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Dogs & Harmful Algal Blooms

What are Harmful Algal Blooms?

Harmful algal blooms (HABs) are overgrowths of algae that cause water quality problems in the Chesapeake Bay and its tributaries. Most often HABs are due to growth of blue-green algae or cyanobacteria, resulting from the flow of nutrients from fertilizers or septic systems into our creeks and rivers. Blue-green algae can produce toxins that can cause liver damage. These toxins can poison people, waterfowl or dogs. HABs often appear as a green or blue-green discoloration of the water. If such a change in color is evident, keep children and pets out of the water. Maryland residents can check for the presence of HABs by going to If you see an algal bloom, you can report it on the Fish Health Hotline, 877-224-7229. There is more information on HABs at

A cluster of dark false mussels that might be found growing on pilings or floats left in the water.  Dogs like Finnegan love to play in the water, but may be at health risk from algal toxins. Although Finnegan did eat some dark false mussels, quick veterinary attention saved his life.

How Can Dogs Be Exposed to Cyanobacterial Toxins?

Because of their behavior, dogs are more susceptible than humans to cyanobacterial poisoning. When toxins are present, dogs can be exposed by drinking the water, by eating debris on the beach, or by licking their coat after returning to the beach. Additionally some parts of the Magothy river watershed, and areas north of the Magothy, are supporting the growth of dark false mussels, Mytilopsis leucophaeata. Although these mussels are contributing to greater water clarity by acting as filter-feeders and removing algae and sediments from the water, they many concentrate toxins from algae as well. Several dogs that have eaten these mussels have been sickened or even killed due to liver failure. Repeated exposures to low levels of toxins can have health consequences for your dog down the road.

How Can You Reduce the Risk of Dog Poisoning?

  • Always keep your dog on a leash near shorelines
  • Don’t let dogs drink creek water or wade in beach debris
  • Don’t let dogs eat mussels or any shellfish on the beach
  • Don’t let dogs near an area where boaters may have scraped mussels off boat bottoms and lines

Common Signs of HAB Toxin Poisoning:

  • Repeated vomiting (green liquid)
  • Diarrhea or tarry (bloody) stool
  • Loss of appetite
  • Jaundice (yellowing of eye whites, gums)
  • Abdominal swelling that may be tender to the touch
  • Cyanosis (bluish coloration of the skin)
  • Dark urine or reduced/no urine output

If you suspect your dog has been exposed to cyanobacterial toxins, seek immediate veterinary care. The Magothy River Association thanks Sea Grant of New York for help in compiling this information.

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