Identifying an outbreak of a novel swine disease using test requests for porcine reproductive and respiratory syndrome as a syndromic surveillance tool

Table 4 The association ^a between the weekly probability of PRRSV PCR positivity and the PCVAD outbreak of the Ontario swine industry after controlling for a PRRSV outbreak, season and year using data from the Animal Health Laboratory from January 2000 and April 2007

Variable		n	OR^b	95% CI	P-value
PCVAD outbreak			0.66	0.58 - 0 .75	0.01
PRRSV outbreak ^c			2.53	2.14 - 2.97	<0.001
Season^c	Fall	91	Referent	-
Season^c	Spring	101	0.92	0.60 - 1.43	0.72
	Summer	91	0.57	0.35 - 0.93	0.02
	Winter	99	0.85	0.55 - 1.33	0.49
Year	2000	52	Referent	-	-
	2001	52	0.81	0.51 - 1.30	0.38
	2002	52	0.95	0.60 - 1.52	0.84
	2003	52	0.90	0.60 - 1.35	0.61
	2004	52	0.66	0.44 - 0.99	0.04
	2005	52	1.04	0.71 - 1.50	0.85
	2006	52	0.49	0.35 - 0.70	<0.001
	2007	18	0.85	0.62 - 1.17	0.32
Year*Season interaction^d	2001*spring		0.58	0.30 - 1.12	0.11
	*2001summer**		2.98	1.51 - 5.86	0.002
	2001*winter		0.66	0.35 - 1.25	0.20
	2002*spring		0.95	0.51 - 1.76	0.86
	2002*summer		0.97	0.46 - 2.01	0.93
	2002*winter		1.34	0.72 - 2.49	0.35
	2003*spring		0.77	0.44 - 1.36	0.37
	2003*summer		0.66	0.34 - 1.31	0.23
	*2003winter**		2.06	1.18 - 3.06	0.01
	2004*spring		0.85	0.51 - 1.40	0.52
	2004*summer		1.22	0.69 - 2.17	0.49
	*2004winter**		2.24	1.34 - 3.74	0.002
	*2005spring**		2.19	1.39 - 3.47	0.001
	*2005summer**		2.06	1.23 - 3.48	0.006
	2005*winter		0.98	0.61 - 1.59	0.94
	*2006spring**		1.61	1.01 - 2.57	0.045
	2006*summer		1.09	0.65 - 1.85	0.74
	2006*winter		1.57	0.97 - 2.54	0.07
	2007*spring		0.93	0.59 - 1.45	0.74

a. Logistic regression using generalized linear model ML estimation with the logit link, binomial distribution and total number of PRRSV PCR tests ordered representing the denominator.
b. Odds ratio.
c. Suspected a priori may be a confounding variable.
d. Interaction term.
bold text – significant variables.

ISSN: 1746-6148