Avoid Harmful Algae and Cyanobacteria

Harmful algae and cyanobacteria (sometimes called blue-green algae) can produce toxins (poisons) that can make people and animals sick and affect the environment. Learn more about them to keep you and your pets safe.

Green algae in a river

Algae and cyanobacteria are simple, plant-like organisms that live in the water. Algae and cyanobacteria can rapidly grow out of control, or “bloom,” when water is warm, slow-moving, and full of nutrients. Blooms can occur in fresh water, salt water, and brackish (a mixture of fresh and salt) water around the world. Blooms sometimes look like foam, scum, mats, or paint on the surface of the water. They can even make the water appear different colors, including green, blue, red, brown, or another color.

Blooms of algae or cyanobacteria can be harmful to people, animals, or the environment if they

  • produce toxins (poisons)
  • become too dense
  • use up the oxygen in the water or
  • release harmful gases

Blooms can occur more often or become more severe with warmer water temperatures or if the amount of nutrients in the water increases. Levels of nutrients increase when fertilizer, sewage, or runoff from cities and industrial buildings get into the water.

How People and Animals Get Sick

Animal Safety Alert

When in doubt, keep animals out! Cyanobacteria can be deadly for animals. See our Animal Safety Alert fact sheet for tips to protect your pets.

People and animals (including pets, livestock, and wildlife) can get sick when they have contact with water or food that contains certain types of algae, cyanobacteria, or their toxins.

People and animals can get sick if they

  • swim, wade, or play in or near contaminated water
  • eat contaminated fish, shellfish or
  • use contaminated drinking water

Illnesses and symptoms can vary depending on how a person or animal was exposed (came into contact with algae, cyanobacteria, or their toxins), how long they were exposed, which type of toxin was present, and how much toxin was present.

Symptoms can include

  • stomach pain, vomiting, or diarrhea
  • general symptoms, like headache
  • skin, eye, nose, or throat irritation
  • neurological symptoms (for example, muscle weakness, dizziness)

Exposure to some algal and cyanobacterial toxins can also harm your liver and kidneys. If you think you may have symptoms caused by harmful algae, cyanobacteria, or their toxins, contact your healthcare provider or Poison Control Centerexternal icon.

Animals can get very sick or even die quickly after exposure to harmful algae and cyanobacteria. Seek veterinary care immediately if your pets or livestock seem sick after going in or near water. Animals can die within hours to days of swallowing toxins. Animals are often the first affected, in part because they are more likely to swim in or drink from bodies of water that contain harmful algae or cyanobacteria.

One Health Harmful Algal Bloom System logo

The One Health Harmful Algal Bloom System (OHHABS) collects information to help CDC and partners better understand harmful algal blooms (HABs) and help prevent human and animal illnesses caused by HABs.

When in Doubt, Stay Out

You can take steps to protect yourself and your pets from getting sick from harmful algae and cyanobacteria.

  • Check for swimming and fishing advisories before visiting lakes, rivers, and oceans. Follow advisories to reduce your chances of getting sick.
  • If you see a bloom, stay out of the water and keep your pets and livestock out of the water. You cannot tell if a bloom is harmful by looking at it, so it is best to use caution and stay away.
  • Do not fish, swim, boat, or play water sports in areas where there is harmful algae or cyanobacteria.
  • Do not go into water that
    • smells bad
    • looks discolored
    • has foam, scum, mats, or paint-like streaks on the surface
    • has dead fish or other animals washed up on its shore or beach
  • If you are notified of harmful algae or cyanobacteria in a nearby body of water or in your public drinking water supply, follow local or state guidance to reduce your chances of getting sick.
  • Check for and follow local shellfish and fish advisories before eating any fish or shellfish you collect.
Page last reviewed: June 28, 2021