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Emergency Medicine & First Aid

What Every Dog Owner Should Know About Harmful Algal Blooms

Learn about life-threatening toxins produced by marine and freshwater algae including blue-green algae, when to keep your dog out of the water and what to do if your dog is exposed.

By Lorraine Backer PhD MPH

When might your wanderings with your dog expose them to harmful algal blooms (HABs)? Read on to learn about the harmful toxins produced by marine and freshwater algae, including how to recognize the dangers, when to keep your dog out of the water, and what to do if your dog is exposed to contaminated water.

Algal blooms are called harmful algal blooms (HABs) when they have negative effects on the health of animals and people or on the environment.

What are algae?

Pond scum, seaweeds, and giant kelp are all types of algae. Like plants, algae can photosynthesize, that is, they use light to make their food. Like animals, some algae possess flagella for movement and can feed on organic matter in their habitat. Algae range in size from single-celled cyanobacteria and marine microalgae to multicellular species. Note that despite their name, cyanobacteria, also known as blue-green algae, are a type of microscopic algae, not a bacteria.

Benthic algae (cyanobacteria) growing on submerged rocks. Photo: California Water Quality website
Algal mat floating in Lake Erie 2023 Photo: Krista Halling

What are harmful algal blooms (HABs)?

Sometimes algae grow very quickly, or bloom. Most algae and algal blooms are harmless. However, some algae can produce poisons that can be in the algal cells or in the water; or they may produce thick mats that block sunlight from getting to other plants. Algal blooms are called harmful algal blooms (HABs) when they have deleterious effects on the health of animals, people or on the environment.

  • Some blooms float on the water surface, some are suspended in the water, and others are benthic, that is, attached to rocks or sediments.
  • Blooms may be different colors, sometimes blooms in freshwaters look like turquoise paint on the water surface.
  • As a bloom dies off, you may smell an odor like that of dying plants or rotten eggs.
Swirls of Cyanobacteria giving this lake water a paint-like appearance. Photo: Lorraine Backer
Cyanobacterial scum on Klamath Lake 2007 Photo: Lorraine Backer

How can dogs be exposed to HAB toxins?

Dogs can be exposed to the toxins and become severely ill:

  • If they drink water contaminated with HAB toxins
  • If they eat fish or shellfish that have accumulated the poisons in their tissues
  • If they eat mats of algae from freshwater blooms
  • We know that some HAB toxins, such as those from red tides that occur in the Gulf of Mexico, can get into the air and cause respiratory problems in people, but we don’t know if that’s true for dogs
  • Dogs are more likely to be exposed to HAB toxins than people are because dogs don’t seem to mind swimming in slimy or stinky waters or eating or chewing on algal mats
Dogs can be exposed to HAB by swimming in or drinking contaminated water.

What are the health effects in humans from exposure to HAB toxins?

We don’t have a lot of information about how dogs are affected by exposure to these toxins, but the health effects are likely similar as those experienced by humans.

Marine Algae

Algae that are found in marine waters, the toxins they make, where they are often found, and what we know about health effects in people are in the table below:

Marine Algae1Toxin ProducedGeographic DistributionHealth Effects in Humans
Karenia brevisBrevetoxinsGulf of Mexico, Southeastern United States, New ZealandNeurotoxic shellfish poisoning (diarrhea, vomiting, tingling and numbness in lips, tongue, and throat) from eating contaminated shellfish; respiratory irritation and asthma exacerbation from contaminated sea breezes
Pseudo-nitzchiaspeciesDomoic acidEast and West Coasts of North America
Amnesiac shellfish poisoning (abdominal pain diarrhea, vomiting, neurologic problems such as confusion and memory loss) from eating contaminated shellfish
GymnodiniumPyrodinium, and Alexandrium speciesSaxitoxinsTemperate oceans worldwideParalytic shellfish poisoning (Diarrhea, nausea, vomiting, numbness or tingling of mouth and lips, possibly respiratory paralysis needing respiratory support)
Dinophysis and Prorocentrum speciesOkadaic acidEurope, JapanDiarrhetic shellfish poisoning (nausea, vomiting, diarrhea, abdominal pain, chills, headache fever) from eating contaminated shellfish
Gambierdiscus toxicus, other speciesCiguatoxin, maitotoxin,
Tropical coral reefs, Gulf of Mexico near oil rigsCiguatera poisoning (Abdominal pain, nausea vomiting, diarrhea, parathesias, more intense hot and cold sensations; symptoms may last for weeks or months
1 Backer LC, Fleming LE.  Background Epidemiology. In: Walsh, P.J., S.L. Smith, L.E. Fleming, H. Solo-Gabriele, W.H. Gerwick (eds.) 2008. Oceans and Human Health: Risks and Remedies from the Sea, Elsevier Science Publishers, New York, 2008, pp.201-218 (Chapter 10).

Freshwater algae

Many of the species of algae found in fresh waters are cyanobacteria. Note that cyanobacteria are not infectious like other bacteria; rather, they are a type of microscopic algae. Cyanobacteria found in fresh waters, the toxins they make, where they are often found, and what we know about health effects in people are in the table below.

