Government specialists have revealed botulism is responsible for a large number of swans and other water birds that have been found dead in a London canal in recent weeks.
Dozens of swans, geese, and other wild birds have been found in the Grand Union Canal in London.
Environment groups were “concerned” by the unusual cluster of deaths, and the government’s Animal and Plant Health Agency (Apha) opened an investigation.
Post-mortems on swans, geese, ducks and coots found dead in the canal have now ruled out fears of an outbreak of avian influenza (bird flu).
The post-mortems found the birds were of a good weight and condition but had not eaten well directly before their deaths, pointing towards toxin poisoning.
Apha says it now believes the birds were poisoned by botulism in the water, after a specialist tests returned a positive result for botulism.
It said avian botulism can lead to “hundreds” of deaths and can lead to recurring outbreaks.
It said some live swans have been found unwell, lying with outstretched wings, unable to lift their heads and, in some instances, gasping - a common symptom of avian botulism.
“We are strongly suspicious that this incident was caused by a toxin from the bacteria Clostridium botulinum,” it said.
“This toxin causes avian botulism, a disease characterised by progressive flaccid paralysis and weakness in affected birds, causing difficulty walking or moving their wings and inability to lift their head to eat or drink.
“The bacteria grow in environments without oxygen and thrive on the protein from putrefying plant and animal material, and is often found in urban lakes, canals, and reservoirs.
“Outbreaks of avian botulism usually involve waterbirds, can last several weeks, and can result in hundreds of deaths.
“Outbreaks can recur as once in the environment; the toxin is relatively stable and persistent.”
Apha said that in the UK, most avian botulism is caused by a type of the toxin that does not affect humans.
“However, care should be taken by both people and their pets where dead birds are found as other health hazards may be present in these environments,” it warned.
Wendy Hermon, operations director at rescue organisation Swan Support, told The Standard in late October the charity had collected more than 25 swans from the Grand Union Canal in around three weeks, from locations including Paddington Arm, Alperton, Greenford, Hayes, Southall, and Northolt.
“We’ve collected over 25 swans but [the number found dead has] been a lot more than that, and we’ve just had a report of more this morning so it’s still ongoing,” she told The Standard.
She said the charity had also collected around 15 dead geese, as well as moorhens, coots and ducks, and has had a report of a dead heron.
More birds have been found dead since then. Swan Support said five dead swans were recovered last Saturday alone, from the Hayes and Southall area.
Swan Support said it was surprised by the botulism diagnosis, writing on Facebook: “In our experience this isn’t normal - outbreaks of bootulism usually follow periods of heat and extreme rainfall.”
“There is no treatment as such for botulism,” it added. “We give [birds] fluids and vitamins to help flush out the toxin but it’s not always successful. Unfortunately in these cases they are usually too far gone when we get called and are dying before we arrive, or in some cases, shortly after.
“We are doing all we can by removing the bodies as soon as we are aware but beyond that we feel helpless,” it added, saying it is hopefully “the worst is over”.
The Grand Union Canal is the longest canal in the UK and connects London and the Midlands. Beginning in the capital, one arm of the waterway runs to Leicester while another, stretches for 137 miles to Birmingham.
A Canal and River Trust spokesperson previously said: “Members of the public should not touch or pick up any dead or visibly sick birds they find, but instead immediately call the Defra helpline on 03459 33 55 77, giving as accurate a location as possible.”
