Do Captive-Reared Waterfowl pose a Disease Threat?

by Gary L. Pearson, D.V.M.

Concern has been expressed about the potential for captive-reared waterfowl to introduce diseases such as avian cholera and duck plague into wild waterfowl populations. When considering this potential, it is necessary to examine the two elements of the threat. First is the risk of introducing diseases into migratory waterfowl, and second is the impact on migratory waterfowl populations if such introductions were to occur. To put these risks into perspective, it is helpful to consider four diseases which generally are regarded to be the most important in wild waterfowl – lead poisoning, botulism, avian cholera, and duck plague.

Lead poisoning of waterfowl results primarily from the ingestion of lead shot while the birds are feeding in areas that have been heavily hunted in the past. Prior to the 1991 ban on the use of lead shot in waterfowl hunting, from 1,600,000 to 2,400,000 waterfowl were estimated to die annually from lead poisoning in the United States, and substantial losses continue to occur today. However, because lead poisoning is not a contagious disease, there is no risk of captive-reared waterfowl introducing it into wild waterfowl populations.

Botulism is reported to have killed millions of waterfowl in California and Utah in the early 20th century, and eight major outbreaks with estimated mortalities ranging from 100,000 to 5,000,000 hve been reported in the U.S. and Canada since 1950. Botulism results from the ingestion of a toxin produced by the bacterium, Clostridium botulinum (usually Type C), which may develop during warm summer weather in any decaying animal protein in wetlands, including dead aquatic invertebrates, and it may concentrate in maggots that have fed on decaying carcasses of birds or fish. Because botulism is not a contagious disease, there also is no risk of captive-reared waterfowl introducing it into wild waterfowl populations.

Avian cholera was not reported in migratory waterfowl in North America until 1944 when it was diagnosed in wild ducks in conjunction with an outbreak in domestic poultry in Texas, but information from the National Wildlife Health Center (NWHC) in Madison, Wisconsin, indicates that the disease likely has been present in wild waterfowl populations at least since the early 20th century. Since 1944, avian cholera outbreaks have occurred periodically in migratory waterfowl from coast to coast in the U.S. and in Canada, with mortalities estimated as high as 80,000.

Although there is no evidence that captive-reared waterfowl are an important source of avian cholera in migratory waterfowl, it is instructive to consider the impacts of avian cholera on wild waterfowl populations. It is believed that the introduction of avian cholera into susceptible waterfowl frequently results in only a few deathes which are not detected, but under conditions favorable for transmission, large mortalities may occasionally occur. Although avian cholera outbreaks killing 60,000 – 80,000 wild waterfowl have been reported, mortalities exceeding 100,000 apparently are rare. The loss of 100,000 waterfowl would be equivalent to about 0.25% of the total annual mortality in a continental population of 80,000,000.

Duck plague is a contagious viral disease that affects only waterfowl. It was first diagnosed on North America in commercial pekin ducks on Long Island, New York, in 1967, but soon was reported in captive avicultural and free-flying wild waterfowl in the area. In 1973, an outbreak at Lake Andes, South Dakota, killed an estimated 42,500 waterfowl, primarily mallards, out of total population of 163,500. Studies conducted during that outbreak showed that up to 31% of the survivors had been exposed to duck plague virus, and band returns showed that mallards from Lake Andes disperse to 26 states and four Canadian provinces in all four flyways. Despite this massive infusion of duck plague carriers into wild waterfowl populations across the continent, the only outbreak reported in wild waterfowl since then occurred in 1994 on the Finger Lakes in New York, where an estimated 1,200 waterfowl died. Several isolated duck plague mortalities averaging 39 birds per year were reported in non-migratory waterfowl from 1967 to 1995. The total reported losses of migratory waterfowl from duck plague since 1967 are approximately 45,000. To put this into perspective, these total reported losses of migratory waterfowl from duck plague over the past 34 years are equivalent to about 0.1% of the total annual mortality in a continental population of 80,000,000 ducks.

A study by the NWHC of non-migratory waterfowl in the Cheasapeake Bay area of Maryland in 1998 using a newly developed polymerase chain reaction procedure showed high rates of duck plague infection in private flocks, in free-flying non-migratory waterfowl and in waterfowl raised and released for hunting. The study did not include migratory waterfowl from the area, but with the high prevalence of infection in these other groups with which they frequently associate, there can be little doubt that duck plague is present in migratory waterfowl populations, as well.

The NWHC study shows two things. First, with the high prevalence of duck plague in free-flying non-migratory waterfowl in the area, the only way that infection could be prevented in captive-reared waterfowl would be to vaccinate them before they have an opportunity to become exposed. Second, the relative paucity of duck plague outbreaks reported in Maryland (29 from 1967 to 1995, most involving muscovy ducks) in the face of this high prevalence of infection in non-migratory waterfowl in the area indicates that the duck plague virus strains circulating in those populations are of low virulence and pose little threat to migratory waterfowl. In fact, it is likely that these low virulence virus strains are producing natural immunity to more virulent duck plague virus strains in both non-migratory and migratory waterfowl.

Five conclusions can be drawn regarding the threat posed by captive waterfowl for introducing diseases such as avian cholera and duck plague into wild waterfowl populations. First, the risk is minor in comparison with the risks posed by chronically infected carriers already present in these populations and in other non-migratory waterfowl with which they commonly associate. Second, in the unlikely event that captive-reared waterfowl should introduce diseases into wild waterfowl populations, the probability of major outbreaks occurring is extremely low. Third, even if disease outbreaks were to occur from such introductions, they would pale in comparison with the losses from non-contagious diseases such as lead poisoning and botulism. Fourth, the impacts of such disease introductions on migratory waterfowl populations, if they should occur, would not be significant. And fifth, any concerns about the introduction of duck plague into wild waterfowl populations could be minimized by vaccinating captive-reared waterfowl before they are released.

Unfortunately, the rhetoric concerning the alleged disease threat to migratory waterfowl posed by the release of captive-reared waterfowl has been dominated by agency bias, personal prejudice, invalid assumptions, and inaccurate and misleading information. However, an objective analysis of the scientific evidence demonstrates that those claims are unfounded.