Urban environments support high concentrations of humans, domestic pets and introduced animals, creating conditions conducive to the transmission of parasites. This study compared patterns of ectoparasite infestation of the common brushtail possum Trichosurus vulpecula in urbanised Sydney (n = 161) to those from a remote woodland site (n = 18) from February 2005 – November 2006. We found diff erences in ectoparasite species prevalence between the two groups: the flea Echidnophaga myrmecobii was only found on urban possums and the tick Ixodes trichosuri was much more prevalent in the urban habitat, while the mite Atellana papilio was more prevalent on woodland possums. E. myrmecobii and I. trichosuri diff ered from other ectoparasites by showing an association with host sex and host age. Potential physiological costs of ectoparasitism to urban-dwelling possums were determined using multivariate analysis of haematology, serum biochemistry and body condition. Changes in serum iron levels were seen in the presence of both the tick Ixodes trichosuri and the flea E. myrmecobii, and E. myrmecobii was associated with elevated serum levels of the liver enzyme ALT. However, ectoparasite-related changes in haematology and serum biochemistry were not indicative of long-term pathology. In this urban possum population, the costs of ectoparasitism appear to be limited and unlikely to pose a major threat to the health of the population.
Urbanisation, the growth of cities to accommodate an increasing human population, is regarded as the most destructive anthropocentric pressure on wildlife, as it results in fragmentation or complete destruction of habitat (McKinney 2002, Garden et al. 2006). Less overt consequences of urbanisation such as changes to food webs, competition for resources and alterations in host—parasite relationships may also threaten the viability of wildlife populations (Patz et al. 2004, Spratt 2005, Bradley and Altizer 2007). In particular, urbanisation leads to a restructuring of species composition that may destabilise wildlife—pathogen relationships. Evidence of this exists in the growing number of studies in which urban areas act as a focal point for the emergence of zoonotic disease, including West Nile virus, Dengue fever (Dauphin et al. 2004, Weaver 2005), toxoplasmosis (Smith and Frenkel 1995, Meireles et al. 2004, Sukthana 2006), Lyme disease (Stanczak et al. 2004, Jobe et al. 2007) and leptospirosis (Ko et al. 1999). Despite the risks to the health of wildlife and humans, few studies have systematically investigated whether wildlife species experience a higher parasite burden as a result of living in an urban environment.
Ectoparasites are arthropods that complete at least one part of their life cycle on the surface of their host and include the ticks, lice, fleas and mites. The survival and spread of ectoparasites may be favoured as a consequence of urbanisation (Spratt 2005, Bradley and Altizer 2007). Increasing population densities of wildlife that exploit anthropogenic sources of food and shelter may facilitate ectoparasite transmission, whilst the co-existence of humans, domestic pets and wildlife provides ectoparasites with a range of novel hosts (Dautel et al. 1991, Comer et al. 2001, Maetzel et al. 2005, Pearce and O'Shea 2007). Urban areas are also associated with warm microclimates due to lack of shade from vegetation, smog and effluent from industry, retention of heat by impervious surfaces (Saaroni et al. 2000, Baker 2002) and the high number of human occupants (Torok et al. 2001). The warmer temperatures and reduced seasonality caused by this ‘urban heat island effect’ may increase the survival, breeding success and activity of arthropod ectoparasites (Bradley and Altizer 2007).
As a consequence of inhabiting a disturbed environment with potentially higher rates of ectoparasite infection, wildlife inhabiting human settlements may be adversely impacted. By acquiring nutrients at the expense of the host, feeding on blood or skin tissue, ectoparasites impose a cost on the host (Price 1977). Ectoparasites may also infl ict physical damage, such as creating skin lesions, injecting salivary toxins into the wound and causing blood loss (Aeschlimann 1991); such physical damage may be costly to repair. Behavioural changes that deter ectoparasites, such as grooming or relocation of nest sites, can diminish reproductive success (Loehle 1995, Moore and Wilson 2002) and immune responses to infestation are energetically demanding (Brossard et al. 1991). Ectoparasites are also vectors and intermediate hosts for a range of highly-pathogenic viral, protozoan, bacterial and rickettsial diseases (Walker et al. 1996). Owing to these detrimental impacts, ectoparasites have the potential to infl uence the growth, survival and reproduction of their host and consequently act as an important selective agent (Renaud et al. 1996).
