July in the northern hemisphere—when outdoor pursuits, particularly those involving water, are really ramping up. This summer, influenced by a lengthy global pandemic, more people than ever own dogs and are taking those dogs on every trip out of the house. As gradual climate change and intensive agricultural methods continue to advance, it’s time to sound the alarm about blue-green algae.

The villain here is a quiet family of microorganisms that pretty much keep to themselves. They make their own food using chlorophyll and sunlight, in the peaceful manner of plants. It’s somewhat hard to see them as a threat, and you can’t see them at all without a microscope. When the conditions are right, however, they reproduce very rapidly, forming a large, often colorful, “algal bloom”. One especially notorious bloom is called Red Tide. It forms periodically along the seacoast, killing dolphins, manatees, and massive numbers of fish, as well as humans (by contaminating the shellfish harvest). The type I want to discuss in this article, though, forms in fresh water and is called blue-green algae. It is of special importance to your dog because exposure is fatal for approximately 90% of non-human mammals.

Algal blooms behave in an ostensibly quiet and peaceful manner, but they can exude a vast library of toxins that affect different animals in wide variety of ways. Humans swimming near blue-green algae may experience skin rashes, labored breathing, and digestive upset; however, toxic effects are more intense for dogs and can manifest within just a few minutes. There’s no antidote for the resulting seizures and liver failure, or even a test to document what has transpired. In any case, many animals do not survive long enough to see the vet.

Blue-green algae reproduces best in slowly moving, warm water with sun exposure. It loves agricultural runoff containing fertilizer, but blooms can also form in small, isolated water sources like livestock troughs and neglected swimming pools. Harmful algal blooms can form and dissipate rapidly based on the weather, which makes water quality tests impractical. Be on the alert if you see discolored zones (any color, even brown or red) or something that looks like spilled paint in the water. Foam may accumulate on the surface, and there could be an unusual odor. This is not actually a plant, but it thrives under the same conditions as true algae (mats of slimy vegetation), so they are often found together. Algal blooms are affected by the prevailing wind, so they are usually thickest on the downwind side of a pond or lake. Your dog won’t be attuned to those subtle clues, though. She’s just going to race ahead and plunge in, grabbing a nice, tasty drink on the way.

Almost every algae you see is perfectly harmless, but the bad ones are especially bad. You can stumble across them in virtually any body of untreated water during the summer months, and the risk is likely to increase slightly with each passing year. Stay alert for the warning signs of harmful algal blooms and keep that adventurous pup close by your side.

Dr. M.S. Regan