Published March 16, 2011, 12:01 PM

Fungal infection results in mallard die-off near Pierre

Government personnel have been investigating a large mallard (Anas platyrhynchos) die-off located approximately 16 miles northwest of Pierre, South Dakota. A recent update by the U.S. Geological Survey’s National Wildlife Health Center (NWHC) indicates respiratory fungal infections caused the deaths of more than 8,000 mallards.

Government personnel have been investigating a large mallard (Anas platyrhynchos) die-off located approximately 16 miles northwest of Pierre, South Dakota. A recent update by the U.S. Geological Survey’s National Wildlife Health Center (NWHC) indicates respiratory fungal infections caused the deaths of more than 8,000 mallards.

The South Dakota Department of Game Fish and Parks and the U.S. Fish and Wildlife Service first responded on January 27, to a mallard die-off on the Okobojo Creek arm of Lake Oahe. A total of 360 mallards were found dead on the lake ice and were removed. The following day, approximately 7,000 additional dead mallards were discovered at an open artesian pond on private land located 3 miles north of the first die-off site. In an effort to keep birds away from the water source, hazing techniques including use of propane canons, scare streamers, and other activities were initiated at the pond after all dead mallards were removed on January 29. An aerial survey of the area on February 3, found more than 600 additional dead mallards at a third site consisting of several small open beaver ponds.

A sample of thirteen mallards either found dead or euthanized were sent to NWHC for diagnostic necropsy. Initial results indicated that at least two of eight mallards necropsied died due to aspergillosis, a respiratory tract infection caused by the inhalation of spores produced by fungi of the genus Aspergillus. Asperillosis has been known to cause previous mallard die-off events, including an event that occurred at the Oahe Seep near Pierre in 1985 (Bair et al. 1988). Further testing, including histopathology analysis of lung tissues, later confirmed that two of the other six mallards necropsied also died from a fungal respiratory infection, possibly caused by a different genus of fungi. Scientists at NWHC are continuing tests to try to identify the fungal pathogen associated with disease in these birds. Fungi can also cause avian morality by producing toxins (mycotoxins) that target the liver; however, lab results did not indicate mortality from liver disease.

The mallards most likely developed the fungal respiratory infections after being exposed to moldy corn; however, lab findings did not definitively determine the source of fungi that infected the mallards. Moldy corn was found in a silage pile at a feedlot located near the site where most of the dead mallards were found and in the digestive tract of several mallards examined. Observations also indicate that mallards are drawn to the feedlot when snow cover keeps them from foraging elsewhere.

The fungal disease identified from the mallards that were tested is not considered contagious (does not spread from bird to bird) and humans are not considered susceptible unless they have an immune system deficiency or develop an allergic reaction. Allergic reactions are rare and generally found in individuals working directly with the fungi and not from exposure to infected bird carcasses.

For more information contact one of the agencies named in this release. More information on aspergillosis and other fungal diseases is available in the National Wildlife Health Center “Field Manual of Wildlife Diseases” available at http://www.nwhc.usgs.gov/publications/field_manual/index.jsp).

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