It can be a temptation for pets to take a quick, refreshing drink from a puddle, but harmful materials can lurk in that water, so there are serious reasons not to allow them to do it.
Leptospirosis: Infected animals, including deer and rodents, can be symptom-free while harboring these organisms. They transmit the organisms through their urine, which can end up in puddles through rain runoff. Symptoms include lethargy, vomiting, diarrhea, and loss of appetite, and the liver and lungs can suffer. Testing and vaccinations are available.
Giardia: These microorganisms live in the intestines of domestic animals. If they contract an infected form of it, the animal’s feces will contaminate water. (This is why people are advised to purify water before drinking from sources in nature such as streams and ponds.) Dog parks are places where pets can become infected. Symptoms include diarrhea, vomiting, and loss of appetite. Testing and medications are available.
Antifreeze: Even though companies were forced to add a bittering agent to offset the attractively sweet taste antifreeze has, a very thirsty pet may drink it anyway. They can find it under cars in parking lots or in open antifreeze containers. It is so toxic that just a tiny amount can lead to serious harm and even death. Symptoms include lethargy, vomiting, diarrhea, loss of appetite, weakness and ultimately coma and/or seizures. Unfortunately, many pets don't survive even with treatment.
For these reasons, when you take your pet for a walk, it's a good idea to bring along water and a bowl for when she or he works up a thirst.
Material from Pet Sitters Associates: petsits.com/