Perspectives on the Diagnosis, Epizootiology, and Control of the 1973 Duck Plague Epizootic in Wild Waterfowl at Lake Andes, South Dakota

Abstract

An epizootic of duck plague occurred in early 1973 in a population of 163,500 wild waterfowl, primarily mallards (Anas platyrhynchos), wintering on Lake Andes and the nearby Missouri River in southeastern South Dakota (USA). The diagnosis was based on pathologic lesions and confirmed by virus isolation. Control measures included quarantine, attempts to reduce virus contamination of the area, dispersal of waterfowl, and monitoring of wild waterfowl populations for mortality. The epizootic resulted in documented mortality of 18% and estimated mortality of 26% of the waterfowl at risk. Prompt implementation of control measures might have limited mortality to approximately 8%. Losses during the epizootic were equivalent to 0.12% of the annual mortality in the North American 1996 fall population of 80,000,000 wild ducks. The most likely sources of the infection were free-flying wild mallard or American black duck (Anas rubripes) carriers from the upper midwestern or northeastern United States. Duck plague serum neutralization antibodies were demonstrated in 31% of 395 apparently healthy mallards sampled prior to dispersal of the flock at Lake Andes, suggesting that tens of thousands of potential duck plague carriers entered the wild waterfowl populations of all four major flyways. Consequently, the absence of major epizootics of duck plague in wild waterfowl in the subsequent two decades is evidence that substantial numbers of duck plague carriers can occur in wild waterfowl populations without resulting in epizootic mortalities. The failure to isolate duck plague virus from apparently healthy mallards sampled during the epizootic raises questions concerning the validity of conclusions regarding the status of duck plague in wild waterfowl based upon negative results of random surveys conducted in the absence of epizootics.

Field Manual of Wildlife Diseases: General Field Procedures and Diseases of Birds

“…Moreover, the NWHC has diagnosed duck plague in migratory waterfowl several times since 1975, including the 1994 epizootic that killed 1,200 wild waterfowl on the Finger Lakes (New York), a mallard (Anas platyrhynchos) in Saskatchewan (Canada) in 1984 (Wobeser and Docherty, 1987), a black duck (Anas rubripes) in Maryland (USA) in 1985 (Brand and Docherty, 1988) and a mallard in North Dakota (USA) in 1988. The claim that duck plague is not an established disease in North American waterfowl not only is unsupported by credible scientific evidence (Pearson and Cassidy, 1997), but it is contrary to prevailing scientific opinion (Wobeser, 1997), including the U.S. Fish and Wildlife Service's Duck Plague (Duck Virus Enteritis) Panel Report (Beard et al, 1984).…”

“…However, Pearson and Cassidy (1997) concluded that the most probable source of the infection in the 1973 Lake Andes epizootic was other migratory waterfowl from the northeastern United States, and surveys of non-migratory waterfowl associated with the 1994 Finger Lakes epizootic failed to show evidence of duck plague in those birds.…”

“…For example, Chapter 16 on Duck Plague mentions the 1973 Lake Andes, South Dakota (USA), epizootic eight times and 18 of the 21 photographs are from that epizootic. However, it does not list either of the two peer-reviewed reports on the epizootic (Proctor et al, 1975;Pearson and Cassidy, 1997), despite the fact that one provides a detailed account of the diagnosis, epizootiology and control of this largest duck plague epizootic reported in free-flying wild waterfowl. On the other hand, it lists a 1999 paper on identifying duck plague virus by polymerase chain reaction.…”

“…However, Pearson and Cassidy (1997) have pointed out that neither serological surveys nor those based on virus isolation provide reliable information on the status of duck plague in waterfowl populations. Moreover, the NWHC has diagnosed duck plague in migratory waterfowl several times since 1975, including the 1994 epizootic that killed 1,200 wild waterfowl on the Finger Lakes (New York), a mallard (Anas platyrhynchos) in Saskatchewan (Canada) in 1984 (Wobeser and Docherty, 1987), a black duck (Anas rubripes) in Maryland (USA) in 1985 (Brand and Docherty, 1988) and a mallard in North Dakota (USA) in 1988.…”

Response to Creekmore's Letter

“…would likely be insignificant for the continental Mallard population,'' and I noted from p. 15 of his book that the total recognized losses of wild waterfowl on North America from duck plague over the past three decades have been fewer than 50,000 birds, about 42,000 of which were mallards. Thus, these loss figures are not my personal views, they are well-documented (Leibovitz, 1968;Friend and Pearson, 1973;Pearson and Cassidy, 1997), and they frequently have been cited by Mr. Creekmore's colleagues at the National Wildlife Health Center (NWHC) (Brand andDocherty, 1984, 1988;Brand, 1987;Friend, 1992;Friend and Cross, 1995). My subsequent statement that nowhere in the book is there the citation of a reference discussing the impact of duck plague on wild waterfowl populations again is not a personal view but a fact that can be verified by reading the book.…”