If you know that Trich is present in the area from where you will select replacements, please use extra caution as you select bulls and females to enter your herd.  You greatly decrease the risk of bringing Trich into your herd if you purchase replacement animals from a herd with excellent reproductive performance and good management practices.  If this information is not available it is best to purchase virgin replacements animals or females in their third trimester of pregnancy. 

BonDurant et. al., Infect Immun, 1993.  Clark et. al., Aus Vet J 1983. Parsonson et. al., J Comp Pathol. 1976.  Skirrow et. al, JAVMA 1987