Hidden danger in the water: Protecting pets from blue-green algae
August 19, 2025
Warm weather adventures with our pets are one of the great joys of the season, and a cool dip in a lake or pond seems like the perfect way to have fun and beat the heat.
But a hidden danger, often looking like simple pond scum or spilled paint on the water's surface, can turn a happy outing into an emergency. This threat is commonly known as blue-green algae, a type of toxic bacteria that can be fatal to pets within minutes of exposure.
Veterinarian and second-year resident of anatomic pathology, Alexandra Reddy, answers questions about blue-green algae and explains how pet owners can help safeguard their pets.
Could you briefly explain what blue-green algae is and why it's a concern for pets?
Blue-green algae poisoning refers to harmful algal blooms caused by an excessive growth of bacteria known as “cyanobacteria.” These are toxin-producing bacteria that are harmful to animals — including pets, livestock, and wildlife — as well as people. It's a major concern because the toxins target the liver and nervous system and can be rapidly fatal.
Where are blue-green algae most commonly found and what do they look like?
Blue-green algae are most commonly found in warm, stagnant fresh, salt, or brackish water like ponds and lakes, especially during hot summer months or after a dry spell.
The blooms often look like a thick, blue-green mat or scum floating on the surface, but they can also be red-brown. The water may also have an odor of rotting plants. Importantly, the algae can also grow below the surface, where they are hard to see.
Since you cannot tell if a bloom is toxic without testing, you should assume all suspicious blooms are dangerous.
How do pets typically get exposed to blue-green algae toxins?
The most common way pets are exposed is by mouth. This includes drinking from contaminated water, licking their fur after a swim, or eating the algal mats or contaminated fish. Pets that swim in lakes and ponds during the summer are particularly at risk. Even just a few mouthfuls of contaminated water can be fatal, and the scums that accumulate along shorelines have the highest concentrations of toxins.
What are the primary signs of exposure?
The clinical signs can appear rapidly. Important signs to look for include:
Vomiting or diarrhea;
Weakness or collapsing;
Excessive drooling;
Jaundice (yellowing of the skin and gums);
Seizures, muscle tremors, or rigidity;
Paralysis or difficulty breathing.
Unfortunately, in some animals, the only clinical “sign” is sudden death.
If you suspect your pet has been exposed, what is the immediate action you should take?
If you suspect your pet has been exposed, seek immediate veterinary attention. Do not wait for clinical signs to develop. Blue-green algae poisoning can be rapidly fatal, so rush your pet to the nearest emergency veterinary hospital.
If your pet swam in contaminated water and is not in immediate distress, washing them with clean water is also crucial to wash off toxins before they can be licked off.
Be sure to also protect yourself when handling an animal exposed to blue-green algae; use gloves and avoid touching your mouth and eyes; and clean clothes and surfaces that have been contaminated.
What is the typical course of treatment? Is there an antidote?
Unfortunately, there is no specific antidote. Treatment consists of supportive care, which may include IV fluids, oxygen therapy, anti-seizure drugs, and muscle relaxants. By the time patients arrive at the hospital, they are often in critical condition. If caught very early, a veterinarian may be able to decontaminate the pet by inducing vomiting or pumping the stomach.
What are the most crucial preventive measures pet owners can take?
The most important step is to prevent your pets from accessing stagnant, murky water. Keep them on a leash around suspicious ponds or lakes. You can also help prevent algae growth around your home by keeping water sources like bird baths and fountains clean.
Are there any "safe" ways for pets to interact with water?
Safer water sources include filtered tap water, supervised access to a clean backyard pool, and non-stagnant bodies of water such as flowing streams and creeks. However, even these sources have some risks, so always be mindful.
How can I check for blue-green algae advisories in my area?
In Virginia, the Virginia Department of Health (VDH) has a Harmful Algal Bloom Dashboard that is regularly updated by their task force. The website will include specific locations and advisories for public water sources in our area. Their harmful algal bloom map doesn’t reflect all possible blooms, just those that have been reported and those under investigation.
Many other state and local health or environmental departments issue public warnings for harmful algal blooms. The best place to start is by searching online for “[Your State] harmful algal bloom advisory” or checking the website for your local public health department. The Environmental Protection Agency (EPA) also provides general guidance.
What is the one most important message you want pet owners to take away?
My goal is to empower owners with the knowledge to protect their pets. If you take away just one message, let it be this: it is best to assume that all algal blooms on a body of water are toxic to your pet, so do NOT let them near it! If they are exposed, it is better to be safe than sorry, so bring them to your nearest veterinary hospital immediately!
A pathologist's perspective
For a deeper dive into the science, watch the companion video with Alex Reddy. She explains the pathology of blue-green algae poisoning and what happens inside a pet's body after exposure.