Intrauterine infections with ncp BVDV between days 40 and 120 of gestation generate persistently infected animals. These PI-animals play a key role in the distribution of BVD. They shed the virus in secretions and excrements (saliva, nasal secretions, urine, semen etc.) in high concentrations. They do not produce detectable antibodies against “their” virus, i.e. they are immunotolerant. In large part PI-animals are clinically inconspicuous and will only be detected using laboratory techniques. Other individuals are poor doers and susceptible to secondary diseases (e.g. respiratory or enteric ones). PI-animals are candidates for Mucosal Disease. Persistently infected females will always produce persistently infected offspring. Whenever possible, PI animals should be culled (also see: Control strategies).