Recreational water and health: Cyanobacteria and their toxins

Learn about the risks of cyanobacteria and their toxins, and how to keep yourself safe.

On this page

• About cyanobacteria
• Health risks
• Signs of cyanobacteria blooms
• Protect yourself and your pets
• Managing recreational water areas

About cyanobacteria

Cyanobacteria are also known as blue-green algae. They're a type of bacteria that occur naturally in fresh and marine bodies of water. There are many types of cyanobacteria, and some can produce harmful toxins called cyanotoxins.

Cyanobacteria can grow quickly in freshwater lakes and rivers under favourable conditions that can include:

• slow moving waters
• higher water temperatures
• an increase of nutrients in the water, such as nitrogen and phosphorus

These conditions can sometimes lead to large cyanobacteria blooms. When a bloom containing toxin-producing cyanobacteria develops, toxins can reach hazardous levels.

Marine waters generally don't have problems with cyanobacteria blooms.

Health risks

You can be exposed to cyanobacteria and their toxins by:

• swallowing contaminated water
• bloom material contacting your skin
• breathing in water droplets that contain toxins

If you're in an area containing blooms, this can happen while you're:

• wading
• playing
• swimming
• tubing or water skiing

Cyanobacteria and their toxins can cause adverse health effects. In people, exposure to water containing blooms can cause:

• skin, eye, nose and throat irritation
• stomach pain, diarrhea and vomiting
• headaches, muscle weakness and fever
• severe illnesses that may result in death, such as:
  • liver or kidney damage
  • paralysis of skeletal and respiratory muscles

The type and severity of illness depends on:

• the length of exposure
• the type and amount of cyanobacteria or toxins present
• how the person was exposed, for example through ingestion or skin contact

Children are more at risk because they're more likely to accidentally swallow water and often spend more time in water than adults.

It's rare for people to die from exposure to cyanobacteria or their toxins. There have only been a few deaths reported worldwide.

There have been many more reports of animals dying after:

• drinking water containing cyanotoxins
• licking or eating bloom material attached to their fur or washed up on the shore

These animals include livestock, wild animals, and pets, particularly dogs. Animal deaths usually result from a rapid onset of respiratory failure.

Signs of cyanobacteria blooms

Not all cyanobacteria blooms look the same. Cyanobacteria blooms on the surface of the water are generally green or blue-green and can look like spilled paint or fine grass clippings. Surface cyanobacteria blooms can also come in different colours, such as red or brown, which are not as common.

It can be difficult to tell the difference between a cyanobacteria bloom and other blooms on the surface of the water, such as green algae blooms.

Some blooms can spread through the water, making it look like the water has changed colour.

Benthic cyanobacteria grow on the bottom surfaces of water and form dense mats of cyanobacterial material. These benthic mats range in colour from black to brown to green. They can become detached from the bottom and accumulate along shorelines.

Other signs of a toxic bloom could include:

• earthy or musty odours
• dead waterfowl or other wildlife along the shoreline

It's impossible to tell if a cyanobacteria bloom or benthic mat contains toxins by looking at it.Treat any cyanobacteria bloom like it's toxic as a precaution.

Learn more about identifying blooms:

• Guide d'identification des fleurs d'eau de cyanobactéries (French only)
• California Freshwater Harmful Algal Bloom Field Guide

Protect yourself and your pets

Check online or at the recreational water area for swimming and contact advisories for cyanobacteria blooms. Follow the advice and recommendations.

Learn what cyanobacteria blooms look like. Not all recreational water areas monitor for blooms, so they may not post advisory signs.

Avoid activities in areas with a bloom. This includes areas where there is visible cyanobacterial material or benthic mats.

Talk to your healthcare provider if you're sick and suspect it's the result of exposure to a bloom.

If you or your pet accidentally touch a bloom, shower or rinse with tap water as soon as you can.

Do not let your pets:

• eat cyanobacterial material on the shore
• swim, play in, or drink water in areas with a bloom
• lick their fur before you shower or rinse them, if they come into contact with a bloom

Talk to your veterinarian if your pet shows abnormal behaviour or signs of distress and you suspect they were exposed to a bloom.

If you suspect the water has a cyanobacteria bloom, report it and stay out of the water. Check with your jurisdiction for online reporting forms or toll-free numbers. If no specific guidance is available, report the suspected bloom to your local public health unit.

When reporting a suspected cyanobacteria bloom, include a description or a picture of the bloom if possible. Report any illnesses in people or animals that you suspect are related to contact with the water.

Managing recreational water areas

Health Canada publishes guidelines for Canadian recreational water quality. They provide information on cyanobacteria and their toxins, including:

• how to reduce the risk of exposure
• strategies for what and where to monitor
• guidelines that can be used to protect public health

To check recreational water areas for cyanobacteria and their toxins, authorities can:

• look for blooms on the surface of the water
• test the transparency or the discoloration of the water
• look for dense mats attached to the bottom (in shallow, clear water)
• collect samples to test for cyanobacteria, toxins or other bloom indicators

Not all recreational water areas in Canada need to be monitored, as some are unlikely to develop blooms. Areas with higher risk for bloom development can include those:

• with increased concentrations of nutrients
• where blooms have developed in the past

In waters were there is a visible bloom, a swimming or contact advisory should be issued as a precaution. Advisories should remain in place until authorities say the area is acceptable for recreational activities again.