The common brushtail possum Trichosurus vulpecula is one of the most abundant native mammals in urban Australia, having adapted to utilise anthropogenic resources (Hill et al. 2007). Despite success in colonising urban habitats, there are limits to the adaptability of this species. Tree clearing and predation by the European red fox Vulpes vulpes has been responsible for the decline of possums from more than 50% of their former range (Kerle 1992, 2004, Statham and Statham 1997). This study sought to determine whether adaptation to the urban environment enhanced ectoparasite infestation and whether such infestations were associated with changes in host physiology including body condition, haematological and serum biochemical parameters. To achieve this, we compared two populations of possums, one from metropolitan Sydney and the other inhabiting woodland remote from urban areas. This is the first detailed investigation into multiple host—ectoparasite relationships of common brushtail possums, providing baseline data against which the health of the species may be compared as humans increasingly modify their original habitat.
Material and methods
The principal urban study area was Taronga Zoological Park (33°50′S, 151°14′E) in the northern Sydney suburb of Mosman, Australia. The zoo is inhabited by an abundant free-ranging possum population that forage nightly on the leftover foodstuff s in the exhibits and on anthropogenic food from garbage bins. Also included in the ‘urban’ field site were the properties of Mosman residents who lived in proximity to the zoo (<1 km) and had experienced possum activity. Possums inhabiting this urban area achieved a population density of approximately 5.03 ha-1, one of the highest recorded in Australia (Hill and Deane unpubl.). For comparison, trapping took place at a non-urban site located within Jenolan Caves Reserve Trust land, Blue Mountains, Australia (33°49′S, 150°01′E). The reserve was characterised by open eucalypt woodland and was inaccessible to the public. A wire fence prevented invasion by exotic species and control of feral animals was routinely performed throughout the enclosure.
Climate information was obtained from the closest Australian Bureau of Meteorology weather station to the urban study site (Observatory Hill, Sydney; station number 066062) and the woodland site (Oberon, Springbank; station number 063063). Records included monthly averages of rainfall, number of rainy days, maximum temperature and minimum temperature for each month of trapping (< www.bom.gov.au/climate/averages>).
At the urban site, trapping was performed on a monthly basis between February 2005 and November 2006 (n = 22 trapping surveys). At the woodland site, the remote location limited trapping to November 2005, May 2006 and October 2006 (n = 3 trapping surveys). Treadle-operated cage traps (59 × 22 × 22 cm) were baited with apples and a rolled oats/peanut butter mixture and set at locations considered to be suitable possum ‘runways’ (ground routes used to travel between trees). The top of each trap was covered in durable plastic sheeting to provide protection from the elements. Captured animals were transported either to the Veterinary and Quarantine Centre, Taronga Zoo or sampled on site at Jenolan Caves. All possums were permanently implanted with a passive integrated transponder to enable identification of recaptured animals. Recaptured possums were included in the survey if the time since original capture exceeded three months (to avoid pseudo-replication). Ethics approval was obtained from the Macquarie University Animal Ethics Committee (no. 2004/16), the Zoological Parks Board of New South Wales Animal Care and Ethics Committee (no. 4c/08/04) and the NSW Dept of Environment and Conservation (no. S11107).
Possums were anaesthetised via a face-mask using 5% isofl urane in oxygen during sampling, eliminating the need for physical restraint and minimising stress for the animal. Age was estimated by observing the degree of molar tooth wear as per Winter (1980). Body measurements, weight and sex were recorded. Males were classified as ‘juvenile’ if their tooth wear class was 1 or 2 (corresponding to an age of <2 years; Winter 1980) and their testis size was less than 15 mm long and less than 10 mm wide; ‘adult’ males had larger testes (i.e. >15 mm long and >10 mm wide) and a tooth wear class of 2 or more. (In this species males can reach sexual maturity as early as 14–15 months; Johnson et al. 2001). Females were classified as ‘juvenile’ if their pouch was undeveloped (indicating a lack of sexual maturity), which in all cases corresponded with a tooth wear class of 1 or 2. Females were considered ‘adult' if they were lactating or had a fully developed (non-breeding) pouch; this typically corresponded with tooth wear classes of 3 or greater (i.e. an age of 2 + years), but a few individuals of tooth wear class 2 were found to be sexually mature. The age at which females reach sexual maturity varies between populations, but most females in most populations studied begin breeding sometime in their second year (How and Hilcox 2000). All females of breeding age were classified as either ‘with young’ if they carried off spring and/or were lactating, or ‘without young’ if they exhibited neither of these features.
