Increased reports of dead fish and waterfowl

Published on
August 12, 2020

The number of reports to the Dutch Wildlife Health Centre (DWHC) of dead ducks and other waterfowl has increased over the last week. Given the unusually warm weather, the chances of botulism playing a role are plausible. Outbreaks of botulism usually occur when the temperature lingers above 25℃ for a number of days.

Birds with botulism become paralysed and will generally die because they can no longer eat or drink, or simply suffocate due to paralysis of the respiratory muscles. In hot summers, botulism can lead to mass mortality of waterfowl. Moreover, reduced water quality (low ogyxen levels) can also play a role in mass mortality of fish stocks.

What kind of disease is botulism?

Botulism is a paralytic disease caused by a powerful toxin which forms under certain circumstances via the bacterium Clostridium botulinum. The bacterium is common in the Netherlands in soils and waterbeds. Fish and fowl contract the bacteria and spores through their food. The bacteria are often present in the intestines of many waterfowl without causing any adverse effects.

Living conditions for the bacteria

When the environmental conditions for this bacterium are less favourable, it forms spores that can survive for long stretches of time in soils and waterbeds. Once the environmental conditions are conducive again, which constitute a protein-rich, oxygen-poor environment and a temperature above 20℃, the spores multiply and release very harmful toxins. If an animal ingests these toxins, botulism develops, paralysis occurs and the animal dies.

Carcasses as a source of botulism

In turn, the carcass itself becomes a protein-rich, oxygen-poor environment. Hence a dead waterfowl or fish, which for whatever reason has died, and which has traces of bacteria in its intestines, in combination with an ambient temperature above 20℃, form the ideal environment for the multiplication of the Clostridium spores and, with them, the dangerous toxins.

Such a carcass can contain huge amounts of toxins and thus be a source of contamination for other waterfowl. When such a carcass disintegrates, the immediate environment can become infected with spores and toxins. Other birds can also ingest toxins through insect larvae (such as maggots of flies).

Insect larvae that feed on an infected carcass absorb the toxins in their bodies at high concentrations. The larvae themselves are not affected by this, but they are called 'toxin bombs' for other birds. By eating infected insect larvae, other waterfowl become infected with the toxins, become paralysed and die. Because they themselves are then a new source of contamination, the number of victims can increase exponentially in a short period of time.

Seven types of botulism

There are seven different toxins identified by the letters A to G. Types C and D, or their intermediate forms, are the ones most commonly found. These are the types to which waterfowl are most susceptible. Fish and fish-eating birds are susceptible to type E, and sometimes type C. Humans are sensitive to type A,B, E and F. Dogs are less susceptible, but can contract the disease from time to time.