As part of a study described in a previous paper observations were made to determine whether and for how long experimentally infected young pigs would transmit their infection to new groups of weanlings maintained in contact with them. When groups of 4 or 5 susceptible weanlings 2-3 months old were placed in contact for a month with infected pigs 42 days or 3, 6, 9 or 12 months after experimental infection, no antibody rises were observed in the contact pigs. However, a strain of virus identical with the infecting strain was isolated from lung suspensions from 2 of the 5 contact pigs exposed to pigs infected 3 months previously. Possible sources of technical error such as laboratory contamination could be almost certainly excluded. It is considered that a shedder state of virus had occurred some time during the fourth month following experimental infection. There was suggestive serological evidence that the shed virus acted as a booster dose to previously infected pigs.
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