FrançaisIs blue-green algae dangerous for dogs?
Yes. Toxins made by blue-green algae can kill a dog, sometimes within minutes, and the outlook is guarded at best [1]. If your dog has swum in, drunk from or played in water that may contain algae, take it out of the water, rinse its coat well with clean fresh water so it cannot lick the algae off, and call your vet now, even if it seems fine [1]. Say where and when the dog was in the water.
Dogs are especially at risk because they swim in and drink from water during blooms and eat algal mats [2]. Dogs and livestock make up most of the reported animal poisonings, and about two thirds of reported cases come from Europe and the United States [16].
What blue-green algae is and where it grows
Cyanobacteria, formerly called blue-green algae, are microscopic organisms of fresh, brackish and salt water [1, 19]. They grow best in warm water (above 20 °C, or 68 °F) rich in nutrients from pollution [1]. Some produce toxins; you cannot tell by looking whether a bloom is toxic [1].
- Floating blooms: in ponds, lakes, reservoirs and slow streams, and even in water tanks and buckets [1]. They can look blue, green or brown, like pea soup or spilled paint, or be invisible [1].
- Mats on river beds: some cyanobacteria grow as mats on stones at the bottom of rivers. In France, dogs died after drinking at the shore of the Loue in 2003 [10] and of the Tarn in 2002, 2003 and 2005 [11]; at least 40 dog deaths in France have been linked to these river mats since the early 2000s [14]. In the Netherlands, 3 dogs died after swimming in a lake where such bottom-growing cyanobacteria had spread [6]. In the summers of 2002 and 2003 alone, 37 dogs along the Tarn and 2 in the Jura showed nerve signs after swimming in or drinking river water, and 28 died [12].
- No visible bloom: at a lake in Berlin in 2017, at least 12 dogs fell ill after swimming or drinking, although no bloom was visible; the toxin came from cyanobacteria living in floating water moss [5].
- Season: most calls to a US animal poison center about these algae came from July to September [13], but dogs have also died after swimming in January and February in Texas [15].
How dogs get poisoned: swimming, drinking, licking
Most poisonings come from swallowing the toxins [1]. In US surveillance data, the most common route was swimming combined with swallowing water or licking algae off the fur [2]. Dogs have also died after grooming algae from their coat after a swim, and some eat dried algal mats or crusts on the shore [2]. In one Dutch outbreak, a dog that had only swum in the scum, without eating it, also died [7].
That is why rinsing the coat with plenty of clean water matters: it removes algae before the dog can lick them off [1]. Nerve toxins from river mats stay inside the cells, so dogs are poisoned mainly by eating pieces of the mats [14].
What blue-green algae does to dogs: two main toxins
| Anatoxin-a (nerve toxin) | Microcystins (liver toxin) | |
|---|---|---|
| What it does | Overstimulates the junctions between nerves and muscles, then paralyzes them [1] | Taken up by liver cells and blocks enzymes they need; the liver fills with blood [1] |
| Signs | Muscle twitching, tremors, stiffness, then paralysis [1] | Vomiting, diarrhea, weakness, pale gums and shock; later jaundice and bleeding [1] |
| Timing | Signs almost immediately; death from breathing failure can occur within minutes [1] | Minutes to days after exposure [1] |
| Blood work | No consistent changes [1] | High ALT, ALP, CK and bilirubin, low glucose, slow clotting [1] |
Signs and timing
Nerve signs can start while the dog is still at the water. In North America, 3 dogs had seizures and died within an hour of swimming in a river, and 3 others died within an hour of swimming in a pond [3]. Two dogs exposed to a backyard pond died 20 to 30 minutes after falling ill [2].
- Nerve toxin: weakness of the hind legs, anxiety, mild drooling, tremors and twitching, then collapse [4].
- Liver toxin: a dog that ate scum on the shore vomited severely, became lethargic, had trouble breathing and died 4 to 5 hours later; a pup that only swam in it died after 12 to 16 hours [7]. Another dog arrived the day after a swim with vomiting, loss of appetite, weakness and lethargy [9].
- In US reports: vomiting or diarrhea in 43% of affected dogs, lethargy in 18% and nerve signs such as stumbling in 9%; 58% of the cases were fatal [2].
Do not wait for signs: call your vet as soon as your dog has been in water that may contain algae.
What the blood tests show, and why
With the liver toxin, blood work early on shows liver cell damage, low blood sugar and clotting problems; later, bilirubin keeps rising and albumin falls [1]. With the nerve toxin, the blood count and chemistry usually show no consistent change, so a normal result does not rule it out [1].
- Abbreviation ALT, the liver enzyme that leaks from damaged liver cells.
