Thanks for your question.
Deworming regimens do not treat pets for every parasite and protozoan that is out there in the environment. This may be why you're having potential issues with parasites even though you have recently dewormed. There are two common dewormers on the market for pets: Pyrantel pamoate and Ivermectin. These dewormers work best against roundworms and hookworms, but do not rid pets of giardia, whipworms and other parasites/protozoan in the environment. Depending on what the fecal says, your vet may wish to add in another medication to help rid one or both pets parasites or protozoan with which they're diagnosed.
If well vaccinated, a 10 year old dog should not acquire parvo from your puppy. Arguably, most dogs have either good enough immunity by 10 through vaccines or by acquiring it. It is said that once a dog acquires parvo, they will not acquire it again and for the most part that rings true. Some research has shown that a low margin of dogs will acquire it again, but have only some soft stool or a few bouts of diarrhea (nothing like the original onset of parvo). In some very rare circumstances, a 10 year old who had not been vaccinated may have avoided parvo for its lifetime through a limited environment and a puppy could potentially pass on the virus (the oldest dog I have ever seen with a confirmed parvo case with typical onset was an 8 year old yorkie who had never been out of her home previously and acquired parvo during a move).
I would recommend holding off on the dewormer unless your vet has directed you to do so. Since he's not feeling well right now anyhow, additional medication given by mouth may cause more vomiting and potentially diarrhea. I would wait until your vet has contacted you with results from the fecal exam before adding any more stress to his body, otherwise he may require future hospitalization.
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