Five questions about botulism

August 22, 2022

During warm summers, the news media frequently reports on mortality among aquatic birds and fish as a result of botulism. Miriam Koene from Wageningen Bioveterinary Research (WBVR) studies this bacterial disease. She answers five frequently asked questions about botulism.

1. Why do many (aquatic) birds and fish die due to botulism in the summer?

The bacterium that causes botulism occurs naturally in water, and in particular in sludge. In carcasses, which provide the ideal circumstances for the bacteria’s growth, it produces toxins. These toxins find their way into the larvae that feed on the cadavers. The larvae form a tasty snack for birds, who thus ingest large quantities of the toxin and frequently die as a result. The dead birds, in turn, form a new source of infection. When outdoor temperatures rise above 25 degrees Celsius, this can easily result in a snowball effect. Both the bacteria and larvae thrive in high temperatures.

2. What can I do if I suspect water contains botulism?

If you encounter dead birds or fish, you can report your findings to the owner of the pond or lake. This will generally be the municipality or the water authorities. An officer will then inspect the site. If there is reason to believe botulism is present, action will be taken. Cadavers will be removed to prevent a further spread, and warning signs will be put in place.

Botulisme warning sign in the Netherlands
Botulisme warning sign in the Netherlands

3. What should I do if I find a dead or sick animal near water?

Never touch a dead or sick animal with your bare hands, as they may be infected with a number of diseases such as avian influenza. Contact the Bird Rescue Society or Animal Ambulance in the area, and have them pick up, treat, or remove the animal.

How can you tell if an animal is suffering from botulism? Paralysis is the most apparent symptom. Birds that are affected have trouble taking off and landing. The bird can also have difficulty standing and walking. At a later stage, its neck and neck and respiratory muscles become paralysed, preventing the animal from lifting its head and causing drowning or asphyxiation. The sooner an animal suffering from botulism is treated, the greater the chance of recovery.

Dead animals must be removed without delay as they pose a potential infection hazard to other animals and humans. In the Netherlands, dead animals are occasionally removed by organisations such as the Dutch Wildlife Health Centre (DWHC) or the Netherlands Food and Consumer Product Safety Authority (NVWA) for an investigation into the cause of death.

4. Can my dog and I still swim in natural waters?

The chance that you, or your dog, will become infected with botulism is slight. However, there is a risk of humans and animals becoming ill from a number of possible other pathogens found in cadavers. Take a good look at the water you want to take a swim in. Does the water look clean, free from dead animals, and are there no warning signs?

5. Is botulism a threat to me or my pets?

Botulism does not frequently occur in humans and domestic animals such as dogs and cats. There are different types of toxins, and humans and pets are not all that sensitive to the toxins that occur in dead birds. Still, in rare cases, a dog may exhibit signs of botulism. Especially if it has ingested a large number of toxins, for example, by eating an infected cadaver. Contact your veterinarian immediately if you suspect a botulism infection.