The RSPCA said it picked up 23 sick swans over the past two weeks, according to local media reports, but only one survived. The animal welfare charity was first alerted by the public on 28 July and has been trying to rescue ill birds from the river ever since, it said.
The charity contacted the Environment Agency and according to the local press, the regulator’s initial findings suggest that avian botulism was the cause of the birds’ deaths.
The Animal and Plant Health Agency (APHA) says avian botulism is usually caused by C. botulinum, an oxygen intolerant bacterium that multiplies in putrefying plant and animal material and is “often found in lakes in periods of anoxic conditions and poor water quality.”
Low oxygen levels can be linked to warm weather. Rivers can also be starved of oxygen when there is an excess of organic matter in the water, which can come from sewage works or farms. Raw sewage discharges into a river are accompanied by a decrease in dissolved oxygen content because bacteria, which use the sewage as a food source, grow in number and respire, using up oxygen in the water and can lead to fish kills.
The disease, caused by the swans eating the bacteria, can lead to the paralysis of birds’ wings, legs and necks and they usually die as a result of respiratory failure or drowning.
Stockton Council’s cabinet member for access, communities and community safety Steve Nelson said he was saddened by the deaths and urged people and their pets to stay out of the water.
“We strongly advise this at all times anyway but this is just another reason to do so, alongside the other dangers like currents and cold water shock,” he said.
“If you’re walking your dog close to the area where the swans have died, we’re advising you to keep them on a short lead and away from the water.”
In a statement, RSPCA chief inspector Mark Gent, who has been leading the rescue operation, said: “Swans affected by the illness usually display a kinked neck and their beaks are more of a grey colour rather than a distinctive orange.
“It is awful to deal with this situation as the birds are in such a state of suffering the chances of them recovering are extremely low and if they are left they will die a lingering death.
“We have been busy rescuing them but sadly 22 were in such a poor condition that a vet decided they needed to be put to sleep to end their suffering.
“We estimate there are still 86 swans on this stretch of the river and will be monitoring their health for as long as needed. We are hoping for some cooler weather and rain to help the water return to normal.”
According to the RSPCA, APHA and DEFRA are testing some of the deceased swans.
The Environment Agency said it was not able to comment because the matter did not involve a pollution incident, but that it was investigating the swans’ deaths alongside the RSPB.