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Take your dog to your veterinarian. Bring a sample of your dog’s feces. Allow your veterinarian to examine the fecal sample. Learn about the limitations of fecal examinations.

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As soon as your dog is showing signs of a roundworm infestation, schedule an appointment with your veterinarian. A potbellied appearance will be the most obvious sign of roundworms, so schedule the appointment when you see the potbelly. Your veterinarian will be able to confirm the diagnosis. If your dog defecates outside of your veterinarian's office, pick up and discard the feces immediately to prevent infecting other dogs. Your veterinarian will be able to definitively diagnose a roundworm infection by examining your dog’s feces under a microscope. If you were not able to collect a fecal sample at home, your veterinarian will take a sample during the appointment. Since a single adult worm can produce up to 85,000 eggs per day, a small sample of fecal material should be sufficient to identify roundworms microscopically. Your veterinarian will prepare a fecal solution (also called a ‘fecal flotation’) that will make it easier to detect the roundworm eggs. Under the microscope, roundworm eggs will be large and stout looking. Roundworm eggs have a very characteristic appearance, so it is unlikely to mistake them for another type of worm egg. Although fecal examinations are the primary way to detect roundworms, they are not foolproof. A dog with a roundworm infection does not continuously shed roundworm eggs, so a single fecal sample may not contain any eggs. In addition, if the sample has only a small number eggs, those eggs may not be detectable. Even if no eggs are detected, your veterinarian may still diagnose a roundworm infection, especially if your dog is a puppy and is showing other clinical signs of infection.