Not all cows are epidemiologically equal: quantifying the risks of bovine viral diarrhoea virus (BVDV) transmission through cattle movements

Table 3 Simulated risk of generating additional PI calves associated with cattle purchased by open beef and dairy herds

	Number of seeding events	Average age related probability of being PI (×10⁻²)	% simulation runs resulting in outbreak^†	OR	95% CI
Beef herds
Open heifers	22 473	1.37	1.32	Ref	-
Pregnant heifers	5205	0.63	2.32	1.76	1.74 – 1.77
Open cows (dry)	1184	0.16	0.16	0.13	0.12 – 0.13
Open cows (calf at foot)	8269	0.28	1.73	1.23	1.22 – 1.24
Pregnant cows (dry)	5131	0.09	0.82	0.68	0.67 – 0.79
Pregnant cows (calf at foot)	2176	0.15	1.19	0.94	0.92 – 0.95
Dairy herds
Open heifers	3188	0.84	2.00	Ref	-
Pregnant heifers	1734	0.54	1.95	0.92	0.90 – 0.93
Open cows (dry)	275	0.11	0.86	0.44	0.42 – 0.46
Open cows (lactating)	3510	0.30	1.79	0.67	0.66 – 0.68
Pregnant cows (dry)	547	0.10	1.20	0.64	0.62 – 0.66
Pregnant cows (lactating)	769	0.16	1.49	0.68	0.67 – 0.70

¹The percentage of simulation replicates that resulted in the generation of at least one additional PI calf in the herd within 2 years of the original seeding event.
The results are based on a dynamic within-herd simulation model that makes use of the explicit demographic structure and movement patterns of 2895 beef herds and 546 dairy herds in Scotland from records stored in the Cattle Tracing System (CTS) database. A total of 1000 replicates were performed for each disease seeding event in the herds to capture the stochastic variation in the probability of purchased replacement breeding cattle being persistently or transiently infected with BVDV and the probability of generating at least one additional PI calf in the herd following disease introduction. The odds of particular types of replacement breeding cattle causing outbreaks were expressed relative to the "open heifer" group, which had the highest probability of being PI based on the average animal age at time of purchase.

ISSN: 1297-9716