The mortality slowed after water was added to the refuge and carcass collection was completed and continued on a daily basis. An estimated 20,000 waterfowl were migrating daily through this refuge and at risk throughout the mortality event. Two mortality events involving approximately 150 western sandpipers, black skimmers, and a few other shorebirds occurred in July and again in October on Marco Island, Collier County, Florida. The cause of death in these events was poisoning by an organophosphorus compound. Subsequent contaminant analysis of birds from the July event confirmed an unusual combination of very high levels of phorate, diazinon, dimethoate, dursban and malathion in one bird and dursban in another.
Western sandpipers are migratory while the black skimmers are resident birds in Florida.
Biologists are baffled as to the route of exposure. To date there has been no analysis of stomach contents on birds collected during the second event to confirm these findings. NWHC and the University of Florida, Gainesville received birds from these events. As in the winter of 1996-97, coots with neurologic clinical signs were detected in late November on DeGray Lake, Arkansas and the presence of vacuolar myelinopathy of the white matter of the brain was confirmed. Despite an incredible amount of field work, diagnostic evaluation and research, the cause of this neurologic disease has not been determined.