Freshwater Algae2Toxin ProducedWhere They are FoundHealth Effects in Humans
Anabaena speciesAnatoxin-aWorld-wide in freshwater and marine habitats, mostly freshwater ponds and saline lakesProgression of muscle weakness, decreased movement, cyanosis, death
Cylindrospermopsis raceborskiiCylindropsermopsinTropical, subtropical, and temperate climatesLiver toxicity, kidney toxicity, malaise, anorexia, vomiting, headache
Microcystis speciesMicrocystinsGlobal distribution except for AntarcticaLiver toxicity, elevated liver enzyme levels in blood
Nodularia spumigenaNodularinsWorldwideLiver toxicity
Aphanizomenon flos-aquae and Anabaena circinalisSaxitoxinsSubtropical and temperate watersNeurotoxicity including incoordination, tingling and numbness of lips, mouth, tongue; possible respiratory failure
Zaias J, Backer LC, Fleming LE.  Harmful Algal Blooms.  In:  Rabinowitz PM, Conti LA (eds.) 2010.  Human-Animal Medicine. Clinical Approaches to Zoonoses, Toxicants, and Other Shared Health Risks.  Sanders, Maryland Heights, Missouri, 2010, pp. 91-104.

You cannot tell just by looking, whether water is safe. When in doubt, keep your dog out!

What are the health effects in dogs from exposure to cyanobacteria toxins?

When dogs are exposed to Cyanobacteria toxins it may result in severe illness or even death.

  • Dogs that go into water will most likely drink it or swallow it while swimming, so don’t let your dog go into water that might have harmful algae in it. 
  • Swallowing water that contains HAB toxins can cause serious illness.
  • Dogs might have worse symptoms than people if they swallow water while swimming or lick their fur after getting wet.
  • Dogs may become ill and collapse or die suddenly if they lick water from their fur that is contaminated with some freshwater algal toxins, such as microcystin or anatoxin.
  • Smaller dogs may be more susceptible to the toxins because of their size.
  • We don’t currently know if any breeds are more susceptible to these toxins.
  • There are no antidotes to these toxins. It is essential that emergency veterinary care be obtained to provide supportive care, which enhances survival outcome.

The table below shows some signs that your dog may exhibit if it is exposed to cyanobacterial toxins. We have much more information about how dogs are affected by exposure to cyanobacterial toxins, particularly anatoxin-a and microcystins, because the blooms can occur in freshwaters used for recreation. The information below is based on reports derived from dog poisonings by cyanobacterial toxins.

Potential Cyanobacterial Toxin Exposure Route3Common Signs in DogsPossible Laboratory or Other Findings in Dogs
Swallowing water contaminated with cyanobacterial toxinsHepatotoxins and nephrotoxins:
Excess drooling
Foaming at the mouth
Jaundice, hepatomegaly
Blood in urine or dark urine
Loss of appetite
Abdominal tenderness
Photosensitization in recovering animals

Progression of muscle twitches
For saxitoxin, high doses may lead to respiratory paralysis and death if artificial ventilation not provided
Elevated bile acids, ALP, AST, GGT
Prolonged clotting time
Presence of toxins in clinical specimens or stomach contents of animals that became ill

Presence of toxins in clinical specimens or stomach contents of animals that became ill
Skin contact with water contaminated with cyanobacterial toxinsDermal toxins:
Allergic reaction
Blue-green staining of fur or hair
Centers for Disease Control and Prevention, https://www.cdc.gov/habs/specific-groups/animal-owners-cyanobacteria.html

Dogs may become exposed to HAB toxins by swallowing water, licking water or algae from their fur or eating a contaminated algal mat, fish or crustaceans.

Other important facts

  • You can not tell if a bloom is making toxins just by looking at it.
  • Even algae that can make toxins only do so under specific environmental conditions, most of which we don’t understand.
  • Scientists continue to identify the genes responsible for producing toxins in different algal species, so it’s not always possible to say if a given bloom could or will produce a toxin.
  • Many states have interactive websites where you can find information about local water bodies, including whether they have a current bloom and whether it is producing toxins. 

Before you go dogpacking on or in water

Check the state or provincial website for any warnings about the presence of algae and/or toxins at your destination.

When you get to your destination

Keep your dog out of the water if you see any of the following signs of a bloom:

  • Foam, scum, mats, or paint-like streaks on the water
  • Different colors (e.g., green, blue, red, or brown) in the water
  • Smell like rotting plants

What to do if you think your dog was exposed to HAB toxins

  • Wash your dog with clean water to keep them from licking algae from their hair or fur.
  • Seek veterinary anttention immediately and let them know what water body your dog was swimming or playing in and that you suspect your dog was exposed to algal toxins.
  • Call your local Animal Poison Control.
Immediately rinse your dog and contact your veterinarian if your dog is exposed to unsafe water

Many dogs love water, and dogpacking in and around water can be immensely fun. Just keep in mind potential toxicity from HABs, and stay safe by monitoring local government websites and watching for any signs of algal blooms in your local waters.

Additional resources for dog owners and veterinarians

About the author

Dr Lorraine Backer led Center for Disease Control (CDC)’s research efforts to understand the association between exposure to HABs and HAB toxins and health outcomes for more than 20 years.  She is now retired from CDC, but continues to work on HAB-related public health research as a consultant.

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