Update on Warning for Dog Owners About Blue-Green Algae in River

Update 7 August

A spokesperson for the Environmental Agency said, “We thought that we’d let your readers know that on Sunday 5 August we investigated reports of possible blue green algae on the River Thames south of Marsh Lock between Henley-on-Thames and Shiplake. We took a water sample from the River for analysis. No blue green algae was found in the sample. Open waters are not considered to be of bathing water standard and we would encourage people to read Public Health England’s advice on reducing the risk of illness when open water swimming https://www.gov.uk/government/publications/swim-healthy-leaflet”

If people do think they see blue green algae in not just the Thames but in any stream, lake, pond etc, we encourage the public to report that on 0800 807060


Reports are coming in about local dogs who have swam in the river becoming ill afterwards.

It is thought they have been affected by the build-up of blue-green algae in the hot weather around the edges of the river.

Animal charity, Blue Cross have issued this following advice:

The bacteria cannot be seen with the naked eye unless they clump together. When this happens, blue-green algae can look like green flakes, greenish bundles or brown dots. When the algae blooms, it can give look like a blue-green scum has appeared on the surface of the water. It sometimes looks a bit like pea soup.

Blooms of blue-green algae can produce harmful toxins which stop a dog’s liver from functioning properly. However, not all types of blue-green algae are dangerous.

Sadly, exposure to toxic blue-green algae is often fatal, and can also cause long term health problems in dogs that survive after drinking or swimming in algae-contaminated water. Some types of blue-green algae can kill a dog just 15 minutes to an hour after drinking contaminated water.

Dogs who have been swimming in water can get the algae caught in their fur, and can ingest it while cleaning themselves later on.

Contact your vet immediately if your dog shows signs of vomiting, Diarrhoea, seizures/fitting, weakness/collapse/unconsciousness, disorientation/confusion, drooling or breathing difficulties after swimming in the river.

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1 comment
  1. Robbie says:

    I think the blue cross comment above is incomplete and therefore a bit misleading and anxiety making. I have a dog so I’m interested in accuracy.

    The blue green algae are always in the river water and are as natural as anything else and not at all harmful in normal concentrations.

    They only become a hazard at times and in places of high temperature and low flow. So e.g the Lake District suffers more than rivers do.

    When Blue Green algae are at harmful concentrations they are clearly visible to the naked eye as described above and ingestion of algae or the water nearby carries the threats outlined above.

    I am part of a group who swim in the river regularly and we have gone specifically to look for blooms after these reports started up without any sightings.

    So far we have found nothing. The correct procedure is if you think you see some is to call the Environment Agency hotline to report it and they will investigate and inform the landowner who will put up signs.

    They answer the hotline straight away. I called on Saturday morning and they had had no reports of sightings here.

    People are speculating because some dogs have been ill but this could be caused by many things. I had a tummy upset after swimming in the river this weekend bit I also ate a couple of bbq burgers. It was only my second ever upset tum after river swimming. I’ve had more trouble with bbqs.

    So I don’t know what caused my tum trouble but it wasn’t blue green algae at the moment.

    Obviously we should all be sensible and look out for the stuff and if we see it report it straightaway and keep us and our dogs away from it.

    However nobody should stop enjoying the river or cause alarm as no sightings have yet been made.

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