Blue green algae, (known as cyanobacteria) are microscopic plants that grow in any type water and are too small to be seen. However, they can experience rapid growth (bloom) in nutrient rich water in late summer or early fall and can be deadly to animals.
When the algae blooms, the water appears cloudy or like foam, scum, or even matts on the surface of ponds or lakes.  Some liken them to the appearance of pea soup. Since the blooms float, they are easily driven towards shore, making them more accessible to livestock and pets.  The poisons produced by some cyanobacteria are among the most potent natural poisons known. There are no known antidotes.  Although most blue green algae blooms don’t produce toxins, it is impossible to determine without testing.
Dogs affected by these poisons may exhibit neurological signs such as weakness, stumbling, difficulty breathing, or seizures 15-20 minutes after exposure. Other signs include severe vomiting and diarrhea, jaundice, pale mucous membranes, blood in the stool, and abdominal pain hours or days later. Livestock drinking from water with algal blooms are often found dead near the source.
Sadly, the prognosis for a pet surviving a toxic exposure to blue green algae is very poor once clinical signs have occurred, so prevention is essential. Don’t let pets drink from or swim in ponds or lakes that are scummy.  If you suspect you or your pets may be affected, rinse with fresh water as soon as possible (don’t let your pet lick themself) and seek immediate veterinary assistance.