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).