Four gull carcasses collected from beaches in Sleeping Bear Dunes National Lakeshore (National Lakeshore) have tested positive for type E botulism toxin (avian botulism). Type E botulism is a powerful neurotoxin that has been linked to large bird die-offs throughout several areas of the Great Lakes in past years. More than 200 birds have already been found dead on National Lakeshore beaches this summer, and based on trends from recent years, that number is expected to increase this fall.
Avian botulism is a paralytic, often fatal disease of birds that results when they ingest toxin produced by the native Clostridium botulinum type E; an anaerobic bacterium found in nutrient-rich lake bottoms. The bacterium spores rest in the gills and digestive tracts of fish living in many North American lakes. The spores can remain viable for years and are harmless until the correct environmental conditions prompt them to germinate. Type E botulism occurs only when the spores germinate and the bacteria multiply and produce toxin. Changes in the Great Lakes ecosystem have increased the growth of the botulism bacterium, which has led to an increase in birds poisoned by eating toxin-tainted fish or invertebrates.
The National Lakeshore is conducting a study of avian botulism to determine whether any steps can be taken to control outbreaks. Dead birds and fish are identified, counted, and collected for analysis. Dedicated National Lakeshore volunteers have helped discover that die-offs begin in the heat of the summer and reach their peak during the fall bird migrations. It is believed that the large flocks of migrating birds ingest the toxin in offshore waters, where it has been accumulating throughout the summer, resulting in the fall die-offs. These are usually detected when strong autumn storms blow the dead birds to shore. At this time last year, nearly 300 birds had been found dead, and then from early October to mid-November, another 385 birds washed ashore.
In addition to actively monitoring the shoreline for sick and dead birds, the park is collaborating with scientists from the U.S. Geological Survey, the University of Wisconsin-Milwaukee, the University of Michigan, and Northwestern Michigan College’s Water Studies Institute, to conduct studies in the Lake Michigan nearshore environment to better understand the mechanisms of toxin transmission. Many of these studies are funded through President Obama’s Great Lakes Restoration Initiative to assure that Great Lakes beaches, fish, and sources of public drinking water are safe, and that the ecosystems that fish and wildlife depend upon are healthy.
Lake Michigan beaches within the National Lakeshore remain safe for swimming and recreation, however, park visitors should exercise caution upon encountering bird or fish carcasses. Type E botulism is not an infectious disease. It is a poison. You must ingest the toxin, usually by eating an undercooked infected fish or animal, to become ill. You are not at risk for contracting botulism by swimming in Lake Michigan. Visitors bringing pets to the park should keep them leashed and away from dead animals on the beach. Pets may be poisoned if they eat dead birds or fish containing botulism.