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Happiness is a dog frolicking in the water on a hot summer day.

But a sign posted along a fence in Napa County warned of the worst outcome of a day playing at the water’s edge: A pair of German Shorthaired Pointers died just one day after playing near a pond in southern Napa County. The sign prompted health officials to test the water and, while still awaiting results, urge people to keep pets away due to the possible presence of potentially harmful blue-green algal blooms.

“I have run my dogs here for years with no problems,” an anonymous writer penned on the sign. “This warning is so you and your pets do not have to go through the ordeal I just experienced.”

Sonoma County health officials have for two summer seasons kept close watch on Russian River water quality after blue-green algae was detected in the water and blamed for the 2015 death of a golden retriever named Posie.

The river appears healthy so far this year, Sonoma County Health Officer Karen Milman said.

“Right now the Russian River is looking good,” Milman said.

Most algae is harmless. But blue-green algae, also called cyanobacteria, can be mixed in with other types of common algae. It thrives in still, warm water with high-nutrient conditions, often collecting along shorelines and areas like boat ramps and docks. Blue-green algae can produce a harmful substance called Anatoxin-a, a neurotoxin that, when ingested, can cause painful symptoms, from abdominal pain and nausea to diarrhea and dizziness. In some cases it can be fatal within minutes of ingestion.

Most deaths and serious symptoms affect pets. The algae can cause problems for people as well, especially children, and cause injuries to the liver, kidney and nervous system.

Hot temperatures and low water levels caused by a yearslong drought created prime conditions for blue-green algae to bloom in previous years.

With high river flows after a wet winter, conditions bode well for the river, Milman said.

Still, river monitors from agencies including the North Coast Regional Water Quality Control Board keep watch for conditions promoting the growth of blue-green algae, such as warmer water temperatures, dissolved oxygen and pH levels and visual signs. If the right combination of conditions persist, they will begin an expensive testing process to detect blue-green algae at public beaches.

“Out of extra caution, don’t let your dog eat algae no matter what,” Milman said. “There’s nothing wrong with normal green algae. It’s when we get the blue-green algae mixed in, that’s when it becomes a concern, that’s what we’re monitoring for.”

Last week, toxic algae was discovered at Lake Temescal in Oakland, and the beach remains closed to swimmers.

In Napa County, the dogs swam in a pond at the end of Milton Road in the Napa-Sonoma Marshes Wildlife Area. Their owner posted signs on a fence in the area stating the dogs went into the water June 23 and died the next day, according to county spokeswoman Kristine Jourdan. County health officials are still trying to contact the owner, she said.

County health staff collected a water sample and sent it to a state lab, and by Monday hadn’t received the results.

County officials opted to warn the public June 30 in an abundance of caution.

Blue-green algae often looks as it sounds, but it can also be present in white or brown foam, scum or mats floating on the water’s surface.

“If the body of water has a lot of algae or scum floating in it, it may be best for you and your pets to avoid the water,” Dr. Karen Relucio, Napa County public health officer, said in a statement.

You can reach Staff Writer Julie Johnson at 707-521-5220 or julie.johnson@pressdemocrat.com. On Twitter @jjpressdem.