Officials in Northern California are warning residents after two dogs died following a swim in a pond where toxic blue-green algae is growing.

People are urged to avoid ponds, rivers and streams in the area of southern Napa County where the dogs died last week.

The San Francisco Chronicle said Saturday that it's the second report of dangerous algae in the Bay Area in a week. Oakland's Lake Temescal was closed following an algae bloom there.

Algae poisoning in people can cause serious injury to the liver, kidney and nervous system if the affected water is swallowed. Exposure requires immediate medical attention.

Officials say the algae blooms are caused by sitting or slow-moving water with an abundance of nutrients.