After three dogs died from toxic algae poisoning from a North Carolina pond, dog owners are worried about algae blooms.

The dogs had seizures shortly after taking a dip in a pond to cool off. Within a day, all three dogs — Izzy and Abby, both West Highland white terriers and Harpo, a doodle — had died.

“What started out as a fun night for them has ended in the biggest loss of our lives,” one of the dogs’ owners wrote in a Facebook post shared 30,000 times.

The dogs were poisoned by blue-green algae in the pond, their veterinarian told CNN.

Harmful algal blooms happen when algae colonies grow out of control. The blooms can have toxic or harmful effects on people, fish, shellfish, marine mammals and birds, according to the National Oceanic and Atmospheric Administration.

Here’s more information about algae blooms:

What are harmful algal blooms?

In lakes and rivers, harmful algal blooms are most commonly cyanobacteria. Because of their color, they are often called blue-green algae, according to the Centers for Disease Control.

There are other types found in salt water.

Where are they found?

Harmful algal blooms have been reported in every U.S. coastal state, according to the National Oceanic and Atmospheric Administration, and their occurrence may be on the rise. They’ve also been spotted in Lake Erie and other Pennsylvania waters and can occur almost anywhere: lakes, ponds, stormwater retention basins, rivers, streams or reservoirs, according to Sea Grant Pennsylvania.

In July, the U.S. Army Corps of Engineers warned heavy rains and hot temperatures could create harmful algae blooms at Blue Marsh Lake in Berks County.

How can you identify harmful algae blooms?

Some colors are green, blue-green, brown, black, white, purple, red and black, according to Sea Grant Pennsylvania. They can look like film, crust or puff balls at the surface, grass clippings, or dots in the water. Some look like spilled paint, pea soup, foam, wool or streaks.

One of the dog owners pointed out on Facebook the pond they visited was largely crystal clear except for what looked like debris from foliage.

How can you prevent illness from a harmful algae bloom?

Look for algae advisories or ask if there has been a recent bloom.

If you see an algal bloom, keep yourself and your pets out of the water, according to the Centers for Disease Control.

If you or your pets do go in water that has an algal bloom, wash immediately with tap water. Do not let pets lick their fur until they are washed with tap water.

How can people or animals get sick?

• Swimming, kayaking, fishing, or wading through water, according to the Centers for Disease Control.

• Breathing contaminated water droplets or mist from recreational activities or wind-blown sea spray.

• Drinking contaminated water.

• Eating contaminated fish or shellfish.

What are the signs of algae bloom poisoning?

• In humans, symptoms can include skin, eye, nose or throat irritation; abdominal pain, headache, vomiting or diarrhea, according to the Centers for Disease Control.

• In animals, signs include staggering, difficulty breathing, convulsions, salivation, weakness and vomiting, according to Sea Grant Pennsylvania.

What causes a bloom?

What causes a bloom is complicated, according to the National Oceanic and Atmospheric Administration. They occur naturally but human activities that disturb ecosystems appear to play a role. “Increased nutrient loadings and pollution, food web alterations, introduced species, water flow modifications and climate change all play a role,” according to the agency.