All About Avian Botulism

Since we’re probably all a little more familiar with disease ecology than we were just a couple of years ago, why not add some information about another wildlife disease to the knowledge bank? Most hunters’ familiarity with avian botulism is dependent upon the region of the country they hunt. While this toxin can affect birds anywhere, it is most common in the western states and the Great Lakes region. This pathogen can have population-level effects on waterfowl so for the conservation-minded hunter, a little awareness can go a long way.  

The Pathogen  

The toxin that causes avian botulism disease in birds is produced by a bacterium, Clostridium botulinum. This bacteria produces a series of toxins labeled A through G, which have varying effects. Yes, some of these are the same toxins that are used in Botox! The toxin classes most important to waterfowl disease are C and E.  

Signs of Disease  

The toxins block the release of acetylcholine, which affects the transmission of nervous system signals. Birds show signs of paralysis, especially in the cardiac and respiratory systems, and the inability to fly, walk or swim. Other classic signs include a limp neck and an elevated third eyelid. Because of the paralysis of the respiratory system and inability to fly, one of the most common causes of mortality in affected birds is actually drowning. Gross lesions are unlikely because the toxin kills affected birds so quickly. 

Toxin C 

The C. botulinum toxin C is most common west of the Mississippi River. Conditions which support outbreaks and high mortality include warmer than usual weather, receding water conditions and shallow water with lots of decaying plant matter. Late summer and early fall is prime time for outbreaks of type C botulism. The bacteria can produce spores which stay dormant for years and are exposed by receding water levels, releasing them into the ecosystem.  

Birds that consume enough of the toxin to kill them attract maggots, which concentrate the toxin in their bodies. Healthy birds that eat the maggots then become affected. The cycle can build rapidly and cause major die-offs, especially when outbreaks occur in areas with high numbers of birds. 

Toxin E 

East of the Mississippi, C. botulinum toxin E is most common in the Great Lakes region among fish-eating species. Notably, the invasive round goby concentrates the toxin and is in turn eaten by fish-eating birds like loons and mergansers. Die-offs are monitored by collecting and counting carcasses on Great Lakes beaches during particularly bad outbreaks.  

Management and current outbreaks 

Recently, the Klamath/Tule Lake region in California, an important migratory area for waterfowl, has experienced very high die-offs of waterfowl species due to avian botulism. In 2020, high temperatures and receding water levels contributed to the loss of approximately 60,000 birds. Thankfully, the population did not experience those same drastic effects in 2021, likely in part to some beneficial changes in water level management of refuge pools.  

There were several management practices used to assist vulnerable waterfowl in these outbreaks. Disposing of dead carcasses prevents the growth of toxin-concentrating maggots that further exacerbate an outbreak. When moving carcasses, it is important to be as thorough as possible because sick ducks will often hide in vegetation and are difficult for people to locate. Additionally, hazing can be used to scare birds away from areas with a high likelihood of toxin exposure. Some birds do recover from botulism poisoning as well if treated early enough!

An avian botulism outbreak will have dead, sick and healthy birds all in the same area. If you do see birds exhibiting signs of botulism, alert the state wildlife agency. Testing birds for the toxin to confirm the cause of mortality is helpful for identifying and managing the site of an outbreak. Knowing a little background on the biology of this important waterfowl disease can really make a difference for conservation!

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