Blue-Green “Algae” and Companion Dogs

Blue-green algae companion dogs

Every summer we hear a smattering of reports about companion dogs dying after swimming in “blue-green algae”. This topic should be top of mind for any dog parent particularly during the summer and fall months.

What are blue-green algae?

  • Blue-green algae are not algae.
  • The U.S. Environmental Protection Agency (EPA) refers to deadly cyanobacteria as CyanoHABs (Cyano-Harmful Algal Blooms) or cyanotoxins.
  • The Centers for Disease Control and Prevention (CDC) call these outbreaks cyanobacteria blooms.
  • Why does the CDC call these cyanobacteria blooms? Because blue-green algae are actually bacteria called cyanobacteria, which produce a blue-green color in freshwater, estuaries and salt water.
  • Cyanobacteria can also create blue, bright green, brown, or red colors.
  • They can look like foam, scum, or mats on the surface of water.
  • Note: the Red Tide Phenomenon that plagued Florida coasts is caused by a type of algae.
  • Some cyanobacterial blooms are toxic and deadly depending on the species of cyanobacteria.
  • Not all cyanobacteria are bad. Spirulina is a genus in the cyanobacteria family; it is nutritious and a rich source of protein, often added to pet foods and supplements.
  • Cyanobacteria are photosynthetic, so they can produce their own food.
  • Many species of cyanobacteria capture nitrogen in the atmosphere and convert it into ammonia. Other bacteria then convert the ammonia into nitrates and nitrites.
  • This nitrogen capture – known as “fixing” – is necessary for plant life and growth.

Where are cyanobacterial blooms found?

Cyanotoxins are found in water. Typically, most cyanotoxin blooms occur in still or slow-moving water as temperatures increase during the summer.

They are commonly believed to be isolated to the Southeastern part of the United States.

But, this is false. The EPA notes, “Harmful algal blooms are a major environmental problem in all 50 states.” Albeit, the EPA’s statement includes algae.

The Woods Hole Oceanographic Institution makes a more definitive statement, “All 50 states are impacted by cyanobacteria HAB.”

Remember: cyanobacteria can grow in many waterways including warm fresh, marine or brackish waters.

It is also incorrect to think that all cyanotoxins create a blue-green color.

Remember: Cyanotoxin blooms can be different colors, but we just colloquially refer to them as blue-green algae because that is the predominant hue and look overall. Lyngbya, a genus of cyanobacteria, creates dark green or nearly black colors. However, the color may become mottled with light green or even white later in summer.

What is the cause of cyanobacteria blooms?

Nitrate-contaminated groundwater – combined with the right amount of phosphate and some other trace elements – can actually be used as a growth medium for some species of cyanobacteria. As this bacteria grows, nitrates are removed from the water.

The major sources of nitrates in water come from wastewater, fertilizers from both homes and farms, livestock farming, sewage, run-off from urban areas and industrial facilities.

Climate change also appears to be a major factor contributing to the increase in cyanobacteria blooms. The CDC states that low water flows, ocean upwelling, increases in water temperature and changes in ocean currents all play roles.

What are the symptoms of cyanobacteria poisoning in dogs?

Simply ingesting water from, or swimming in, cyanobacteria toxic waterways can be deleterious to companion dogs.

The Environmental Protection Agency (EPA) has identified the four most commonly found toxins produced by certain cyanobacteria:

  • Microcystins
  • Cylindrospermopsin
  • Anatoxins
  • Saxitoxins

All four toxins are clinically significant, but the two researchers know the most about regarding dogs are microcystins and anatoxins.


Microcystins damage the liver leading to organ failure. Death can occur between 12-24 hours after ingestion or within a few days.

  • Bleeding
  • Collapse and coma
  • Death
  • Diarrhea (dark, tarry stool)
  • Jaundice (yellow tint to gums and skin)
  • Lack of appetite
  • Lethargy
  • Pale gums
  • Seizures
  • Shock
  • Vomiting
  • Weakness


Anatoxins can kill companion dogs faster than microcystins – within minutes to hours of exposure. Anatoxins attack the nervous system.


