Quantification of the increase in the frequency of early calving associated with late exposure to bluetongue virus serotype 8 in dairy cows: implications for syndromic surveillance

Table 3 Effect of adjustment variables on the frequency of early calving estimated with mixed logistic models

Variable and class	Number of pregnancies	Odds Ratio	95% CI	p value
Parity
1	96 048	1	NA
2	62 470	1.18	1.15–1.21	<0.001
3	37 036	1.11	1.08–1.15	<0.001
4 or more	37 293	1.10	1.07–1.14	<0.001
Month of calving
August	17 525	1	NA	<0.001
September	24 525	0.59	0.57–0.62	<0.001
October	41 507	0.63	0.61–0.66	<0.001
November	52 305	0.49	0.47–0.51	<0.001
December	44 618	0.52	0.5–0.55	<0.001
January	26 490	0.51	0.48–0.53	<0.001
February	14 341	0.53	0.5–0.56	<0.001
March	11 536	0.57	0.54–0.61	<0.001

The unexposed population considered in this analysis corresponded to cows belonging to herds exposed in 2007 that calved in 2005 and 2006 (17 139 cows in 2616 case herds, 20 139 cows in 4008 herds located in the 2007 outbreak area but not reported and 189 359 cows in 8015 reference herds).
NA not applicable.

ISSN: 1297-9716