Discussion
This study assessed, for the first time, the prevalence of seven parasite genera in four different wild deer species across Australia. A total of 243 blood samples were analysed by PCR and the presence ofTrypanosoma , Plasmodium , Neospora ,Sarcocystis , Toxoplasma , Babesia andTheileria was not detected. These findings provide a wide perspective of the current disease status of the four deer species investigated (rusa, sambar, chital and fallow deer), which is important information given that deer coexist and closely interact with humans, domestic animals and other wildlife species.
Although the limited knowledge of parasitic infections in Australian wild deer populations is restricted to helminths (McKenzie et al., 1985; Moriarty, 2004), reports from other countries (Cripps et al., 2019) indicate that the deer species tested in our study are susceptible to the pathogens screened in this survey. Interestingly, experimental infection of white-tailed deer with Babesia bovis (Ueti, Olafson, Freeman, Johnson, & Scoles, 2015), a pathogenic parasite in cattle and endemic in Australia (Bock et al., 2006), was reported unsuccessful. This finding raises the question about the epidemiological role of specific wild deer species in the maintenance of pathogens in livestock populations.
Theileria , Babesia and Neospora are endemic parasites in livestock in the geographical areas covered in our study and with particularly high prevalence in Queensland (Bock et al., 2006; Reichel, 2000). Babesiosis is a well-documented disease of cattle and endemic areas have been reported in northern Australia (Queensland, Western Australia and Northern Territory) (Bock et al., 2006). Bovine theileriosis currently occurs throughout coastal New South Wales and Victoria, Queensland and some isolated parts of South Australia and Western Australia (Jenkins, 2018). The prevalence of Neospora caninum , identified as a major cause of infectious abortion in Australian cattle, was estimated to be around 20% in one study (Reichel, 2000). It is therefore conceivable that wild deer might carry some of these parasites. However, the low number of samples collected in Queensland in this study, 1.6% (4/243), decreases the probability of detecting infected animals despite the high prevalence of ​​the parasite in livestock species in this region.
Reports of Australian wildlife infections with Plasmodiumparasites are limited and restricted to Leadbeater’s possum (Gymnobelideus leadbeateri ) (Scheelings, McLaren, Tatarczuch, & Slocombe, 2016), birds (Grim, McCutchan, Sullivan, & Cranfield, 2008; Spratt & Beveridge, 2018) and reptiles (Spratt & Beveridge, 2018; Telford, 1979). In contrast, Trypanosoma , Sarcocystis andToxoplasma have been widely identified in Australian wildlife (Spratt & Beveridge, 2018). For instance, Trypanosoma has been detected in indigenous mammalian fauna living in the same Australian regions targeted in this study. In total, eight native species ofTrypanosoma have been described and exotic trypanosomes have been identified from introduced mammals (Thompson et al., 2014). In Australia, wild deer coexist with livestock but are also sympatric with other wildlife species. Thus, considering the diversity and distribution of intermediate hosts for pathogens such as trypanosomes, wild deer constitute an obvious biosecurity concern. Our study has found no evidence of trypanosome infections in the blood of Australian wild deer, nor of the other six parasites tested, which may be dependent on several factors, including season of the sampling, low parasitaemia at the time of blood collection and fluctuation of parasitaemia during the parasite’s life cycle.
One of the limitations of this study is that samples analysed were mostly collected during the Australian winter and spring seasons, with 69% of the samples being collected between June and October. Most of the sampling occurred during cold weather months due to logistical reasons outside of our control. Cold weather conditions negatively influence the transmission rates of vector-borne diseases (Caminade, McIntyre, & Jones, 2019); therefore, sampling season may impact the chances of detecting infected animals.
Extremely low parasite loads have been previously reported in wild deer.Plasmodium odocoilei was estimated to infect ~1/65,000 red blood cells of white-tailed deer (Martinsen et al., 2016; Templeton, Asada, et al., 2016; Templeton, Martinsen, Kaewthamasorn, & Kaneko, 2016), and molecular tests have previously determined Plasmodium parasitaemia levels (percentage of infected red blood cells) in cervids to be as low as 0.003% (Martinsen et al., 2016; Templeton, Asada, et al., 2016). This low blood parasite burden observed in adult individuals has led to the hypothesis that blood-stage ungulate malaria is best characterised as a chronic, occult infection without major health consequences (Templeton, Martinsen, et al., 2016). Further, it is important to highlight that molecular screening such as conducted in this study can only identify an active infection (i.e. animals that have recovered from infection will not be identified as they would be via serology). A variety of PCR-based assays are now widely used for detection of parasite DNA in blood (Garcia-Sanmartin et al., 2007; Li et al., 2014; Remesar et al., 2019; Yang et al., 2014). In our study, we minimised the effects of PCR inhibitors and the presence of false negatives by obtaining high-quality DNA for each sample, however molecular assays are not infallible.
