What is avian botulism?
Botulism is a neuromuscular disease caused by the bacterium Clostridium botulinum. There are several different types of botulism. Type C and type E can kill waterfowl and fish. 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. Under the right conditions, the spores germinate and begin growing the toxin-producing bacterial cells.
Botulism has been responsible for over 80,000 bird deaths on the Great Lakes since 1999. One theory is that infected fish, partially paralyzed by the toxin, became easy prey for flocks of migrating waterbirds.
Scientists believe that outbreaks of Type E botulism occur only when particular ecological factors happen simultaneously, such as warmer water temperatures, anoxic (oxygen-deprived) conditions, and abundant nutrients from 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.
Botulism has been identified as a problem for fish and birds in Lake Ontario, Lake Erie, and Lake Michigan.
What you can do
Remove dead birds and fish immediately to prevent the spread of botulism, as the bacteria in the carcasses can serve as the source of outbreaks for months. Please review the following guidelines for handling carcasses and monitoring your beach area:
- Do not handle dead fish or birds with your bare hands.
- Properly dispose of carcasses by double bagging and placing them in the trash.
- Beware of fish that are floating — if they are not fighting, they are likely not healthy and should not be consumed.
- Do not eat undercooked or improperly prepared fish or waterfowl.
- Hunters should never harvest birds that appear to be sick or are dying.
- Do not let your pets eat dead fish or birds.
- Look for carcasses at two peak times: in mid-late summer and in the fall.
FAQs about botulism
What is botulism?
Botulism is a serious neuromuscular illness caused by a toxin that is produced by the bacterium Clostridium botulinum. Avian botulism has been recognized as a major cause of mortality in migratory birds since the 1900s. Human botulism is typically caused by eating improperly canned or stored foods. The bacterium is classified into 7 types (A-G) by characteristics of the neurotoxins that are produced. Four of these types (A, B, E, and rarely F) cause human botulism, while types C, D, and E cause illness in mammals, birds, and fish. All types of botulism are paralytic to some degree, due to the nature of the neurotoxins produced by the bacteria. The following are the four most common types of botulism:
Type A or Type B botulism is most commonly caused by the consumption of bacteria in improperly home-canned foods. Diluted and purified forms of the type A and B toxins are also used in certain facial aesthetic products.
Type C botulism and Type E botulism are responsible for extensive waterfowl and some fish kills. They are both brought on by the consumption of these particular types of the botulinum toxin through food web interactions. Type C botulism mostly impacts waterfowl (especially ducks) and is typically restricted to marshes and wetlands in prairie regions, primarily found west of the Mississippi River. Type E botulism is more prevalent in the Great Lakes, but has also been documented in California.
What species are affected by Type E and C botulism?
A large number of bird and fish species are susceptible to the Type E botulinum toxin, as are some amphibians, like mudpuppies, and most mammals. A few cases of Type E botulism in humans have been reported in North America and were the result of eating improperly smoked or cooked fish, but these types of cases in humans are rare.
Loons, mergansers, long tail ducks, grebes, scaup, cormorants, and gulls are the bird species commonly affected by Type E botulism. Commonly affected fish species include freshwater drum (sheepshead), smallmouth bass, rock bass, stonecats, round gobies, channel catfish, and lake sturgeon.
Type C botulism outbreaks on prairie wetlands have mainly affected ducks, coots, grebes, gulls, and other shorebirds.
Is Type E botulism responsible for Great Lakes bird and fish kills?
Where does botulism come from?
Botulism spores (the resting stage of the bacteria) are abundant in anaerobic habitats, such as soils, and aquatic sediments of many lakes, and can be readily found in the gills and digestive tracts of fish living in those lakes. The spores can remain in the ecosystem for extended periods of time, even years, and are quite resistant to temperature changes and drying. These spores themselves are harmless until the correct environmental factors and anaerobic conditions prompt them to germinate and begin vegetative growth of the toxin-producing bacterial cells.
The active bacteria that cause botulism only grow in a nutrient-rich substrate, such as areas with large amounts of decaying plant growth, which are free of oxygen (anaerobic). Fish that die for any reason and that contain the bacterial spores in their tissues are also suitable substrates for growth and toxin production by the bacteria.
How do birds end up dying as a result of the botulism toxin?
Fish-eating birds that died of Type E botulism were poisoned by eating fish that contained the toxin. However, it is not clear exactly how this happens. Birds such as loons and mergansers normally capture and eat only live fish. Yet, Clostridium botulinum Type E should not grow and produce the actual toxin in living fish.