Ectoparasite collection and identification
Ectoparasites were collected from six sites on the body: eyes, ears, nose, shoulders, rump and belly. To maximize consistency in sampling eff ort, a single researcher (NJH) performed all ectoparasite collections. The number of ticks and fleas present at each site were counted and samples stored in sealed containers. Feeding mites were detected by parting the pelage on the rump in 5–6 locations and dousing with paraffin oil to allow collection with forceps. Fur-clasping mites were collected in hair samples removed with a fi ne-toothed comb. The small size of mites and the time-consuming methods necessary to locate them meant counts could not be performed. Ectoparasites were stored in 70% ethanol and later identified under a stereo microscope to species-level on the basis of shared morphology with published descriptions of ticks (Roberts 1970), fleas (Dunnet and Mardon 1974) and mites (Domrow 1987, 1992).
Estimation of body condition
Body mass (Mb) was measured on electronic scales to the nearest single gram. Skeletal body length (L) was measured from the tip of the nose to the cloaca. To ensure consistency, only a single researcher (NJH) performed measurements of possum skeletal structures. A typical body condition index (BCI) estimate, such as that used for the closely related mountain brushtail possum, Trichosurus caninus (Viggers et al. 1998), is to divide body mass by length: BCI = Mb/ L. Possums are sexually dimorphic, with males significantly heavier than females (but not significantly longer), so the typical BCI estimation could lead to over-estimating the body condition of males. Therefore, we instead calculated a “scaled mass index”, SMI (Peig and Green 2009, 2010), for each population sub-group (adult males, adult females, juvenile males and juvenile females), where the body mass was scaled to the mean body length for each sub-group.
Blood collection and analyses
Approximately 5 ml of blood was drawn from the lateral tail vein and distributed between two vacuette tubes; one lined with ‘EDTA’ for haematological analysis and a serum clot activator tube for biochemical analysis. After clotting, the serum clot activator tube was centrifuged at 200 × g for 10 min and serum collected in a microtube. All testing was conducted within 72 h of blood collection at PaLMS (Pacific Laboratory Medicine Services, NSW, Australia).
Haematological parameters were measured on a Beckman—Coulter counter. To limit erroneous measurements arising from the use of equipment designed for human blood cells (Whittington and Comer 1984), instrumentation was optimised for use with marsupial cells. Electronic white cell counts were verifi ed against differential counts from blood fi lms to confi rm accuracy of the equipment. Red cell parameters measured included haemoglobin concentration, erythrocyte count, mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH) and mean corpuscular haemoglobin concentration (MCHC). White cell parameters measured were lymphocytes, neutrophils and eosinophils. Also measured were mean platelet volume (MPV) and iron parameters including iron and iron saturation.
Serum biochemical parameters were measured on a modular analyzer, optimised for use with marsupial serum. General biochemistry measurements included sodium, potassium, creatinine, urate, total protein, albumin, total bilirubin, calcium, phosphate, magnesium, lipase and hormones, alkaline phosphatase (ALP), aspartate aminotransferase (AST), alanine transaminase (ALT) and g-glutamyl transferase (GGT). Assays for bilirubin and GGT were colorimetric, whereas those for ALP, AST and ALT were UV based protocols. Also measured were lipid chemistry parameters including cholesterol and triglyceride.
Possum population demographics and climate data
Pearson's χ2-tests were used to investigate whether the urban possum sub-groups (zoo and backyard) were similar with respect to age, gender and maternal status. The same test was used to assess similarity between the possum populations in urban and woodland habitats. One-way ANOVA was used to assess whether body condition (as estimated by SMI) varied between urban and woodland populations.
The average rainfall, number of rainy days, maximum temperature and minimum temperature at each habitat was determined for the period of trapping and compared with a one-way ANOVA.