The opportunistic nature of our study allowed us to collect many samples, but each sample was collected at a single time point (i.e. no serial sampling of animals). Hence, the parasite life cycle at the time of blood collection may have had a direct impact on the negative findings of our study.
Despite the lack of evidence of current infection in the 243 blood samples analysed in this study, the maximum possible prevalence was calculated for each deer species (Table 2) and found to range between 3.5 and 11.4% for fallow, rusa and sambar deer with a 99% confidence level. Since the maximum possible prevalence is directly influenced by the number of samples screened, the maximum prevalence obtained for chital deer (n = 4) greatly exceeds the values obtained for the other three deer species given the limited number of animals sampled. We acknowledge our very limited sample size and recommend further studies to expand on our initial screening of chital deer blood parasites in Queensland.
Importantly, the impact of climate change might be considered as a factor affecting the spread of wildlife diseases in new areas. Modelling suggests that, by 2100, the average global temperature will have risen by 1.0–3.5ºC (Githeko, Lindsay, Confalonieri, & Patz, 2000). Climate change will strongly affect the distribution, abundance and transmission rates, the intensity and temporal pattern of vector activity; and the survival and reproduction of pathogens within vectors. This will increase the likelihood of vector-borne diseases in new areas (Duncan, Backus, Lynn, Powers, & Salman, 2008; Guberti, Stancampiano, & Ferrari, 2014). The impact of climate change is evident in Europe withIxodes ticks, vectors of several pathogens includingBabesia and Anaplasma , where, over the past decade, an expansion in vectors’ geographical range (spread to northern areas) and increase in activity were observed as consequence of milder winters and prolonged spring and autumn seasons, combined with increased vegetative cover and the spread of animals carrying ticks into newly suitable regions (Caminade et al., 2019). Although all the samples screened in this study were negative for the parasites tested, the presence of suitable vectors and previous evidence of infection in domestic and wild animals (Bock et al., 2006; Spratt & Beveridge, 2018; Thompson et al., 2014) indicates that climate change could transform current pathogen-free regions into ‘new habitats’ for vectors and pathogens.
In the context of animal health, wildlife disease surveillance is an important tool to obtain information of morbidity and mortality, changes in patterns of disease occurrence over time, and early detection of disease outbreaks, including those linked to emerging diseases (Duncan et al., 2008; Grogan et al., 2014). For example, surveillance programs in Europe resulted in detection of a new disease in rabbits, the European brown hare syndrome caused by a calicivirus (Artois et al., 2009). However, detection of a new disease depends on its prevalence, patterns of transmission and disease-induced mortality. Therefore, sampling effort is crucial. Our sampling methodology involved recreational hunters and staff in deer control programs, who provided samples and data, but obtaining information on the health status of deer at the time of sampling was challenging. Considering the likelihood of chronic and low burden parasitic infections of deer, a key factor that would increase the efficacy and efficiency of wildlife disease surveillance is a clearer definition of the term ’suspect case’. This description could be available and shared with the wide range of stakeholders including hunters and wildlife rangers who could be involved in retrieving important information during their traditional activities (Grogan et al., 2014; Guberti et al., 2014). In Australia, recreational hunting is an economically important activity, contributing AU$335 to the economy, with deer among the primary species hunted (Australian Government Department of Health, 2019). In Victoria alone there were over 36,000 licensed deer hunters that harvested ~100,000 deer in 2018 (Victoria State Government, 2019). This large hunter population, if utilised to enable wildlife disease surveillance (Ryser-Degiorgis, 2013), represents an opportunity to implement a passive surveillance program to detect and identify endemic and emerging infections in new areas.
The focus of this study was on parasites, but the approach taken could be readily expanded to consider bacterial and viral pathogens. Our data suggest that it is unlikely that a large proportion of Australian deer are involved in maintaining the life cycle of Trypanosoma ,Plasmodium , Neospora , Sarcocystis ,Toxoplasma , Babesia and Theileria , as our maximum possible prevalence of infection are lower than those reported in Italy (Zanet et al., 2014) and Canada (Milnes et al., 2019). Importantly, this survey represents the first molecular study of its type in Australian deer and provides important baseline information about the disease status of wild deer in eastern Australia. Despite our best efforts, we could not conduct extensive deer sampling in warmer months or in tropical areas. Considering the additional limitations of our study discussed above, further studies combining serology assays and high-throughput sequencing are desirable. This would enhance parasite detection, and ultimately characterise the epidemiology of such pathogens in Australian wild deer populations. Assessment of the infection status of invasive species such as deer is necessary for future planning and successful implementation of disease eradication programs in livestock (Gortazar et al., 2015).