Duck plague: a carrier state in waterfowl.

Healthy waterfowl were found to be carriers of duck plague (DP) virus. Black ducks (Anas rubripes) and Canada geese (Branta canadensis) surviving a natural outbreak of DP at Coloma, Wisconsin, in 1973 yielded DP virus in cloacal swabs taken four years postinfection. Experimental infection of previously unexposed mallard ducks (Anas platyrhynochos) with the Coloma strain of DP virus CO-WI (73) also produced cloacal virus shedding for up to four years after infection. A second DP virus strain, LA-SD (73) from the Lake Andes, South Dakota, epornitic, was detected from cloacal swabs of pintail ducks (Anas acuta), gadwall ducks (Anas strepera), wood ducks (Aix sponsa), and Canada geese infected experimentally one year before. The frequency of swabs positive for DP virus varied between individuals within each of the tested species. The amount of detectable DP virus shed was about 100 plaqueforming units of virus percloacal swab. Oral erosions were present in all species tested except Canada geese and gadwall ducks. Erosions occurred at the openings of the sublingual salivary gland ducts. DP virus was isolated from erosions. All ducks with lesions proved to shed DP virus, although not necessarily at the time they had the lesion.