Historical attributes of matched pairs of infected and uninfected ewes.

aAll 130 pairs were matched for breed composition by comparing sire and dam line records. Each pair was birthed together in the same group of ewes in the same location over a 45-day period. In addition, all 260 sheep selected were born without difficulty, had a high level of vigor after birth, and were raised by ewes until weaning. The matching success rate for 5- to 9-year-old sheep with these criteria was approximately one matched pair per 8.9 sheep tested. Sibling relationships within pairs were analyzed retrospectively and identified two pairs of fraternal twins and 12 pairs of paternal half-sibs sharing on the sire side.
bThese sheep were not born at USMARC but rather were purchased together as a group from a single Texas flock in May 1998.
cNot applicable.