  • Blue discoloration of skin and mucous membranes
  • Coma
  • Death
  • Difficulty breathing
  • Disorientation
  • Excessive salivation
  • Excessive tearing
  • Muscle rigidity
  • Paralysis
  • Seizures
  • Tremors
  • Weakness or inability to walk


In response to the uptick in cyanobacteria poisoning incidents, the CDC’s National Center for Environmental Health (NCEH) launched a reporting system called the Harmful Algal Bloom-related Illness Surveillance System (HABISS) between 2007 and 2011 for both animals and humans.

The Departments of Health and/or Environment of 13 states across the country reported 67 canine cases of cyanobacterial poisoning to this system.

67 is awfully low, but the results are sadly astonishing:

  • 58% of the canine intoxications were fatal. That is 38 out of 67 cases.
  • 32% of deaths were attributed to anatoxin poisoning – 12/67.
  • 8% of deaths were attributed to microcystin poisoning – 3/67.
  • 45% of deaths were not attributed to a specific cause and were unknown – 17/67.
  • 58 (87%) dogs were exposed to fresh waters, 1 to marine waters, the source of exposure was unknown in 9 canine cases.
  • 29 of the 67 (43%) dogs experienced gastrointestinal symptoms, including vomiting and diarrhea.
  • 12 reports (18%) noted lethargy.
  • 9% (6) of the reports mentioned neurological signs such as stumbling or changes in behavior.

HABISS is now shut down, but appears to have been replaced by the One Health Harmful Algal Bloom System (OHHABS).


The best medicine is prevention when it comes to cyanobacterial poisoning in companion dogs. However, if you do not want to deprive your dog of a good swim during the summer, the EPA provides some guidance. Avoid water if:

  • It is slimy or looks like foam, scum or mats are present on the surface.
  • The color is weird.
  • It stinks. Some (but not all) cyanobacterial blooms produce a nauseating smell.

Even if you made sure the water does not meet the above avoidance criteria, we suggest that you thoroughly rinse your companion dog off immediately with fresh and clean water. As well, please wear gloves to protect yourself.

Watch for the symptoms mentioned above for several days. If any symptoms appear, please take pets to your veterinarian immediately.

If diagnosis sadly confirms cyanobacterial poisoning, please report it to OHHABS.


Backer, Lorraine C et al. “Canine cyanotoxin poisonings in the United States (1920s-2012): review of suspected and confirmed cases from three data sources.” Toxins vol. 5,9 1597-628, 24 Sep. 2013, doi:10.3390/toxins5091597,

Buzhardt, Lynn. Algae Poisoning. VCA Hospitals,

“Cyanobacteria: Life History and Ecology.” University of California Museum of Paleontology, University of California Berkeley,

The Editors of Encyclopaedia Britannica. “Blue-Green Algae.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 29 Dec. 2017,

The Editors of Encyclopaedia Britannica. “Spirulina.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 20 June 2018,

Ernst, Anneliese et al. “Nitrate and phosphate affect cultivability of cyanobacteria from environments with low nutrient levels.” Applied and Environmental Microbiology vol. 71,6 (2005): 3379-83, doi:10.1128/AEM.71.6.3379-3383.2005,

“Facts about Cyanobacterial Harmful Algal Blooms for Poison Center Professionals.” Centers for Disease Control and Prevention, Centers for Disease Control and Prevention, 24 Aug. 2018,

Hu, Q et al. “Removal of nitrate from groundwater by cyanobacteria: quantitative assessment of factors influencing nitrate uptake.” Applied and Environmental Microbiology vol. 66,1 (2000): 133-9, doi:10.1128/aem.66.1.133-139.2000,

“Introduction to the Cyanobacteria: Architects of Earth’s Atmosphere.” University of California Museum of Paleontology, University of California Berkeley,

“Learn about Cyanobacteria and Cyanotoxins.” EPA, Environmental Protection Agency, 6 Feb. 2020,

Merel, Sylvain et al. “State of knowledge and concerns on cyanobacterial blooms and cyanotoxins.” Environment International vol. 59, 303-327. Sept. 2013,,

“Protect Your Pooch: How to Keep Your Dog Safe from Toxic Algae.” EPA, Environmental Protection Agency, 10 June 2019,

Scroll to Top