3.2. Contact structure: domestic dogs
For the transmission of lyssavirus (the cause of rabies) between dogs,
the probability of transmission given contact (a bite) has been
estimated to be 49% (95% CI, 45−52%) (Hampson et al., 2009). The
epidemiology of transmission is not subject to great uncertainty because
the parameter estimate is not highly variable: this is information that
can be obtained through controlled laboratory studies and detailed field
observations. Therefore, understanding rabies spread at the
wild−domestic interface depends on knowledge of the pattern of contacts
between domestic and wild dogs. Canine rabies lyssavirus can only be
transmitted between live animals, so the role of the environment in
indirect transmission is not relevant. Thus, with documented knowledge
of the probability of transmission given contact, the challenge to model
rabies spread within populations and between populations at the
wild−domestic interface is to characterise and measure effective
contact.
Within Indigenous communities across Australia, it is common practice to
allow domestic dogs to roam freely both within and around the
communities, with some individuals traveling to more distant bushland
areas (Bombara et al., 2017a; Hudson et al., 2017; Maher et al., 2019).
Such roaming behaviours increase the likelihood of disease spread within
and between communities, as well as creating an opportunity for disease
transmission beyond communities via interactions with wild dogs.
In northern Australia, and specifically within the NPA, the roaming
distributions of domestic dog − and inferences regarding contacts − have
been studied extensively. For domestic dogs, the approach has used GPS
units attached to collars (Figure 1A). With periods of monitoring from 3
days up to 2 months, GPS data (Figure 1B) allow dogs’ home ranges (HR)
and utilisation distributions (UD) to be estimated (Figure 1C). The
Biased Random Bridge (BRB) analytical method has application to such GPS
data in which the period between GPS fixes is semi-regular but there can
be considerable time periods when no fix is recorded (for example, a dog
lying underneath a car). The BRB method places kernel functions over
each movement of the dog between two consecutive GPS fixes, therefore
more realistically tracing the dog’s movements. Kernel smoothing is
carried out considering not only recorded GPS fixes, but also
interpolated locations. The dog is assumed to move between two
successive fixes according to a biased random walk, i.e. the roaming
pattern has a drift along each track, represented by a diffusion
parameter (Dürr and Ward, 2014). The estimated UD has a probabilistic
interpretation; for example, at the 95% isopleth, this is the area in
which the dog spent 95% of its time during the monitoring period.
In initial studies in the NPA, Dürr and Ward (2014) estimated UDs for
roaming dogs of up to 20 ha (95% isopleth). In addition, within this
short-term study dogs were recorded to be taken in vehicles on hunting
trips of up to 25km in distance and 6 hrs in duration (Dürr and Ward,
2014). In another study, seven community dogs were identified with an
estimated HR > 20 ha (maximum 40 ha), which included the
bushland surrounding these Indigenous communities, areas in which many
dogs were observed to spend considerable periods of their time (Bombara
et al., 2017a). In a study using longer monitoring periods (up to 67
days), three classes of domestic dogs were characterised: stay-at-home,
roamers and explorers (Hudson et al., 2017). Explorer dogs showed large
variability in their roaming patterns, but their HRs continued to
increase as the monitoring period increased. In further analyses, Maher
et al. (2019) examined the temporal activity of roaming domestic dogs
and found that dogs roamed further during the dry season and that their
daily roaming behaviour peaked between 1700 and 1800 hours. Some dogs
were recorded up to 2.5 km from their residence in this study. The
studies conducted in the NPA suggested that mostly, domestic dogs
remained within their communities with ample opportunities to contact
other domestic dogs. However, it was also observed that some of these
dogs could roam considerable distances from their communities into
bushland areas, demonstrating a potential wildlife−domestic interface.