Sampling

The study period spanned two winter periods without midge activity (vector-free period) and one summer in between with midge activity (vector season). During the study period there were four to seven visits to the herds or flocks in which blood samples were taken from all animals present; these visits were timed such as to obtain information on changes in prevalence during both the vector season and the vector-free period. Samples were labelled with individual animal identification number. The first visit was on December 28^th, 2006, (cattle herd 5) and the last on July 10^th, 2008, (cattle herd 3); for a complete list of sampling dates we refer to (Tables ​(Tables11 and ​and2).2). According to official estimates of the Dutch Food and Consumer Product Safety Authority (NVWA) the vector-free period in the first winter lasted from 30 December 2006 until 27 March 2007 and the second vector-free period from 12 December 2007 until 22 April 2008. In the present study, no information was recorded on the age of the sheep animals. In two of the cattle herds, information on the age of the animals present was recorded at the first sampling date. The mean age of the animals in these herds was 3 years in both herds and the age ranges were 0–9 and 0–13 years.

Shaded rows: sampling dates within vector-free periods as defined by the official estimates of the Dutch Food Safety Authority (NVWA) (vector-free periods lasting from 30 December 2006 until 27 March 2007 and from 12 December 2007 until 22 April 2008).

Shaded rows: sampling dates within vector-free periods as defined by the Dutch Food and Product Safety Authority (NVWA) (vector-free periods lasting from 30 December 2006 until 27 March 2007 and from 12 December 2007 until 22 April 2008).

In Flock 5, part of the second round of sampling was performed on 13 April 2007, part on 18 May 2007.