Flock-level risk factors for outbreaks of infectious arthritis in lambs, Norway 2018

Table 1 Description of flock data variables tested in univariable screening (Chi²-test)

Variable	Categories	Total		Case flocks (n = 84)		Control flocks (n = 1414)
Variable	Categories	n	%	n	%	n	%	P-value
Number of lambs (ear-tagged, categorized)^a	< 200	1014	67.7	36	42.9	978	69.2	< 0.0001
	200–500	427	28.5	36	42.9	391	27.7
	> 500	57	3.8	12	14.3	45	3.2
Outbreak of infectious arthritis before 2018^c	No	1286	85.9	18	21.4	1268	89.7	< 0.0001
Outbreak of infectious arthritis before 2018^c	Yes	212	14.2	66	78.6	146	10.3
Lambing percentage^b	≤ 200	592	39.7	19	22.6	573	39.7	0.001
Lambing percentage^b	> 200	898	60.3	65	77.4	833	60.3
Breed^b	Breed other than Norwegian White Sheep	301	20.5	7	8.9	294	21.1	0.009
Breed^b	Norwegian White Sheep	1171	79.6	72	91.1	1099	78.9
Start of lambing season^b	April	1181	79.2	62	73.8	1119	73.8	0.038
	May	189	12.7	9	10.7	180	12.7
	March	121	8.1	13	15.5	108	7.7
Length of lambing season^b	< 4 weeks	921	61.5	44	52.4	877	62.1	0.076
Length of lambing season^b	> 4 weeks	576	38.5	40	47.6	536	37.9

^aNot tested in the multivariable model due to collinearity with number of ear-tagged lambs (continuous, not shown)
^bTested in multivariable model
^cIntervening variable, not tested in the multivariable model

ISSN: 1751-0147