Is vaginal discharge in a dog normal?

For the same reason a human does. The dog is changing on the inside. Things are being pushed in every direction. The discharge sometimes may be urine. If you are really worried, contact your vet.

狗的阴道分泌物是正常的，但其颜色、质地和量可以因狗的年龄、健康状况和生理周期而有所不同。一般来说，健康的成年雌性狗在发情期间会有阴道分泌物流出，这是其生育周期的一部分。如果分泌物的颜色突然发生变化，或者有异味，量也异常增多，这可能是健康问题的迹象，建议咨询兽医。