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Table 2 Univariable associations for variables considered for multivariable analysis, including interaction between high-risk local exposure and time of matching

From: Spread of porcine circovirus associated disease (PCVAD) in Ontario (Canada) swine herds: Part II. Matched case-control study

Variable Odds ratio 95% CI P a Overall Pb
Estimates based on the maximum likelihood method using assumption of Gaussian distribution
Herd PRRSv-positive status (no = refc) 1.56 (0.89, 2.75) 0.121 0.107
Herd type     0.090
   Farrow-to-finish - - - -  
   Farrow-to-grow 1.23 (0.70, 2.18) 0.473  
   Finisher 1.47 (0.96, 2.26) 0.077  
   Nursery 0.74 (0.43, 1.27) 0.275  
   Nursery and finisher 0.59 (0.23, 1.51) 0.274  
Easting (100 kms)     0.002
   linear 1.54 (1.17, 2.03) 0.002  
Cumulative local exposure (CLE) (no = ref) 1.05 (0.64, 1.70) 0.852 0.852
High-risk local exposure(HRLE) (no = ref) 2.26 (1.06, 4.83) 0.035 0.040
HRLE*Time interaction model     0.005
   HRLE 0.04 (0.00, 1.74) 0.094 -
   HRLE*Time (Month) 1.21 (1.02, 1.44) 0.031 0.0128
Local # of high-risk herds LNHRH (continuous) 2.16 (1.06, 4.39) 0.033 0.036
Local # of high-risk herds LNHRH (nominal; 0 = ref)     0.111
   One 2.20 (1.02, 4.76) 0.045  
   Two 4.03 (0.24, 67.50) 0.333  
Ownership-adjusted high-risk local exposure (OWHRLE; No high-risk local exposure = ref)     0.001
   High-risk local exposure within ownership 1.0E+7 (not estimated) 0.984  
   High-risk local exposure outside of ownership 1.31 (0.54, 3.17) 0.555  
Number of direct contactsd/month 0.95 (0.88, 1.03) 0.221 0.205
Number of indirect contacts/month 1.02 (0.98, 1.07) 0.256 0.263
Number of nursery contacts/month 0.93 (0.85, 1.02) 0.106 0.086
Number of breeding contacts/monthe 1.03 (0.96, 1.10) 0.407 0.407
Number of feed contacts/month 1.06 (1.00, 1.12) 0.043 0.045
Estimates of the high-risk local exposure obtained using exact logistic regression
Herd PRRSv-positive status (no = ref) 1.56 (0.87, 2.95) 0.151 0.151
Cumulative local exposure (CLE) (no = ref) 1.05 (0.62, 1.74) 0.901 0.901
High-risk local exposure(HRLE) (no = ref) 2.26 (0.97, 5.18) 0.049 0.049
Local # of high-risk herds LNHRH (continuous) 2.16 (0.99, 4.69) 0.043 0.043
Local # of high-risk herds LNHRH (nominal; 0 = ref)     0.108
   One 2.18 (0.92, 5.04) 0.049  
   Two 4.03 (0.05, 330.32) 0.367  
Ownership-adjusted high-risk local exposure (OWHRLE; No high-risk local exposure = ref)     0.001
   High-risk local exposure within ownership 20.17f (2.75, +inf) 0.001  
   High-risk local exposure outside of ownership 1.31 (0.46, 3.39) 0.634  
  1. a Based on the Wald test for conditional maximum likelihood.
  2. b Based on the Likelihood ratio test for conditional maximum likelihood.
  3. c ref = referent group.
  4. d Frequency of all contacts was calculated on the basis of visits to herds.
  5. e Based on 230 observations where such information was applicable.
  6. f Median unbiased estimator.
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