Blue Green Algae Poisoning in Dogs: the complete guide


Blue-green algae poisoning in dogs is so serious that it could kill your dog in as little as 15 minutes.

Toxic algae blooms are typically a late summer or early autumn danger that can affect lakes, ponds, and slow-flowing rivers. When water levels become low and slow, as well as warm, the conditions are ideal for the toxic algae’s growth to explode.

If your dog enjoys swimming, or if you walk your dog near any water, then you need to know about the risk of blue-green algae poisoning in your dog, as well as how to recognize when blue-green algae are present and the symptoms of poisoning.

Blue-green algae poisoning is just one of the hot weather summer risks that I discuss in my free hot weather dog care ebook. Keep your dog safe this summer and download your free copy here.

Blue-green algae are actually, despite their name, a group of bacteria known as cyanobacteria. They are found in freshwater rivers, streams and ponds as well as brackish, or slightly salty, water. 

While they are normally present throughout the year, they are microscopic and only found in very low numbers. At these times they are completely invisible (unless you’re carrying around a microscope!) and don’t normally pose any kind of danger.

When conditions are right however, there can be an explosion in growth, resulting in huge numbers of cyanobacteria known as an algal bloom.

Dog blue green algae poisoning  guide

Why is Blue-Green Algae Poisonous to Dogs?

Not all cyanobacteria produce toxins, but if you see the tell-tale algal blooms in the water there is simply no way to distinguish safe from dangerous forms by sight alone. If you see any sign that blue-green algae are present then you must assume that the water is toxic and keep yourself and your dog well clear.

The toxins they produce really do have the potential to be rapidly fatal. In some cases, depending on the toxin type, death can take place within 15 minutes of exposure.

To make matters worse, only a really small amount of water containing the toxin needs to be drunk.  A couple of mouthfuls may be all that is needed to result in fatal poisoning.


One group of cyanobacteria produce toxins that interfere with nerve function. If ingested they cause rapid muscular paralysis that ultimately prevents a dog from being able to breathe.


The other major toxin affects the liver. Proteins essential for normal liver function are disrupted leading to liver damage, failure, and even death.

Other toxins

Although (potentially) less severe, there are several other toxins produced by blue-green algae that can cause your dog to become very unwell, although these are not typically fatal.

Dermatotoxins can cause severe rashes and other skin reactions that develop hours to days after exposure.

Separate endotoxins result in inflammation of the gut, resulting in fever and immune stimulation. Although unusual, these toxins can also be fatal as a result of septic shock developing.

Symptoms of poisoning in dogs

Nerve Toxicity

If the neurotoxin is present in the water, then poisoning can take place within 15 minutes of exposure! This means that you may notice your dog becoming unwell while you are still on your walk, or shortly after getting back home.

Signs of poisoning with blue-green algae neurotoxin can include:

  • excessive salivation

  • muscle tremors and twitching

  • rigidity with the legs becoming stiff

  • seizures and fitting

  • Paralysis

  • difficulty breathing as the chest muscles and diaphragm stop working

  • Blue-tinged gums

Death may only take minutes to hours.  This poison works very quickly and is deadly.

If you notice any of these blue-green algae poisoning symptoms minutes to several hours after your dog swims or drinks from an outdoor water source then you need to take them to the vet immediately. Even then the prognosis for survival is poor.

Liver Toxicity

If the cyanobacteria are producing hepatotoxins then the symptoms of liver failure take longer to show and death is more drawn out, taking a couple of days to weeks.

Signs of poisoning with a cyanobacteria liver toxin are:

  • Vomiting

  • Diarrhea

  • bloody or black stools

  • pale or yellow gums

  • Weakness

  • Seizures

Blue green algae poisoning only takes a couple of mouthfuls and can kill in minutes

What Causes Blue-Green Algae to Bloom?

In some parts of the world, blue-green algae may be a year-round threat to your dog. In temperate regions, however, favorable conditions are typically found in late summer and early autumn. Knowing the risk period for your local area is very important but, especially with climate change, understanding what triggers an algal bloom will help prevent you from being caught out by unseasonal conditions.

Blue-Green Algae needs several conditions to be right to bloom:


The ideal water temperature for cyanobacteria is 25°C (77°F) or warmer. This is very different from most algae which prefer cooler temperatures of less than 15°C.

As a result, when water temperatures rise, the blue-green algae has a competitive advantage and is able to outgrow any other algae present.


Nitrogen and Phosphorus are key nutrients needed for algae growth. These are found in the water at higher levels around farmland especially due to fertilizer run-off. Sewage also releases high levels and so storm-water run-off can also introduce high levels of nutrients into the water.

