PERSPECTIVES ON THE DIAGNOSIS, EPIZOOTIOLOGY, AND CONTROL OF THE 1973DUCK PLAGUE EPIZOOTIC IN WILD WATERFOWL AT LAKE ANDES, SOUTH-DAKOTA

An epizootic of duck plague occurred in early 1973 in a population of 163,500 wild waterfowl, primarily mallards (Anas platyrhynchos), winte ring on Lake Andes and the nearby Missouri River in southeastern South Dakota (USA). The diagnosis was based on pathologic lesions and confi rmed by virus isolation. Control measures included quarantine, attempt s to reduce virus contamination of the area, dispersal of waterfowl, a nd monitoring of wild waterfowl populations for mortality. The epizoot ic resulted in documented mortality of 18% and estimated mortality of 26% of the waterfowl at risk. Prompt implementation of control measure s might have limited mortality to approximately 8%. Losses during the epizootic were equivalent to 0.12% of the annual mortality in the Nort h American 1996 fall population of 80,000,000 wild ducks. The most lik ely sources of the infection were free-flying wild mallard or American black duck (Anas rubripes) carriers from the upper midwestern or nort heastern United States. Duck plague serum neutralization antibodies we re demonstrated in 31% of 395 apparently healthy mallards sampled prio r to dispersal of the flock at Lake Andes, suggesting that tens of tho usands of potential duck plague carriers entered the wild waterfowl po pulations of all four major flyways. Consequently, the absence of majo r epizootics of duck plague in wild waterfowl in the subsequent two de cades is evidence that substantial numbers of duck plague carriers can occur in wild waterfowl populations without resulting in epizootic mo rtalities. The failure to isolate duck plague virus from apparently he althy mallards sampled during the epizootic raises questions concernin g the validity of conclusions regarding the status of duck plague in w ild waterfowl based upon negative results of random surveys conducted in the absence of epizootics.