Purulent liquid in the dog's urethral orifice may be caused by urethritis, cystitis, urinary tract infection, etc. It is best to take the dog to the pet hospital for examination to confirm the cause and then carry out targeted treatment to avoid delaying the condition. You should pay attention to the hygiene of your dog's reproductive organs and clean them regularly. Pay special attention when your dog is in estrus to avoid bacterial infection and disease.