Harmful Algal Blooms

Beach polluted with blue-green algae bloom

Harmful algal blooms can produce poisons that are dangerous to people, animals, and the environment. Learn how to keep you and your pets safe.

Warm weather brings many happy occasions: picnics, ballgames, and back yard fun. It also is a time when microscopic plant-like organisms – algae and cyanobacteria – are more likely to overgrow in rivers, lakes, and oceans.

These overgrowths, called algal blooms, occur across the country. Sometimes they are just eyesores – an unpleasant scum or thick green, red, blue, or brown layers in the water that look or smell bad. However, sometimes they contain poisons that hurt people, animals, and the environment. In this case, they are known as harmful algal blooms. But you can’t tell if a bloom is harmful just by looking at it. Also, not all blooms are easy to see—poisons can be present even when you can’t see the bloom.

Be Aware of Harmful Algal Blooms

Harmful algal blooms can produce poisons that can make people and animals sick. They also can block sunlight in a body of water or use up a lot of the oxygen, which kills fish and plants in the water. Harmful algal bloom poisons have caused the shutdown of the water supply in a major U.S. city, killed wildlife and pets, and sickened hundreds of people with a variety of skin, respiratory, neurological, and abdominal symptoms. Evidence suggests that harmful algal blooms are increasing in number and severity because of farming practices, storm water runoff, wastewater overflows, and increasing temperatures.

People and Animals Can be Exposed to Algal Poisons in Many Ways:

  • Swimming or coming in direct contact with the poisons
  • Breathing in the poisons
  • Swallowing food or water contaminated with poisons

Animals are often the first affected because they are more likely to swim in or drink from bodies of water that contain algal blooms.

Don’t let pets play in discolored or scummy water

Don’t let your pets swim or play in bodies of water that smell bad, look discolored, or have scum on the surface. Download more buttons and badges.

Tips for You and Your Pets to Stay Healthy

To protect yourself and your pets, avoid entering or playing in bodies of water that:

  • Smell bad
  • Look discolored
  • Have foam, scum, or thick layers of algae on the surface
  • Contain or are near dead fish or other dead animals

Check for beach warnings that might be posted online or on signs near the water. Follow guidance if you learn about a harmful algal bloom in bodies of water, like the beach or the lake, or if you are notified that your tap water contains algal poisons. Know the health risks of eating contaminated fish and shellfish and follow warnings.

If you think you or your pet have become ill because of a harmful algal bloom, see your healthcare provider or veterinarian. Animals can get very sick, so don’t delay contacting your veterinarian. If you have immediate questions about your symptoms, call your local or state poison center.External

Collecting Data to Protect Health
OH HABS logo

Because harmful algal blooms affect people, animals, and the environment, it’s important to track when they happen, where they happen, and whether they cause illness. CDC’s One Health Harmful Algal Bloom System (OHHABS), is a web-based national tracking system for harmful algal blooms. Health departments and their designated environmental health or animal health partners can use this voluntary system to report harmful algal bloom events and associated illnesses in people and animals. The data collected helps CDC and its partners learn more about these blooms and understand how to best prevent them and the illnesses they cause.

Harmful algal blooms are an example of a One Health issue. One Health is based on the fact that human, animal, and environmental health are connected. Using this approach to track harmful blooms highlights the opportunity for human health, animal health, and environmental health experts to work together to effectively address the problems that harmful algal blooms can cause. Learn more about harmful algal blooms and One Health in action from a story about poisoned sea otters in California.