mac 2011, version 14.5.2, and IBM® SPSS® Statistics 2013 software, version 22 (Armonk, NY, USA). During the analysis, dogs were considered small if their weight was ≤ 10 kg, medium if 11–19 kg, and large if ≥ 20 kg, and the scale of tick paralysis was recoded into a binary variable (present or absent).

Results

Morphological identification of Ixodidae

Overall, 220 larvae, 805 nymphs, 1404 males, and 2336 females were identified from 837 companion animals in 7/8 Australian states and territories. The number and location (state) of ixodids that were identified on dogs, cats, and horses are presented in Tables 2, 3 and 4. Photographs of a single female for each species identified, except for Bothriocroton sp., are displayed in Additional file 2.

Table 2 Tick species, location and number of instars collected from dogs
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Table 3 Tick species, location and number of instars collected from cats
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Table 4 Tick species, location and number of instars collected from horses
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Host records

One novel host record was obtained for I. myrmecobii; one female I. myrmecobii was collected from Felis catus (domestic cat) in the town of Esperance, WA (Additional file 3). All other host records of dogs, cats, and horses for the various tick species identified were consistent with previous host records (Table 1).

Ixodidae collection locations

The collection locations for each ixodid species identified from companion animal hosts are presented in Fig. 1.

Fig. 1
figure 1

Collection locations of ticks removed from dogs, cats, and horses in Australia. Each point represents a unique collection location for the corresponding tick species. Overlapping points were displaced with a point displacement renderer around a centre symbol (denoted in legend); point displacement distance was defined by number of map units (kilometres)

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The individual geographic collection locations, including the coordinates that were geo-referenced and displayed in Fig. 1, for the instars identified on dogs, cats, and horses are listed in Additional file 3. Several collection locations occurred outside of the previously recorded distribution ranges for the following species: H. longicornis (one in the suburb of Sancrox, NSW) [14]; I. holocyclus (two in TAS, and one in WA) [40]; and R. sanguineus (72 in southwest WA, 17 canine hosts; 410 in SA, 15 canine hosts) [45].

Questionnaires

The majority of samples were received in the years 2013 (28 %) and 2014 (65 %), and most were collected from companion animal hosts during the months of spring (September-November) and summer (December-February) (Fig. 2). The data gained from responses to questionnaires is summarised in Table 5.

Fig. 2
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Number of sample submissions in each month from 2013–2015

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Table 5 Summary of questionnaire responses
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In the vast majority of cases where signs of tick paralysis were reported in companion animals, I. holocyclus was identified from the host (97 %; 226/232) (data not shown). In one case, R. sanguineus was removed from a cat with tick paralysis. The remaining five cases of tick paralysis were reported in dogs that were infested with Bothriocroton sp. (n = 1), I. tasmani (n = 1), Haemaphysalis bancrofti (n = 1) and R. sanguineus (n = 2) (data not shown).

Discussion

This report describes the first comprehensive nationwide survey of ticks associated with companion animals in Australia and the results are generally consistent with the individual geographical distributions and host records [14, 40, 46–54], with a few exceptions. Interestingly, one novel host record was obtained in this study for I. myrmecobii on F. catus in Esperance, WA. Although native Australian ticks primarily feed on native wildlife species [14], they also feed on a variety of introduced mammals and birds [46–58]. The primary hosts of the introduced species H. longicornis and R. australis are cattle, but these ticks have been recorded on other livestock, introduced and native wildlife, and companion animals [14, 54, 58].

The collection locations obtained for the vast majority of ticks in this study adhered to previously described Australian distribution ranges, or to previous collection locations [14, 40, 45, 47, 52, 54, 59]. The records of two I. holocyclus in TAS, and one I. holocyclus in the city of Wagga Wagga, NSW, most likely occurred due to travel to I. holocyclus endemic areas [40] prior to tick removal, which was documented by the companion animal owners. Given that the distribution of ticks is affected by climate, vegetation, and the presence of the primary host species [60], it is also likely that the single I. holocyclus recorded from a dog in southwest WA is a result of interstate travel from I. holocyclus endemic areas. The collection locations that occurred outside of the previously recorded distribution ranges for H. longicornis and R. sanguineus [14, 45] may also be attributable to travel, since people and their companion animals can readily travel with, and potentially disperse, ticks outside of their endemic range.

