Table 4.

Results of the univariable Cox regression analysis of factors associated with the rate of subsequent incidents.

Variable	Level	HRa	95% CIb		P-value
First WHT status in 2012	Clear	1.00
	IRs only	3.58	3.41	3.76	<0.001
Season in which first WHT took place	Spring	1.00
	Summer	1.06	0.99	1.13	0.105
	Autumn	1.08	1.02	1.14	0.007
	Winter	1.06	1.01	1.11	0.031
Number of incidents in the previous 10 years	<3	1.00
	3 or more	1.50	1.49	1.52	<0.001
Geographical risk area	England high risk	1.00
	England Edge	0.43	0.40	0.47	<0.001
	Wales	0.47	0.44	0.49	<0.001
Annual rolling county level incidence at end of 2012	0–14.6 per 100 herd years at risk	1.00
	>14.6 per 100 herd years at risk	1.07	1.07	1.07	<0.001
Herd type	Beef	1.00
	Dairy	2.26	2.16	2.36	<0.001
	Other	0.44	0.33	0.58	<0.001
Herd size	1–10	1.00
	11–50	2.21	1.99	2.45	<0.001
	51–100	3.82	3.44	4.23	<0.001
	101–200	5.74	5.19	6.35	<0.001
	201–300	7.71	6.92	8.59	<0.001
	>300	10.49	9.45	11.63	<0.001

^aHazard ratio.

^bConfidence interval.

Ratios in italics represent the reference groups.