Prevalence, defined as the number of infected individuals expressed as a percentage of the study population (Bush et al. 1997), was determined for each ectoparasite species in each trapping area. Abundance, defined as the number of individual parasite specimens occurring on the host (Bush et al. 1997), was determined for ticks and fleas, but not mites (since mites were scored as present/absent but not counted). The abundance of each ectoparasite species was not normally distributed (Shapiro—Wilk test of normality, p<0.05) and when plotted revealed an aggregated distribution, whereby the majority of animals were not infected with an individual species of ectoparasite. It was therefore more informative to use either prevalence or intensity (the number of parasite specimens per ‘infected’ individual; always a number >0) for comparative purposes (e.g. across trapping sites).
Pearson's χ2-tests were used to identify diff erences in prevalence of ectoparasites between the trapping sites. For three ectoparasite species with prevalence greater than 25% (in any trapping site), variables infl uencing the prevalence of ectoparasite species were investigated using generalized linear mixed models (GLMMs). In this analysis the individual microchip ID of each possum was included as subject variable, to control for possible effects of re-trapped individuals on the analysis. Factors included in each model were trapping site (woodland, zoo or backyard), host demography (sex, age and maternal status of females), host body condition (scaled mass index, SMI), and season. Factors were centred around their respective mean values (‘grand mean centering’) unless there was more variation within animals than between animals, in which case factors were centred around individual animal means (‘group mean centering’).
Species richness by host individual was defined as the total number of parasite species occurring on the host individual (Magurran 1988). The species richness of ectoparasites per host individual was compared across habitats using ANOVA. In addition, the variables infl uencing species richness per host individual were examined using a GLMM with the factors trapping site, host sex, host age, maternal status and SMI.
The large number of health variables measured (29 in total; 11 haematological measurements, 17 serum biochemical parameters and scaled mass index), were reduced to limit statistical analysis to biologically meaningful groups of variables. We hypothesised that ectoparasite infestation was most likely to aff ect red blood cell parameters, immune function, iron levels and body condition, and that severe infestation could potentially compromise host liver function. Selection of the specifi c variables for investigation was assisted by using a Pearson's correlation matrix to identify variables that were correlated, although in some cases groupings were selected for their biological importance, rather than solely on correlations data (e.g. variables within both liver function parameters and immune parameters were only weakly, though signifi—cantly, correlated). The following groupings or single variables were selected for analysis: red blood cell parameters (variables: haemoglobin, erythrocyte count), iron status (variables: iron levels, iron saturation), immune parameters (variables: lymphocytes, neutrophils and eosinophils), liver function (variables: ALT, AST, total bilirubin) and scaled mass index.
Multivariate ANOVA (MANOVA) was used to examine the effect of ectoparasite prevalence on red blood cell parameters, iron status, immune parameters and liver function of urban possums only (the small study population in the woodland habitat precluded use of such statistics). The independent variables in this analysis were presence/absence of each of six ectoparasite species (analysed simultaneously to account for potential interactions between ectoparasites), species richness (to allow for potential interactions between species prevalence and the number of species parasitising a host to be assessed), age, a combined sex/female lactation status variable, season and number of re-captures (these variables are known to aff ect haematology and blood biochemistry of possums; Barnett et al. 1979, Presidente and Correa 1981, O'Keefe and Wickstrom 1998). Two-way interactions between ectoparasites were only included in the model if the number of individuals hosting both parasites was greater than twelve. MANOVA was also used to examine the effect of the abundance of the most abundant ectoparasite (a flea) on red blood cell parameters and iron status.
Univariate ANOVA was used to examine the effect of ectoparasite prevalence on scaled mass index of urban possums. Again, the independent variables were presence/ absence of each of six ectoparasite species, species richness, age, combined sex/lactation status, season and number of re-captures. A seventh ectoparasite species (the flea Ctenocephalides felis) was excluded from these analyses as it was only found on a single possum in the urban habitat. Statistical analysis was performed using SPSS Statistics, ver. 20. For all analyses, an alpha value ≤ 0.05 indicated a signifi cant diff erence. All data are reported as mean = SEM (standard error of the mean), unless otherwise noted.
In total, across all trapping areas, there were 278 capture events of 179 individual possums. The majority of these were at Taronga Zoo, where 140 individuals were trapped over 219 capture events. The backyard group consisted of 21 individual possums, captured on 22 occasions. There were 37 capture events at the woodland site, consisting of 18 individual possums. Because of the large number of recapture events, only first capture data was used for comparisons of the sex structure of the populations. However, because both the age of individual possums and maternal status of females changed over the time-course of the study, all data points were used in comparisons of age and maternal status across the different possum populations. A summary of the demographic data is shown in Table 1.