- Result 1250 U/L in this illustrative line: far above the interval. A high ALT is one of the early changes of microcystin poisoning [1].
- Unit U/L: units of enzyme activity per liter of serum.
- Laboratory interval The example uses the dog interval of the Merck Veterinary Manual [18]; each laboratory sets its own.
- Flag H: above the interval. The vet will read it with bilirubin, glucose and clotting times, and repeat it [1].
| Test | What can be found | Why it matters |
|---|---|---|
| ALT and ALP | High | Microcystins damage liver cells, which release these enzymes [1] |
| Bilirubin | High, rising over time | The liver fails to clear it; survivors can become jaundiced [1]. See bilirubin in dogs |
| Glucose | Low (hypoglycemia) | Seen early in microcystin poisoning [1] |
| Clotting times | Prolonged | The failing liver can lead to bleeding [1]; a dog in one report had liver failure with clotting failure [8] |
| Potassium, phosphorus, CK | High | Also reported early in microcystin poisoning [1] |
| Albumin | Low, later on | Reported later in the course [1] |
| Blood count and chemistry after nerve signs | Often normal | Anatoxin-a causes no consistent changes [1] |
Signs and liver damage from microcystins look much like those of mushroom poisoning, and the two cannot be told apart without toxin testing [2].
How the vet confirms it: toxins in water and vomit
The diagnosis usually rests on the signs and a history of contact with water that may contain cyanobacteria [1]. Confirming it means finding the toxin. Some veterinary and environmental laboratories can measure the toxins in water by specialized methods (liquid chromatography with mass spectrometry), and quicker antibody tests (ELISA) exist for microcystins and anatoxin-a [1]. Seeing cyanobacteria in the water under a microscope does not prove that toxins are present [1].
Samples from the dog help too. Microcystins have been found in a dog’s vomit and liver [8], and anatoxin-a in stomach contents, bile and urine [4]. In Berlin, the nerve toxin was found in the dogs’ stomach contents at up to 8,700 µg/L [5].
Tell your vet exactly where the dog swam or drank. The vet or local authorities may sample the water the dog swallowed, including the bloom itself [1]; in one case, a water sample from the lake supported the diagnosis [9]. Few cases are confirmed by testing, and many are likely missed [2].
Outlook and prevention
There is no antidote. Care is hospital care: removing the toxin from the skin and gut, fluids, liver support, and control of tremors and seizures, sometimes with breathing support [1]. The outlook is often poor, but some dogs have survived with intensive care, and survivors of the nerve toxin may recover fully [1]. A dog with liver failure after swimming in a lake survived with supportive care including plasma and blood, the first such survivor reported [9].
Prevention matters most [1]. Keep your dog out of water that looks green, scummy or discolored and away from areas where algae grow or wash up, and bring fresh water for it to drink [1, 12]. A dog’s illness after a swim can also warn of a risk to people using the same water [17]. For another disease that dogs can catch from water, see leptospirosis in dogs.
Related articles
Plants and outdoor hazards for pets: toxic plants for dogs and cats.
En français : cyanobactéries et chien.
Questions to ask your veterinarian
- Which toxin do you suspect, given my dog’s signs?
- Are the ALT, bilirubin, glucose and clotting times normal, and when will you recheck them?
- Can the vomit or stomach contents be sent to a laboratory for toxin testing?
- Should the water where my dog swam be sampled, and who should I alert?
- Could another poison, such as mushrooms, explain these results?
Get your dog’s emergency blood results explained.
AI DiagAnimals reads ALT, bilirubin, glucose, clotting times and the other values together with your laboratory’s intervals, and prepares the questions for your vet. The service opens soon: leave your email to be told first.
Frequently asked questions
What to do if a dog swims in blue-green algae?
Take it out of the water and rinse its coat with plenty of clean fresh water, so it cannot lick the algae off, then call your vet now, even if it seems fine [1]. Tell the vet where and when it swam. Nerve signs can start within minutes [1].
What are the first signs of toxic poisoning in dogs?
After blue-green algae, the nerve toxin causes muscle twitching, tremors and stiffness within minutes [1], sometimes with drooling and weak hind legs [4]. The liver toxin causes vomiting, diarrhea and weakness, over minutes to days [1]. Call your vet as soon as your dog has been exposed, before signs appear.
How much blue-green algae is toxic to dogs?
There is no known safe amount, and you cannot tell by looking whether a bloom is toxic [1]. A pup that only swam in scum died [7], and dogs have died within an hour of a swim [3]. Call your vet or a pet poison line whatever the amount.
What does blue-green algae do to dogs?
