My Yorkie is lethargic, coughing, and sick. When should I see a vet?

Depending on your location, there may be confirmed cases of Canine Influenza in your area. This is a potential cause for the cough. The flu will typically present similar to as it does in people, with a fever, cough, lethargy, inappetance, and often a nasal discharge. It has the potential to progress to pneumonia. There is no specific treatment for the flu, only supportive care and antibiotics if secondary pneumonia develops. I use the rule if a cough does not improve, or worsens, over the course of 48 hours, and is accompanied by lethargy, decreased appetite, difficulty breathing, and vomiting or diarrhea, then the pet should be seen. With the recent outbreaks of Canine Influenza I would certainly try to have him seen by his veterinarian.