During the summer, Americans often head out to enjoy the great outdoors and seek relief from the heat by swimming in rivers, lakes, and oceans across the country.
But before you or your pets dive into a body of water, you should be sure to look out for blue-green algae — and know what to do if exposure occurs.
Harmful algal blooms are spreading across the world, leading to toxic algae poisoning.
Blue-green algae poisoning was even speculated as a possible cause of death for a family of three and their dog who died under mysterious circumstances while hiking in central California in 2021, but officials later determined that they died of heat exhaustion.
So, what are these harmful algae, how can you spot them, and what should you do if exposed?
Cyanobacteria, more commonly known as blue-green algae, are some of the oldest living organisms on the planet. These tiny aquatic organisms are photosynthetic bacteria, meaning they convert light into chemical energy, and they can be found in fresh water or in oceans.
“Blue-green algae are not new. They are not something that has just come on the scene. They literally are the oldest photosynthetic organisms going. They’re billions of years old,” University of Southern California biological sciences professor David Caron told BuzzFeed News. “So they have been around a long time, but they have not been dominant in most freshwater bodies for most of that time.”
Big clusters of blue-green algae, so-called cyanobacterial harmful algal blooms, occur when the algae grow quickly in water that is warm and slow moving and has undergone a process called eutrophication, meaning it is full of excessive nutrients. This usually happens in late summer or early fall but can occur anytime under the right conditions.
You might notice these blooms as a foamy layer of scum on the water’s surface or as pigment that turns water red, green, or yellow brown. Sometimes they have an odor like rotting plants.
What makes them poisonous?
Cyanobacteria compete with other species, including other forms of nonharmful algae, for survival in their ecosystem.
“An ancient group of organisms, they have been battling it out fighting for nutrients for literally billions of years, and they have developed mechanisms for doing that,” Caron explained. “It’s essentially a competition. The one who grows the best or grows the easiest is the one who wins that competition.”
To take out their competitors, cyanobacteria evolved to produce poisons called cyanotoxins.
“It seems unlikely that blue-green algae are trying to toxify dogs or humans or cattle,” Caron said. “That may be simply something that happens, but really, those compounds are being produced for a very specific reason that is pertinent to the water body where they’re living. Maybe they’re poisoning the plankton that may be eating them or the competing algae.”
There are a variety of cyanotoxins that can be produced, depending on the species of cyanobacteria. Some of the most common types in the US are called microcystins, anatoxins, and saxitoxins.
One variant of anatoxin, anatoxin-a, is so toxic that it is known in science circles as “very fast death factor.”
“If you were exposed to a sufficient quantity, yes, it could kill you in a matter of minutes to hours,” UC Santa Cruz ocean sciences professor Raphael Kudela told BuzzFeed News.
One study from 2021 even found anatoxin-a being released into the air above a harmful algal bloom on Nantucket Island in Massachusetts two years prior. Scientists collected samples on glass fiber filters from the air above a bloom in a pond, discovering evidence of anatoxin-a molecules. No one was harmed, but the scientists said their findings suggested the emission “presents a potential human health exposure not previously examined.”
How do they affect humans and animals?
People or animals can be exposed to cyanotoxins when they touch or swim in contaminated water, when they inhale droplets of aerosolized toxins, or when they consume seafood or water that has been contaminated.
Dogs are especially at risk because they’re more likely to play in or drink contaminated water without hesitation, or consume other dead animals found near a bloom.
According to the CDC, symptoms vary depending on a multitude of factors: Was it fresh or salt water? How were they exposed and for how long? Which type of toxin was present, and how much?
People who are exposed from touching or swimming in contaminated water, or by breathing in contaminated droplets, may experience irritation of the eyes, lungs, noise, skin, and throat.
Consumption of contaminated water, from drinking it or eating contaminated seafood, may cause stomach pain, headache, vomiting, diarrhea, liver damage, or neurological symptoms such as muscle weakness or dizziness, depending on the specific cyanotoxin.
