Last week, a North Carolina dog owner took her three pets for a swim in a pond. Mere hours later, all three pets died due to exposure to toxic blue-green algae.

This is every dog owner's nightmare — and unfortunately, it's not an isolated incident. A 2013 study identified 368 cases of toxic algae poisoning of dogs throughout the country, although researchers believe this is only "a small fraction of cases that occur throughout the U.S. each year."

And it's not just a North Carolina thing, either. Algal blooms (cyanobacteria) with harmful effects can happen in any body of fresh water, including in the Bay Area.

The toxins typically occur when temperatures are high and water flows are at their lowest, so spring and summer are times to be watchful when swimming. Look out for green, black, orange or brown foam floating on the water that produces a nauseating smell.

However, as Dr. Tami Reynolds, a Humboldt County veterinarian, wrote in the Times-Standard, "Not all blooms contain toxins — most algal blooms in California contain harmless green algae."

But species that do produce toxins can sicken or even kill people, pets and wildlife. Dogs are especially vulnerable due to their tendency to lick their fur and drink the water they are playing in. Symptoms, which usually come on fast, include diarrhea, vomiting, weakness, drooling, difficulty breathing, and seizures, according to the EPA.

The California State Water Resources Control Board monitors harmful algal blooms, and offers an incident report map showing locations where they have been voluntarily reported.

"In recent years, there has been an increased frequency and severity of cyanoHABs around the world, including the San Francisco Bay Region," its website says.

According to the incident map, several bodies of water in the Bay Area have had reports of harmful algal blooms this summer. Lake Anza in Berkeley and Lake Temescal in Oakland both have "Caution Advisory" signs posted, meaning that blue-green algae is present in the lake, but no toxins have been detected. Thus, humans are allowed to swim in these lakes, but dogs are not.

Lake Chabot and Big Break, which are also part of the East Bay Regional Park District system, are permanently closed to swimmers due to cyanobacteria. The Quarry Lakes in Fremont have been closed to swimmers since early April due to a harmful algal bloom.

Lake Almaden in San Jose has also experienced toxic algae problems in the past and has warning signs posted.

As the temperatures rise in the Bay Area this weekend and taking your dog for a swim sounds enticing, be sure to do your research first.

Instructions for reporting a harmful algal bloom in the San Francisco Bay Region can be found here.

Madeline Wells is an SFGate editorial assistant. Email: [email protected] | Twitter: @madwells22