The urban possum sub-groups (zoo and backyard) displayed no signifi cant diff erences in the demographic profi les of the trapped individuals with respect to sex (χ2 = 0.054, DF = 1, n = 172, p = 0.82), age (χ2 = 0.464, DF = 1, n = 252, p = 0.50) or maternal status (lactating vs non-lactating; χ2 = 0.152, DF = 1, n = 123, p = 0.70). Therefore, the data from the two sub-groups were combined into a single ‘urban’ group for comparison with the woodland population. There was no signifi cant diff erence between possums trapped in urban and woodland habitats with respect to sex (χ2 = 1.812, DF = 1, n = 190, p = 0.18) or age (χ2 = 0.144, DF = 1, n = 289, p = 0.71). Nor was there any signifi cant diff erence between possums with respect to maternal status (χ2 = 3.430, DF = 1, n = 148, p = 0.06), even though the woodland population contained fewer lactating females than was expected (48% of females lactating, compared to 67% of females in the urban habitat).
The mean (± SEM) scaled mass index (SMI) of zoo possums was 2297 ± 52 g (n = 227); possums trapped in backyards had a mean SMI of 2617 ± 155 g (n = 22); and woodland possums had mean SMI of 2320 ± 145 g (n = 27). This index of body condition was not significantly different across the three trapping sites (one-way ANOVA; F2,273 = 1.711, p = 0.183).
Demographic data from possums trapped in urban (zoo or backyard) and woodland habitats. Sex structure data is from first captures only; all other data includes recaptures of the same individuals (because age and maternal status changed over the timecourse of the study).
Ectoparasite species richness per individual possum: (a) woodland versus urban animals, (b) females versus males, and (c) seasonal changes.
Ectoparasite fauna of possums
In total, seven species of ectoparasites were identified from 179 possums (161 from urban and 18 from woodland habitat). The ectoparasite fauna of possums included three species of tick, Ixodes trichosuri, Ixodes tasmani and Ixodes holocyclus; two species of mite, one astigmatid, Atellana papilio and one mesostigmatid mite, Trichosurolaelaps crassipes; and two species of flea, Echidnophaga myrmecobii and Ctenocephalides felis. The two flea species were unique to urban-dwelling possums, but all other ectoparasite species were found in both urban and woodland possum populations. All ticks, fleas and mites detected in this study had been previously identified from this species of possum.
Species richness, prevalence and mean intensity of ectoparasites
Mean species richness of ectoparasites per host individual was not significantly different between the two urban subgroups (zoo and backyard; one-way ANOVA F1,239 = 0.246, p = 0.62). Therefore further comparisons of ectoparasite species richness per host individual were made between the urban and woodland populations. Urban possums were more likely than woodland possums to host three or more ectoparasite species (Table 2a), but the mean ectoparasite species richness per host was not significantly different between possums inhabiting urban and woodland habitat (F1,276 = 1.648, p = 0.200). The GLMM analysis of factors aff ecting the species richness per host confi rmed that there was no signifi cant effect of habitat, as well as no effect attributable to host age or host scaled mass index (SMI). However, species richness per host individual was significantly aff ected by host sex (GLMM F1,145.5 = 4.315, p = 0.028) and season (GLMM F1,79.6 = 6.859, p = 0.011). More females than males were ectoparasite-free (35%, compared with 18% of males); and one or more ectoparasite species were more common on males than females (Table 2b). In most seasons, possums were most likely to host a single ectoparasite species (45–55% of the population aff ected; Table 2c). However, in spring the proportion of the population that was ectoparasite-free was equal to that hosting a single species (35%). Winter appeared to be the peak season for trapping possums hosting two species of ectoparasite, but possums hosting three or more parasites were most commonly trapped in spring or summer (Table 2c).
Prevalence of ectoparasite species in the different trapping areas. Prevalence is presented as the percentage of the population with each ectoparasite species present.