Its toxins either paralyze the nerves and muscles, which can stop breathing within minutes (anatoxin-a), or destroy the liver, causing liver failure and bleeding over hours to days (microcystins) [1].
Sources
- Bischoff K. Algal Poisoning of Animals. Merck Veterinary Manual, reviewed 2021, updated 2024. merckvetmanual.com/toxicology/algal-poisoning/algal-poisoning-of-animals
- Backer LC, Landsberg JH, Miller M, Keel K, Taylor TK. Canine cyanotoxin poisonings in the United States (1920s–2012): review of suspected and confirmed cases from three data sources. Toxins. 2013;5(9):1597-1628. doi.org/10.3390/toxins5091597
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- Puschner B, Pratt C, Tor ER. Treatment and diagnosis of a dog with fulminant neurological deterioration due to anatoxin-a intoxication. J Vet Emerg Crit Care. 2010;20(5):518-522. doi.org/10.1111/j.1476-4431.2010.00578.x
- Fastner J, Beulker C, Geiser B, et al. Fatal Neurotoxicosis in Dogs Associated with Tychoplanktic, Anatoxin-a Producing Tychonema sp. in Mesotrophic Lake Tegel, Berlin. Toxins. 2018;10(2):60. doi.org/10.3390/toxins10020060
- Faassen EJ, Harkema L, Begeman L, Lurling M. First report of (homo)anatoxin-a and dog neurotoxicosis after ingestion of benthic cyanobacteria in The Netherlands. Toxicon. 2012;60(3):378-384. doi.org/10.1016/j.toxicon.2012.04.335
- Lürling M, Faassen EJ. Dog poisonings associated with a Microcystis aeruginosa bloom in the Netherlands. Toxins. 2013;5(3):556-567. doi.org/10.3390/toxins5030556
- van der Merwe D, Sebbag L, Nietfeld JC, et al. Investigation of a Microcystis aeruginosa cyanobacterial freshwater harmful algal bloom associated with acute microcystin toxicosis in a dog. J Vet Diagn Invest. 2012;24(4):679-687. doi.org/10.1177/1040638712445768
- Sebbag L, Smee N, van der Merwe D, Schmid D. Liver failure in a dog following suspected ingestion of blue-green algae (Microcystis spp.): a case report and review of the toxin. J Am Anim Hosp Assoc. 2013;49(5):342-346. doi.org/10.5326/JAAHA-MS-5913
- Gugger M, Lenoir S, Berger C, et al. First report in a river in France of the benthic cyanobacterium Phormidium favosum producing anatoxin-a associated with dog neurotoxicosis. Toxicon. 2005;45(7):919-928. doi.org/10.1016/j.toxicon.2005.02.031
- Cadel-Six S, Peyraud-Thomas C, Brient L, et al. Different genotypes of anatoxin-producing cyanobacteria coexist in the Tarn River, France. Appl Environ Microbiol. 2007;73(23):7605-7614. doi.org/10.1128/AEM.01225-07
- Anses (French Agency for Food, Environmental and Occupational Health & Safety). Évaluation des risques liés aux cyanobactéries et leurs toxines dans les eaux douces. Opinion and expert report, May 2020 (in French). anses.fr/fr/system/files/EAUX2016SA0165Ra.pdf
- Bloch RA, Faulkner G, Hilborn ED, Wismer T, Martin N, Rhea S. Geographic Variability, Seasonality, and Increase in ASPCA Animal Poison Control Center Harmful Blue-Green Algae Calls—United States and Canada, 2010–2022. Toxins. 2023;15(8):505. doi.org/10.3390/toxins15080505
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- Fredrickson A, Richter A, Perri KA, Manning SR. First Confirmed Case of Canine Mortality Due to Dihydroanatoxin-a in Central Texas, USA. Toxins. 2023;15(8):485. doi.org/10.3390/toxins15080485
- Wood R. Acute animal and human poisonings from cyanotoxin exposure: a review of the literature. Environ Int. 2016;91:276-282. doi.org/10.1016/j.envint.2016.02.026
- Hilborn ED, Beasley VR. One health and cyanobacteria in freshwater systems: animal illnesses and deaths are sentinel events for human health risks. Toxins. 2015;7(4):1374-1395. doi.org/10.3390/toxins7041374
- Merck Veterinary Manual. Serum biochemical analysis reference ranges. merckvetmanual.com/reference-values-and-conversion-tables/reference-guides/serum-biochemical-analysis-reference-ranges
- Pet Poison Helpline. Blue-green Algae (Cyanobacteria). petpoisonhelpline.com/poison/blue-green-algae
Laboratory methods and reference intervals vary. This article explains; it does not diagnose. For a decision about your pet, use the original report and talk to your veterinarian. Our editorial policy.