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It is not unusual to see a touch of diarrhea or loose stools at this age. Frequently it is due to parasites or protozoan infections such as coccidia and giardia. Deworming on pups needs to be done at least twice two weeks apart to be effective and all fecal matter in the yard has to be cleaned up. In addition often eggs and cysts are in dirt and grass to be ingested again which reinfects the pup. What I do since I do breed is to use a bleach solution on my grass of 30 parts water to 1 part bleach to help kill any eggs, cysts and disease that might be in the yard. This may damage certain types of grass so test a spot first if you do try this. Be sure to keep the dogs off of it until it dries.
For additional information about worms, see this site
Common dewormers are Panacur, which is a prescription product so you need to obtain it from your Vet and it works on roundworms and whipworms. Wormers labeled as Pyrantel Pamoate are the best over the counter medication for roundworms. Drontal plus is also a prescription product and is used for the treatment of Tapeworms, though dewormers with praziquantel work well against tapeworms. Many heartworm preventatives also help control whipworms and roundworms. Another reason to keep your dog on Heartworm medication.
Coccidia manifests as mild or severe diarrhea depending on how infected they are. If it is severe, there may be blood and mucous in the stool along with voming and loss of appetitie leading to dehydration You can read about it here:
Information on giardia can be found on these sites:
Symptoms of giardia are diarrhea with possible terrible smell, mucousy stool and sometimes bouts of vomiting.
Now dogs can also have loose stools as the result of ingesting unusual items but I'd get a stool sample of the pups and have it tested. Since rabies vaccinations are due between three and four months of age, he likely needs that done anyway.
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