Table 5.

Multivariable extended Cox regression model of factors associated with a subsequent incident amongst clear and IR only herds, including time varying coefficients.

Variable	Level	HRa	95% CIb		P-value
MAIN COVARIATES
First WHT status in 2012	Clear	1.00
	IRs only	2.69	2.54	2.84	<0.001
Herd size	1–10	1.00
	11–50	1.92	1.70	2.17	<0.001
	51–100	3.00	2.66	3.39	<0.001
	101–200	3.93	3.49	4.43	<0.001
	201–300	4.65	4.09	5.30	<0.001
	>300	6.18	5.45	7.02	<0.001
Number of incidents in the previous 10 years		1.19	1.17	1.21	<0.001
Herd type	Beef	1.00
	Dairy	0.98	0.93	1.04	0.547
	Other	0.61	0.45	0.82	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	1.05	1.05	1.06	<0.001
Geographical risk area	England high risk	1.00
	England Edge	0.90	0.80	1.02	0.088
	Wales	0.80	0.75	0.86	<0.001
TIME-VARYING COEFFICIENTS
First WHT status in 2012	Clear	1.00
	IRs only	0.37	0.34	0.39	<0.001
Herd size	1–10	1.00
	11–50	1.20	1.05	1.38	0.008
	51–100	1.26	1.10	1.44	0.001
	101–200	1.32	1.16	1.51	<0.001
	201–300	1.46	1.26	1.69	<0.001
	>300	1.40	1.21	1.61	<0.001
Number of incidents in the previous 10 years		1.02	1.01	1.04	0.008
Geographical risk area	England high risk	1.00
	England Edge	1.04	0.93	1.17	0.464
	Wales	0.88	0.83	0.94	<0.001
Herd type	Beef	1.00
	Dairy	1.14	1.07	1.21	<0.001
	Other	0.62	0.44	0.88	0.007

^aHazard ratio.

^bConfidence interval.

Ratios in italics represent the reference groups.