An 11-week-old 5.8-kg (12.8-lb) sexually intact female Golden Retriever–Poodle crossbred dog was referred for evaluation because of progressive respiratory distress of 5 hours’ duration. Before the onset of respiratory distress, the dog was examined and vaccinated by the referring veterinarian. Vaccinations included a third booster vaccination (SC) against distemper, parvovirus, adenovirus, and coronavirus; an initial vaccination (SC) against