It is possible there are circumstances that can cause toxin production in the tissues and digestive tracts of live, perhaps dying, fish. Alternatively, it may be that the fish captured alive and eaten by the birds had themselves fed on a source of Type E toxin. In these cases, it would be the toxin in the digestive tracts of the live fish that was the source of toxin for the birds in these outbreaks.
It is also possible that the live fish captured by the birds were already partially paralyzed by the Type E toxin and were therefore particularly easy prey for the birds. This might account for preferential feeding on toxin-containing fish by the affected birds.
Scientists also think that ingestion of maggots from the carcass of an infected animal can continue the spread of botulism, which may be responsible for large kills of shorebirds.
Why are we so concerned about avian botulism outbreaks?
Has botulism always been in the Great Lakes?
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 in 1964 regarding 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 coming to about 61,630 type E botulism-attributed deaths for 1999 through 2006 (according to databases maintained by the USGS – National Wildlife Health Center).
Different types of avian botulism have had destructive effects on birds throughout the U.S. for a considerable time and probably predate written records. One of the earliest major reported die-offs of a large number of waterfowl was encountered in the Great Salt Lake area of the United States in the early 1900s. Since early observations occurred on alkaline lakes in areas of western North America, the phenomenon was suspected of being a form of alkali poisoning and became known as Western Duck Sickness. It wasn’t until a quarter of a century later that the cause of these die-offs was determined to be Type C botulism poisoning.
Why are botulism outbreaks occurring now?
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 adequate levels of bacterial substrate. 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 or eat maggots from the decaying carcasses of infected individuals.
Invasive species may also play a role. 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.
Is there a link between botulism outbreaks and fluctuating water levels?
What are some possible symptoms that an animal with botulism could display?
As Type E botulism results in paralysis, infected species begin to exhibit unusual behavior. Water birds may not be able to hold their head up and, as a result, often drown. Gulls can often walk but not fly. Other birds may drag one or both wings while standing.
Once infected with Type E botulism, fish may flounder or swim erratically near the surface of the water. Their equilibrium may be affected, and they may have trouble staying right-side up. “Breaching” may also occur, during which a fish will float with its head near the surface and tail end lowered below. Infected fish usually die quickly and are most likely to be seen washed up on shore.
Note: ANY fish or waterfowl that seem sick should not be harvested or eaten.
Why is it difficult for scientists to positively determine that birds have died from Type E botulism?
Do blooms of the filamentous algae Cladophora play a role in botulism outbreaks?
The recent increases in the growth of Cladophora ultimately result in increased decaying plant matter in some areas of the Great Lakes. This decomposition can create an oxygen-deprived environment that is suitable to the bacterium that produces the Type E botulism toxin.
Are inland lakes susceptible to Type E botulism outbreaks?
The Michigan Department of Natural Resources has cited rare reports of Type E botulism on the state’s inland lakes. Scientists believe that there is a minimal threat of botulism outbreak transfer from the Great Lakes to inland lakes, since the disease itself is not transferable from a transportation standpoint. The likelihood of an infected animal getting from the Great Lakes to an inland lake is small, since it will probably be too incapacitated by the toxin to travel.
With botulism spores already existing everywhere, the most likely way an outbreak would occur in a new location is if the optimal environmental factors exist that allow the bacteria to enter a vegetative state and produce the toxin.
Human and pet health
Is it safe to eat fish and wildfowl during a botulism outbreak?
Can I get botulism?
Botulism in humans is usually caused by the consumption of improperly home-canned foods and is most often a result of the Type A or Type B botulinum toxin. A few cases of Type E botulism in humans have been reported in North America as the result of eating improperly smoked or cooked fish, but these cases are very rare.
Thorough cooking is necessary to destroy the bacteria and bacterial toxins. Consult your local health agency for recommended cooking temperatures.
As a precaution, any fish or waterfowl that are sick or act abnormally should not be harvested or eaten because cooking may not destroy the botulism Type E toxin.
Can I swim in the water if there's an outbreak?
Is it safe to walk dogs on the beach after a bird kill?
Will my dog get sick if it eats a dead bird?
Should I wash my hands after I touch a dead bird?
What steps should I take when preparing healthy fish or birds for consumption to ensure maximum safety?
Wear rubber or plastic protective gloves while filleting, field dressing, skinning, or butchering.
Remove the intestines of birds soon after harvest; don’t eat the intestines and avoid direct contact with intestinal contents. Fish should be filleted, and contact with any gut material should be avoided. Hands, utensils, and work surfaces should be washed before and after handling any raw food, including fish and game meat.
