Table 2. ORs for a pooled sample (in the first cycle) and an individual animal (in the second cycle) to be MRSA-positive in an intervention study performed in 51 veal calf farms to reduce MRSA carriage, the Netherlands 2010–2012.

	1st production cycle: ORs for a pooled sample to be MRSA-positive				2nd production cycle: ORs for an individual animal sample to be MRSA-positive
Sampling moment-Arm of study	MRSA-positive/N total a	OR	95% CI	Wald P-value b	MRSA-positive/N total c	OR	95% CI	Wald P-value b
Week 12—Control	95/170	9.70	3.62–26.03	<0.01	183/478	13.69	4.07–46.00	<0.01
Week 12—RAB-CD	79/170	5.97	2.24–15.87	<0.01	171/465	14.72	4.69–49.31	<0.01
Week 12—RAB	57/170	3.08	1.76–5.39	<0.01	111/488	6.35	3.95–10.20	<0.01
Week 0—Control	33/170	1.17	0.42–3.25	0.77	26/482	0.56	0.15–2.02	0.37
Week 0—RAB-CD	32/170	1.31	0.48–3.62	0.60	37/465	1.44	0.41–5.03	0.57
Week 0—RAB	27/170	Ref.	-	-	26/488	Ref.	-	-

Results from generalized linear mixed models accounting for clustering at farm level in which study arm and sampling moment were grouped in a single determinant to evaluate interaction effects (i.e. differential effects by arm of intervention and time). MRSA, methicillin resistant Staphylococcus aureus; RAB, farms reducing antimicrobials by protocol; RAB-CD, farms reducing antimicrobials by protocol and applying a cleaning and disinfection program; Control, farms without interventions; Ref., reference category of the variable.

^a 17 farms per arm of intervention, 2 sampling moments and 10 pooled samples per farm.

^b Overall p<0.01.

^c 17 farms per arm of intervention, 2 sampling moments and a mean of 28 animals sampled per farm.