Anatoxin-a and saxitoxins, for example, are neurotoxins that will shut down a person’s nervous system, leading to nausea, tingling, and potentially death. Conversely, microcystins are hepatotoxins, meaning they degrade the liver, but they can also affect the kidneys and reproductive system.
Have people died from blue-green algae in the US before?
Human deaths in the US from cyanotoxins have been practically nonexistent because of the extremely large amount of contaminated water that would need to be ingested to cause death. But that doesn’t mean people can’t still get sick, or that their furry friends can’t die, from toxic algae poisoning.
“As far as I know, nobody in the US has ever died from cyanotoxins,” Kudela said. “We have a lot of dog deaths and a lot of cattle deaths, and people are exposed and get sick, but no [human] deaths.”
Two decades ago, a Wisconsin coroner cited toxic algae poisoning as the cause of death for a 17-year-old boy who died in 2002 two days after he swam in water and became infected with the cyanotoxin known as anatoxin-a. However, this finding was disputed by some experts who believed that if the toxin were to blame, it would have killed the teen much quicker — potentially within hours.
In 2019, 14 states reported 242 harmful algal blooms, which sickened 63 people, according to CDC data.
In 1996, two cyanotoxins were discovered in the water source for a dialysis center in Caruaru, Brazil. Scientists estimated that the water used for dialysis treatments contained toxins that were 19.5 times higher than the level set by the World Health Organization for drinking water. The outbreak caused the deaths of 76 patients from liver failure.
What precautions can I take to avoid algae poisoning?
Before swimming or fishing in lakes, rivers, or oceans, check local water advisories.
You can’t tell whether an algae bloom is toxic just by looking at it, so the CDC advises, “When in doubt, stay out.” Don’t go into water that smells bad, is discolored, has dead animals floating in it, or has a foamy scum or paintlike streaks on its surface.
“You can tell a lake that has the algal blooms, that has that film on the top,” Erika Holland, an assistant professor of biological sciences at California State University, Long Beach, told BuzzFeed News. “As an experienced hiker, you’re always told not to drink from stagnant water.”
If you do come into contact with contaminated water, the CDC recommends that you rinse yourself as quickly as possible with clean fresh water. Seek medical treatment as soon as possible if you show any symptoms, or call the Poison Control Center.
For animals, be sure to also keep pets or livestock away from water experiencing harmful algal blooms.
If your dog does swim in a bloom, use clean fresh water to immediately wash them down, and stop them from licking any cyanobacteria that may still be on their fur. Call a veterinarian if the dog exhibits any stumbling, convulsions, foaming at the mouth, vomiting, or other unexplained symptoms.
Why are blue-green algae spreading?
Several human-induced factors are contributing to a spread in water conditions that lead to harmful algal blooms around the world.
The first is land management. Agricultural runoff, such as fertilizer, and urban runoff, such as sewage, are depositing extra nutrients into bodies of water, stimulating eutrophication. The construction of dams has also slowed water movement in some areas, creating stagnant bodies that foster cyanobacteria.
But climate change is also leading to hotter temperatures and warmer bodies of water that stimulate the growth of cyanobacteria. Droughts also lead to water and nutrients condensing in depressed lake levels.
“If we continue to see extreme warmth, if we continue to see droughts,” Caron said, “we will probably see more of these toxins produced and the potential for more animals and humans to get in the way of those toxins.”
What can be done to stop the spread?
With harmful algal blooms rising around the world in both frequency and intensity, scientists are calling for more public awareness and increased government investment in water testing.
But to truly reverse eutrophication and curb the spread of cyanobacteria, we need to work as a planet to limit global heating due to climate change.
We must also find ways to reduce runoff and remove nutrients from wastewater discharge. This needs to happen at the industrial agricultural and urban levels, but also at home. Holland recommended that people reduce their use of fertilizers or laundry products that contain nitrogen and phosphorus, which can lead to eutrophication. She also suggested that people push officials to invest in better wastewater-control programs.
“It will take intestinal fortitude. It will take good land management,” Caron said. “We all need farming. We all need ranching. We all need urbanization to some degree. But we need to figure out how to do those things in a way that will not degrade our water quality.
“We are heading down a road right now that is not being reversed.”