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 outbreak in Australia occurred in 1938 — one of the earliest major reported die-offs of a large number of waterfowls.

Botulism risks for humans and pets

Humans can become sick from botulism typically by eating improperly canned or stored foods. Aside from this, humans and pets are primarily at risk only 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 or acting abnormally. Prevent pets from eating potentially infected fish, birds or maggots.

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 is 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 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 the vegetative growth of the toxin-producing bacterial cells.

The active bacteria that cause avian botulism grow only in a nutrient-rich substrate, such as in areas with large amounts of decaying plant or animal materials. 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.

Fish-eating birds that die of botulism are poisoned by eating fish that contain the toxin. Ingestion of maggots from the carcass of an infected animal can continue the spread of avian botulism, which may be responsible for large kills of birds.

Outbreaks of avian botulism occur only when a variety of particular ecological factors occur concurrently. This typically involves:

  • warmer water temperatures
  • anoxic (oxygen-deprived) conditions
  • bacterial substrate in decaying plants, algae or animal materials.

As average air and water temperatures have risen on a global scale, warmer temperatures and anoxic conditions have been occurring more frequently on the Victorian wetlands, lakes and ponds, possibly resulting in an increase of avian botulism incidents. Once these factors lead to production of the toxin in material eaten by fish, the toxin can be passed up the food chain as wild birds consume the contaminated fish or eat maggots from the decaying carcasses of infected individuals.

Signs and symptoms of avian botulism

Avian botulism typically results in paralysis — this results in the infected species exhibiting unusual behaviour. For example, 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, exhibiting poor posture while standing.

Once infected, fish may flounder or swim erratically near the surface of the water 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. Infected fish usually die quickly and are most likely to be seen washed up on shore.

More information

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Page last updated: 31 May 2021