Hello and thanks for researching this very important question!
Pets can ingest and develop an intestinal colony of tapeworms in the following ways:
1) Eating an animal that has tapeworms (mice, etc).
2) Swallowing tapeworm segments (when grooming another animal, or when licking the floor, eating out of the litter box, eating dirt containing tapeworm eggs, etc.).
3) Swallowing a FLEA! Fleas are an integral part of the tapeworm lifecycle. Immature fleas eat animal feces, and can transfer tapeworms between animals through their own digestive systems.
If your two pets have had these possible points of contact, the dog should be treated as well.
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