Spatial and temporal variation in proximity networks of commercial dairy cattle in Great Britain

Abstract

The nature of contacts between hosts can be important in facilitating or impeding the spread of pathogens within a population. Networks constructed from contacts between hosts allow examination of how individual variation might influence the spread of infections. Studying the contact networks of livestock species managed under different conditions can additionally provide insight into their influence on these contact structures. We collected high-resolution proximity and GPS location data from nine groups of domestic cattle (mean group size = 85) in seven dairy herds employing a range of grazing and housing regimes. Networks were constructed from cattle contacts (defined by proximity) aggregated by different temporal windows (2 h, 24 h, and approximately 1 week) and by location within the farm. Networks of contacts aggregated over the whole study were highly saturated but dividing contacts by space and time revealed substantial variation in cattle interactions. Cows showed statistically significant variation in the frequency of their contacts and in the number of cows with which they were in contact. When cows were in buildings, compared to being on pasture, contact durations were longer and cows contacted more other cows. A small number of cows showed evidence of consistent relationships but the majority of cattle did not. In one group where management allowed free access to all farm areas, cows showed asynchronous space use and, while at pasture, contacted fewer other cows and showed substantially greater between-individual variation in contacts than other groups. We highlight the degree to which variations in management (e.g. grazing access, milking routine) substantially alter cattle contact patterns, with potentially major implications for infection transmission and social interactions. In particular, where individual cows have free choice of their environment, the resulting contact networks may have a less-risky structure that could reduce the likelihood of direct transmission of infections.

Keywords: Cattle; Contact; Disease transmission; Livestock; Social network.

Fig. 1

Examples of how contacts were grouped by different locations and times of day (i.e. Pasture, Buildings, and Split networks by day or night) to analyse the repeatability of dyads being in the same community over time. Each number represents a separate network formed from the contacts below it (represented by multiple overlapping points), the location is denoted by the y-axis and the colour represents the time of day (yellow = day, blue = night), where day is defined as 07:00–19:00. For example, the yellow ‘1’ in the top left of the plot represents the first Pasture, day network for the Strip-grazed group. We calculated communities in each of these networks, noted for each dyad if they were in the same community in each network and then tested the repeatability of dyads being in the same community across the sequential networks for each network category, (e.g. the pasture day category compared yellow 1, 2, 3, 4, 5, 6, 7 in the Pasture group – circled in yellow on schematic). As the Dry group stayed in one location for the majority of the study, the networks were only divided by time of day, not location.

Fig. 2

Temporal variation in numbers of contacts recorded between cows in nine groups of dairy cattle. Contacts during the day (yellow shading) and night (blue shading) were recorded by proximity loggers and contact frequency is averaged per hour per dyad (cow-cow pair) and aggregated into varying time periods; the whole study period (mean = 7 days; black solid line), daily (red circles and dashed lines), and 2-hourly (blue circles and dotted lines). Daily and 2-hourly values are selected from the middle four days of the study period for brevity. Milking regimes (light red shading) varied by group and timings plotted here are those reported by farmers. There is little variation between days but strong within-day patterns are evident in most farms, with higher contacts around dawn and dusk, frequently aligned with milking time.

Fig. 3

Relative amount of time and number of between-cow contacts in different farm areas recorded in seven groups of dairy cattle. Bar charts show the relative proportion of time and contacts recorded by proximity loggers in Buildings (brown) and at Pasture (white). Groups are defined as ‘Split’ when less than 75 % of cows are in either Buildings or Pasture (purple). Most cattle only had access to buildings or pasture at specified times, however, the Free group was allowed access to all farm areas and elected to spend the majority of time split into two groups.

Fig. 4

Distributions of the time cows spent together and the numbers of cows contacted in nine groups of dairy cattle at pasture and in buildings. a) Mean amount of time cows spent with other cows. b) Proportion of cows in the group with which they came into contact during the whole study period. Values are calculated for Pasture networks (black) and Buildings networks (brown). Groups are ordered by ascending group size from left to right and * denotes groups only in buildings for milking. Boxplots show the median, 25th and 75th percentiles of values and the upper and lower whiskers extend to the largest or smallest value no further than 1.5 times the interquartile range, data beyond this range are plotted as outlying points. In all comparable groups, less time is spent in proximity at pasture and on most farms, fewer cows are in proximity at pasture compared to in buildings. Housed and dry groups were only in buildings and at pasture respectively, therefore only appear on the corresponding plot.

Fig. 5

Variation between cows in the number and duration of their contacts (triangles) compared to variation in random networks in nine groups of dairy cattle at pasture and in buildings. a) Coefficient of Variation (CV) of degree showing variation in number of other cows each cow contacted b) CV of strength showing variation in duration of contact between-cows. Triangles indicate observed values above the upper 95 % bound of random network values calculated for contacts on pasture (‘Pasture network’; black) and in buildings (‘Buildings network’; brown). Boxplots represent the distribution of CV values calculated from randomised networks (n = 4999). Median, 25th and 75th percentiles of values are shown and the upper and lower whiskers extend to the largest or smallest value no further than 1.5 times the interquartile range, data beyond this range are not plotted. Groups are ordered by ascending group size from left to right and * denotes groups only in housing for milking. Housed and dry groups were only in buildings and at pasture respectively, therefore only appear on the corresponding plot.

Fig. 6

Social preference based on cow-cow interactions in nine groups of dairy cattle. Plots show the variation in time spent with other cows (CV_cddyad), compared with the maximum time a cow spent with a single group member. Each cow is represented by a point, coloured by the P value for the coefficient of variation (CV) compared to CV values calculated on random networks (P values < 0.025 are shown as triangles and > 0.025 are shown as crosses). A small number of cows in each group show preference for spending more time with particular other cows, however, the variation in the interactions of most cows did not differ significantly from random. To aid visualisation of more sociable cows, 37 cows with mean contact time with other cows below one minute were removed from this plot.