If your puppy has mucus in its stool, it can be a sign of irritation of its stomach and large intestine. Mucousy diarrhea can be caused by factors such as sudden changes in diet, eating something unusual like spoiled or rotten food, food allergies or infection by intestinal parasites such as giardia and coccidia. These parasites cause colitis in your puppy and cause it to become sick very quickly upon becoming infected.
The first thing to do for your puppy once you notice mucus in its stool is to take it to the veterinarian, who can determine the cause of the mucus and if it's related to a parasite. Since puppies can die within 24 hours from dehydration of getting diarrhea, it is important to treat giardia and coccidia if these are the causes of the illness. The veterinarian will prescribe proper treatment, which may include a medication that you will give your puppy for up to five days and temporary changes in its diet. You may have to give the puppy's stomach some rest by not feeding it for a day and then giving it foods that will ease the lining of its stomach, such as cooked rice. Make sure your puppy is properly hydrated by giving it water, Pedia-lite or even chicken broth. If your puppy is not drinking on its own, you will have to fill a syringe with Pedia-lite and squeeze it down the side of its mouth. Gradually return your puppy to its normal diet and watch it while it eats. If the puppy doesn't eat, it can be an indicator that it is still sick and may need to go back to the vet. After a few days, check the puppy's stool to make sure it's clear of mucus. If it isn't, take a sample to the veterinarian to be checked.
To prevent your puppy from becoming sick again or contaminating other puppies in the same litter, immediately separate your sick puppy. Place it in a crate by itself and be sure to sanitize the crate regularly. Line the crate with newspapers and change them every time your puppy urinates or passes stool. Before replacing the newspaper, wipe down the crate with a mixture of bleach and water that you can apply with a spray bottle. Wash the puppy's water and food bowls regularly as well. Watch your other puppies and look for signs of dehydration such as a dry nose, sad eyes or fatigue. If there is mucus in their stools or they develop diarrhea, immediately take them to the vet.