Avian Botulism Claims Waterbirds on Lake Michigan

Loons, ring-billed gulls, grebes, and red-breasted mergansers were among the 2,985 waterbirds that died of type E botulism last November along the shores of northern Lake Michigan. The birds washed up on beaches along a 12-mile stretch of Sleeping Bear Dunes National Lakeshore.

Though the outbreak is the first one in Lake Michigan in more than 20 years, it is one of many such episodes that have recently occurred in the Great Lakes, particularly in Lake Erie and Lake Ontario. Researchers, state and federal agencies and a variety of non-government organizations are tracking the occurrences of botulism in the Great Lakes and are investigating the possible causes behind these outbreaks.

What is botulism?
Botulism is a neuromuscular disease caused by the bacterium Clostridium botulinum. There are several different types of botulism. Type C and type E are responsible for extensive waterfowl die-offs and some fish kills. Type E is more prevalent in the Great Lakes. Botulism in humans is usually caused by type A or B and results from consuming improperly home-canned foods.

In the Great Lakes, botulism spores (the resting stage of the bacteria) are abundant in anaerobic habitats, such as soils, and aquatic sediments of many lakes. When the correct environmental factors are present, the spores germinate and begin vegetative growth of the toxin-producing bacterial cells.

The birds that died on Lake Michigan last November were poisoned by eating fish that carried the toxin. One theory is that infected fish, partially paralyzed by the toxin, became easy prey for flocks of migrating waterbirds.

While botulism has been around for a long time, records of it did not appear on the Great Lakes until recently. Type C botulism was first identified in the Great Lakes in 1936 on Lake Michigan, and Type E botulism in the Great Lakes was first documented on Lake Michigan related to a 1963 outbreak.

Since 1999, significant die-offs of birds and fish have occurred regularly in Lake Erie and Lake Ontario, with estimated avian mortalities due to type E botulism totaling about 61,630 from 1999 through 2006, according to data maintained by the USGS - National Wildlife Health Center. About 52,140 avian deaths were attributed to Type E botulism from 2002 to 2006 on the Great Lakes, in addition to the nearly 3,000 deaths reported from Sleeping Bear Dunes National Lakeshore.

Scientists believe that there are outbreaks of type E botulism only when a variety of particular ecological factors occur simultaneously, such as warmer water temperatures, anoxic (oxygen deprived) conditions, and nutrient-rich substrate, or areas with large amounts of decaying plant growth. As average air and water temperatures have been rising on a global scale, warmer temperatures and anoxic conditions are occurring more frequently. Once these factors lead to the production of the toxin in food material eaten by fish, the toxin can be passed up the food chain as birds consume the infected fish.

Role of Aquatic Invasive Species
Aquatic invasive species may be contributing to the environmental conditions that promote botulism outbreaks. Current hypotheses under study suggest that zebra and quagga mussel beds may create additional habitat for the bacterium that causes botulism. Many scientists believe that quagga mussels also have the potential for filtering the bacteria and passing it up the food chain when the quagga mussels are eaten by fish such as the round goby.

Invasive mussels may also be responsible for the increase in growth of the algae Cladophora (which is also potentially tied to botulism outbreaks), since the mussel’s filtration of the water makes it clearer, therefore prompting increased algal growth. This increase in algal growth and the subsequent decay of the algae can increase the oxygen demand in the ecosystem leading to possible anaerobic conditions necessary for botulism toxin production.

“It seems like a number of factors came together last fall in this pristine stretch of the National Lakeshore as these migrating waterfowl were passing through,” says Mark Breederland, Northwest District Sea Grant Educator based in Traverse City. “The combination of a changed food web due to invasives along with certain weather conditions may have added together, setting up this loss. Researchers will be watching more closely for possible type E losses in 2007.”

Source: Botulism in the Great Lakes – Frequently Asked Questions, edited by Chiara Zuccarino-Crowe. U.S. EPA Great Lakes National Program Office. 2007.

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