Infected litters are weak, unthrifty, or splay-legged. Increasing incidence of bruising and hemorrhaging during processing has been reported. Preweaning mortality has been reported as high as 80 percent. Most consistent clinical signs observed are associated with respiratory disease or ill-thrift. Actinobacillus pleuropneumoniae , swine influenza virus, porcine respiratory coronavirus, and pseudorabies virus.
Direct contact is the most prominent route of exposure.
Persistence of virus in the respiratory tract and lymphoid tissues, predominantly tonsil, may occur for up to 157 days post infection. Further investigation into other strains is needed to provide a firm grasp of persistence of the virus. Logical means of transmission include movement of the infected pigs into a naive unit and local airborne spread. Fresh semen from an infected boar used to inseminate gilts was shown to cause infection and failure to conceive. Alternately, extended semen from an infected boar did not cause infection of gilts.