Prevalence of scab based on clinical diagnosis
In 2020, 17.4% (± confidence interval[CI]4.6%; n= 48) of the herds in the study reported a scab outbreak. There was a very significant difference in the prevalence of scab between the three regions in 2020 (χ2= 13.6, df= 2, P.= 0.001), with the highest prevalence in the Midlands (26.9% ± 10.5%; n= 25), followed by the North (19.3% ± 9%; n= 16) and the lowest in the southwest (7.0% ± 5.6%; n= 7) (Table 1). At least one outbreak of scab at 34.4% (± 5.7%; n= 95) of farms in all regions. There was a significant regional difference (χ2= 13.5, df= 2, P.= 0.001), with the number of farms reporting scabs between 2012 and 2019 highest in the Midlands (45.2% ± 10.1%; n= 42), followed by the Southwest (37% ± 9.5%; n= 37) and lower in the North (19.2 ± 8.5%; n= 16).
Four farms have reported a scab outbreak every year since 2012 (Fig. 2a). Of the farms that reported a scab outbreak in 2020, 54.2% (± 14.5%; n= 26) also reported having had at least one scab outbreak in the previous 8 years (Fig. 1a). Farmers who reported having had at least one outbreak in the previous 8 years were significantly more likely to have reported a scab outbreak in 2020 (χ2= 9, df= 1, P.= 0.002; Fig.1a). The number of years that a scab outbreak had previously been reported for a farm significantly increased the likelihood of an outbreak being reported in 2020 (coefficient: 0.426, standard error [SE]: 0.106, P.= 0.001). Farms that reported at least 1 year with an outbreak in the previous 8 years were 2.72 times more likely to report an outbreak in 2020. Farms that reported an outbreak in 2019 were 8.61 times more likely to report an outbreak. outbreak in 2020. 228 farms that did not have a scab outbreak in 2020, 69.7% (± 6.1; n= 159) reported no scab outbreaks in the previous 8 years, while 59.2% (± 6.6%; n= 135) reported never having had a scab outbreak (Fig. 2b).
Prevalence of scab based on serological tests
The serological positives were determined in 25.6% (± 5.5%; n= 65) of farms. There were no significant differences in regional prevalence (χ2= 4.3, df= 2, P.= 0.12). Prevalence was highest in the North (33.7% ± 10.5%; n= 28), followed by the Southwest (22% ± 8.9%; n= 20) and the Midlands (21.2% ± 9.4%; n= 17). Overall, significantly more scab outbreaks were detected in 2021/2022 using ELISA compared to self-reported clinical cases in 2020 (χ2= 4.8, df= 1, P.= 0.03). The number of reported outbreaks was significantly higher in the North (χ2= 3.7, df= 1, P.= 0.05) and the southwest (χ2= 7.6, df= 1, P.= 0.005). There was no significant difference in the number reported between 2020 and 2021/2022 in the Midlands (χ2= 0.5, df= 1, P.= 0.5).
Farms that reported having had at least one outbreak in the previous 8 years were not more likely to report a positive ELISA test (χ2= 0.05, df= 1, P.= 0.82). A total of 65 herds had a positive ELISA result, with the majority (69.2% ± 11.2%, n= 45) reporting that they have not had any outbreaks of scab in the last 8 years (Fig. 2a). A negative ELISA result was reported in 189 farms, with 66.7% (± 6.7; n= 126) reporting that they had not had a scab outbreak in the previous 8 years (Fig. 2b).
Self-reported scab outbreaks in 2020 on individual farms were compared with the 2021/2022 ELISA results (Table 1). Most of the farms that did not have self-reported outbreaks in 2020 had negative ELISA results in 2021/2022 (62.2% ± 6.1%; n= 158). A relatively small number of farms with a self-reported outbreak in 2020 tested positive for ELISA in 2021/2022 (5.1% ± 3%; n= 13). Overall, in terms of scab testing, with the ELISA, more herds went from negative to positive (20.5% ± 5%; n= 52), which from positive to negative (12.2% ± 4.2%, n= 31). Of the 52 herds that tested negative for scab in 2020 based on clinical signs but serologically positive in 2021/2022, there was a difference between regions, with a significantly greater difference than expected in the North compared to the Midlands (χ2= 3.9, df= 1, P.= 0.05). Furthermore, in the North, twice as many herds that changed status from no reported outbreaks in 2020 to positive for scab in 2021/2022 as determined by ELISA were those using common pastures (Table 1), a pattern not seen in other regions. .
Spatial distribution of outbreaks
Based on self-reported data on the 2020 epidemic, 38.4% (± 5.7%; n= 106) of farms had a neighboring contiguous holding that reported having a scab outbreak, but only 18.9% (± 7.5%; n= 20) of these reported having an outbreak themselves. Of the 48 farms that reported a scab outbreak in 2020, 41.7% (± 13.9%; n= 20) had at least one adjoining neighboring farm that also reported an outbreak. Farms with outbreaks in the Midlands were significantly more likely to have a neighboring farm which also reported an outbreak (64% ± 19.8%, n= 16) compared to their Northern counterparts (25% ± 23.1%, n= 4) or southwestern regions (n= 0) (P.= 0.072). Overall, having a nearby farm with an outbreak did not significantly change the likelihood of a farm reporting a scab outbreak in 2020 (χ2= 0.12, df= 1, P.= 0.73).
As anticipated, the underlying distribution of herds in the study areas was significantly different from the random one (as determined by the G-envelope function analyzes). There were no significantly elevated risk areas within any of the three regions, as identified by the relative risk estimate of kernel density. This was supported by SatScan, which did not identify any significant clusters within each region. Case-control comparison with L function in the north and southwest regions showed that the case points had a similar grouping to the control points. However, in the Midlands, cases were more clustered than controls within a radius of <1.5km.
Based on the 2021/2022 ELISA serological data, kernel density and SatScan analyzes were similarly unable to identify any significant clustering. However, in the L function case-control comparison, the Southwest and Midlands cases were more clustered than the controls at a radius <4.5 km and <3 km, respectively. Mirroring the 2020 analysis, there was no significant relationship between contiguous adjacent farms and the likelihood of reporting an outbreak using ELISA (χ2= 1.75, df= 2, P.= 0.42). Just under half (46.9% ± 6.1%; n= 120) of contiguous neighboring farms with a positive ELISA result, but only 18.3% (± 6.9%; n= 22) of these had a positive ELISA result. The number of positive farms with a positive contiguous farm was highest in the Midlands (41.2% ± 25.3%; n= 7) and the Southwest (45 %% ± 23.2%; n= 9), but much lower than in the North (21.4% ± 11.6%; n= 6).