Local officials issued a county-wide warning on Friday, and closed at least two bodies of water in the area due to an out break of toxic algae, reported SFGate.
The dogs swam and likely drank the water in a pond off Huichica Creek, according to officials.
Friday's warning was issued by the county's board of supervisors, and urges swimmers and others to avoid ponds, rivers and streams in the area - as they could be contaminated with the toxic blue-green algae.
There are also signs posted by the bodies of water warning of the danger.
If humans swallow the infected water, the algae poisoning can lead to liver, kidney and nervous system damage. Anyone who is exposed to the toxins are told to acquire immediate medical attention.
Camping filters, purification tablets and boiling temperatures do not make the water safe.
(Daily Mail - July 1, 2017)