Hello,
Based on the details you've provided, I can't really speculate on the possible causes as I don't have enough information. I recommend booking an appointment with your vet so they can check for parasites through a physical exam and further tests. It's only by identifying the specific parasite involved that an effective and appropriate treatment can be chosen.
Furthermore, the symptoms you mentioned aren't specific to a parasitic infection. Even though the lack of worming makes this the most likely theory, you should keep in mind that viruses can also cause these symptoms and can be passed from dog to dog. However, even if it doesn't turn out to be a parasite, it's still best to get both your dogs back on a regular worming routine.
Kind regards,
Quitterie
Translated from French
