Bird Diseases/pathology; Ducks/virology; Enteritis/veterinary; Nephritis/veterinary; Polyomavirus Infections/veterinary; Polyomavirus/pathogenicity; Animals; Base Sequence; Bird Diseases/epidemiology; Bird Diseases/virology; Carrier State/transmission; Carrier State/veterinary; Carrier State/virology; Enteritis/epidemiology; Enteritis/pathology; Enteritis/virology; Geese/virology; Genome, Viral/genetics; Intestine, Small/pathology; Kidney/pathology; Molecular Sequence Data; Nephritis/epidemiology; Nephritis/pathology; Nephritis/virology; Polyomavirus/genetics; Polyomavirus Infections/epidemiology; Polyomavirus Infections/pathology; Polyomavirus Infections/virology; Sequence Analysis, DNA; Viral Load/veterinary; Virulence