When water levels drop, as well as becoming warmer, the nutrient content of the water also rises. To make matters worse (or better if you happen to be a cyanobacterium), when water oxygen levels fall, something that happens as the water warms, even more phosphorus is released from the sediment. This further encourages algae growth.


Sun is great, with light the final piece of the algal growth puzzle.

Summer and autumn not only typically bring more sunlight, but the water conditions are again better suited to the blue-green algae taking advantage of the extra UV.

As flow levels drop and water slows in rivers, or stable weather allows ponds and lakes to be calm for prolonged periods, and sediment in the water will slowly fall to the bottom. This allows the light to penetrate the water, optimizing growing conditions below the surface. Exactly the place blue-green algae thrive.

blue green algae blooms are most common in late summer and early autumn

What Does Blue-Green Algae Look Like?

Blooms of cyanobacteria can appear in two different forms.

The first is where the algae clump together and float on the surface. These clumps form bluey-greeny-brown flakes or mats.

Rather than solid mats, the water may also take on a discolored pea-soup appearance or develop into a blue-green-brown foamy scum that builds up especially on the water's surface at the edges of lakes and ponds.

Finally, the algae can also look like spilled paint, swirling on the surface of the water.

These mats or foam most commonly develop in still water, so lakes, ponds, and even swimming pools that aren’t in use or kept maintained appropriately. Rivers can definitely be affected as well, especially if the water is slow-flowing or there are backwaters and other pockets of still water present.

In fact, several of the rivers near me develop blue-green algae most years toward the end of the summer.


What Blue-Green Algae Does Not Look Like

Duckweed is a really common plant found on the surface of lakes and ponds. From a distance, this can look very similar to blue-green algae. Up close, however, it is very different. Duckweed looks like miniature lily pads floating on the surface of the water.

Similarly, if the algae present looks like hair or threads then it is unlikely to be blue-green algae and instead harmless filamentous algae.


Testing the Water

There are 2 free tests you can use to help determine if blue-green algae is present in a body of water. While they can both give some indication, neither is perfect and I would hesitate to recommend that you rely on them completely to tell if the water is safe for your dog to swim in or drink.

The jar test involves taking a sample of water in a regular jam jar and letting it sit in the fridge overnight. If the algae settles to the bottom then the water is likely safe. If however the algae rise to the surface then cyanobacteria is likely present.

The stick test can be useful if mats of algae are present. Simply push a sturdy stick into the mat of algae and carefully pull it back out. If it looks like it is covered in paint, then blue-green algae is forming the mats you are seeing. If instead the stick is covered in green strands then filamentous algae is highly likely.

There is one strain of blue-green algae that forms threads. You can tell this apart from harmless filamentous algae by the smell, as the blue-green algae smell is putrid and sewage-like. It will also occasionally cause a purple discoloration of the surrounding water.

You can read more details about the jar and stick tests here.

There are commercial tests available that give a rapid answer to the question is blue-green algae toxins present or not, however they only test for the common liver toxins, not the neurotoxins. As such, I wouldn’t recommend relying on them as an absolute marker of safety.

Poisoning Treatment and Prevention

How then can blue-green algae poisoning in dogs be treated?

Unfortunately, there is no antidote once poisoning had occurred.

Treatment immediately after exposure is vital but even then, the prognosis is poor if a fatal amount of poison has been consumed.  Your vet may make your dog sick to remove any toxin still present in the stomach (although this won’t be done if your dog is showing nervous signs of poisoning), and supportive treatment will be given to try and limit the damage and effect of any toxin.

For those dogs that don’t die, the likelihood of a complete recovery is very good, but given the fact that only such a small amount of poison is needed to be fatal, even with immediate treatment death is very likely.

once poisoned, unfortunately there is no specific treatment for blue green algae toxicity in dogs

As with most things, prevention of blue-green algae poisoning is always the best course of action:

  • Remember what blue-green algae look like and avoid letting your dog into any water that looks remotely suspicious.

  • Be aware of the local risks - certain waterways are more likely to be a poison risk than others.

  • Local councils and environmental agencies are very likely to monitor and sample local water conditions and publish where blue-green algae is present. If you are in the USA, then you can find your state's report here.

  • Use the Bloomwatch app to become familiar with cyanobacteria blooms and report any suspect blooms you come across.

  • Your vet may know which local areas to avoid.

  • If in doubt, during hot weather with low rainfall do not let your dog swim, paddle or drink any water found outdoors, especially lakes, ponds, and slow-flowing rivers.

Also, be aware that blue-green algae is toxic to humans and other animals.  You should not swim or even paddle in suspect water.

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