It is probable that the distribution of R. sanguineus has extended further south of the NT border into northern and central SA, which is comprised of the same terrestrial ecoregion as southern and central NT (deserts and xeric shrub lands) [61]. Investigations of R. sanguineus group ticks overseas have found two paraphyletic lineages of R. sanguineus: the tropical (northern) lineage [R. sanguineus (sensu lato)]; and the temperate (southern) lineage [possibly R. sanguineus (sensu stricto)] [62–66], and these lineages may represent two different species [66]. These paraphyletic groupings remain to be investigated across different climatic regions of Australia.

The collection localities of I. myrmecobii along the southern coastline of WA obtained in this study are novel. The information pertaining to the distribution range of this enigmatic tick species is limited, with very few studies of I. myrmecobii conducted [14, 67, 68]. Formal geographical distribution data for many of the Australian tick species we report in this study is either non-existent, or requires a systematic study.

Bothriocroton ticks collected from dogs in TAS and VIC (n = 10) could not be identified to the species level as the morphological features were too damaged in the male and female specimens, and there is currently no key for the identification of Bothriocroton nymphal and larval species. These specimens are likely Bothriocroton auruginans, which is distributed in TAS and VIC [14, 40], and is the only species of Bothriocroton that parasitises dogs in Australia. The current Australian tick morphology keys [14, 40] also lack a complete description of I. cornuatus instars; therefore, the I. cornuatus nymphs examined in this study have been only tentatively identified, pending further species confirmation by molecular techniques.

There were no male H. longicornis ticks identified in this study, which was expected, as the populations of H. longicornis in Australia (as well as in north-eastern Russia, northern Japan, New Zealand, New Caledonia, and Fiji) are parthenogenetic [69], and represent the only known example of triploidy in ticks [70]. In Australia, very few males have ever been reported [50, 71].

The use of standard Australian tick morphology keys to identify ticks collected in Australia seems appropriate given the context of the study, however, there are species found elsewhere with similar morphology to those that are present in Australia. It is possible that other tick species could be inadvertently introduced into this country as a result of international movements of animals and humans, thus future studies could include molecular phylogenetic analyses of genetic markers (e.g. mitochondrial cytochrome c oxidase subunit 1 (cox1), 12S ribosomal RNA (rRNA), and 16S rRNA genes) to increase the confidence and accuracy of tick identification.

As expected, the majority of the ticks examined in this study were collected during the warmer months of spring and summer, when ixodids are generally more abundant [72–76] (Fig. 2). There is limited data pertaining to ownership of companion animals in Australia. A 2013 survey of 1089 pet owners reported that 76 % of dogs are kept exclusively or partly indoors [10]. Conversely in this study, 54 % of dogs usually lived outdoors (i.e. in a kennel), and 13 % were confined to the home. Increased exposure to tick habitats likely increases the chance of tick attachment, which could explain our observations, as only dogs with ticks were sampled in this study. Overseas studies have reported that factors such as host species, breed, and habitat significantly affect the likelihood of tick species attachment [75, 77]. Explanatory variables for tick species attachment in this study could not be fairly assessed, as questionnaire data was skewed towards companion animals that were infested with I. holocyclus on the eastern coast of Australia.

Several tick species identified in this study are of potential concern to the health of companion animals according to the current literature. Importantly, R. sanguineus is a well-known vector of B. vogeli [78], the cause of canine babesiosis. Most of the animals that were infested with I. holocyclus had clinical signs of tick paralysis (77 %; 226/293) (data not shown), and this condition can be fatal [79, 80]. The reports of tick paralysis in one cat infested with R. sanguineus, and in five dogs infested with Bothriocroton sp., I. tasmani, H. bancrofti and R. sanguineus are unusual. These may have been reported erroneously on the questionnaire, or Ixodes spp. known to cause tick paralysis might have attached to the animal, but were not collected.

There are few reports of TBD associated with I. cornuatus [81] and virtually nothing is known about pathogens transmitted by I. myrmecobii and I. hirsti. Although Amblyomma triguttatum triguttatum, H. bancrofti, H. longicornis, I. tasmani and R. australis have been associated with TBD in other host species [40, 82–87], it remains to be investigated whether these species carry pathogens that could impact the health of companion animals.

Conclusions

This first nationwide study of ticks on companion animals in Australia has provided a comprehensive snapshot of the current tick-host associations in dogs, cats, and horses that should be of interest to pet owners and carers, veterinarians, and manufacturers of ectoparasiticides. The species that were most commonly found on these animals are well-known vectors of pathogens, or cause neurological disease. However, the vector competency of several species identified has not been widely investigated. Such knowledge is required to better understand the risks of TBD transmission to pets and potentially, to their owners. Further investigations are required to establish the environmental and host factors that influence tick species infestations on companion animals, which may help to develop prevention strategies against tick infestations.