Avian botulism
Avian botulism is a serious neuromuscular illness of birds caused by a toxin that is produced by the bacterium Clostridium botulinum. Avian botulism has been recognised as a major cause of mortality in wild-birds since the 1900s.
The first recorded outbreak in Australia occurred in 1938 — where major die-offs of a large number of waterfowls were reported.
Botulism risks for humans and pets
The most common way humans and pets can become sick from botulism is by eating improperly canned or stored foods. Aside from this, humans and pets are primarily at risk if they eat infected fish, birds or maggots.
Precautions should be taken when harvesting fish or waterfowl — they should not be harvested or eaten if they are sick, acting abnormally or decaying.
Contact a vet if you suspect your pet has been infected.
Cause of avian botulism
Botulism spores, (the resting stage of the bacteria) are abundant in habitats where there are low oxygen levels such as:
- soils
- aquatic sediments of many wetlands and lakes
- in the gills and digestive tracts of healthy fish living in those lakes.
The spores can remain in the ecosystem for extended periods of time, even years, and are resistant to temperature changes and drying. The spores are harmless until the correct environmental conditions prompt them to germinate and begin the vegetative growth of the toxin-producing bacteria.
The active bacteria that cause avian botulism grow only in a nutrient-rich substrate, such as:
- areas with large amounts of decaying plant or animal material
- dead fish that contain the bacterial spores in their tissues
- maggots in decaying plant or animal material.
Fish-eating birds that die of botulism are generally poisoned by eating fish that contain the toxin. Ingestion of maggots from the carcass of an infected animal can also continue the spread of avian botulism, which may be responsible for large die-offs in birds.
Large outbreaks of avian botulism occur when favourable ecological factors occur concurrently. This typically involves:
- warmer water temperatures
- anoxic (oxygen-deprived) conditions
- large amount of decaying plants, algae or animal materials for bacteria to grow in.
As average air and water temperatures have risen on a global scale, warmer temperatures and anoxic conditions have been occurring more frequently on Victorian wetlands, lakes and ponds, possibly resulting in an increase of avian botulism incidents. Toxin material can be passed up the food chain where small amounts of toxins are consumed by microscopic invertebrates, fish, and then fish-eating birds.
Signs and symptoms of avian botulism
Avian botulism typically results in paralysis, with the infected species exhibiting unusual behaviour. For example:
- water birds not able to hold their head up (they may drown as a result)
- birds being unable to fly
- dragging of one or both wings, exhibiting poor posture while standing.
Infected fish may flounder or swim erratically near the surface of the water and may have trouble staying right-side up allowing them to be caught by birds more easily. '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.
More information
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