The most common cause for a skin rash in dogs is a bacterial skin infection; demodex mites or ringworm (fungal) skin infection are also possible causes. On the belly and armpits, areas with very thin skin and hair, a contact reaction to chemicals on the floor, carpet or bedding is also a possibility. However none of these issues will cause lethargy, or decreased eating/drinking--in a puppy these symptoms can be caused by a viral infection such as parvovirus, or by an intestinal obstruction, and serious dehydration occurs rapidly in such a small animal. I recommend taking your puppy to a veterinarian today for exam and treatment.
Kimberly Coyner, DVM DACVD
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