Bessie and her bovine companions can do more than provide milk, they can be a major source of infections for your dog. Cows are a possible reservoir for illness such as giardia and leptospirosis, and ponds in their fields can attract potentially infectious wildlife.
The CDC lists giardia as contagious protozoan causing diarrhea. Any mammal can be infected and shed the cysts in its stool. Cryptosporidiosis is another infection caused by a gastrointestinal parasite, which is commonly found in cows. Cryptosporidia is a coccidial infection, causing severe diarrhea and can infect dogs and people.
Wild animals and farm animals are sources for leptospirosis, a bacterial infection that can lead to kidney failure. If your dog drinks the water, gets any in his mouth or has an exposed cut, he could be infected. Run-off from infected animals can accumulate in cow ponds. E. coli is another bacteria that can cause illness in dogs. Your dog can be infected when he goes for a swim in contaminated water.
The layer of green algae covering that pond can trigger illness in your pooch. As this blue-green algae dies, it releases toxins that can trigger gastrointestinal complications, lethargy and seizures. Your dog is considered prone to infections, since the algae can stick to her coat, which she can ingest while cleaning.