Napa Valley health officials are urging people planning to go swimming or boating to avoid close contact with lakes and other bodies of water containing blue-green algae.
"Be aware of posted signs that indicate the presence of blue-green algae. Also, if the body of water has algae or scum floating in it, it may be best for you and your pets to avoid the water," Dr. Karen Relucio, Napa County's public health officer, said in a statement.
People should avoid wading, swimming or waterskiing in water containing algae blooms, scums or mats, Relucio said.
The algae produce toxins that can cause eye irritation, skin rashes, mouth ulcers, vomiting, diarrhea and flu-like symptoms, according to Relucio.
Blue-green algae poisoning is most common in pets. "Symptoms including death can be more severe in pets because they tend to drink the water from affected lakes and reservoirs," Relucio said.
Children and adults can experience serious injury to the liver, kidney and nervous system if affected water is swallowed, according to Relucio. Medical treatment should be sought immediately, the doctor said.
Blue-green algae are microscopic bacteria found in freshwater lakes, rivers and streams, Relucio said. These algal blooms appear as blue-green, white or brown foam, scum or mats that typically float on the water's surface.