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 1 Number of herds, number of gestations and raw frequency of early calvings according to exposure statuses of the herd to Bluetongue virus serotype 8 (BTV-8)

	Number of herds	Number of gestations	Frequency of early calving (%)
Reported case herds with clinical signs in 2007	2616	17 139^a	24.4
Non-reported herds located in the 2007 outbreak area	4008	20 139^a	21.8
Unexposed herds in 2007	11 038	148 857^b	19.5
Herds in 2005/2006 that were located in the 2007 outbreak area	8015	189 359	19.4

Early calving was defined as a calving after a pregnancy length between expected percentiles 1 and 25 of the Gaussian distribution of normal gestation length. 2005–2006: unexposed population, 2007: exposed population, France
^aGestations for which the date of herd exposure (observed or interpolated) occurred during the last 10 weeks of pregnancy (see “Materials and methods” section for more details).
^bGestations with calving occurring during the same period in the year than herds located in the outbreak area.

ISSN: 1297-9716