Please remember that proper and thorough cooking is necessary to destroy disease-causing organisms that occur naturally or that can be introduced during handling, storage, or preparation.
Contact your local health agency for more detailed information on suggested cooking temperatures and other possible health and fish consumption advisories. General information on food preparation is also available through the Food Safety Research Information Office of the USDA, Department of Agriculture
What can I do to help?
What steps can I take to help stop the spread of botulism?
Identifying possible cases of avian botulism at the early stages is the key to effective control. Public awareness of the conditions that lead to avian botulism and prompt corrective action can greatly reduce the epidemics which now claim hundreds of thousands of birds each year. Sick and dead birds in areas of avian botulism epidemics should be reported immediately to state and federal wildlife agencies. Contact your local Michigan Sea Grant Extension educator for specific contact information in your region.
Immediate removal of dead birds and fish can also help prevent the spread of botulism, as the bacteria in the carcasses can serve as the source of outbreaks for months. Follow appropriate safety and disposal methods.
How should I notify authorities of a potential botulism-related fish or bird kill?
How can I stay safe while retrieving and disposing of dead fish or birds?
Be sure to follow local wildlife agency (e.g., Natural Resources, Fish and Wildlife, etc.) recommendations in handling dead fish and wildlife. In certain areas, burying of the carcasses is allowed, while in other areas incineration may be recommended. If birds are to be collected, they should be placed in heavy plastic bags to avoid the spread of botulism-containing maggots.
The major goal should be to protect yourself, while also ensuring that the dead birds or fish are not available for consumption by other wildlife.
Is rehabilitation of sick birds possible? If so, how, and who should I contact about it?
Rehabilitation is unusual but may be possible in cases where birds did not ingest a large amount of the toxin. Recovery can be aided by providing these birds with rest, fresh water, and shade. They should be protected from predators during this process.
A botulism antitoxin is available, but it requires special handling and must be given early on. Surviving an outbreak will NOT give birds immunity to botulism.
Please remember that extreme caution should be practiced when handling wildlife. Contact the local office of your state’s wildlife management agency for more information about rehabilitation possibilities.
Reporting in Canada
Reporting in the United States
For additional information about botulism or any other federal wildlife health issue, contact the USGS National Wildlife Health Center, (608) 270-2400.
Botulism in Michigan
- Avian Botulism and Grand Traverse Bay (2014 news article by Mark Breederland)
- Michigan DNR botulism website
- State of Michigan website on Emerging Disease Issues
- Sleeping Bear National Lakeshore
Citations for information in FAQs:
- Pennsylvania Sea Grant factsheet on botulism
Citation: Pennsylvania Sea Grant and Penn State Erie. 2003. Botulism Factsheet
- Canadian Cooperative Wildlife Health Centre website on botulismCitation: Leighton, F.A. Wildlife Health Topics, “Botulism”. Canadian Cooperative Wildlife Health Centre (CCWHC), March 2000, (accessed December 2006).
- Mortality figures from internal databases maintained by the USGS – National Wildlife Health Center. Estimate totals compiled in February, 2007.
- Sleeping Bear Dunes mortality estimates supplied via personal communication with Ken Hyde, Sleeping Bear Dunes National Lakeshore, 1 March 2007.
- Michigan DEQ website on botulism
Citation: Michigan State Department of Natural Resources. Michigan Wildlife Disease Manual, “Botulism”. State of Michigan, 2001-2006, (accessed December 2006).
- Kaufmann, O.W. and L.D. Fay. 1964. Clostridium botulinum type E toxin in tissues of dead loons and gulls. Michigan State University Agricultural Experiment Station Quarterly Bulletin. 47(2):236-242.
- Environment Canada, Prairie and Northern Region website’s page on Type C avian botulismCitation: Avian Botulism Task Force. Avian Botulism. Environment Canada, Prairie and Northern Region, 27 November 2006, (accessed December 2006).
- NY DEC website’s botulism FAQ
Citation: New York State Department of Environmental Conservation. Type E Botulism in Lakes Erie and Ontario – Q & A, (accessed December 2006).
- NY-PA-OH Sea Grant website on botulism
Citation: New York, Pennsylvania and Ohio Sea Grant. NY/PA/OH Sea Grant: Botulism in Lakes Erie, Ontario, 30 November 2006, (accessed December 2006).
- USGS National Wildlife Health Center website on avian botulism
Citation: USGS National Wildlife Health Center. Disease Information, “Avian Botulism,” 7 November 2006, (accessed December 2006).