What Is 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 botulism? Type 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 in particular are the bird species affected by Type E botulism. Commonly affected fish species include, Freshwater Drum (Sheepshead), Smallmouth Bass, Rock Bass, Stonecats, Round Gobies, Channel Catfish and Sturgeon. (see sources, i)
Type C botulism outbreaks on prairie wetlands have mainly affected ducks, coots, grebes, gulls and other shorebirds. (see sources, ii)
Is Type E botulism responsible for the recent bird and fish kills?
Yes. Pathology conducted on fish and birds during recent die-offs points to Type E botulism as the cause. Type E Botulism is contracted by ingesting invertebrates, fish or birds contaminated with the toxin (Clostridium botulinum).
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. (see sources, ii)
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. (see sources, ii)
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. (see sources, ii)
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. (see sources, ii)
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. (see sources, ii)
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?
Natural resource managers, environmental organizations and others are concerned about the thousands of migrating birds that have died, including loons, and other species. According to estimates compiled from the USGS, National Wildlife Health Center's databases, about 52,140 avian deaths were attributed to Type E botulism from 2002 to 2006 in the Great Lakes. (see sources, iii)
Recent reports from the Sleeping Bear Dunes National Lakeshore also estimate that an additional 3,000 avian botulism-related mortalities occurred in 2006 on Lake Michigan. (see sources, iv) Some fish species, such as Lake Sturgeon, that have been listed as threatened, endangered or of special concern, are also now at an increased risk because of botulism. Additionally, dead wildlife may contain toxin levels that could harm other animals, including pets that might ingest beached carcasses.
Has botulism always been in the Great Lakes?
While Botulism has been around since the early 1900s in other areas of the country, records of it did not appear on the Great Lakes until much later. Type C botulism was first identified in the Great Lakes in 1936 on Lake Michigan, (see sources, v) and Type E botulism in the Great Lakes was first documented on Lake Michigan in 1964, regarding a 1963 outbreak. (see sources, vi) 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 thru 2006, (mortality figures from databases maintained by the USGS - National Wildlife Health Center.) (see sources, iii)
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 was not until a quarter of a century later that the cause of these die-offs was determined to be Type C botulism poisoning. (see sources, vii)
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. (see sources, ii)
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, a non-native bottom-dwelling fish.
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?
There is some evidence that outbreaks correspond to low water level events. Historically, larger bird die-offs as a result of Type E botulism have occurred during periods of low or rapidly declining water levels, and water level fluctuations and draw down events in wetlands have also correlated with Type C botulism outbreaks. The mechanism behind this possible link still needs to be researched but is likely to be related to warmer water and sediment temperatures during low water events.
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 (poor posture) while standing. In birds, a specialized eyelid (nictitating membrane), will often become paralyzed which impacts the bird’s vision.
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.
Researchers in Canada, (Moccia, et. al), have discovered that several species of fish show dramatic color changes or patterns of color when the toxin is ingested. Infected fish usually die quickly and are most likely to be seen washed up on shore. It should be noted that round gobies lack a swimbladder that other fish possess, so they normally remain on the bottom after death. (see sources, viii)
Note: ANY fish or waterfowl that behave unusually or appear ill or injured should not be harvested or eaten.
Why is it difficult for scientists to positively determine that birds have died from Type E botulism?
Both Type C and Type E botulism, as well as a few other types of poisoning, can produce similar symptoms in affected wildlife. Definitive diagnosis of Type E botulism requires that the Type E botulinum toxin be found in the blood of a live, sick bird. Although finding the toxin in a recently dead bird may be evidence that the bird died of botulism, it is also possible that the toxin detected was produced after death, during putrefaction, and may not have been the cause of the bird’s death. (see sources, ii)
Do the recent algal blooms of the macro algae Cladophora play any role in the botulism outbreaks?
The recent increases in the growth of Cladophora ultimately results 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 move great distances.
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 & Pet Health
Is it safe to eat fish/waterfowl?
When fishing or hunting on the Great Lakes, you should only harvest fish and waterfowl that behave and look healthy. Do not harvest any fish or game that show signs of illness, and follow good sanitary practices when preparing the harvest. (see sources, viii) It is especially important to stay away from the gut area when cleaning fish; filleting is recommended. Similarly, when preparing waterfowl, the gut should be immediately removed and care should be taken to not disturb the gut contents.
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. (see sources, ii)
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. (see sources, viii) (More information on botulism from a human health and food safety standpoint can be obtained through the USDA Food Safety Research Information Office’s Clostridium botulinum resource list at, http://fsrio.nal.usda.gov/document_reslist.php?product_id=137).
Can I swim in the water?
You are not at risk for botulism poisoning by swimming in Great Lakes waters. Botulism is only contracted by ingesting fish or birds contaminated with the toxin. If you have concerns about water quality, contact your local health department or swim in a regulated beach area. Remember that beaches sometimes close for other reasons such as fecal contamination. (More information regarding beach advisories can be found through the U.S. EPA’s Beaches website at, www.epa.gov/beaches.)
Is it safe to walkdogs on the beach after a bird kill?
If you bring pets to the shore, keep them away from dead animals on the beach.
Will my dog get sick if it eats a dead bird?
Dead wildlife may contain potentially harmful bacteria or toxins. In cases where you think your pet may have ingested a contaminated carcass, monitor them for signs of sickness and contact a veterinarian if you suspect changes in their health.
Do I have to wash my hands after I touch a dead bird?
Yes, you should always wash your hands after handling any wildlife. Ideally, you should also wear gloves or use an inverted plastic garbage bag to handle any dead animal.
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 and do not eat the intestines. Also, 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 thoroughly 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, (http://fsrio.nal.usda.gov/document_reslist.php?product_id=130).
Information on Great Lakes fish consumption advisories is available through the Great Lakes Information Network (GLIN) website, (http://www.great-lakes.net/envt/flora-fauna/wildlife/fishadv.html).
What can I do to help?
What steps I can 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 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. (see sources, v) Contacting your local education and outreach organizations, such as Sea Grant programs, may also aid in their efforts to track outbreaks, and they can provide you with answers to additional questions.
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?
In case of a die-off, individuals are urged to contact local agencies responsible for fish and wildlife management to report fish and bird mortalities.
It is important to record the location, type of birds or fish, and number of carcasses found. (see sources, ix) By reporting accurate information about botulism, you will assist natural resources managers and others involved in wildlife conservation planning.
How can people who want to help clean up the beach after a bird kill best protect themselves?
People who handle dead wildlife should wear protective gear, such as disposable rubber gloves or an inverted plastic bag over their hands. In cases where a diseased or dead bird is handled without gloves, hands should be thoroughly washed with hot, soapy water or an anti-bacterial cleaner.
What is the best way to dispose of dead fish/birds in my area, especially after a botulism outbreak?
Be sure to follow local wildlife agency (e.g., Natural Resources, Fish and Wildlife, etc.) recommendations in handling dead fish and wildlife. Wear disposable, rubber or plastic gloves or invert a plastic bag over your hands when handling sick, dead, or dying fish, birds or other animals. 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. (see sources, ix)
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. Contact a local wildlife rehabilitator to check on their willingness to accept birds that may have been impacted by botulism or other appropriate recommended actions.
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 (see sources, x).
Please remember that extreme caution should be practiced when handling wildlife. Contact the local office of your state’s wildlife management agency or approved local wildlife rehabilitators for more information about rehabilitation possibilities.