A summary of the prevalence of ectoparasite species in each trapping area (i.e. zoo. backyard or woodland) is presented in Table 3. χ2-analysis revealed signifi cant differences in prevalence between trapping areas for the flea E. myrmecobii, which was not present in woodland habitat (χ2 = 26.104, DF = 2, n = 278, p < 0.001); for the mite A. papilio, which was more prevalent in woodland habitat (χ2 = 24.196, DF = 2, n = 278, p < 0.001); and for the tick I. trichosuri, which was most prevalent on backyard possums (χ2 = 7.939, DF = 2, n = 278, p = 0.019).
The prevalence of most ectoparasite species in most areas was low (<20%; Table 3). Species with prevalence greater than 25% of the population were the tick I. trichosuri (41% prevalence in backyard possums), the flea E. myrmecobii in urban possums (27% prevalence in backyards and 43% at the zoo) and the mite A. papilio (43% prevalence in woodland possums).
The mean intensity (mean number of parasites per infested individual) of each of the tick and flea species is presented in Table 4 (frequency histograms of intensity are available in Supplementary material Appendix 1 Fig. A1–A4). The tick species all had mean intensities in the range of one to two ticks per possum host (Table 4), with one tick per host being the most frequent intensity for all tick species in all trapping areas. The flea E. myrmecobii had the highest mean intensities, of 18.54 fleas per possum in the zoo habitat and 35.67 fleas per possum in backyards. However, the very high mean intensity of E. myrmecobii on backyard possums must be treated with caution, as only six individuals were hosts of this ectoparasite (Table 4), and the high mean intensity was principally due to a single possum hosting 115 individual fleas (hence the very high variance: mean ratio for the intensity of this ectoparasite). For possums trapped at Taronga Zoo, E. myrmecobii intensity was skewed towards lower intensities, with 60 out of 84 individual hosts (71%) infested with fewer than 15 fleas per possum, and 47 individuals (56% of infested hosts) having fewer than 10 fleas.
Mean intensity of ectoparasite species (ticks and fleas) in the different trapping areas. Mean intensity is defined as the mean number of conspecifi c parasites living on an infected host.
Factors affecting infestation by the most prevalent ectoparasite species
The GLMM analysis of factors affecting the presence or absence of I. trichosuri indicated that the presence of this species was aff ected by host sex (GLMM F1,116.11 = 4.389, p = 0.038), with males nearly twice as likely to harbour I. trichosuri than females (Table 5a). The presence of the flea E. myrmecobii was significantly affected by trapping site (GLMM F1,188.99 = 18.283, p<0.001) and an interaction between host sex and host age (GLMM F1,229.91 = 4.733, p = 0.031). Approximately half of adult male possums hosted E. myrmecobii, compared to one fi fth of juvenile males, and around one quarter of juvenile and adult females (Table 5b). E. myrmecobii was found only on urban possums, with a higher prevalence on possums trapped at Taronga Zoo than in suburban backyards (Table 3). A trapping site effect was also found for the mite A. papilio (F1,179.33 = 5.477, p = 0.020), with this parasite significantly more prevalent in woodland habitat (43%) than in either urban habitat (Table 3). Separate analyses of females only indicated no effect of maternal status on prevalence of any of these three parasites.
Factors significantly affecting the prevalence of (a) I. trichosuri (host sex); and (b) E. myrmecobii (interaction between host sex and host age, in addition to trapping site effect shown in Table 3).
Effect of ectoparasite prevalence and abundance on the health of urban-dwelling possums
A summary of the haematological and serum biochemical parameters of possums according to habitat type (urban or woodland) is presented in the Supplementary material Appendix 2 Table A1. Because of the low sample numbers for woodland possums, only the health parameters of urban possums were investigated using MANOVA or ANOVA.
Host sex and lactation status interacted with host age to significantly aff ect the red blood cell parameters haemoglobin and erythrocyte count (analysed together using MANOVA; Wilks λ = 0.945, F2,187 = 5.407, p = 0.005, partial eta-squared = 0.055). Adult male possums had the highest haemoglobin and erythrocyte counts, and juvenile males had the lowest (Table 6a). Females, regardless of age or lactation status, had similar red blood cell parameter values, which were intermediate between adult and juvenile male values (Table 6a). Red blood cell parameters were unaff ected by the presence of any of the haematophagus ectoparasites.
Factors affecting the health parameters of common brushtail possums. (a) Mean haemoglobin concentration and erythrocyte count of urban-dwelling possums