PGD is a multifactorial worldwide disease with both infectious and non-infectious causes.  PGD leads to significant problems in Atlantic salmon (Salmo salar) and many other species. Among the infectious causes are viruses (Atlantic salmon paramyxovirus-ASPV, Salmon gill poxvirus-SGPV), bacteria (Tenacibaculum maritimus, Piscichlamydia salmones, Candidatus Clavochlamydia salmonis), parasites (Neoparamoeba perurans, Loma salmonae, Ichthyobodo spp., Trichodina sp.). Non-infectious causes include phytoplankton and zooplankton. Gross pathological changes include “clubbing”, in which thickened white areas of hyperplastic epithelium can be easily seen, mainly at the ends or distal third...