A variant has arisen which is antigenically very closely related to TGEV but lacks one epitope (= antigenic determinant) on the TGEV spike protein. Is now enzootic in Europe. The variant replicates to high titre in the porcine respiratory tract but replicates poorly in the enteric tract (in contrast to TGEV). It is not associated with disease in either tract but can confuse serodiagnosis. It is thought that immunity to this virus may have provided immunity to TGEV.